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<indexterm><primary>man page</primary><see>reference guide</see></indexterm>
<indexterm class="startofrange" id="man-nmap1-indexterm"><primary>reference guide (man page)</primary></indexterm>
<refentry id='man-nmap1'>
<refmeta>
<refentrytitle>nmap</refentrytitle>
<manvolnum>1</manvolnum>
<refmiscinfo class="source">Nmap</refmiscinfo>
<refmiscinfo class="manual">Nmap Reference Guide</refmiscinfo>
</refmeta>
<refnamediv id='man-name'>
<refname>nmap</refname>
<refpurpose>Network exploration tool and security / port scanner</refpurpose>
</refnamediv>
<!-- body begins here -->
<refsynopsisdiv id='man-synopsis'>
<cmdsynopsis>
<command>nmap</command>
<arg choice='opt' rep='repeat'>
<replaceable>Scan Type</replaceable>
</arg>
<arg choice='opt'>
<replaceable>Options</replaceable>
</arg>
<arg choice='req'>
<replaceable>target specification</replaceable>
</arg>
</cmdsynopsis>
</refsynopsisdiv>
<refsect1 id='man-description'>
<title>Description</title>
<indexterm><primary>Nmap</primary><secondary>description of</secondary></indexterm>
<web>
<note><para>This document describes the very latest version of
Nmap available from <ulink
url="https://nmap.org/download.html" /> or <ulink
url="https://nmap.org/dist/?C=M&amp;O=D" />. Please
ensure you are using the latest version before reporting that a
feature doesn't work as described.</para></note>
</web>
<para>Nmap (<quote>Network Mapper</quote>) is an open source tool for network
exploration and security auditing. It was designed to rapidly
scan large networks, although it works fine against single
hosts. Nmap uses raw IP packets in novel ways to determine what
hosts are available on the network, what services (application
name and version) those hosts are offering, what operating systems
(and OS versions) they are running, what type of packet
filters/firewalls are in use, and dozens of other
characteristics. While Nmap is commonly used for security audits,
many systems and network administrators find it useful for routine
tasks such as network inventory, managing service upgrade
schedules, and monitoring host or service uptime.</para>
<para>The output from Nmap is a list of scanned targets, with
supplemental information on each depending on the options
used. Key among that information is the <quote>interesting ports
table</quote>.<indexterm><primary>ports</primary><secondary sortas="interesting">&ldquo;interesting&rdquo;</secondary></indexterm>
That table lists the port number and protocol,
service name, and state. The state is either
<literal>open</literal>, <literal>filtered</literal>,
<literal>closed</literal>, or <literal>unfiltered</literal>.
<literal>Open</literal><indexterm><primary><literal>open</literal> port state</primary></indexterm>
means that an application on the target machine is listening for
connections/packets on that port.
<literal>Filtered</literal><indexterm><primary><literal>filtered</literal> port state</primary></indexterm>
means that a firewall, filter, or other network
obstacle is blocking the port so that Nmap cannot tell whether it is
<literal>open</literal> or <literal>closed</literal>.
<literal>Closed</literal><indexterm><primary><literal>closed</literal> port state</primary></indexterm>
ports have no application listening on them,
though they could open up at any time.
Ports are classified as
<literal>unfiltered</literal><indexterm><primary><literal>unfiltered</literal> port state</primary></indexterm>
when they are
responsive to Nmap's probes, but Nmap cannot determine whether they are
open or closed.
Nmap reports the state combinations
<literal>open|filtered</literal><indexterm><primary><literal>open|filtered</literal> port state</primary></indexterm>
and <literal>closed|filtered</literal><indexterm><primary><literal>closed|filtered</literal> port state</primary></indexterm>
when it cannot determine which
of the two states describe a port. The port table may also
include software version details when version detection has been
requested. When an IP protocol scan is requested
(<option>-sO</option>), Nmap provides information on supported IP
protocols rather than listening ports.</para>
<para>In addition to the interesting ports table, Nmap can provide
further information on targets, including reverse DNS names,
operating system guesses, device types, and MAC addresses.</para>
<para>A typical Nmap scan is shown in <xref
linkend="man-ex-repscan" xrefstyle="select: label nopage" />. The only Nmap arguments used in
this example are <option>-A</option>, to enable OS and version
detection, script scanning, and traceroute; <option>-T4</option> for
faster execution; and then the hostname.</para>
<example id="man-ex-repscan"><title>A representative Nmap scan</title>
<indexterm><primary><option>-A</option></primary><secondary>example of</secondary></indexterm>
<screen>
# <userinput>nmap -A -T4 scanme.nmap.org</userinput>
Nmap scan report for scanme.nmap.org (74.207.244.221)
Host is up (0.029s latency).
rDNS record for 74.207.244.221: li86-221.members.linode.com
Not shown: 995 closed ports
PORT STATE SERVICE VERSION
22/tcp open ssh OpenSSH 5.3p1 Debian 3ubuntu7 (protocol 2.0)
| ssh-hostkey: 1024 8d:60:f1:7c:ca:b7:3d:0a:d6:67:54:9d:69:d9:b9:dd (DSA)
|_2048 79:f8:09:ac:d4:e2:32:42:10:49:d3:bd:20:82:85:ec (RSA)
80/tcp open http Apache httpd 2.2.14 ((Ubuntu))
|_http-title: Go ahead and ScanMe!
646/tcp filtered ldp
1720/tcp filtered H.323/Q.931
9929/tcp open nping-echo Nping echo
Device type: general purpose
Running: Linux 2.6.X
OS CPE: cpe:/o:linux:linux_kernel:2.6.39
OS details: Linux 2.6.39
Network Distance: 11 hops
Service Info: OS: Linux; CPE: cpe:/o:linux:kernel
TRACEROUTE (using port 53/tcp)
HOP RTT ADDRESS
[Cut first 10 hops for brevity]
11 17.65 ms li86-221.members.linode.com (74.207.244.221)
Nmap done: 1 IP address (1 host up) scanned in 14.40 seconds
</screen>
</example>
<!-- This para is a bit jumbled together for man page rendering reasons -->
<para>The newest version of Nmap can be obtained from
<ulink url="https://nmap.org" />. The newest version of this man page
is available at <ulink url="https://nmap.org/book/man.html"/>.
<notbook>It is also included as a chapter of <web><ulink url="https://nmap.org/book/"><citetitle>Nmap Network Scanning: The Official Nmap Project Guide to Network Discovery and Security Scanning</citetitle></ulink>.</web><notweb><citetitle>Nmap Network Scanning: The Official Nmap Project Guide to Network Discovery and Security Scanning</citetitle> (see <ulink url="https://nmap.org/book/"/>).</notweb></notbook>
</para>
</refsect1>
<refsect1 id='man-briefoptions'>
<title>Options Summary</title>
<para>This options summary is printed when Nmap is run
with no arguments, and the latest version is always available at
<ulink url="https://svn.nmap.org/nmap/docs/nmap.usage.txt" />.
It helps people remember the most common options, but is no
substitute for the in-depth documentation in the rest of this
manual. Some obscure options aren't even included here.</para>
<!-- sortas="#" puts it before the entries that start with '-' in the options
section. -->
<indexterm class="startofrange" id="nmap-usage-indexterm"><primary sortas="#">summary of options</primary></indexterm>
<indexterm class="startofrange" id="nmap-usage-nmap-indexterm"><primary>command-line options</primary><secondary>of Nmap</secondary></indexterm>
&nmap-usage;
<indexterm class="endofrange" startref="nmap-usage-nmap-indexterm"/>
<indexterm class="endofrange" startref="nmap-usage-indexterm"/>
</refsect1>
<refsect1 id='man-target-specification'>
<title>Target Specification</title>
<indexterm><primary>target specification</primary></indexterm>
<para>Everything on the Nmap command-line that isn't an option (or
option argument) is treated as a target host specification. The
simplest case is to specify a target IP address or hostname for scanning.</para>
<para>Sometimes you wish to scan a whole network of adjacent hosts. For
this, Nmap supports CIDR-style<indexterm><primary>CIDR (Classless
Inter-Domain Routing)</primary></indexterm> addressing. You can append
<literal>/<replaceable>numbits</replaceable></literal> to an IPv4
address or hostname and Nmap will scan every IP address for which the
first <replaceable>numbits</replaceable> are the same as for the
reference IP or hostname given. For example,
<literal>192.168.10.0/24</literal> would scan the 256 hosts
between 192.168.10.0
(binary: <literal>11000000 10101000 00001010 00000000</literal>)
and 192.168.10.255
(binary: <literal>11000000 10101000 00001010 11111111</literal>),
inclusive.
<literal>192.168.10.40/24</literal> would scan exactly the same targets. Given
that the host
scanme.nmap.org<indexterm><primary>scanme.nmap.org</primary></indexterm>
is at the IP address 64.13.134.52, the specification
<literal>scanme.nmap.org/16</literal> would scan the 65,536 IP addresses
between 64.13.0.0 and 64.13.255.255. The smallest allowed value is
<literal>/0</literal>, which targets the whole Internet. The largest
value is <literal>/32</literal>, which scans just the named host or IP
address because all address bits are fixed.</para>
<indexterm><primary>address ranges</primary></indexterm>
<para>CIDR notation is short but not always flexible enough. For example, you
might want to scan 192.168.0.0/16 but skip any IPs ending with .0 or
.255 because they may be used as subnet network and broadcast addresses. Nmap supports
this through octet range addressing. Rather than specify a normal IP
address, you can specify a comma-separated list of numbers or ranges
for each octet. For example, <literal>192.168.0-255.1-254</literal> will skip all
addresses in the range that end in .0 or .255, and <literal>192.168.3-5,7.1</literal> will
scan the four addresses 192.168.3.1, 192.168.4.1, 192.168.5.1, and
192.168.7.1. Either side of a range may be omitted; the default values
are 0 on the left and 255 on the right. Using <literal>-</literal> by
itself is the same as <literal>0-255</literal>, but remember to use
<literal>0-</literal> in the first octet
so the target specification doesn't look like a command-line option.
Ranges need not be limited to the final octets: the specifier
<literal>0-255.0-255.13.37</literal> will perform an Internet-wide scan for all IP
addresses ending in 13.37. This sort of broad sampling can be useful
for Internet surveys and research.</para>
<indexterm><primary>IPv6</primary></indexterm>
<para>IPv6 addresses can only be specified by their fully qualified IPv6
address or hostname. CIDR and octet ranges aren't yet supported for
IPv6.</para>
<indexterm><primary>link-local IPv6 address</primary><see>IPv6 address, link-local</see></indexterm>
<indexterm><primary>IPv6 address</primary><secondary>link-local</secondary></indexterm>
<indexterm><primary>zone ID (IPv6 address)</primary></indexterm>
<para>
IPv6 addresses with non-global scope need to have a zone ID suffix. On
Unix systems, this is a percent sign followed by an interface name; a
complete address might be <literal>fe80::a8bb:ccff:fedd:eeff%eth0</literal>.
On Windows, use an interface index number in place of an interface name:
<literal>fe80::a8bb:ccff:fedd:eeff%1</literal>. You can see a list of
interface indexes by running the command
<command>netsh.exe interface ipv6 show interface</command>.
</para>
<para>Nmap accepts multiple host specifications on the command line,
and they don't need to be the same type. The command <command>nmap
scanme.nmap.org 192.168.0.0/8 10.0.0,1,3-7.-</command> does what
you would expect.</para>
<para>While targets are usually specified on the command lines, the following options are also available to control target selection:</para>
<variablelist>
<varlistentry>
<term>
<option>-iL <replaceable>inputfilename</replaceable></option> (Input from list)
<indexterm><primary><option>-iL</option></primary></indexterm>
<indexterm><primary>target specification</primary><secondary>from list</secondary></indexterm>
</term>
<listitem>
<para>Reads target specifications from
<replaceable>inputfilename</replaceable>. Passing a huge
list of hosts is often awkward on the command line, yet it
is a common desire. For example, your DHCP server might
export a list of 10,000 current leases that you wish to
scan. Or maybe you want to scan all IP addresses
<emphasis>except</emphasis> for those to locate hosts using
unauthorized static IP addresses. Simply generate the list
of hosts to scan and pass that filename to Nmap as an
argument to the <option>-iL</option> option. Entries can be
in any of the formats accepted by Nmap on the command line
(IP address, hostname, CIDR, IPv6, or octet ranges). Each
entry must be separated by one or more spaces, tabs, or
newlines. You can specify a hyphen (<literal>-</literal>)
as the filename if you want Nmap to read hosts from standard
input rather than an actual file.</para>
<para>The input file may contain comments that start with
<literal>#</literal> and extend to the end of the line.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-iR <replaceable>num hosts</replaceable></option> (Choose random targets)
<indexterm><primary><option>-iR</option></primary></indexterm>
<indexterm><primary>random targets</primary></indexterm>
<indexterm><primary>target specification</primary><secondary>at random</secondary></indexterm>
</term>
<listitem>
<para>For Internet-wide surveys
and other research, you may want to choose targets at
random. The <replaceable>num hosts</replaceable> argument
tells Nmap how many IPs to generate. Undesirable IPs such
as those in certain private, multicast, or unallocated
address ranges are automatically skipped. The argument <literal>0</literal>
can be specified for a never-ending scan. Keep in mind that
some network administrators bristle at unauthorized scans of
their networks and may complain. Use this option at your
own risk! If you find yourself really bored one rainy
afternoon, try the command
<command>nmap -Pn -sS -p 80 -iR 0 --open</command><indexterm><primary><option>-sS</option></primary><secondary>example of</secondary></indexterm><indexterm><primary><option>-PS</option></primary><secondary>example of</secondary></indexterm><indexterm><primary><option>-iR</option></primary><secondary>example of</secondary></indexterm><indexterm><primary><option>--open</option></primary><secondary>example of</secondary></indexterm>
to locate random web servers for browsing.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--exclude
<replaceable>host1</replaceable><optional>,<replaceable>host2</replaceable><optional>,...</optional></optional></option> (Exclude hosts/networks)
<indexterm significance="preferred"><primary><option>--exclude</option></primary></indexterm>
<indexterm><primary>excluding targets</primary></indexterm>
</term>
<listitem>
<para>Specifies a comma-separated list of targets to be
excluded from the scan even if they are part of the overall
network range you specify. The list you pass in uses normal
Nmap syntax, so it can include hostnames, CIDR netblocks,
octet ranges, etc. This can be useful when the network you
wish to scan includes untouchable mission-critical servers,
systems that are known to react adversely to port scans,
or subnets administered by other people.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--excludefile <replaceable>exclude_file</replaceable></option> (Exclude list from file)
<indexterm significance="preferred"><primary><option>--excludefile</option></primary></indexterm>
</term>
<listitem>
<para>This offers the same functionality as the <option>--exclude</option>
option, except that the excluded targets are provided in a
newline-, space-, or tab-delimited
<replaceable>exclude_file</replaceable> rather than on the
command line.</para>
<para>The exclude file may contain comments that start with
<literal>#</literal> and extend to the end of the line.</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1 id='man-host-discovery'>
<title>Host Discovery</title>
<indexterm class="startofrange" id="man-host-discovery-indexterm"><primary>host discovery</primary></indexterm>
<para>One of the very first steps in any network reconnaissance
mission is to reduce a (sometimes huge) set of IP ranges into a
list of active or interesting hosts. Scanning every port of
every single IP address is slow and usually unnecessary. Of
course what makes a host interesting depends greatly on the
scan purposes. Network administrators may only be interested in
hosts running a certain service, while security auditors may
care about every single device with an IP address. An
administrator may be comfortable using just an ICMP ping to
locate hosts on his internal network, while an external
penetration tester may use a diverse set of dozens of probes in
an attempt to evade firewall restrictions.</para>
<para>Because host discovery needs are so diverse, Nmap offers a
wide variety of options for customizing the techniques used. Host
discovery is sometimes called ping scan, but it goes well beyond
the simple ICMP echo request packets associated with the
ubiquitous <application>ping</application> tool. Users can skip
the ping step entirely with a list scan (<option>-sL</option>) or
by disabling ping (<option>-Pn</option>), or engage the network
with arbitrary combinations of multi-port TCP SYN/ACK, UDP, SCTP
INIT and ICMP probes. The goal of these probes is to solicit
responses which demonstrate that an IP address is actually active
(is being used by a host or network device). On many networks,
only a small percentage of IP addresses are active at any given
time. This is particularly common with private address space
such as 10.0.0.0/8. That network has 16 million IPs, but I have
seen it used by companies with less than a thousand machines. Host
discovery can find those machines in a sparsely allocated sea of
IP addresses.</para>
<para>If no host discovery options are given, Nmap sends an ICMP
echo request, a TCP SYN packet to port 443, a TCP ACK packet to
port 80, and an ICMP timestamp request. (For IPv6, the ICMP
timestamp request is omitted because it is not part of ICMPv6.)
These defaults are
equivalent to the <option>-PE -PS443 -PA80 -PP</option> options.
The exceptions to this are the ARP (for IPv4) and Neighbor
Discovery<indexterm><primary>Neighbor Discovery</primary><secondary>for host discovery</secondary></indexterm>
(for IPv6)
scans which are used for any targets on a local ethernet network.
For unprivileged Unix shell
users, the default probes are a SYN packet to ports 80 and 443 using
the <function>connect</function> system
call.<indexterm><primary>unprivileged users</primary><secondary>limitations of</secondary></indexterm>
This host discovery is often sufficient when scanning local
networks, but a more comprehensive set of discovery probes is
recommended for security auditing.</para>
<para>The <option>-P*</option> options (which select
ping types) can be combined. You can increase your odds of
penetrating strict firewalls by sending many probe types using
different TCP ports/flags and ICMP codes. Also note that
ARP/Neighbor Discovery
(<option>-PR</option>)<indexterm><primary><option>-PR</option></primary></indexterm>
is done by default against
targets on a local ethernet network even if you specify other
<option>-P*</option> options, because it is almost always faster
and more effective.</para>
<para>By default, Nmap does host discovery and then performs a
port scan against each host it determines is online. This is true
even if you specify non-default host discovery types such as UDP
probes (<option>-PU</option>). Read about the
<option>-sn</option> option to learn how to perform
only host discovery, or use <option>-Pn</option> to skip host
discovery and port scan all target hosts. The following options
control host discovery:</para>
<variablelist>
<varlistentry>
<term>
<option>-sL</option> (List Scan)
<indexterm><primary><option>-sL</option></primary></indexterm>
<indexterm><primary>list scan</primary></indexterm>
</term>
<listitem>
<para>The list scan is a degenerate form of host discovery
that simply lists each host of the network(s) specified,
without sending any packets to the target hosts. By
default, Nmap still does reverse-DNS resolution on the hosts
to learn their names. It is often surprising how much
useful information simple hostnames give out. For example,
<literal>fw.chi</literal> is the name of one company's Chicago firewall.
<indexterm><primary>DNS</primary><secondary>records as source of information</secondary></indexterm>
Nmap also reports the total number of
IP addresses at the end. The list scan is a good sanity
check to ensure that you have proper IP addresses for your
targets. If the hosts sport domain names you do not
recognize, it is worth investigating further to prevent
scanning the wrong company's network.</para>
<para>Since the idea is to simply print a list of target
hosts, options for higher level functionality such as port
scanning, OS detection, or ping scanning cannot be combined
with this. If you wish to disable ping scanning while still
performing such higher level functionality, read up on the
<option>-Pn</option> (skip ping) option.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-sn</option> (No port scan)
<indexterm><primary><option>-sn</option></primary></indexterm>
<indexterm><primary>ping scan</primary></indexterm>
<indexterm><primary>port scan</primary><secondary>disabling with <option>-sn</option></secondary></indexterm>
</term>
<listitem>
<para>This option tells Nmap not to do a port scan after host
discovery, and only print out the available hosts that
responded to the host discovery probes. This is often known as a <quote>ping
scan</quote>, but you can also request that traceroute and
NSE host scripts be run. This is by default
one step more intrusive than the list scan, and can often
be used for the same purposes. It allows light
reconnaissance of a target network without attracting much
attention. Knowing how many hosts are up is more valuable
to attackers than the list provided by list scan of every
single IP and host name.</para>
<para>Systems administrators often find this option
valuable as well. It can easily be used to count available
machines on a network or monitor server availability. This
is often called a ping sweep, and is more reliable than
pinging the broadcast address because many hosts do not
reply to broadcast queries.</para>
<para>The default host discovery done with <option>-sn</option> consists of an ICMP echo
request, TCP SYN to port 443, TCP ACK to port 80, and an ICMP
timestamp request by default. When
executed by an unprivileged user, only SYN packets are sent
(using a <function>connect</function> call) to ports 80 and 443 on
the target. When a privileged user tries to scan targets
on a local ethernet network, ARP requests
are used unless
<option>--send-ip</option> was specified.
The <option>-sn</option> option can be combined with any of the
discovery probe types (the <option>-P*</option> options,
excluding <option>-Pn</option>) for greater flexibility.
If any of those probe type and port number options are
used, the default probes are
overridden. When strict firewalls are in place between the
source host running Nmap and the target network, using
those advanced techniques is recommended. Otherwise hosts
could be missed when the firewall drops probes or their
responses.</para>
<para>In previous releases of Nmap, <option>-sn</option> was
known as
<option>-sP</option>.<indexterm><primary><option>-sP</option></primary><seealso><option>-sn</option></seealso></indexterm></para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-Pn</option> (No ping)
<indexterm><primary><option>-Pn</option></primary></indexterm>
<indexterm><primary>host discovery</primary><secondary>disabling</secondary></indexterm>
</term>
<listitem>
<para>This option skips the Nmap discovery stage altogether.
Normally, Nmap uses this stage to determine active machines
for heavier scanning. By default, Nmap only performs heavy
probing such as port scans, version detection, or OS
detection against hosts that are found to be up. Disabling
host discovery with <option>-Pn</option> causes Nmap to
attempt the requested scanning functions against
<emphasis>every</emphasis> target IP address specified. So
if a class B target address space (/16) is specified
on the command line, all 65,536 IP addresses are scanned.
Proper host discovery is skipped as with the list scan, but
instead of stopping and printing the target list, Nmap
continues to perform requested functions as if each target
IP is active. To skip ping scan <emphasis>and</emphasis> port
scan, while still allowing NSE to run, use the two options
<option>-Pn -sn</option> together.</para>
<para>For machines on a local ethernet network, ARP
scanning will still be performed (unless
<option>--disable-arp-ping</option> or <option>--send-ip</option> is specified) because Nmap needs
MAC addresses to further scan target hosts. In previous
versions of Nmap, <option>-Pn</option> was
<option>-P0</option><indexterm><primary><option>-P0</option></primary><seealso><option>-Pn</option></seealso></indexterm>
and
<option>-PN</option>.<indexterm><primary><option>-PN</option></primary><seealso><option>-Pn</option></seealso></indexterm>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-PS <replaceable>port list</replaceable></option> (TCP SYN Ping)
<indexterm><primary><option>-PS</option></primary></indexterm>
<indexterm><primary>SYN ping</primary></indexterm>
</term>
<listitem>
<para>This option sends an empty TCP packet with the SYN
flag set. The default destination port is 80 (configurable
at compile time by changing <varname>DEFAULT_TCP_PROBE_PORT_SPEC</varname><indexterm><primary><varname>DEFAULT_TCP_PROBE_PORT_SPEC</varname></primary></indexterm>
in <filename>nmap.h</filename>).<indexterm><primary><filename>nmap.h</filename></primary></indexterm>
Alternate ports can be
specified as a parameter. The syntax is the same as for the
<option>-p</option> except that port type specifiers like
<literal>T:</literal> are not allowed. Examples are
<option>-PS22</option> and
<option>-PS22-25,80,113,1050,35000</option>. Note that there
can be no space between <option>-PS</option> and the port
list. If multiple probes are specified they will be sent in
parallel.</para>
<para>The SYN flag suggests to the remote system that you
are attempting to establish a connection. Normally the
destination port will be closed, and a RST (reset) packet
sent back. If the port happens to be open, the target will
take the second step of a TCP
three-way-handshake<indexterm><primary>three-way handshake</primary></indexterm>
by responding
with a SYN/ACK TCP packet. The machine running Nmap then
tears down the nascent connection by responding with a RST
rather than sending an ACK packet which would complete the
three-way-handshake and establish a full
connection. The RST packet is sent by the
kernel of the machine running Nmap in response to the
unexpected SYN/ACK, not by Nmap itself.</para>
<para>Nmap does not care whether the port is open or closed.
Either the RST or SYN/ACK response discussed previously tell
Nmap that the host is available and responsive.</para>
<para>On Unix boxes, only the privileged user
<literal>root</literal><indexterm><primary>privileged users</primary></indexterm>
is generally able to send and receive
raw TCP packets.<indexterm><primary>raw packets</primary></indexterm>
For unprivileged users, a
workaround is automatically employed<indexterm><primary>unprivileged users</primary><secondary>limitations of</secondary></indexterm>
whereby the <function>connect</function> system call is initiated against each
target port. This has
the effect of sending a SYN packet to the target host, in an
attempt to establish a connection. If <function>connect</function> returns
with a quick success or an ECONNREFUSED failure, the
underlying TCP stack must have received a SYN/ACK or RST and
the host is marked available. If the connection attempt
is left hanging until a timeout is reached, the host is
marked as down.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-PA <replaceable>port list</replaceable></option> (TCP ACK Ping)
<indexterm><primary><option>-PA</option></primary></indexterm>
<indexterm><primary>ACK ping</primary></indexterm>
</term>
<listitem>
<para>The TCP ACK ping is quite similar to the
just-discussed SYN ping. The difference, as you could
likely guess, is that the TCP ACK flag is set instead of the
SYN flag. Such an ACK packet purports to be acknowledging
data over an established TCP connection, but no such
connection exists. So remote hosts should always respond
with a RST packet, disclosing their existence in the
process.</para>
<para>The <option>-PA</option> option uses the same default
port as the SYN probe (80) and can also take a list of
destination ports in the same format. If an unprivileged
user tries this, the
<function>connect</function> workaround discussed previously is used. This
workaround is imperfect because <function>connect</function> is actually
sending a SYN packet rather than an ACK.</para>
<para>The reason for offering both SYN and ACK ping probes
is to maximize the chances of bypassing firewalls. Many
administrators configure routers and other simple firewalls
to block incoming SYN packets except for those destined for
public services like the company web site or mail server.
This prevents other incoming connections to the
organization, while allowing users to make unobstructed
outgoing connections to the Internet. This non-stateful
approach takes up few resources on the firewall/router and
is widely supported by hardware and software filters. The
Linux Netfilter/iptables<indexterm><primary>iptables</primary></indexterm>
firewall software offers the
<option>--syn</option> convenience option to implement this
stateless approach. When stateless firewall rules such as
this are in place, SYN ping probes (<option>-PS</option>)
are likely to be blocked when sent to closed target ports.
In such cases, the ACK probe shines as it cuts right through
these rules.</para>
<para>Another common type of firewall uses stateful rules
that drop unexpected packets. This feature was initially
found mostly on high-end firewalls, though it has become
much more common over the years. The Linux
Netfilter/iptables system supports this through the
<option>--state</option> option, which categorizes packets
based on connection state. A SYN probe is more likely to
work against such a system, as unexpected ACK packets are
generally recognized as bogus and dropped. A solution to this quandary is
to send both SYN and ACK probes by specifying
<option>-PS</option> and <option>-PA</option>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-PU <replaceable>port list</replaceable></option> (UDP Ping)
<indexterm><primary><option>-PU</option></primary></indexterm>
<indexterm><primary>UDP ping</primary></indexterm>
</term>
<listitem>
<indexterm><primary>payloads, protocol-specific</primary><see>protocol-specific payloads</see></indexterm>
<para>Another host discovery option is the UDP ping, which
sends a UDP packet to the given ports. For most ports, the
packet will be empty, though some use a protocol-specific
payload that is more likely to elicit a
response.
<man>The payload database is described at <ulink url="https://nmap.org/book/nmap-payloads.html" />.</man>
<notman>See <xref linkend="nmap-payloads"/> for a description of the database of payloads.</notman>
<indexterm><primary>protocol-specific
payloads</primary><secondary>UDP</secondary></indexterm>. Packet
content can also be affected with the <option>--data</option>,
<option>--data-string</option>, and <option>--data-length</option> options.</para>
<para>The port list
takes the same format as with the previously discussed
<option>-PS</option> and <option>-PA</option> options. If
no ports are specified, the default is
40125.<indexterm><primary>UDP scan</primary><secondary>default port of</secondary></indexterm>
This default
can be configured at compile-time by changing
<varname>DEFAULT_UDP_PROBE_PORT_SPEC</varname><indexterm><primary><varname>DEFAULT_UDP_PROBE_PORT_SPEC</varname></primary></indexterm>
in <filename>nmap.h</filename>.<indexterm><primary><filename>nmap.h</filename></primary></indexterm>
A highly uncommon port is used by default because sending to
open ports is often undesirable for this particular scan
type.</para>
<para>Upon hitting a closed port on the target machine, the
UDP probe should elicit an ICMP port unreachable packet in
return. This signifies to Nmap that the machine is up and
available. Many other types of ICMP errors, such as
host/network unreachables or TTL exceeded are indicative of
a down or unreachable host. A lack of response is also
interpreted this way. If an open port is reached, most
services simply ignore the empty packet and fail to return
any response. This is why the default probe port is 40125,
which is highly unlikely to be in use. A few services, such
as the Character Generator (chargen) protocol, will respond to an empty UDP packet, and thus
disclose to Nmap that the machine is available.</para>
<para>The primary advantage of this scan type is that it
bypasses firewalls and filters that only screen TCP. For
example, I once owned a Linksys BEFW11S4 wireless broadband
router. The external interface of this device filtered all
TCP ports by default, but UDP probes would still elicit port
unreachable messages and thus give away the device.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-PY <replaceable>port list</replaceable></option> (SCTP INIT Ping)
<indexterm><primary><option>-PY</option></primary></indexterm>
<indexterm><primary>SCTP INIT ping</primary></indexterm>
</term>
<listitem>
<para>This option sends an SCTP packet containing a minimal
INIT chunk. The default destination port is 80 (configurable
at compile time by changing
<varname>DEFAULT_SCTP_PROBE_PORT_SPEC</varname><indexterm><primary><varname>DEFAULT_SCTP_PROBE_PORT_SPEC</varname></primary></indexterm>
in <filename>nmap.h</filename>).
Alternate ports can be specified as a parameter. The syntax
is the same as for the
<option>-p</option> except that port type specifiers like
<literal>S:</literal> are not allowed. Examples are
<option>-PY22</option> and
<option>-PY22,80,179,5060</option>. Note that there
can be no space between <option>-PY</option> and the port
list. If multiple probes are specified they will be sent in
parallel.</para>
<para>The INIT chunk suggests to the remote system that you
are attempting to establish an association. Normally the
destination port will be closed, and an ABORT chunk will be
sent back. If the port happens to be open, the target will
take the second step of an SCTP
four-way-handshake<indexterm><primary>four-way handshake</primary></indexterm>
by responding with an INIT-ACK chunk. If the machine running
Nmap has a functional SCTP stack, then it tears down the
nascent association by responding with an ABORT chunk rather
than sending a COOKIE-ECHO chunk which would be the next step
in the four-way-handshake. The ABORT packet is sent by the
kernel of the machine running Nmap in response to the
unexpected INIT-ACK, not by Nmap itself.</para>
<para>Nmap does not care whether the port is open or closed.
Either the ABORT or INIT-ACK response discussed previously tell
Nmap that the host is available and responsive.</para>
<para>On Unix boxes, only the privileged user
<literal>root</literal><indexterm><primary>privileged users</primary></indexterm>
is generally able to send and receive raw SCTP
packets.<indexterm><primary>raw packets</primary></indexterm>
Using SCTP INIT Pings is currently not possible for unprivileged
users.<indexterm><primary>unprivileged users</primary><secondary>limitations of</secondary></indexterm>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-PE</option>;
<option>-PP</option>;
<option>-PM</option> (ICMP Ping Types)
<indexterm><primary><option>-PE</option></primary></indexterm>
<indexterm><primary><option>-PP</option></primary></indexterm>
<indexterm><primary><option>-PM</option></primary></indexterm>
<indexterm><primary>ICMP ping</primary></indexterm>
</term>
<listitem>
<para>In addition to the unusual TCP, UDP and SCTP host
discovery types discussed previously, Nmap can send the
standard packets sent by the ubiquitous
<application>ping</application> program. Nmap sends an ICMP
type 8 (echo request) packet to the target IP addresses,
expecting a type 0 (echo reply) in return from available
hosts.<indexterm><primary>ICMP echo</primary></indexterm>
Unfortunately for network explorers, many hosts and
firewalls now block these packets, rather than responding as
required by <ulink
role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc1122.txt">RFC
1122</ulink>.<indexterm><primary>RFC 1122</primary></indexterm>
For this reason, ICMP-only scans are rarely
reliable enough against unknown targets over the Internet.
But for system administrators monitoring an internal
network, they can be a practical and efficient approach.
Use the <option>-PE</option> option to enable this echo
request behavior.</para>
<para>While echo request is the standard ICMP ping query,
Nmap does not stop there. The ICMP standards
(<ulink role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc792.txt">RFC 792</ulink><indexterm><primary>RFC 792</primary></indexterm>
and
<ulink role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc950.txt">RFC 950</ulink><indexterm><primary>RFC 950</primary></indexterm>
)
also specify timestamp request, information
request, and address mask request packets as codes 13, 15,
and 17, respectively. While the ostensible purpose for
these queries is to learn information such as address masks
and current times, they can easily be used for host
discovery. A system that replies is up and available. Nmap
does not currently implement information request packets, as
they are not widely supported. RFC 1122 insists that
<quote>a host SHOULD NOT implement these messages</quote>.
Timestamp and address mask queries can be sent with the
<option>-PP</option> and <option>-PM</option> options,
respectively. A timestamp reply (ICMP code 14) or address
mask reply (code 18) discloses that the host is available.
These two queries can be valuable when administrators specifically
block echo request packets while forgetting that other ICMP
queries can be used for the same purpose.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-PO <replaceable>protocol list</replaceable></option> (IP Protocol Ping)
<indexterm><primary><option>-PO</option></primary></indexterm>
<indexterm><primary>IP protocol ping</primary></indexterm>
</term>
<listitem>
<para>One of the newer host discovery options is the IP protocol ping,
which sends IP packets with the specified protocol number
set in their IP header. The protocol list
takes the same format as do port lists in the
previously discussed TCP, UDP and SCTP host discovery options.
If no protocols are specified, the default is to send multiple
IP packets for ICMP (protocol 1), IGMP (protocol 2), and
IP-in-IP (protocol 4). The default protocols can be
configured at compile-time by changing
<varname>DEFAULT_PROTO_PROBE_PORT_SPEC</varname><indexterm><primary><varname>DEFAULT_PROTO_PROBE_PORT_SPEC</varname></primary></indexterm>
in <filename>nmap.h</filename>.
Note that for the ICMP, IGMP, TCP (protocol 6), UDP
(protocol 17) and SCTP (protocol 132), the packets are sent
with the proper protocol
headers<indexterm><primary>protocol-specific payloads</primary><secondary>IP</secondary></indexterm>
while other protocols are
sent with no additional data beyond the IP header (unless any of
<option>--data</option>,
<option>--data-string</option>, or
<option>--data-length</option>
options are specified).</para>
<para>This host discovery method looks for either responses
using the same protocol as a probe, or ICMP protocol
unreachable messages which signify that the given protocol
isn't supported on the destination host. Either type of
response signifies that the target host is alive.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-PR</option> (ARP Ping)
<indexterm><primary><option>-PR</option></primary></indexterm>
<indexterm><primary>ARP ping</primary></indexterm>
</term>
<listitem>
<para>One of the most common Nmap usage scenarios is to scan
an ethernet LAN. On most LANs, especially those using
private address ranges specified by <ulink role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc1918.txt">RFC 1918</ulink>, the vast majority of
IP addresses are unused at any given time. When Nmap tries
to send a raw IP packet such as an ICMP echo request, the
operating system must determine the destination hardware
(ARP) address corresponding to the target IP so that it can
properly address the ethernet frame. This is often slow and
problematic, since operating systems weren't written with
the expectation that they would need to do millions of ARP
requests against unavailable hosts in a short time
period.</para>
<para>ARP scan puts Nmap and its optimized algorithms in
charge of ARP requests. And if it gets a response back,
Nmap doesn't even need to worry about the IP-based ping
packets since it already knows the host is up. This makes
ARP scan much faster and more reliable than IP-based scans.
So it is done by default when scanning ethernet hosts that Nmap
detects are on a local ethernet network. Even if different
ping types (such as <option>-PE</option> or
<option>-PS</option>) are specified, Nmap uses ARP instead
for any of the targets which are on the same LAN. If you
absolutely don't want to do an ARP scan, specify
<option>--disable-arp-ping</option>.</para>
<para>For IPv6 (-6 option), <option>-PR</option> uses ICMPv6
Neighbor Discovery instead of ARP. Neighbor Discovery, defined
in RFC 4861, can be seen as the IPv6 equivalent of ARP.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--disable-arp-ping</option> (No ARP or ND Ping)
<indexterm><primary><option>--disable-arp-ping</option></primary></indexterm>
</term>
<listitem>
<para>Nmap normally does ARP or IPv6 Neighbor Discovery (ND)
discovery of locally connected ethernet hosts, even if other
host discovery options such as <option>-Pn</option> or
<option>-PE</option> are used. To disable this implicit
behavior, use the <option>--disable-arp-ping</option>
option.</para>
<para>The default behavior is normally faster, but this option
is useful on networks using proxy ARP, in which a router
speculatively replies to all ARP requests, making every target
appear to be up according to ARP scan.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--traceroute</option> (Trace path to host)
<indexterm significance="preferred"><primary><option>--traceroute</option></primary></indexterm>
<indexterm significance="preferred"><primary>traceroute</primary></indexterm>
</term>
<listitem>
<para>
Traceroutes are performed post-scan using information from the scan results to determine the port and protocol most likely to reach the target. It works with all scan types except connect scans (<option>-sT</option>) and idle scans (<option>-sI</option>). All traces use Nmap's dynamic timing model and are performed in parallel.
</para>
<para>
Traceroute works by sending packets with a low TTL (time-to-live) in an attempt to elicit ICMP Time Exceeded messages from intermediate hops between the scanner and the target host. Standard traceroute implementations start with a TTL of 1 and increment the TTL until the destination host is reached. Nmap's traceroute starts with a high TTL and then decrements the TTL until it reaches zero. Doing it backwards lets Nmap employ clever caching algorithms to speed up traces over multiple hosts. On average Nmap sends 5&ndash;10 fewer packets per host, depending on network conditions. If a single subnet is being scanned (i.e. 192.168.0.0/24) Nmap may only have to send two packets to most hosts.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-n</option> (No DNS resolution)
<indexterm><primary><option>-n</option></primary></indexterm>
</term>
<listitem>
<para>Tells Nmap to <emphasis>never</emphasis> do reverse
DNS
<indexterm><primary>reverse DNS</primary><secondary>disabling with <option>-n</option></secondary></indexterm>
resolution on the active IP addresses it finds. Since
DNS can be slow even with Nmap's built-in parallel stub
resolver, this option can slash scanning times.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-R</option> (DNS resolution for all targets)
<indexterm><primary><option>-R</option></primary></indexterm>
</term>
<listitem>
<para>Tells Nmap to
<emphasis>always</emphasis> do reverse DNS
resolution on the target IP addresses. Normally reverse DNS is
only performed against responsive (online) hosts.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--system-dns</option> (Use system DNS resolver)
<indexterm significance="preferred"><primary><option>--system-dns</option></primary></indexterm>
</term>
<listitem>
<para>By default, Nmap resolves IP addresses by sending
queries directly to the name servers configured on your host
and then listening for responses. Many requests (often
dozens) are performed in parallel to improve performance.
Specify this option to use your system resolver instead (one
IP at a time via the <function>getnameinfo</function> call). This is slower
and rarely useful unless you find a bug in the Nmap parallel
resolver (please let us know if you do). The system
resolver is always used for IPv6 scans.
<indexterm><primary>IPv6</primary><secondary>limitations of</secondary></indexterm>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--dns-servers <replaceable>server1</replaceable><optional>,<replaceable>server2</replaceable><optional>,...</optional></optional>
</option> (Servers to use for reverse DNS queries)
<indexterm significance="preferred"><primary><option>--dns-servers</option></primary></indexterm>
</term>
<listitem>
<para>By default, Nmap determines your DNS servers
(for rDNS resolution) from your resolv.conf file (Unix) or
the Registry (Win32). Alternatively, you may use this
option to specify alternate servers. This option is not
honored if you are using <option>--system-dns</option> or an
IPv6 scan. Using multiple DNS servers is often faster,
especially if you choose authoritative servers for your
target IP space. This option can also improve stealth, as
your requests can be bounced off just about any recursive
DNS server on the Internet.</para>
<para>This option also comes in handy when scanning private
networks. Sometimes only a few name servers provide
proper rDNS information, and you may not even know where
they are. You can scan the network for port 53 (perhaps
with version detection), then try Nmap list scans
(<option>-sL</option>) specifying each name server one at a
time with <option>--dns-servers</option> until you find one
which works.</para>
</listitem>
</varlistentry>
</variablelist>
<indexterm class="endofrange" startref="man-host-discovery-indexterm"/>
</refsect1>
<refsect1 id='man-port-scanning-basics'>
<title>Port Scanning Basics</title>
<para>While Nmap has grown in functionality over the years,
it began as an efficient port scanner, and that remains its
core function. The simple command <command>nmap
<replaceable>target</replaceable></command> scans
1,000 TCP ports on the host
<replaceable>target</replaceable>. While many port scanners
have traditionally lumped all ports into the open or closed
states, Nmap is much more granular. It divides ports into
six states: <literal>open</literal>,
<literal>closed</literal>, <literal>filtered</literal>,
<literal>unfiltered</literal>,
<literal>open|filtered</literal>, or
<literal>closed|filtered</literal>.</para>
<para>These states are not intrinsic
properties of the port itself, but describe how Nmap sees them. For
example, an Nmap scan from the same network as the target may show
port <literal>135/tcp</literal> as open, while a scan at the same time with the same
options from across the Internet might show that port as <literal>filtered</literal>.</para>
<variablelist><title>The six port states recognized by Nmap</title>
<varlistentry><term>
<indexterm><primary><literal>open</literal> port state</primary></indexterm>
open</term>
<listitem><para>An application is actively accepting TCP
connections, UDP datagrams or SCTP associations on this port.
Finding these is often the primary goal of port scanning.
Security-minded people know that each open port is an avenue for attack.
Attackers and pen-testers want to exploit the open ports, while
administrators try to close or protect them with firewalls without
thwarting legitimate users.
Open ports are also interesting for non-security scans because they show
services available for use on the network.
</para></listitem></varlistentry>
<varlistentry><term>
<indexterm><primary><literal>closed</literal> port state</primary></indexterm>
closed</term>
<listitem><para>A closed port is accessible (it receives and
responds to Nmap probe packets), but there is no application
listening on it. They can be helpful in showing that a host is up
on an IP address (host discovery, or ping scanning), and as part
of OS detection. Because closed ports are reachable, it may be
worth scanning later in case some open up. Administrators may want
to consider blocking such ports with a firewall. Then they would
appear in the filtered state, discussed next.
</para></listitem></varlistentry>
<varlistentry><term>
<indexterm><primary><literal>filtered</literal> port state</primary></indexterm>
filtered</term>
<listitem><para>Nmap cannot determine whether the port is open
because packet filtering prevents its probes from reaching the port.
The filtering could be from a dedicated firewall device, router
rules, or host-based firewall software. These ports frustrate
attackers because they provide so little information. Sometimes
they respond with ICMP error messages such as type 3 code 13
(destination unreachable: communication administratively
prohibited), but filters that simply drop probes without responding
are far more common. This forces Nmap to retry several times just
in case the probe was dropped due to network congestion rather than
filtering. This slows down the scan dramatically.</para></listitem></varlistentry>
<varlistentry><term>
<indexterm><primary><literal>unfiltered</literal> port state</primary></indexterm>
unfiltered</term>
<listitem><para>The unfiltered state means that a port is accessible,
but Nmap is unable to determine whether it is open or closed. Only
the ACK scan, which is used to map firewall rulesets, classifies
ports into this state. Scanning unfiltered ports with other scan
types such as Window scan, SYN scan, or FIN scan, may help resolve
whether the port is open.
</para></listitem></varlistentry>
<varlistentry><term>
<indexterm><primary><literal>open|filtered</literal> port state</primary></indexterm>
open|filtered</term>
<listitem><para>Nmap places ports in this state when it is unable to
determine whether a port is open or filtered. This occurs for scan
types in which open ports give no response. The lack of
response could also mean that a packet filter dropped the probe or
any response it elicited. So Nmap does not know for sure whether
the port is open or being filtered. The UDP, IP protocol,
FIN, NULL, and Xmas scans classify ports this
way.</para></listitem></varlistentry>
<varlistentry><term>
<indexterm><primary><literal>closed|filtered</literal> port state</primary></indexterm>
closed|filtered</term>
<listitem><para>This state is used when Nmap is unable to determine
whether a port is closed or filtered. It is only used for the IP ID
idle scan.</para></listitem></varlistentry>
</variablelist>
</refsect1>
<refsect1 id='man-port-scanning-techniques'>
<title>Port Scanning Techniques</title>
<para>As a novice performing automotive repair, I can struggle
for hours trying to fit my rudimentary tools (hammer, duct tape,
wrench, etc.) to the task at hand. When I fail miserably and tow my
jalopy to a real mechanic, he invariably fishes around in a huge tool chest until
pulling out the perfect gizmo which makes the job seem effortless. The
art of port scanning is similar. Experts understand the dozens of
scan techniques and choose the appropriate one (or combination) for a
given task. Inexperienced users and
script kiddies,<indexterm><primary>script kiddies</primary></indexterm>
on the other
hand, try to solve every problem with the default SYN scan. Since Nmap is
free, the only barrier to port scanning mastery is knowledge. That
certainly beats the automotive world, where it may take great skill to
determine that you need a strut spring compressor, then you still
have to pay thousands of dollars for it.</para>
<para>Most of the scan types are only available to
privileged users.<indexterm><primary>privileged users</primary></indexterm>
This is because they send and receive
raw packets,<indexterm><primary>raw packets</primary></indexterm>
which requires root
access on Unix systems. Using an administrator account on Windows is
recommended, though Nmap sometimes works for unprivileged users on that
platform when WinPcap has already been loaded into the OS. Requiring
root privileges was a serious limitation when Nmap was released in
1997, as many users only had access to shared shell accounts. Now,
the world is different. Computers are cheaper, far more people have
always-on direct Internet access, and desktop Unix systems (including
Linux and Mac OS X) are prevalent. A Windows version of Nmap is now
available, allowing it to run on even more desktops. For all these
reasons, users have less need to run Nmap from limited shared shell accounts.
This is fortunate, as the privileged options make Nmap far more
powerful and flexible.</para>
<para>While Nmap attempts to produce accurate results, keep in mind
that all of its insights are based on packets returned by the target
machines (or firewalls in front of them). Such hosts may be
untrustworthy and send responses intended to confuse or mislead Nmap.
Much more common are non-RFC-compliant hosts that do not respond as
they should to Nmap probes. FIN, NULL, and Xmas scans are
particularly susceptible to this problem. Such issues are specific to
certain scan types and so are
discussed in the individual scan type entries.</para>
<para>This section documents the dozen or so port scan
techniques supported by Nmap. Only one method may be used at a time,
except that UDP scan (<option>-sU</option>) and any one of the
SCTP scan types (<option>-sY</option>, <option>-sZ</option>)
may be combined with any one of the TCP scan types.
As a memory aid, port scan type options
are of the form <option>-s<replaceable>C</replaceable></option>, where
<replaceable>C</replaceable> is a prominent character in the scan
name, usually the first. The one exception to this is the deprecated
FTP bounce scan (<option>-b</option>). By default, Nmap performs a
SYN Scan, though it substitutes a connect scan if the user does not
have proper privileges to send raw packets (requires root access on
Unix). Of the scans listed in this
section, unprivileged users can only execute connect and FTP bounce
scans.</para>
<variablelist>
<varlistentry>
<term>
<option>-sS</option> (TCP SYN scan)
<indexterm><primary><option>-sS</option></primary></indexterm>
<indexterm><primary>SYN scan</primary></indexterm>
</term>
<listitem>
<para>SYN scan is the default and most popular scan option for good
reasons. It can be performed quickly, scanning thousands of ports per
second on a fast network not hampered by restrictive firewalls. It is also
relatively unobtrusive and stealthy since it never completes TCP
connections. SYN scan works against any compliant TCP stack rather
than depending on idiosyncrasies of specific platforms as Nmap's
FIN/NULL/Xmas, Maimon and idle scans do. It also allows clear,
reliable differentiation between the <literal>open</literal>,
<literal>closed</literal>, and <literal>filtered</literal>
states.</para>
<para>This technique is often referred to as half-open scanning,
because you don't open a full TCP connection. You send a SYN packet,
as if you are going to open a real connection and then wait for a
response. A SYN/ACK indicates the port is listening (open), while a
RST (reset) is indicative of a non-listener. If no response is
received after several retransmissions, the port is marked as
filtered. The port is also marked filtered if an ICMP unreachable
error (type 3, code 0, 1, 2, 3, 9, 10, or 13) is received. The port is also considered open if a SYN packet (without the ACK flag) is received in response. This can be due to an extremely rare TCP feature known as a simultaneous open or split handshake connection (see <ulink url="https://nmap.org/misc/split-handshake.pdf"/>).</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-sT</option> (TCP connect scan)
<indexterm><primary><option>-sT</option></primary></indexterm>
<indexterm><primary>connect scan</primary></indexterm>
</term>
<listitem>
<para>TCP connect scan is the default TCP scan type when SYN scan is
not an option. This is the case when a user does not have raw packet
privileges. Instead of writing raw
packets as most other scan types do, Nmap asks the underlying
operating system to establish a connection with the target machine and
port by issuing the <function>connect</function> system call. This is
the same high-level system call that web browsers, P2P clients, and
most other network-enabled applications use to establish a connection.
It is part of a programming interface known as the Berkeley Sockets
API. Rather than read raw packet responses off the wire, Nmap uses
this API to obtain status information on each connection attempt.
</para>
<para>When SYN scan is available, it is usually a better choice. Nmap
has less control over the high level <function>connect</function> call
than with raw packets, making it less efficient. The system call
completes connections to open target ports rather than performing the
half-open reset that SYN scan does. Not only does this take longer
and require more packets to obtain the same information, but target
machines are more likely to log the connection. A decent IDS will
catch either, but most machines have no such alarm system. Many
services on your average Unix system will add a note to syslog, and
sometimes a cryptic error message, when Nmap connects and then closes
the connection without sending data. Truly pathetic services crash
when this happens, though that is uncommon. An administrator who sees
a bunch of connection attempts in her logs from a single system should
know that she has been connect scanned.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-sU</option> (UDP scans)
<indexterm><primary><option>-sU</option></primary></indexterm>
<indexterm><primary>UDP scan</primary></indexterm>
</term>
<listitem>
<para>While most popular services on the Internet run over the TCP
protocol, <ulink
role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc768.txt">UDP</ulink> services
are widely deployed. DNS, SNMP, and DHCP
(registered ports 53, 161/162, and 67/68) are three of the most
common. Because UDP scanning is generally slower and more difficult
than TCP, some security auditors ignore these ports. This is a mistake, as
exploitable UDP services are quite common and attackers certainly
don't ignore the whole protocol. Fortunately, Nmap can help inventory
UDP ports.</para>
<para>UDP scan is activated with the <option>-sU</option> option. It
can be combined with a TCP scan type such as SYN scan
(<option>-sS</option>) to check both protocols during the same
run.</para>
<para>UDP scan works by sending a UDP packet to every
targeted port. For some common ports such as 53 and 161, a
protocol-specific payload is sent to increase response rate, but for most ports the packet is
empty unless the <option>--data</option>,
<option>--data-string</option>, or <option>--data-length</option>
options are specified.
If an ICMP port unreachable error (type 3, code 3) is
returned, the port is <literal>closed</literal>. Other ICMP unreachable errors (type 3,
codes 0, 1, 2, 9, 10, or 13) mark the port as <literal>filtered</literal>. Occasionally, a
service will respond with a UDP packet, proving that it is <literal>open</literal>. If
no response is received after retransmissions, the port is classified
as <literal>open|filtered</literal>. This means that the port could be open, or perhaps
packet filters are blocking the communication. Version detection
(<option>-sV</option>) can be used to help differentiate the truly
open ports from the filtered ones.</para>
<para>A big challenge with UDP scanning is doing it quickly.
Open and filtered ports rarely send any response, leaving Nmap to time
out and then conduct retransmissions just in case the probe or
response were lost. Closed ports are often an even bigger problem.
They usually send back an ICMP port unreachable error. But unlike the
RST packets sent by closed TCP ports in response to a SYN or connect
scan, many hosts rate limit<indexterm><primary>rate limiting</primary></indexterm>
ICMP port unreachable messages by default.
Linux and Solaris are particularly strict about this. For example, the
Linux 2.4.20 kernel limits destination unreachable messages to one per
second (in <filename>net/ipv4/icmp.c</filename>).</para>
<para>Nmap detects rate limiting and slows down accordingly to avoid
flooding the network with useless packets that the target machine will
drop. Unfortunately, a Linux-style limit of one packet per second
makes a 65,536-port scan take more than 18 hours. Ideas for speeding
your UDP scans up include scanning more hosts in parallel, doing a
quick scan of just the popular ports first, scanning from behind the
firewall, and using <option>--host-timeout</option> to skip slow
hosts.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-sY</option> (SCTP INIT scan)
<indexterm><primary><option>-sY</option></primary></indexterm>
<indexterm><primary>SCTP INIT scan</primary></indexterm>
</term>
<listitem>
<para>
<ulink role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc4960.txt">SCTP</ulink>
is a relatively new alternative to the TCP and UDP protocols,
combining most characteristics of TCP and UDP, and also adding
new features like multi-homing and multi-streaming. It is mostly
being used for SS7/SIGTRAN related services but has the potential
to be used for other applications as well.
SCTP INIT scan is the SCTP equivalent of a TCP SYN scan.
It can be performed quickly, scanning thousands of ports per
second on a fast network not hampered by restrictive firewalls.
Like SYN scan, INIT scan is relatively unobtrusive and stealthy,
since it never completes SCTP associations. It also allows clear,
reliable differentiation between the <literal>open</literal>,
<literal>closed</literal>, and <literal>filtered</literal>
states.</para>
<para>This technique is often referred to as half-open scanning,
because you don't open a full SCTP association. You send an INIT
chunk, as if you are going to open a real association and then wait
for a response. An INIT-ACK chunk indicates the port is listening
(open), while an ABORT chunk is indicative of a non-listener. If no
response is received after several retransmissions, the port is
marked as filtered. The port is also marked filtered if an ICMP
unreachable error (type 3, code 0, 1, 2, 3, 9, 10, or 13) is
received.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-sN</option>; <option>-sF</option>; <option>-sX</option> (TCP NULL, FIN, and Xmas scans)
<indexterm><primary><option>-sN</option></primary></indexterm>
<indexterm><primary><option>-sF</option></primary></indexterm>
<indexterm><primary><option>-sX</option></primary></indexterm>
<indexterm><primary>NULL scan</primary></indexterm>
<indexterm><primary>FIN scan</primary></indexterm>
<indexterm><primary>Xmas scan</primary></indexterm>
</term>
<listitem>
<para>These three scan types (even more are possible with the
<option>--scanflags</option> option described in the next section)
exploit a subtle loophole in the <ulink
role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc793.txt">TCP RFC</ulink> to
differentiate between <literal>open</literal> and
<literal>closed</literal> ports. Page 65 of RFC 793 says that <quote>if the
[destination] port state is CLOSED .... an incoming segment not
containing a RST causes a RST to be sent in response.</quote> Then the next
page discusses packets sent to open ports without the SYN, RST, or ACK
bits set, stating that: <quote>you are unlikely to get here, but if you do, drop the
segment, and return.</quote></para>
<para>When scanning systems compliant with this RFC text, any packet
not containing SYN, RST, or ACK bits will result in a returned RST if
the port is closed and no response at all if the port is open. As
long as none of those three bits are included, any combination of the
other three (FIN, PSH, and URG) are OK. Nmap exploits this with three
scan types:</para>
<variablelist>
<varlistentry><term>Null scan (<option>-sN</option>)</term>
<listitem><para>Does not set any bits (TCP flag header is 0)</para></listitem></varlistentry>
<varlistentry><term>FIN scan (<option>-sF</option>)</term>
<listitem><para>Sets just the TCP FIN bit.</para></listitem></varlistentry>
<varlistentry><term>Xmas scan (<option>-sX</option>)</term>
<listitem><para>Sets the FIN, PSH, and URG flags, lighting the
packet up like a Christmas tree.</para></listitem></varlistentry>
</variablelist>
<para>These three scan types are exactly the same in behavior except
for the TCP flags set in probe packets. If a RST packet is received,
the port is considered <literal>closed</literal>, while no response
means it is <literal>open|filtered</literal>. The port is marked
<literal>filtered</literal> if an ICMP unreachable error (type 3, code
0, 1, 2, 3, 9, 10, or 13) is received.</para>
<para>The key advantage to these scan types is that they can sneak
through certain non-stateful firewalls and packet filtering
routers. Another advantage is that these scan types are a little more
stealthy than even a SYN scan. Don't count on this though&mdash;most
modern IDS products can be configured to detect them. The big
downside is that not all systems follow RFC 793 to the letter. A
number of systems send RST responses to the probes regardless of
whether the port is open or not. This causes all of the ports to be
labeled <literal>closed</literal>. Major operating systems that do
this are Microsoft Windows, many Cisco devices, BSDI, and IBM OS/400.
This scan does work against most Unix-based systems though. Another
downside of these scans is that they can't distinguish <literal>open</literal> ports from
certain <literal>filtered</literal> ones, leaving you with the response
<literal>open|filtered</literal>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-sA</option> (TCP ACK scan)
<indexterm><primary><option>-sA</option></primary></indexterm>
<indexterm><primary>ACK scan</primary></indexterm>
</term>
<listitem>
<para>This scan is different than the others discussed so far in that
it never determines <literal>open</literal> (or even
<literal>open|filtered</literal>) ports. It is used to map out
firewall rulesets, determining whether they are stateful or not and
which ports are filtered.</para>
<para>The ACK scan probe packet has only the ACK flag set (unless you
use <option>--scanflags</option>). When scanning unfiltered systems,
<literal>open</literal> and <literal>closed</literal> ports will both
return a RST packet. Nmap then labels them as
<literal>unfiltered</literal>, meaning that they are reachable by the
ACK packet, but whether they are <literal>open</literal> or
<literal>closed</literal> is undetermined. Ports that don't respond,
or send certain ICMP error messages back (type 3, code 0, 1, 2, 3, 9, 10,
or 13), are labeled <literal>filtered</literal>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-sW</option> (TCP Window scan)
<indexterm><primary><option>-sW</option></primary></indexterm>
<indexterm><primary>window scan</primary></indexterm>
</term>
<listitem>
<para>Window scan is exactly the same as ACK scan except that it
exploits an implementation detail of certain systems to differentiate
open ports from closed ones, rather than always printing
<literal>unfiltered</literal> when a RST is returned. It does this by
examining the TCP Window field of the RST packets returned. On some
systems, open ports use a positive window size (even for RST packets)
while closed ones have a zero window. So instead of always listing a
port as <literal>unfiltered</literal> when it receives a RST back,
Window scan lists the port as <literal>open</literal> or
<literal>closed</literal> if the TCP Window value in that reset is
positive or zero, respectively.</para>
<para>This scan relies on an implementation detail of a minority of
systems out on the Internet, so you can't always trust it. Systems
that don't support it will usually return all ports
<literal>closed</literal>. Of course, it is possible that the machine
really has no open ports. If most scanned ports are
<literal>closed</literal> but a few common port numbers (such as 22,
25, 53) are <literal>filtered</literal>, the system is most likely
susceptible. Occasionally, systems will even show the exact opposite
behavior. If your scan shows 1,000 open ports and three closed or filtered
ports, then those three may very well be the truly open ones.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-sM</option> (TCP Maimon scan)
<indexterm><primary><option>-sM</option></primary></indexterm>
<indexterm><primary>Maimon scan</primary></indexterm>
</term>
<listitem>
<para>The Maimon scan is named after its discoverer,
Uriel Maimon.<indexterm><primary>Maimon, Uriel</primary></indexterm>
He described the technique in
<citetitle>Phrack</citetitle> Magazine issue #49 (November 1996).<indexterm><primary><citetitle>Phrack</citetitle></primary></indexterm>
Nmap, which included this technique, was released two issues later.
This technique is exactly the same as NULL, FIN, and Xmas scans, except
that the probe is FIN/ACK. According to <ulink role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc793.txt">RFC 793</ulink> (TCP), a RST packet
should be generated in response to such a probe whether the port is
open or closed. However, Uriel noticed that many BSD-derived systems
simply drop the packet if the port is open.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--scanflags</option> (Custom TCP scan)
<indexterm><primary><option>--scanflags</option></primary></indexterm></term>
<listitem>
<para>Truly advanced Nmap users need not limit themselves to the
canned scan types offered. The <option>--scanflags</option> option allows
you to design your own scan by specifying arbitrary
TCP flags.<indexterm><primary>TCP flags</primary></indexterm>
Let your creative juices flow, while evading
intrusion detection systems<indexterm><primary>intrusion detection systems</primary><secondary>evading</secondary></indexterm>
whose vendors simply paged through the Nmap man page adding specific rules!</para>
<para>The <option>--scanflags</option> argument can be a numerical
flag value such as 9 (PSH and FIN), but using symbolic names is
easier. Just mash together any combination of <literal>URG</literal>,
<literal>ACK</literal>, <literal>PSH</literal>,
<literal>RST</literal>, <literal>SYN</literal>, and
<literal>FIN</literal>. For example, <option>--scanflags
URGACKPSHRSTSYNFIN</option> sets everything, though it's not very
useful for scanning. The order these are specified in is
irrelevant.</para>
<para>In addition to specifying the desired flags, you can specify a
TCP scan type (such as <option>-sA</option> or <option>-sF</option>).
That base type tells Nmap how to interpret responses. For
example, a SYN scan considers no-response to indicate a
<literal>filtered</literal> port, while a FIN scan treats the same as
<literal>open|filtered</literal>. Nmap will behave the same way it
does for the base scan type, except that it will use the TCP flags you
specify instead. If you don't specify a base type, SYN scan is
used.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-sZ</option> (SCTP COOKIE ECHO scan)
<indexterm><primary><option>-sZ</option></primary></indexterm>
<indexterm><primary>SCTP COOKIE ECHO scan</primary></indexterm>
</term>
<listitem>
<para>
SCTP COOKIE ECHO scan is a more advanced SCTP scan. It takes
advantage of the fact that SCTP implementations should silently
drop packets containing COOKIE ECHO chunks on open ports, but
send an ABORT if the port is closed.
The advantage of this scan type is that it is not as obvious a
port scan than an INIT scan. Also, there may be non-stateful
firewall rulesets blocking INIT chunks, but not COOKIE ECHO
chunks. Don't be fooled into thinking that this will make a
port scan invisible; a good IDS will be able to detect SCTP
COOKIE ECHO scans too.
The downside is that SCTP COOKIE ECHO scans cannot differentiate
between <literal>open</literal> and <literal>filtered</literal>
ports, leaving you with the state <literal>open|filtered</literal>
in both cases.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-sI <replaceable>zombie host</replaceable><optional>:<replaceable>probeport</replaceable></optional></option> (idle scan)
<indexterm><primary><option>-sI</option></primary></indexterm>
<indexterm><primary>idle scan</primary></indexterm>
</term>
<listitem>
<para>This advanced scan method allows for a truly blind TCP
port scan of the target (meaning no packets are sent to the
target from your real IP address). Instead, a unique
side-channel attack exploits predictable IP fragmentation ID
sequence generation on the zombie host to glean information
about the open ports on the target. IDS systems will
display the scan as coming from the zombie machine you
specify (which must be up and meet certain criteria).
<man>
This fascinating scan type is too complex to fully describe in this
reference guide, so I wrote and posted an informal paper with full
details at <ulink url="https://nmap.org/book/idlescan.html" />.
</man>
<notman>
Full details of this fascinating scan type are in
<xref linkend="idlescan"/>.
</notman>
</para>
<para>Besides being extraordinarily stealthy (due to its
blind nature), this scan type permits mapping out
IP-based trust relationships between machines. The port
listing shows open ports
<emphasis>from the perspective of the zombie
host.</emphasis> So you can try scanning a target using
various zombies that you think might be
trusted<indexterm><primary>trust relationships</primary></indexterm>
(via router/packet filter rules).
</para>
<para>You can add a colon followed by a port number to the
zombie host if you wish to probe a particular port on the
zombie for IP ID changes. Otherwise Nmap will use the port it
uses by default for TCP pings (80).</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-sO</option> (IP protocol scan)
<indexterm><primary><option>-sO</option></primary></indexterm>
<indexterm><primary>IP protocol scan</primary></indexterm>
</term>
<listitem>
<para>IP protocol scan allows you to determine which IP protocols
(TCP, ICMP, IGMP, etc.) are supported by target machines. This isn't
technically a port scan, since it cycles through IP protocol numbers
rather than TCP or UDP port numbers. Yet it still uses the
<option>-p</option> option to select scanned protocol numbers, reports
its results within the normal port table format, and even uses the same
underlying scan engine as the true port scanning methods. So it is
close enough to a port scan that it belongs here.</para>
<para>Besides being useful in its own right, protocol scan
demonstrates the power of open-source software. While the fundamental
idea is pretty simple, I had not thought to add it nor received any
requests for such functionality. Then in the summer of 2000,
Gerhard Rieger<indexterm><primary>Rieger, Gerhard</primary></indexterm>
conceived the idea, wrote an excellent patch implementing it,
and sent it to the
<citetitle>announce</citetitle> mailing list<indexterm><primary><citetitle>announce</citetitle> mailing list</primary></indexterm>
(then called <citetitle>nmap-hackers</citetitle>).<indexterm><primary><citetitle>nmap-hackers</citetitle> mailing list</primary><see><citetitle>announce</citetitle> mailing list</see></indexterm>
I incorporated that patch into the Nmap tree and released a new
version the next day. Few pieces of commercial software have users
enthusiastic enough to design and contribute their own
improvements!</para>
<para>Protocol scan works in a similar fashion to UDP scan. Instead
of iterating through the port number field of a UDP packet, it sends
IP packet headers and iterates through the eight-bit IP protocol field.
The headers are usually empty, containing no data and not even the
proper header for the claimed protocol. The exceptions are TCP,
UDP, ICMP, SCTP, and IGMP. A proper protocol header for those is included since
some systems won't send them otherwise and because Nmap already has
functions to create them. Instead of watching for ICMP port
unreachable messages, protocol scan is on the lookout for ICMP
<emphasis>protocol</emphasis> unreachable messages. If Nmap receives
any response in any protocol from the target host, Nmap marks that
protocol as <literal>open</literal>. An ICMP protocol unreachable
error (type 3, code 2) causes the protocol to be marked as
<literal>closed</literal> while port unreachable (type 3, code 3)
marks the protocol <literal>open</literal>. Other ICMP unreachable errors (type 3, code
0, 1, 9, 10, or 13) cause the protocol to be marked
<literal>filtered</literal> (though they prove that ICMP is
<literal>open</literal> at the same time). If no response is received
after retransmissions, the protocol is marked
<literal>open|filtered</literal></para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-b <replaceable>FTP relay host</replaceable></option> (FTP bounce scan)
<indexterm><primary><option>-b</option></primary></indexterm>
<indexterm><primary>FTP bounce scan</primary></indexterm>
</term>
<listitem>
<para>An interesting feature of the FTP protocol (<ulink
role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc959.txt">RFC 959</ulink>) is
support for so-called proxy FTP connections. This allows a user to
connect to one FTP server, then ask that files be sent to a
third-party server. Such a feature is ripe for abuse on many levels,
so most servers have ceased supporting it. One of the abuses this
feature allows is causing the FTP server to port scan other hosts.
Simply ask the FTP server to send a file to each interesting port of a
target host in turn. The error message will describe whether the port
is open or not. This is a good way to bypass firewalls because
organizational FTP servers are often placed where they have
more access to other internal hosts than any old Internet host would. Nmap supports FTP
bounce scan with the <option>-b</option> option. It takes an argument
of the form
<replaceable>username</replaceable>:<replaceable>password</replaceable>@<replaceable>server</replaceable>:<replaceable>port</replaceable>.
<replaceable>Server</replaceable> is the name or IP address of a
vulnerable FTP server. As with a normal URL, you may omit
<replaceable>username</replaceable>:<replaceable>password</replaceable>,
in which case anonymous login credentials (user:
<literal>anonymous</literal> password:<literal>-wwwuser@</literal>)
are used. The port number (and preceding colon) may be omitted as
well, in which case the default FTP port (21) on
<replaceable>server</replaceable> is used.</para>
<para>This vulnerability was widespread in 1997 when Nmap was
released, but has largely been fixed. Vulnerable servers are still
around, so it is worth trying when all else fails. If bypassing a
firewall is your goal, scan the target network for port 21 (or
even for any FTP services if you scan all ports with version
detection) and use the
<filename>ftp-bounce</filename><indexterm><filename>ftp-bounce</filename> script</indexterm>
NSE script. Nmap will tell you
whether the host is vulnerable or not. If you are just trying to
cover your tracks, you don't need to (and, in fact, shouldn't) limit
yourself to hosts on the target network. Before you go scanning
random Internet addresses for vulnerable FTP servers, consider that
sysadmins may not appreciate you abusing their servers in this
way.</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1 id='man-port-specification'>
<title>Port Specification and Scan Order</title>
<indexterm><primary>port specification</primary></indexterm>
<para>In addition to all of the scan methods discussed previously,
Nmap offers options for specifying which ports are scanned and
whether the scan order is randomized or sequential. By default, Nmap scans the most common 1,000 ports for each protocol.
<indexterm><primary>default ports</primary></indexterm>
</para>
<variablelist>
<varlistentry>
<term>
<option>-p <replaceable>port ranges</replaceable></option> (Only scan specified ports)
<indexterm><primary><option>-p</option></primary></indexterm>
</term>
<listitem>
<para>This option specifies which ports you want to scan and
overrides the default. Individual port numbers are OK, as
are ranges separated by a hyphen (e.g. <literal>1-1023</literal>). The
beginning and/or end values of a range may be omitted,
causing Nmap to use 1 and 65535, respectively. So you can
specify <option>-p-</option> to scan ports from 1 through
65535. Scanning
port zero<indexterm><primary>port zero</primary></indexterm>
is allowed if you specify it
explicitly. For IP protocol scanning (<option>-sO</option>), this option
specifies the protocol numbers you wish to scan for
(0&ndash;255).</para>
<para>When scanning a combination of protocols (e.g. TCP and UDP), you can
specify a particular protocol by preceding the port numbers by
<literal>T:</literal> for TCP, <literal>U:</literal> for UDP,
<literal>S:</literal> for SCTP, or <literal>P:</literal> for IP Protocol.
The qualifier lasts until you specify another
qualifier. For example, the argument <option>-p
U:53,111,137,T:21-25,80,139,8080</option> would scan UDP
ports 53, 111,and 137, as well as the listed TCP ports. Note
that to scan both UDP and TCP, you have to specify
<option>-sU</option> and at least one TCP scan type (such as
<option>-sS</option>, <option>-sF</option>, or
<option>-sT</option>). If no protocol qualifier is given,
the port numbers are added to all protocol lists.</para>
<indexterm><primary>port specification</primary><secondary>wildcards in</secondary></indexterm>
<indexterm><primary>wildcards</primary><see>port selection, wildcards in</see></indexterm>
<para>
Ports can also be specified by name according to what the
port is referred to in the <filename>nmap-services</filename>. You
can even use the wildcards <literal>*</literal> and
<literal>?</literal> with the names. For example, to scan
FTP and all ports whose names begin with <quote>http</quote>, use <option>-p ftp,http*</option>.
Be careful about shell expansions and quote the argument to <option>-p</option> if unsure.</para>
<para>Ranges of ports can be surrounded by square brackets to indicate
ports inside that range that appear in <filename>nmap-services</filename>.
For example, the following will scan all ports in <filename>nmap-services</filename>
equal to or below 1024: <option>-p [-1024]</option>. Be careful with shell
expansions and quote the argument to <option>-p</option> if unsure.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--exclude-ports <replaceable>port ranges</replaceable></option> (Exclude the specified ports from scanning)
<indexterm><primary><option>--exclude-ports</option></primary></indexterm>
</term>
<listitem>
<para>This option specifies which ports you do want Nmap to exclude
from scanning. The <replaceable>port ranges</replaceable>
are specified similar to <option>-p</option>. For IP protocol
scanning (<option>-sO</option>), this option specifies the protocol
numbers you wish to exclude (0&ndash;255).</para>
<para>When ports are asked to be excluded, they are excluded from all
types of scans (i.e. they will not be scanned under any circumstances).
This also includes the discovery phase.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-F</option> (Fast (limited port) scan)
<indexterm significance="preferred"><primary><option>-F</option></primary></indexterm>
<indexterm><primary>fast scan</primary><see><option>-F</option></see></indexterm>
</term>
<listitem>
<para>Specifies that you wish to scan fewer ports than the
default. Normally Nmap scans the most common 1,000 ports for
each scanned protocol. With <option>-F</option>, this is
reduced to 100.</para>
<para>Nmap needs an <filename>nmap-services</filename> file
with frequency information in order to know which ports are
the most common<notman> (see <xref linkend="nmap-services"/>
for more about port frequencies)</notman>. If port frequency
information isn't available, perhaps because of the use of a
custom <filename>nmap-services</filename> file, Nmap scans
all named ports plus ports 1-1024. In that case,
<option>-F</option> means to scan only ports that are named
in the services file.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-r</option> (Don't randomize ports)
<indexterm><primary><option>-r</option></primary></indexterm>
<indexterm><primary>randomization of ports</primary></indexterm>
</term>
<listitem>
<para>By default, Nmap randomizes the scanned port order
(except that certain commonly accessible ports are moved
near the beginning for efficiency reasons). This
randomization is normally desirable, but you can specify
<option>-r</option> for sequential (sorted from lowest to highest) port scanning
instead.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--port-ratio <replaceable>ratio</replaceable>&lt;decimal number between 0 and 1&gt;</option>
</term>
<listitem>
<indexterm><primary>--port-ratio</primary></indexterm>
<para>Scans all ports in <filename>nmap-services</filename> file
with a ratio greater than the one given.
<replaceable>ratio</replaceable> must be between 0.0 and 1.1.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--top-ports <replaceable>n</replaceable></option>
</term>
<listitem>
<indexterm><primary>--top-ports</primary></indexterm>
<para>Scans the <replaceable>n</replaceable> highest-ratio ports
found in <filename>nmap-services</filename> file after excluding all
ports specified by <option>--exclude-ports</option>.
<replaceable>n</replaceable> must be 1 or greater.</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1 id='man-version-detection'>
<title>Service and Version Detection</title>
<indexterm class="startofrange" id="man-version-detection-indexterm"><primary>version detection</primary></indexterm>
<para>Point Nmap at a remote machine and it might tell you
that ports <literal>25/tcp</literal>, <literal>80/tcp</literal>, and <literal>53/udp</literal> are open. Using its
<filename>nmap-services</filename><indexterm><primary><filename>nmap-services</filename></primary></indexterm>
database of about 2,200
well-known services,<indexterm><primary>well-known ports</primary></indexterm>
Nmap would report that those ports probably correspond to a
mail server (SMTP), web server (HTTP), and name server (DNS)
respectively. This lookup is usually accurate&mdash;the vast
majority of daemons listening on TCP port 25 are, in fact, mail
servers. However, you should not bet your security on this!
People can and do run services on
strange ports.<indexterm><primary>non-standard ports</primary></indexterm>
</para>
<para>Even if Nmap is right, and the hypothetical server above is
running SMTP, HTTP, and DNS servers, that is not a lot of
information. When doing vulnerability assessments (or even simple
network inventories) of your companies or clients, you really want
to know which mail and DNS servers and versions are
running. Having an accurate version number helps dramatically in
determining which exploits a server is vulnerable to. Version
detection helps you obtain this information.
</para>
<para>After TCP and/or UDP ports are discovered using one of the
other scan methods, version detection interrogates those ports to
determine more about what is actually running. The
<filename>nmap-service-probes</filename><indexterm><primary><filename>nmap-service-probes</filename></primary></indexterm>
database contains probes
for querying various services and match expressions to recognize
and parse responses. Nmap tries to determine the service protocol
(e.g. FTP, SSH, Telnet, HTTP), the application name (e.g. ISC
BIND, Apache httpd, Solaris telnetd), the version number,
hostname, device type (e.g. printer, router), the OS family
(e.g. Windows, Linux). When possible, Nmap also gets the
Common Platform Enumeration (CPE)<indexterm><primary>Common Platform Enumeration</primary><secondary>service</secondary></indexterm>
representation of this information. Sometimes miscellaneous details like
whether an X server is open to connections, the SSH protocol
version, or the KaZaA user name, are available. Of course, most services don't
provide all of this information. If Nmap was compiled with
OpenSSL support, it will connect to SSL servers to deduce the
service listening behind that encryption layer.<indexterm><primary>SSL</primary><secondary>in version detection</secondary></indexterm>
Some UDP ports are left in the
<literal>open|filtered</literal> state after a UDP port scan is
unable to determine whether the port is open or filtered. Version
detection will try to elicit a response from these ports (just as
it does with open ports), and change the state to open if it
succeeds. <literal>open|filtered</literal> TCP ports are treated
the same way. Note that the Nmap <option>-A</option> option
enables version detection among other things.
<man>
A paper documenting the workings, usage, and customization of version
detection is available at
<ulink url="https://nmap.org/book/vscan.html" />.
</man>
<notman>
Version detection is described in detail in <xref linkend="vscan"/>.
</notman>
</para>
<para>When RPC services are discovered, the Nmap RPC
grinder<indexterm><primary>RPC grinder</primary></indexterm>
is automatically used to determine the RPC program and version
numbers. It takes all the TCP/UDP ports detected as RPC and floods
them with SunRPC program NULL commands in an attempt to determine
whether they are RPC ports, and if so, what program and version
number they serve up. Thus you can effectively obtain the same info
as <command>rpcinfo -p</command> even if the target's portmapper is
behind a firewall (or protected by TCP wrappers). Decoys do not
currently work with
RPC scan.<indexterm><primary>decoys</primary><secondary>which scans use</secondary></indexterm>
</para>
<para>When Nmap receives responses from a service but cannot match
them to its database, it prints out a special fingerprint and
a URL for you to submit if to if you know for sure what is running
on the port. Please take a couple minutes to make the submission
so that your find can benefit everyone. Thanks to these
submissions, Nmap has about 6,500 pattern matches for more than
650 protocols such as SMTP, FTP, HTTP, etc.<indexterm><primary>submission of service fingerprints</primary></indexterm>
</para>
<para>Version detection is enabled and controlled with the
following options:</para>
<variablelist>
<varlistentry>
<term>
<option>-sV</option> (Version detection)
<indexterm><primary><option>-sV</option></primary></indexterm></term>
<listitem>
<para>Enables version detection, as discussed above.
Alternatively, you can use <option>-A</option>, which enables
version detection among other things.</para>
<para><option>-sR</option><indexterm><primary><option>-sR</option></primary></indexterm>
is an alias for <option>-sV</option>. Prior to March 2011, it
was used to active the RPC grinder separately from version
detection, but now these options are always combined.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--allports</option> (Don't exclude any ports from
version detection)
<indexterm><primary><option>--allports</option></primary></indexterm>
</term>
<listitem>
<para>By default, Nmap version detection skips TCP port 9100
because some printers simply print anything sent to that
port, leading to dozens of pages of HTTP GET requests, binary
SSL session requests, etc. This behavior can be changed by
modifying or removing the <literal>Exclude</literal>
directive in <filename>nmap-service-probes</filename>, or
you can specify <option>--allports</option> to scan all
ports regardless of any <literal>Exclude</literal>
directive.
<indexterm><primary><literal>Exclude</literal> directive (<filename>nmap-service-probes</filename>)</primary></indexterm>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--version-intensity <replaceable>intensity</replaceable></option> (Set
version scan intensity)
<indexterm><primary><option>--version-intensity</option></primary></indexterm>
</term>
<listitem>
<para>When performing a version scan (<option>-sV</option>), Nmap sends a
series of probes, each of which is assigned a rarity value
between one and nine. The lower-numbered probes are effective
against a wide variety of common services, while the higher-numbered
ones are rarely useful. The intensity level
specifies which probes should be applied. The higher the
number, the more likely it is the service will be correctly
identified. However, high intensity scans take longer. The
intensity must be between 0 and 9.<indexterm><primary>version detection</primary><secondary>intensity</secondary></indexterm>
The default is 7.<indexterm><primary>version detection</primary><secondary>default intensity</secondary></indexterm>
When a probe is registered to the target port via the
<filename>nmap-service-probes</filename> <literal>ports</literal> directive, that probe is tried
regardless of intensity level. This ensures that the DNS
probes will always be attempted against any open port 53,
the SSL probe will be done against 443, etc.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--version-light</option> (Enable light mode)
<indexterm><primary><option>--version-light</option></primary></indexterm>
</term>
<listitem>
<para>This is a convenience alias for
<option>--version-intensity 2</option>. This light mode
makes version scanning much faster, but it is slightly less
likely to identify services.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--version-all</option> (Try every single probe)
<indexterm><primary><option>--version-all</option></primary></indexterm>
</term>
<listitem>
<para>An alias for <option>--version-intensity 9</option>,
ensuring that every single probe is attempted against each
port.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--version-trace</option> (Trace version scan activity)
<indexterm significance="preferred"><primary><option>--version-trace</option></primary></indexterm>
</term>
<listitem>
<para>This causes Nmap to print out extensive debugging info
about what version scanning is doing. It is a subset of
what you get with <option>--packet-trace</option>.</para>
</listitem>
</varlistentry>
</variablelist>
<indexterm class="endofrange" startref="man-version-detection-indexterm"/>
</refsect1>
<refsect1 id='man-os-detection'>
<title>OS Detection</title>
<indexterm class="startofrange" id="man-os-detection-indexterm"><primary>OS detection</primary></indexterm>
<indexterm><primary>CPE</primary><see>Common Platform Enumeration</see></indexterm>
<para>One of Nmap's best-known features is remote OS detection
using TCP/IP stack fingerprinting. Nmap sends a series of TCP and
UDP packets to the remote host and examines practically every bit
in the responses. After performing dozens of tests such as TCP
ISN sampling, TCP options support and ordering, IP ID sampling, and
the initial window size check, Nmap compares the results to its
<filename>nmap-os-db</filename><indexterm><primary><filename>nmap-os-db</filename></primary></indexterm>
database of more than 2,600 known
OS fingerprints and prints out the OS details if there is a match.
Each fingerprint includes a freeform textual description of the
OS, and a classification which provides the vendor name
(e.g. Sun), underlying OS (e.g. Solaris), OS generation (e.g. 10),
and device type (general purpose, router, switch, game console,
etc). Most fingerprints also have a Common Platform Enumeration
(CPE)<indexterm><primary>Common Platform Enumeration</primary><secondary>operating system</secondary></indexterm>
representation, like
<literal>cpe:/o:linux:linux_kernel:2.6</literal>.</para>
<para>If Nmap is unable to guess the OS of a machine, and
conditions are good (e.g. at least one open port and one closed
port were found), Nmap will
provide a URL you can use to submit the fingerprint if you know
(for sure) the OS running on the machine. By doing this you
contribute to the pool of operating systems known to Nmap and thus
it will be more accurate for everyone.</para>
<para>OS detection enables some other tests which make use
of information that is gathered during the process anyway.
One of these is
TCP Sequence Predictability Classification. This measures
approximately how hard it is to establish a forged
TCP connection against the remote host. It is useful for
exploiting source-IP based trust relationships (rlogin, firewall
filters, etc) or for hiding the source of an attack. This sort of
spoofing is rarely performed any more, but many machines are still
vulnerable to it. The actual
difficulty number is based on statistical sampling and may
fluctuate. It is generally better to use the English
classification such as <quote>worthy challenge</quote> or <quote>trivial joke</quote>. This
is only reported in normal output in verbose (<option>-v</option>)
mode. When verbose mode is enabled along with <option>-O</option>, IP ID sequence
generation is also reported. Most machines are in the
<quote>incremental</quote> class, which means that they increment the ID
field in the IP header for each packet they send. This makes them
vulnerable to several advanced information gathering and
spoofing attacks.</para>
<indexterm><primary>uptime guess</primary></indexterm>
<para>
Another bit of extra information enabled by OS detection is a guess
at a target's uptime. This uses the TCP timestamp option (<ulink role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc1323.txt">RFC 1323</ulink>) to guess when a machine was last rebooted.
The guess can be inaccurate due to the timestamp counter not being
initialized to zero or the counter overflowing and wrapping around,
so it is printed only in verbose mode.
</para>
<para>
<man>
A paper documenting the workings, usage, and customization of OS
detection is available at
<ulink url="https://nmap.org/book/osdetect.html" />.
</man>
<notman>
OS detection is covered in <xref linkend="osdetect"/>.
</notman>
</para>
<para>OS detection is enabled and controlled with the following options:</para>
<variablelist>
<varlistentry>
<term>
<option>-O</option> (Enable OS detection)
<indexterm significance="preferred"><primary><option>-O</option></primary></indexterm>
</term>
<listitem>
<para>Enables OS detection, as discussed above.
Alternatively, you can use <option>-A</option> to enable
OS detection along with other things.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--osscan-limit</option> (Limit OS detection to
promising targets)
<indexterm significance="preferred"><primary><option>--osscan-limit</option></primary></indexterm>
</term>
<listitem>
<para>OS detection is far more effective if at least one
open and one closed TCP port are found. Set this option
and Nmap will not even try OS detection against hosts
that do not meet this criteria. This can save substantial
time, particularly on <option>-Pn</option> scans against many hosts. It
only matters when OS detection is requested with <option>-O</option> or <option>-A</option>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--osscan-guess</option>; <option>--fuzzy</option> (Guess OS detection results)
<indexterm significance="preferred"><primary><option>--osscan-guess</option></primary></indexterm>
<indexterm><primary><option>--fuzzy</option></primary><see><option>--osscan-guess</option></see></indexterm>
</term>
<listitem>
<para>When Nmap is unable to detect a perfect OS match, it
sometimes offers up near-matches as possibilities. The
match has to be very close for Nmap to do this by default.
Either of these (equivalent) options make Nmap guess more
aggressively. Nmap will still tell you when an imperfect
match is printed and display its confidence level
(percentage) for each guess.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--max-os-tries</option> (Set the maximum number of OS detection tries against a target)
<indexterm significance="preferred"><primary><option>--max-os-tries</option></primary></indexterm>
</term>
<listitem>
<para>When Nmap performs OS detection against a target and
fails to find a perfect match, it usually repeats the
attempt. By default, Nmap tries five times if conditions
are favorable for OS fingerprint submission, and twice when
conditions aren't so good. Specifying a lower
<option>--max-os-tries</option> value (such as 1) speeds
Nmap up, though you miss out on retries which could
potentially identify the OS. Alternatively, a high value
may be set to allow even more retries when conditions are
favorable. This is rarely done, except to generate better
fingerprints for submission and integration into the Nmap OS
database.</para>
</listitem>
</varlistentry>
</variablelist>
<indexterm class="endofrange" startref="man-os-detection-indexterm"/>
</refsect1>
<refsect1 id='man-nse'>
<title>Nmap Scripting Engine (NSE)</title>
<indexterm class="startofrange" id="man-nse-indexterm"><primary>Nmap Scripting Engine (NSE)</primary></indexterm>
<para>The Nmap Scripting Engine (NSE) is one of Nmap's most
powerful and flexible features. It allows users to write (and
share) simple scripts (using the <ulink url="http://lua.org">Lua programming language</ulink>
<indexterm><primary>Lua programming language</primary></indexterm>
) to automate a wide variety of
networking tasks. Those scripts are executed in parallel with the
speed and efficiency you expect from Nmap. Users can rely on the
growing and diverse set of scripts distributed with Nmap, or write
their own to meet custom needs.</para>
<para>Tasks we had in mind when creating the system include
network discovery, more sophisticated version detection,
vulnerability detection. NSE can even be used for vulnerability
exploitation.</para>
<para>
To reflect those different uses and to simplify the choice of which
scripts to run, each script contains a field associating it with one or more categories. Currently defined categories are
<literal>auth</literal>,
<literal>broadcast</literal>,
<literal>default</literal>.
<literal>discovery</literal>,
<literal>dos</literal>,
<literal>exploit</literal>,
<literal>external</literal>,
<literal>fuzzer</literal>,
<literal>intrusive</literal>,
<literal>malware</literal>,
<literal>safe</literal>,
<literal>version</literal>, and
<literal>vuln</literal>.
These are all described
<man>at <ulink url="https://nmap.org/book/nse-usage.html#nse-categories" />.</man>
<notman>in <xref linkend="nse-categories"/>.</notman></para>
<para>
Scripts are not run in a sandbox and thus could accidentally or
maliciously damage your system or invade your privacy. Never run
scripts from third parties unless you trust the authors or have
carefully audited the scripts yourself.
</para>
<para>The Nmap Scripting Engine is described in detail
<man>at <ulink url="https://nmap.org/book/nse.html" /></man>
<notman>in <xref linkend="nse"/></notman> and is controlled by the following options:</para>
<variablelist>
<varlistentry>
<term><option>-sC</option>
<indexterm significance="preferred">
<primary><option>-sC</option></primary>
</indexterm>
</term>
<listitem>
<para>Performs a script scan using the default set of scripts. It is
equivalent to <option>--script=default</option>. Some of the
scripts in this category are considered intrusive and should
not be run against a target network without permission. </para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--script <replaceable>filename</replaceable>|<replaceable>category</replaceable>|<replaceable>directory</replaceable>|<replaceable>expression</replaceable><optional>,...</optional></option>
<indexterm><primary><option>--script</option></primary></indexterm>
</term>
<listitem>
<para>
Runs a script scan using the comma-separated list of filenames, script
categories, and directories. Each element in the list may also be a
Boolean expression describing a more complex set of scripts. Each
element is interpreted first as an expression, then as a category, and
finally as a file or directory name.</para>
<para>There are two special features for advanced users only.
One is to prefix script names and expressions with
<literal>+</literal> to force them to run even if they normally
wouldn't (e.g. the relevant service wasn't detected on the target
port). The other is that the argument <literal>all</literal> may be
used to specify every script in Nmap's database. Be cautious with
this because NSE contains dangerous scripts such as exploits, brute
force authentication crackers, and denial of service attacks.
</para>
<para>
File and directory names may be relative or absolute. Absolute names are
used directly. Relative paths are looked for in the
<filename>scripts</filename> of each of the following places until
found:
<indexterm><primary>data files</primary><secondary>directory search order</secondary></indexterm><indexterm><primary>scripts, location of</primary></indexterm>
<simplelist>
<member><option>--datadir</option></member>
<member><envar>$NMAPDIR</envar><indexterm><primary><envar>NMAPDIR</envar> environment variable</primary></indexterm></member>
<member><filename>~/.nmap</filename> (not searched on Windows)<indexterm><primary sortas="nmap directory"><filename>.nmap</filename> directory</primary></indexterm></member>
<member><filename><replaceable>HOME</replaceable>\AppData\Roaming\nmap</filename> (only on Windows)<indexterm><primary sortas="nmap directory"><filename>.nmap</filename> directory</primary></indexterm></member>
<member>the directory containing the <filename>nmap</filename>
executable</member>
<member>the directory containing the <filename>nmap</filename>
executable, followed by <filename>../share/nmap</filename></member>
<member><varname>NMAPDATADIR</varname><indexterm><primary><varname>NMAPDATADIR</varname></primary></indexterm></member>
<member>the current directory.</member>
</simplelist>
</para>
<para>
When a directory name is given, Nmap loads every file in the directory
whose name ends with <filename>.nse</filename>. All other files are
ignored and directories are not searched recursively. When a filename is
given, it does not have to have the <filename>.nse</filename> extension;
it will be added automatically if necessary.
</para>
<indexterm><primary>script database</primary><see><filename>script.db</filename></see></indexterm>
<indexterm><primary>scripts, location of</primary></indexterm>
<para>Nmap scripts are stored in a <filename>scripts</filename>
subdirectory of the Nmap data directory by default
<man>(see <ulink url="https://nmap.org/book/data-files.html"/>).</man>
<notman>(see <xref linkend="data-files"/>).</notman>
For efficiency, scripts are indexed in
a database stored
in <filename>scripts/script.db</filename>,<indexterm><primary><filename>script.db</filename></primary></indexterm>
which lists the category or categories in which each script belongs.</para>
<indexterm><primary>wildcards</primary><secondary>in script selection</secondary></indexterm>
<para>
When referring to scripts from <filename>script.db</filename> by
name, you can use a shell-style &lsquo;<literal>*</literal>&rsquo;
wildcard.
</para>
<variablelist>
<varlistentry>
<term><command>nmap --script "http-*"</command></term>
<listitem>
<para>Loads all scripts whose name starts with
<filename>http-</filename>, such as
<filename>http-auth</filename> and
<filename>http-open-proxy</filename>. The argument to
<option>--script</option> had to be in quotes to protect the
wildcard from the shell.</para>
</listitem>
</varlistentry>
</variablelist>
<indexterm><primary>Boolean expressions in script selection</primary></indexterm>
<para>
More complicated script selection can be done using the
<literal>and</literal>, <literal>or</literal>, and
<literal>not</literal> operators to build Boolean expressions. The
operators have the same
<ulink role="hidepdf" url="http://www.lua.org/manual/5.1/manual.html#2.5.3">precedence</ulink>
as in Lua: <literal>not</literal> is the highest, followed by
<literal>and</literal> and then <literal>or</literal>. You can
alter precedence by using parentheses. Because expressions contain
space characters it is necessary to quote
them.
</para>
<variablelist>
<varlistentry>
<term><command>nmap --script "not intrusive"</command></term>
<listitem>
<para>Loads every script except for those in the
<literal>intrusive</literal> category.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><command>nmap --script "default or safe"</command></term>
<listitem>
<para>This is functionally equivalent to
<command>nmap --script "default,safe"</command>. It loads all
scripts that are in the <literal>default</literal> category or
the <literal>safe</literal> category or both.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><command>nmap --script "default and safe"</command></term>
<listitem>
<para>Loads those scripts that are in
<emphasis>both</emphasis> the <literal>default</literal> and
<literal>safe</literal> categories.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><command>nmap --script "(default or safe or intrusive) and not http-*"</command></term>
<listitem>
<para>Loads scripts in the <literal>default</literal>,
<literal>safe</literal>, or <literal>intrusive</literal>
categories, except for those whose names start with
<filename>http-</filename>.</para>
</listitem>
</varlistentry>
</variablelist>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--script-args <replaceable>n1</replaceable>=<replaceable>v1</replaceable>,<replaceable>n2</replaceable>={<replaceable>n3</replaceable>=<replaceable>v3</replaceable>},<replaceable>n4</replaceable>={<replaceable>v4</replaceable>,<replaceable>v5</replaceable>}</option>
<indexterm significance="preferred"><primary><option>--script-args</option></primary></indexterm>
<indexterm><primary>script arguments</primary><seealso><option>--script-args</option></seealso></indexterm></term>
<listitem>
<para>
Lets you provide arguments to NSE scripts. Arguments are a comma-separated list
of <literal>name=value</literal> pairs. Names and values may be strings not
containing whitespace or the characters
&lsquo;<literal>{</literal>&rsquo;,
&lsquo;<literal>}</literal>&rsquo;,
&lsquo;<literal>=</literal>&rsquo;, or
&lsquo;<literal>,</literal>&rsquo;.
To include one of these characters in a string, enclose the string in single or
double quotes. Within a quoted string, &lsquo;<literal>\</literal>&rsquo;
escapes a quote. A backslash is only used to escape quotation marks in this
special case; in all other cases a backslash is interpreted literally. Values
may also be tables enclosed in <literal>{}</literal>, just as in Lua. A table
may contain simple string values or more name-value pairs, including nested
tables. Many scripts qualify their arguments with the script name, as in <literal>xmpp-info.server_name</literal>. You may use that full qualified version to affect just the specified script, or you may pass the unqualified version (<literal>server_name</literal> in this case) to affect all scripts using that argument name. A script will first check for its fully qualified argument name (the name specified in its documentation) before it accepts an unqualified argument name. A complex example of script arguments is
<option>--script-args 'user=foo,pass=",{}=bar",whois={whodb=nofollow+ripe},xmpp-info.server_name=localhost'</option>. The online NSE Documentation Portal at <ulink url="https://nmap.org/nsedoc/"/>
lists the arguments that each script accepts.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--script-args-file <replaceable>filename</replaceable></option>
<indexterm significance="preferred"><primary><option>--script-args-file</option></primary></indexterm>
<indexterm><primary>script arguments from file</primary><seealso><option>--script-args-file</option></seealso></indexterm></term>
<listitem>
<para>
Lets you load arguments to NSE scripts from a file. Any arguments on the
command line supersede ones in the file. The file can be an absolute path,
or a path relative to Nmap's usual search path (NMAPDIR, etc.)
Arguments can be comma-separated or newline-separated, but otherwise follow
the same rules as for <option>--script-args</option>, without requiring
special quoting and escaping, since they are not parsed by the shell.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--script-help <replaceable>filename</replaceable>|<replaceable>category</replaceable>|<replaceable>directory</replaceable>|<replaceable>expression</replaceable>|all<optional>,...</optional></option>
<indexterm><primary><option>--script-help</option></primary></indexterm>
</term>
<listitem>
<para>
Shows help about scripts. For each script matching the given
specification, Nmap prints the script name, its categories, and its
description. The specifications are the same as those accepted by
<option>--script</option>; so for example if you want help about
the <literal>ftp-anon</literal> script, you would run
<command>nmap --script-help ftp-anon</command>. In addition to getting
help for individual scripts, you can use this as a preview of what
scripts will be run for a specification, for example with <command>nmap
--script-help default</command>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--script-trace</option>
<indexterm significance="preferred"><primary><option>--script-trace</option></primary></indexterm></term>
<listitem>
<para>
This option does what <option>--packet-trace</option> does,
just one ISO layer higher. If this option is specified all incoming
and outgoing communication performed by a script is printed. The
displayed information includes the communication protocol, the
source, the target and the transmitted data. If more than 5% of all
transmitted data is not printable, then the trace output is in a hex
dump format. Specifying <option>--packet-trace</option> enables script
tracing too.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--script-updatedb</option>
<indexterm significance="preferred"><primary><option>--script-updatedb</option></primary></indexterm></term>
<listitem>
<para>This option updates the script database found
in <filename>scripts/script.db</filename> which is used by
Nmap to determine the available default scripts and
categories. It is only necessary to update the database if
you have added or removed NSE scripts from the
default <filename>scripts</filename> directory or if you
have changed the categories of any script. This option is
generally used by
itself: <command>nmap --script-updatedb</command>.</para>
</listitem>
</varlistentry>
</variablelist>
<indexterm class="endofrange" startref="man-nse-indexterm"/>
</refsect1>
<refsect1 id='man-performance'>
<title>Timing and Performance</title>
<indexterm class="startofrange" id="man-performance-timing-indexterm"><primary>timing</primary></indexterm>
<indexterm class="startofrange" id="man-performance-indexterm"><primary>performance</primary></indexterm>
<para>One of my highest Nmap development priorities has always been
performance. A default scan (<command>nmap
<replaceable>hostname</replaceable></command>) of a host on my local
network takes a fifth of a second. That is barely enough time to
blink, but adds up when you are scanning hundreds or thousands
of hosts. Moreover, certain scan options such as UDP scanning and
version detection can increase scan times substantially. So can
certain firewall configurations, particularly response rate limiting.
While Nmap utilizes parallelism and many advanced algorithms to
accelerate these scans, the user has ultimate control over how Nmap
runs. Expert users carefully craft Nmap commands to obtain only the
information they care about while meeting their time
constraints.</para>
<para>Techniques for improving scan times include omitting
non-critical tests, and upgrading to the latest version of Nmap
(performance enhancements are made frequently). Optimizing timing
parameters can also make a substantial difference. Those options are
listed below.</para>
<para>Some options accept a <literal>time</literal> parameter. This
is specified in seconds by default, though you can append
&lsquo;ms&rsquo;, &lsquo;s&rsquo;, &lsquo;m&rsquo;, or &lsquo;h&rsquo; to the value to
specify milliseconds, seconds, minutes, or hours. So the
<option>--host-timeout</option> arguments <literal>900000ms</literal>,
<literal>900</literal>, <literal>900s</literal>, and <literal>15m</literal> all do the same thing.</para>
<variablelist>
<varlistentry>
<term>
<option>--min-hostgroup <replaceable>numhosts</replaceable></option>;
<option>--max-hostgroup
<replaceable>numhosts</replaceable></option> (Adjust parallel scan group sizes)
<indexterm><primary><option>--min-hostgroup</option></primary></indexterm>
<indexterm><primary><option>--max-hostgroup</option></primary></indexterm>
</term>
<listitem>
<para>Nmap has the ability to port scan or version scan multiple hosts
in parallel. Nmap does this by dividing the target IP space into
groups and then scanning one group at a time. In general, larger
groups are more efficient. The downside is that host results can't be
provided until the whole group is finished. So if Nmap started out
with a group size of 50, the user would not receive any reports
(except for the updates offered in verbose mode) until the first 50
hosts are completed.</para>
<para>By default, Nmap takes a compromise approach to this conflict.
It starts out with a group size as low as five so the first results
come quickly and then increases the groupsize to as high as 1024. The
exact default numbers depend on the options given. For efficiency
reasons, Nmap uses larger group sizes for UDP or few-port TCP
scans.</para>
<para>When a maximum group size is specified with
<option>--max-hostgroup</option>, Nmap will never exceed that size.
Specify a minimum size with <option>--min-hostgroup</option> and Nmap
will try to keep group sizes above that level. Nmap may have to use
smaller groups than you specify if there are not enough target hosts
left on a given interface to fulfill the specified minimum. Both may
be set to keep the group size within a specific range, though this is
rarely desired.</para>
<para>These options do not have an effect during the host discovery
phase of a scan. This includes plain ping scans (<option>-sn</option>).
Host discovery always works in large groups of hosts to improve speed
and accuracy.</para>
<para>The primary use of these options is to specify a large minimum
group size so that the full scan runs more quickly. A common choice
is 256 to scan a network in Class C sized chunks. For a scan with
many ports, exceeding that number is unlikely to help much. For scans
of just a few port numbers, host group sizes of 2048 or more may be
helpful.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--min-parallelism <replaceable>numprobes</replaceable></option>;
<option>--max-parallelism
<replaceable>numprobes</replaceable></option> (Adjust probe parallelization)
<indexterm><primary><option>--min-parallelism</option></primary></indexterm>
<indexterm><primary><option>--max-parallelism</option></primary></indexterm>
</term>
<listitem>
<para>These options control the total number of probes that may
be outstanding for a host group. They are used for port scanning and
host discovery. By default, Nmap calculates an ever-changing ideal
parallelism based on network performance. If packets are being dropped,
Nmap slows down and allows fewer outstanding probes. The ideal probe
number slowly rises as the network proves itself worthy. These
options place minimum or maximum bounds on that variable. By default,
the ideal parallelism can drop to one if the network proves unreliable
and rise to several hundred in perfect conditions.</para>
<para>The most common usage is to set
<option>--min-parallelism</option> to a number higher than one to
speed up scans of poorly performing hosts or networks. This is a
risky option to play with, as setting it too high may affect accuracy.
Setting this also reduces Nmap's ability to control parallelism
dynamically based on network conditions. A value of 10 might be
reasonable, though I only adjust this value as a last resort.</para>
<para>The <option>--max-parallelism</option> option is sometimes set to one
to prevent Nmap from sending more than one probe at a time to hosts.
The <option>--scan-delay</option> option, discussed later, is another
way to do this.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--min-rtt-timeout <replaceable>time</replaceable></option>,
<option>--max-rtt-timeout <replaceable>time</replaceable></option>,
<option>--initial-rtt-timeout
<replaceable>time</replaceable></option> (Adjust probe timeouts)
<indexterm><primary><option>--min-rtt-timeout</option></primary></indexterm>
<indexterm><primary><option>--max-rtt-timeout</option></primary></indexterm>
<indexterm><primary><option>--initial-rtt-timeout</option></primary></indexterm>
</term>
<listitem>
<para>Nmap maintains a
running timeout value for determining how long it will wait for a
probe response before giving up or retransmitting the probe. This is
calculated based on the response times of previous probes.
<notman>The exact formula is given in <xref linkend="port-scanning-algorithms" />.</notman>
If the network latency shows itself to be significant and variable,
this timeout can grow to several seconds. It also starts at a
conservative (high) level and may stay that way for a while when Nmap
scans unresponsive hosts.</para>
<para>
Specifying a lower <option>--max-rtt-timeout</option> and
<option>--initial-rtt-timeout</option> than the defaults can cut scan
times significantly. This is particularly true for pingless
(<option>-Pn</option>) scans, and those against heavily filtered
networks. Don't get too aggressive though. The scan can end up
taking longer if you specify such a low value that many probes are
timing out and retransmitting while the response is in transit.</para>
<para>If all the hosts are on a local network, 100 milliseconds
(<option>--max-rtt-timeout 100ms</option>) is a
reasonable aggressive value. If
routing is involved, ping a host on the network first with the ICMP
ping utility, or with a custom packet crafter such as
Nping<indexterm><primary>Nping</primary></indexterm>
that is
more likely to get through a firewall. Look at the maximum round trip
time out of ten packets or so. You might want to double that for the
<option>--initial-rtt-timeout</option> and triple or quadruple it for
the <option>--max-rtt-timeout</option>. I generally do not set the
maximum RTT below 100&nbsp;ms, no matter what the ping times are. Nor do I
exceed 1000&nbsp;ms.</para>
<para><option>--min-rtt-timeout</option> is a rarely used option that
could be useful when a network is so unreliable that even Nmap's
default is too aggressive. Since Nmap only reduces the timeout down to
the minimum when the network seems to be reliable, this need is
unusual and should be reported as a bug to the
<citetitle>nmap-dev</citetitle> mailing list.<indexterm><primary><citetitle>nmap-dev</citetitle> mailing list</primary></indexterm>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--max-retries <replaceable>numtries</replaceable></option> (Specify the
maximum number of port scan probe retransmissions)
<indexterm><primary><option>--max-retries</option></primary></indexterm>
</term>
<listitem>
<para>When Nmap receives no response to a port scan probe, it could
mean the port is filtered. Or maybe the probe or response was simply
lost on the network. It is also possible that the target host has
rate limiting enabled that temporarily blocked the response. So Nmap
tries again by retransmitting the initial probe. If Nmap detects poor
network reliability, it may try many more times before giving up on a
port. While this benefits accuracy, it also lengthen scan times.
When performance is critical, scans may be sped up by limiting the
number of retransmissions allowed. You can even specify
<option>--max-retries 0</option> to prevent any retransmissions,
though that is only recommended for situations such as informal
surveys where occasional missed ports and hosts are acceptable.
</para>
<para>The default (with no <option>-T</option> template) is to allow
ten retransmissions. If a network seems reliable and the target hosts
aren't rate limiting, Nmap usually only does one retransmission. So
most target scans aren't even affected by dropping
<option>--max-retries</option> to a low value such as three. Such
values can substantially speed scans of slow (rate limited) hosts.
You usually lose some information when Nmap gives up on ports early,
though that may be preferable to letting the
<option>--host-timeout</option> expire and losing all information
about the target.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--host-timeout <replaceable>time</replaceable></option> (Give
up on slow target hosts)
<indexterm><primary><option>--host-timeout</option></primary></indexterm>
</term>
<listitem>
<para>Some hosts simply take a <emphasis>long</emphasis> time to scan.
This may be due to poorly performing or unreliable networking hardware
or software, packet rate limiting, or a restrictive firewall. The
slowest few percent of the scanned hosts can eat up a majority of the
scan time. Sometimes it is best to cut your losses and skip those
hosts initially. Specify
<option>--host-timeout</option> with the maximum amount of time you
are willing to wait. For example,
specify <literal>30m</literal> to ensure that Nmap doesn't waste
more than half an hour on a single host. Note that Nmap may be
scanning other hosts at the same time during that half an hour, so it isn't a complete loss. A host that times out is skipped.
No port table, OS detection, or version detection results are printed
for that host.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--scan-delay <replaceable>time</replaceable></option>;
<option>--max-scan-delay
<replaceable>time</replaceable></option> (Adjust delay between probes)
<indexterm><primary><option>--scan-delay</option></primary></indexterm>
<indexterm><primary><option>--max-scan-delay</option></primary></indexterm>
</term>
<listitem>
<para>This option causes Nmap to wait at least the given amount of
time between each probe it sends to a given host. This is
particularly useful in the case of rate limiting.<indexterm><primary>rate limiting</primary></indexterm> Solaris machines
(among many others) will usually respond to UDP scan probe packets
with only one ICMP message per second. Any more than that sent by
Nmap will be wasteful. A <option>--scan-delay</option> of
<literal>1s</literal> will keep Nmap at that slow rate. Nmap tries to
detect rate limiting and adjust the scan delay accordingly, but it
doesn't hurt to specify it explicitly if you already know what rate
works best.</para>
<para>When Nmap adjusts the scan delay upward to cope with rate
limiting, the scan slows down dramatically. The
<option>--max-scan-delay</option> option specifies the largest delay
that Nmap will allow. A low <option>--max-scan-delay</option>
can speed up Nmap, but it is risky. Setting this value too low can lead to wasteful
packet retransmissions and possible missed ports when the target
implements strict rate limiting.</para>
<para>Another use of <option>--scan-delay</option> is to evade
threshold based intrusion detection and prevention systems
(IDS/IPS).<indexterm><primary>intrusion detection
systems</primary><secondary>evading</secondary></indexterm> <notman>This
technique is used in <xref linkend="defeating-ids-snort-portscan" />
to defeat the default port scan detector in Snort IDS. Most other
intrusion detection systems can be defeated in the same way.</notman>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--min-rate <replaceable>number</replaceable></option>; <option>--max-rate <replaceable>number</replaceable></option>
(Directly control the scanning rate)
<indexterm><primary><option>--min-rate</option></primary></indexterm>
<indexterm><primary><option>--max-rate</option></primary></indexterm>
</term>
<listitem>
<para>Nmap's dynamic timing does a good job of finding an appropriate
speed at which to scan. Sometimes, however, you may happen to know an
appropriate scanning rate for a network, or you may have to guarantee
that a scan will be finished by a certain time. Or perhaps you must keep
Nmap from scanning too quickly. The <option>--min-rate</option> and
<option>--max-rate</option> options are designed for these
situations.</para>
<para>When the
<option>--min-rate</option> option is given Nmap will do its best to
send packets as fast as or faster than the given rate. The argument is a
positive real number representing a packet rate in packets per second.
For example, specifying <option>--min-rate 300</option> means that
Nmap will try to keep the sending rate at or above 300 packets per
second. Specifying a minimum rate does not keep Nmap from going faster
if conditions warrant.</para>
<para>Likewise, <option>--max-rate</option> limits a scan's sending rate to a
given maximum. Use <option>--max-rate 100</option>, for example, to
limit sending to 100 packets per second on a fast network. Use
<option>--max-rate 0.1</option> for a slow scan of one packet every ten
seconds. Use <option>--min-rate</option> and <option>--max-rate</option>
together to keep the rate inside a certain range.</para>
<para>These two options are global, affecting an entire scan, not
individual hosts. They only affect port scans and host discovery scans.
Other features like OS detection implement their own timing.</para>
<para>There are two conditions when the actual scanning rate may fall
below the requested minimum. The first is if the minimum is faster than
the fastest rate at which Nmap can send, which is dependent on hardware.
In this case Nmap will simply send packets as fast as possible, but be
aware that such high rates are likely to cause a loss of accuracy. The
second case is when Nmap has nothing to send, for example at the end of
a scan when the last probes have been sent and Nmap is waiting for them
to time out or be responded to. It's normal to see the scanning rate
drop at the end of a scan or in between hostgroups. The sending rate may
temporarily exceed the maximum to make up for unpredictable delays, but
on average the rate will stay at or below the maximum.</para>
<para>Specifying a minimum rate should be done with care. Scanning
faster than a network can support may lead to a loss of accuracy. In
some cases, using a faster rate can make a scan take
<emphasis>longer</emphasis> than it would with a slower rate. This is
because Nmap's
<notman><link linkend="scan-methods-adaptive-retransmission">adaptive
retransmission</link><indexterm><primary>adaptive retransmission</primary><see>retransmission</see></indexterm><indexterm><primary>retransmission</primary></indexterm>
</notman>
<man>adaptive retransmission</man>
algorithms will detect the network congestion caused by an excessive scanning rate
and increase the number of retransmissions in order to improve accuracy.
So even though packets are sent at a higher rate, more packets are sent
overall. Cap the number of retransmissions with the
<option>--max-retries</option> option if you need to set an upper limit on total scan
time.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--defeat-rst-ratelimit</option>
<indexterm><primary><option>--defeat-rst-ratelimit</option></primary></indexterm></term>
<listitem>
<para>Many hosts have long used
rate limiting<indexterm><primary>rate limiting</primary></indexterm>
to reduce the number
of ICMP error messages (such as port-unreachable errors) they send.
Some systems now apply similar rate limits to the RST (reset)
packets they generate. This can slow Nmap down dramatically as it
adjusts its timing to reflect those rate limits. You can tell Nmap to
ignore those rate limits (for port scans such as SYN scan which
<emphasis>don't</emphasis> treat non-responsive ports as
<literal>open</literal>) by specifying
<option>--defeat-rst-ratelimit</option>.</para>
<para>Using this option can reduce accuracy, as some ports will appear
non-responsive because Nmap didn't wait long enough for a rate-limited
RST response. With a SYN
scan, the non-response results in the port being labeled
<literal>filtered</literal> rather than the <literal>closed</literal>
state we see when RST packets are received. This option is useful
when you only care about open ports, and distinguishing between
<literal>closed</literal> and <literal>filtered</literal> ports isn't
worth the extra time.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--nsock-engine
epoll|kqueue|poll|select</option>
<indexterm><primary><option>--nsock-engine</option></primary></indexterm>
<indexterm><primary>Nsock IO engine</primary></indexterm>
</term>
<listitem>
<para>Enforce use of a given nsock IO multiplexing engine. Only the
<literal>select(2)</literal>-based fallback engine is guaranteed to be
available on your system. Engines are named after the name of the IO
management facility they leverage. Engines currently implemented are
<literal>epoll</literal>, <literal>kqueue</literal>, <literal>poll</literal>,
and <literal>select</literal>, but not all will be present on any platform.
Use <command>nmap -V</command> to see which engines are supported.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-T
paranoid|sneaky|polite|normal|aggressive|insane</option>
(Set a timing template)
<indexterm><primary><option>-T</option></primary></indexterm>
<indexterm><primary>timing templates</primary><seealso><literal>paranoid</literal>, <literal>sneaky</literal>, <literal>polite</literal>, <literal>normal</literal>, <literal>aggressive</literal>, and <literal>insane</literal></seealso></indexterm>
</term>
<listitem>
<indexterm><primary><option>-T0</option></primary><see><literal>paranoid</literal> timing template</see></indexterm>
<indexterm><primary><option>-T1</option></primary><see><literal>sneaky</literal> timing template</see></indexterm>
<indexterm><primary><option>-T2</option></primary><see><literal>polite</literal> timing template</see></indexterm>
<indexterm><primary><option>-T3</option></primary><see><literal>normal</literal> timing template</see></indexterm>
<indexterm><primary><option>-T4</option></primary><see><literal>aggressive</literal> timing template</see></indexterm>
<indexterm><primary><option>-T5</option></primary><see><literal>insane</literal> timing template</see></indexterm>
<para>While the fine-grained timing controls discussed in the previous
section are powerful and effective, some people find them confusing.
Moreover, choosing the appropriate values can sometimes take more time
than the scan you are trying to optimize. So Nmap offers a simpler
approach, with six timing templates. You can specify them with the
<option>-T</option> option and their number (0&ndash;5) or their name.
The template names are
<option>paranoid</option>&nbsp;(<option>0</option>),
<option>sneaky</option>&nbsp;(<option>1</option>),
<option>polite</option>&nbsp;(<option>2</option>),
<option>normal</option>&nbsp;(<option>3</option>),
<option>aggressive</option>&nbsp;(<option>4</option>),
and <option>insane</option>&nbsp;(<option>5</option>).
The first two are for IDS evasion.
Polite mode slows down the scan to use less bandwidth
and target machine resources. Normal mode is the default and so
<option>-T3</option> does nothing. Aggressive mode speeds scans up by
making the assumption that you are on a reasonably fast and reliable
network. Finally
insane mode<indexterm><primary><literal>insane</literal> (<option>-T5</option>) timing template</primary></indexterm>
assumes that you are on an
extraordinarily fast network or are willing to sacrifice some accuracy
for speed.</para>
<para>These templates allow the user to specify how aggressive they
wish to be, while leaving Nmap to pick the exact timing values. The
templates also make some minor speed adjustments for which
fine-grained control options do not currently exist. For example,
<option>-T4</option><indexterm><primary><literal>aggressive</literal> (<option>-T4</option>) timing template</primary></indexterm>
prohibits the dynamic scan delay from exceeding
10&nbsp;ms for TCP ports and <option>-T5</option> caps that value at 5&nbsp;ms.
Templates can be used in combination with fine-grained
controls, and the fine-grained controls will you specify will take
precedence over the timing template default for that parameter. I
recommend using <option>-T4</option> when scanning reasonably modern
and reliable networks. Keep that option even when you add
fine-grained controls so that you benefit from those extra minor
optimizations that it enables.</para>
<para>If you are on a decent broadband or ethernet connection, I would
recommend always using <option>-T4</option>. Some people love
<option>-T5</option> though it is too aggressive for my taste. People
sometimes specify <option>-T2</option> because they think it is less
likely to crash hosts or because they consider themselves to be polite
in general. They often don't realize just how slow <option>-T
polite</option><indexterm><primary><literal>polite</literal> (<option>-T2</option>) timing template</primary></indexterm>
really is. Their scan may take ten times longer than a
default scan.
Machine crashes and bandwidth problems are rare with the
default timing options (<option>-T3</option>) and so I normally
recommend that for cautious scanners. Omitting version detection is
far more effective than playing with timing values at reducing these
problems.</para>
<para>While
<option>-T0</option><indexterm><primary><literal>paranoid</literal> (<option>-T0</option>) timing template</primary></indexterm>
and <option>-T1</option><indexterm><primary><literal>sneaky</literal> (<option>-T1</option>) timing template</primary></indexterm>
may be
useful for avoiding IDS alerts, they will take an extraordinarily long
time to scan thousands of machines or ports. For such a long scan,
you may prefer to set the exact timing values you need rather than
rely on the canned <option>-T0</option> and <option>-T1</option>
values.</para>
<para>The main effects of <option>T0</option> are serializing the scan
so only one port is scanned at a time, and waiting five minutes
between sending each probe. <option>T1</option> and
<option>T2</option> are similar but they only wait 15 seconds and 0.4
seconds, respectively, between probes. <option>T3</option><indexterm><primary><literal>normal</literal> (<option>-T3</option>) timing template</primary></indexterm> is Nmap's
default behavior, which includes parallelization.
<option>-T4</option>
does the equivalent of <option>--max-rtt-timeout 1250ms --min-rtt-timeout 100ms
--initial-rtt-timeout 500ms --max-retries 6</option> and sets the maximum TCP scan delay
to 10 milliseconds. <option>T5</option>
does the equivalent of
<option>--max-rtt-timeout 300ms --min-rtt-timeout 50ms
--initial-rtt-timeout 250ms --max-retries 2 --host-timeout 15m</option> as well as
setting the maximum TCP scan delay to 5&nbsp;ms.</para>
</listitem>
</varlistentry>
</variablelist>
<indexterm class="endofrange" startref="man-performance-indexterm"/>
<indexterm class="endofrange" startref="man-performance-timing-indexterm"/>
</refsect1>
<refsect1 id='man-bypass-firewalls-ids'>
<title>Firewall/IDS Evasion and Spoofing</title>
<indexterm class="startofrange" id="man-bypass-filewalls-indexterm"><primary>firewalls</primary><secondary>bypassing</secondary></indexterm>
<indexterm class="startofrange" id="man-bypass-ids-indexterm"><primary>intrusion detection systems</primary><secondary>evading</secondary></indexterm>
<para>Many Internet pioneers envisioned a global open network with a
universal IP address space allowing virtual connections between any
two nodes. This allows hosts to act as true peers, serving and
retrieving information from each other. People could access all of
their home systems from work, changing the climate control settings or
unlocking the doors for early guests. This vision of universal
connectivity has been stifled by address space shortages and security
concerns. In the early 1990s, organizations began deploying
firewalls for the express purpose of reducing connectivity. Huge
networks were cordoned off from the unfiltered Internet by application
proxies, network address translation, and packet filters. The
unrestricted flow of information gave way to tight regulation of
approved communication channels and the content that passes over
them.</para>
<para>Network obstructions such as firewalls can make mapping a
network exceedingly difficult. It will not get any easier, as
stifling casual reconnaissance is often a key goal of implementing the
devices. Nevertheless, Nmap offers many features to help understand these
complex networks, and to verify that filters are working as intended.
It even supports mechanisms for bypassing poorly implemented
defenses. One of the best methods of understanding your
network security posture is to try to defeat it. Place yourself in
the mind-set of an attacker, and deploy techniques from this section
against your networks. Launch an FTP bounce scan, idle scan,
fragmentation attack, or try to tunnel through one of your own
proxies.</para>
<para>In addition to restricting network activity, companies are
increasingly monitoring traffic with intrusion detection systems
(IDS). All of the major IDSs ship with rules designed to detect Nmap
scans because scans are sometimes a precursor to attacks. Many of
these products have recently morphed into intrusion
<emphasis>prevention</emphasis> systems
(IPS)<indexterm><primary>intrusion prevention systems</primary><seealso>intrusion detection systems</seealso></indexterm>
that actively block
traffic deemed malicious. Unfortunately for network administrators
and IDS vendors, reliably detecting bad intentions by analyzing packet
data is a tough problem. Attackers with patience, skill, and the help
of certain Nmap options can usually pass by IDSs undetected.
Meanwhile, administrators must cope with large numbers of false
positive results where innocent activity is misdiagnosed and alerted
on or blocked.</para>
<para>Occasionally people suggest that Nmap should not offer features
for evading firewall rules or sneaking past IDSs. They argue
that these features are just as likely to be misused by attackers as
used by administrators to enhance security. The problem with this
logic is that these methods would still be used by attackers, who
would just find other tools or patch the functionality into Nmap.
Meanwhile, administrators would find it that much harder to do their
jobs. Deploying only modern, patched FTP servers is a far more
powerful defense than trying to prevent the distribution of tools
implementing the FTP bounce attack.
</para>
<para>There is no magic bullet (or Nmap option) for detecting and
subverting firewalls and IDS systems. It takes skill and experience.
A tutorial is beyond the scope of this reference guide, which only
lists the relevant options and describes what they do.</para>
<variablelist>
<varlistentry>
<term>
<option>-f</option> (fragment packets);
<option>--mtu</option> (using the specified MTU)
<indexterm significance="preferred"><primary><option>-f</option></primary></indexterm>
<indexterm significance="preferred"><primary><option>--mtu</option></primary></indexterm>
</term>
<listitem>
<para>The <option>-f</option> option causes the requested scan (including
ping scans) to use tiny fragmented IP packets. The idea
is to split up the TCP header over several packets to
make it harder for packet filters, intrusion detection
systems, and other annoyances to detect what you are
doing. Be careful with this! Some programs have trouble
handling these tiny packets. The old-school sniffer named
Sniffit segmentation faulted immediately upon receiving
the first fragment. Specify this option once, and Nmap
splits the packets into eight bytes or less after the IP
header. So a 20-byte TCP header would be split into three
packets. Two with eight bytes of the TCP header, and one
with the final four. Of course each fragment also has an
IP header. Specify <option>-f</option> again to use 16 bytes per fragment
(reducing the number of fragments).<indexterm><primary><option>-f</option></primary><secondary>giving twice</secondary></indexterm>
Or you can specify
your own offset size with the <option>--mtu</option> option. Don't also
specify <option>-f</option> if you use <option>--mtu</option>. The offset must be a
multiple of eight. While fragmented packets won't get by
packet filters and firewalls that queue all IP fragments,
such as the <varname>CONFIG_IP_ALWAYS_DEFRAG</varname> option in the Linux
kernel, some networks can't afford the performance hit
this causes and thus leave it disabled. Others can't enable
this because fragments may take different routes into their
networks. Some source
systems defragment outgoing packets in the kernel. Linux
with the
iptables<indexterm><primary>iptables</primary></indexterm>
connection tracking module is one such
example. Do a scan while a sniffer such as
<application>Wireshark</application><indexterm><primary><application>Wireshark</application></primary></indexterm>
is running to ensure that sent packets are fragmented. If your host
OS is causing problems, try the
<option>--send-eth</option><indexterm><primary><option>--send-eth</option></primary></indexterm>
option to bypass the IP layer and send raw ethernet frames.</para>
<para>Fragmentation is only supported for Nmap's raw packet features,
which includes TCP and UDP port scans (except connect scan and FTP
bounce scan) and OS detection. Features such as version detection and
the Nmap Scripting Engine generally don't support fragmentation
because they rely on your host's TCP stack to communicate with target
services.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-D <replaceable>decoy1</replaceable><optional>,<replaceable>decoy2</replaceable></optional><optional>,ME</optional><optional>,...</optional></option>
(Cloak a scan with decoys)
<indexterm><primary><option>-D</option></primary></indexterm>
<indexterm><primary>decoys</primary></indexterm>
</term>
<listitem>
<para>Causes a decoy scan to be performed, which makes it
appear to the remote host that the host(s) you specify as
decoys are scanning the target network too. Thus their IDS
might report 5&ndash;10 port scans from unique IP addresses, but
they won't know which IP was scanning them and which were
innocent decoys. While this can be defeated through router
path tracing, response-dropping, and other active
mechanisms, it is generally an effective technique for
hiding your IP address.</para>
<para>Separate each decoy host with commas, and you can
optionally use
<literal>ME</literal><indexterm><primary><literal>ME</literal> (decoy address)</primary></indexterm>
as one of the decoys to
represent the position for your real IP address. If you put
<literal>ME</literal> in the sixth position or later, some
common port scan detectors (such as
Solar Designer's<indexterm><primary>Solar Designer</primary></indexterm>
excellent Scanlogd)<indexterm><primary><application>Scanlogd</application></primary></indexterm>
are unlikely to show your IP address at
all. If you don't use <literal>ME</literal>, Nmap will put
you in a random position. You can also use
<literal>RND</literal><indexterm><primary><literal>RND</literal> (decoy address)</primary></indexterm>
to generate
a random, non-reserved IP address, or <literal>RND:<replaceable>number</replaceable></literal> to
generate <replaceable>number</replaceable> addresses.</para> <para>Note that the hosts
you use as decoys should be up or you might accidentally SYN
flood your targets. Also it will be pretty easy to determine
which host is scanning if only one is actually up on the
network. You might want to use IP addresses instead of names
(so the decoy networks don't see you in their nameserver
logs).</para>
<para>Decoys are used both in the initial ping scan (using
ICMP, SYN, ACK, or whatever) and during the actual port
scanning phase. Decoys are also used during remote OS
detection (<option>-O</option>). Decoys do not work with
version detection or TCP connect scan. When a scan delay is
in effect, the delay is enforced between each batch of
spoofed probes, not between each individual probe. Because
decoys are sent as a batch all at once, they may temporarily
violate congestion control limits.</para>
<para>It is worth noting that using too many decoys may
slow your scan and potentially even make it less
accurate. Also, some ISPs will filter out your spoofed
packets, but many do not restrict
spoofed IP packets at all.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-S <replaceable>IP_Address</replaceable></option> (Spoof source address)
<indexterm><primary><option>-S</option></primary></indexterm>
<indexterm><primary>spoofing source address</primary></indexterm>
</term>
<listitem>
<para>In some circumstances,
Nmap may not be able to determine your
source address (Nmap will tell you if this is the
case). In this situation, use <option>-S</option> with the IP address of
the interface you wish to send packets through.</para>
<para>Another possible use of this flag is to spoof the scan
to make the targets think that <emphasis>someone
else</emphasis> is scanning them. Imagine a company being
repeatedly port scanned by a competitor! The
<option>-e</option> option and <option>-Pn</option> are
generally required for this sort of usage. Note that you
usually won't receive reply packets back (they will be
addressed to the IP you are spoofing), so Nmap won't produce
useful reports.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-e <replaceable>interface</replaceable></option> (Use specified interface)
<indexterm><primary><option>-e</option></primary></indexterm>
<indexterm><primary>interface</primary><seealso><option>-e</option></seealso></indexterm>
</term>
<listitem>
<para>Tells Nmap what interface to send and receive
packets on. Nmap should be able to detect this
automatically, but it
will tell you if it cannot.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--source-port <replaceable>portnumber</replaceable>;</option>
<option>-g <replaceable>portnumber</replaceable></option> (Spoof source port number)
<indexterm significance="preferred"><primary><option>--source-port</option></primary></indexterm>
<indexterm significance="preferred"><primary><option>-g</option></primary></indexterm>
<indexterm><primary>source port number</primary></indexterm>
</term>
<listitem>
<para>One surprisingly common misconfiguration is to trust traffic
based only on the source port number. It is easy to understand how
this comes about. An administrator will set up a shiny new firewall,
only to be flooded with complaints from ungrateful users whose
applications stopped working. In particular, DNS may be broken
because the UDP DNS replies from external servers can no longer enter
the network. FTP is another common example. In active FTP transfers,
the remote server tries to establish a connection back to the client
to transfer the requested file.</para>
<para>Secure solutions to these problems exist, often in the form of
application-level proxies or protocol-parsing firewall modules.
Unfortunately there are also easier, insecure solutions. Noting that
DNS replies come from port 53 and active FTP from port 20, many administrators
have fallen into the trap of simply allowing incoming traffic from
those ports. They often assume that no attacker would notice and
exploit such firewall holes. In other cases, administrators consider this a
short-term stop-gap measure until they can implement a more secure
solution. Then they forget the security upgrade.
</para>
<para>Overworked network administrators are not the only ones to fall
into this trap. Numerous products have shipped with these insecure
rules. Even Microsoft has been guilty. The IPsec filters that
shipped with Windows 2000 and Windows XP contain an implicit rule that
allows all TCP or UDP traffic from port 88 (Kerberos). In another well-known
case, versions of the Zone Alarm personal firewall up to 2.1.25
allowed any incoming UDP packets with the source port 53 (DNS) or 67
(DHCP).</para>
<para>Nmap offers the <option>-g</option> and
<option>--source-port</option> options (they are equivalent) to exploit these
weaknesses. Simply provide a port number and Nmap will send packets
from that port where possible. Most scanning operations that use raw sockets,
including SYN and UDP scans, support the option completely. The option notably
doesn't have an effect for any operations that use normal operating system
sockets, including DNS requests, TCP <function>connect</function>
scan,<indexterm><primary>connect scan</primary></indexterm> version detection,
and script scanning. Setting the source port also doesn't work for OS detection,
because Nmap must use different port numbers for certain OS detection tests to
work properly.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--data <replaceable>hex string</replaceable></option> (Append custom binary data to sent packets)
<indexterm><primary><option>--data</option></primary></indexterm>
</term>
<listitem>
<para>
This option lets you include binary data as payload in sent packets.
<replaceable>hex string</replaceable> may be specified in any of
the following formats: <literal>0xAABBCCDDEEFF<replaceable>...</replaceable></literal>,
<literal>AABBCCDDEEFF<replaceable>...</replaceable></literal> or
<literal>\xAA\xBB\xCC\xDD\xEE\xFF<replaceable>...</replaceable></literal>.
Examples of use are <option>--data 0xdeadbeef</option> and
<option>--data \xCA\xFE\x09</option>. Note that if you specify a
number like <literal>0x00ff</literal>
no byte-order conversion is performed. Make sure you specify
the information in the byte order expected by the receiver.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--data-string <replaceable>string</replaceable></option> (Append custom string to sent packets)
<indexterm><primary><option>--data-string</option></primary></indexterm>
</term>
<listitem>
<para>
This option lets you include a regular string as payload in
sent packets. <replaceable>string</replaceable> can
contain any string. However, note that some characters
may depend on your system's locale and the receiver may not
see the same information. Also, make sure you enclose the string
in double quotes and escape any special characters from the shell.
Examples: <option>--data-string "Scan conducted by Security Ops, extension 7192"</option>
or <option>--data-string "Ph34r my l33t skills"</option>.
Keep in mind that nobody is likely to actually see any comments left by this option
unless they are carefully monitoring the network with a sniffer or custom IDS rules.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--data-length <replaceable>number</replaceable></option> (Append random
data to sent packets)
<indexterm><primary><option>--data-length</option></primary></indexterm>
</term>
<listitem>
<para>Normally Nmap sends minimalist packets containing only
a header. So its TCP packets are generally 40
bytes and ICMP echo requests are just 28. Some
UDP ports<indexterm><primary>protocol-specific payloads</primary><secondary>UDP</secondary></indexterm>
and IP protocols<indexterm><primary>protocol-specific payloads</primary><secondary>IP</secondary></indexterm>
get a custom payload by default.
This option
tells Nmap to append the given number of random bytes to
most of the packets it sends, and not to use any
protocol-specific payloads. (Use <option>--data-length 0</option>
for no random or protocol-specific
payloads.<indexterm><primary>protocol-specific payloads</primary><secondary>disabling with <option>--data-length</option></secondary></indexterm>
OS detection (<option>-O</option>) packets
are not affected<indexterm><primary><option>--data-length</option></primary><secondary>no effect in OS detection</secondary></indexterm>
because accuracy there requires probe consistency, but most pinging and portscan packets
support this. It slows things down a little, but can make a scan slightly less
conspicuous.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--ip-options <replaceable>S|R [route]|L [route]|T|U ... </replaceable>;</option>
<option>--ip-options <replaceable>hex string</replaceable></option> (Send packets with specified ip options)
<indexterm><primary><option>--ip-options</option></primary></indexterm>
<indexterm><primary>IP options</primary></indexterm>
</term>
<listitem>
<para>The <ulink role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc791.txt">IP
protocol</ulink> offers several options which may be placed in
packet headers. Unlike the ubiquitous TCP options, IP options
are rarely seen due to practicality and security concerns. In
fact, many Internet routers block the most dangerous options
such as source routing. Yet options can still be useful in some
cases for determining and manipulating the network route to
target machines. For example, you may be able to use the record
route option to determine a path to a target even when more
traditional traceroute-style approaches fail. Or if your
packets are being dropped by a certain firewall, you may be able
to specify a different route with the strict or loose source
routing options.</para>
<para>The most powerful way to specify IP options is to simply
pass in values as the argument to
<option>--ip-options</option>. Precede each hex number with
<literal>\x</literal> then the two digits. You may repeat
certain characters by following them with an asterisk and then
the number of times you wish them to repeat. For example,
<literal>\x01\x07\x04\x00*36\x01</literal> is a hex string
containing 36 NUL bytes.</para>
<para>Nmap also offers a shortcut mechanism for specifying
options. Simply pass the letter <literal>R</literal>,
<literal>T</literal>, or <literal>U</literal> to request
record-route,<indexterm><primary>record route IP option</primary></indexterm>
record-timestamp,<indexterm><primary>record timestamp IP option</primary></indexterm>
or both options together,
respectively.
Loose or strict source routing<indexterm><primary>source routing</primary></indexterm>
may be specified
with an <literal>L</literal> or <literal>S</literal> followed by
a space and then a space-separated list of IP addresses.</para>
<para>If you wish to see the options in packets sent and
received, specify <option>--packet-trace</option>. For more
information and examples of using IP options with Nmap, see
<ulink url="http://seclists.org/nmap-dev/2006/q3/52"/>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--ttl <replaceable>value</replaceable></option> (Set IP time-to-live field)
<indexterm><primary><option>--ttl</option></primary></indexterm>
<indexterm><primary>time to live (TTL)</primary></indexterm>
</term>
<listitem>
<para>Sets the IPv4 time-to-live field in sent packets to
the given value.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--randomize-hosts</option> (Randomize target host order)
<indexterm significance="preferred"><primary><option>--randomize-hosts</option></primary></indexterm>
<indexterm><primary>randomization of hosts</primary></indexterm>
</term>
<listitem>
<para>Tells Nmap to shuffle each group of up to 16384 hosts
before it scans them. This can make the scans less obvious
to various network monitoring systems, especially when you
combine it with slow timing options. If you
want to randomize over larger group sizes, increase
<varname>PING_GROUP_SZ</varname><indexterm><primary><varname>PING_GROUP_SZ</varname></primary></indexterm>
in <filename>nmap.h</filename><indexterm><primary><filename>nmap.h</filename></primary></indexterm>
and recompile.
An alternative solution is to generate the target IP list
with a list scan (<option>-sL -n -oN
<replaceable>filename</replaceable></option>), randomize it
with a Perl script, then provide the whole list to Nmap with
<option>-iL</option>.<indexterm><primary><option>-iL</option></primary><secondary>randomizing hosts with</secondary></indexterm>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--spoof-mac <replaceable>MAC address, prefix, or vendor
name</replaceable></option> (Spoof MAC address)
<indexterm significance="preferred"><primary><option>--spoof-mac</option></primary></indexterm>
<indexterm><primary>spoofing MAC address</primary></indexterm>
</term>
<listitem>
<para>Asks Nmap to use the given MAC address
<indexterm><primary>MAC address</primary></indexterm>
for all of the raw ethernet frames it sends. This option implies
<option>--send-eth</option><indexterm><primary><option>--send-eth</option></primary><secondary>implied by <option>--spoof-mac</option></secondary></indexterm>
to ensure that Nmap actually sends
ethernet-level packets. The MAC given can take several formats. If
it is simply the number <literal>0</literal>, Nmap chooses a completely random MAC address
for the session. If the given string is an even number of hex
digits (with the pairs optionally separated by a colon), Nmap will
use those as the MAC. If fewer than 12 hex digits are provided, Nmap
fills in the remainder of the six bytes with random values. If the
argument isn't a zero or hex string, Nmap looks through
<filename>nmap-mac-prefixes</filename> to find a vendor name containing the given string
(it is case insensitive). If a match is found, Nmap uses the
vendor's OUI (three-byte prefix)<indexterm><primary>organizationally unique identifier (OUI)</primary><seealso><filename>nmap-mac-prefixes</filename></seealso></indexterm>
and fills out the remaining three bytes
randomly. Valid <option>--spoof-mac</option> argument examples are <literal>Apple</literal>, <literal>0</literal>,
<literal>01:02:03:04:05:06</literal>, <literal>deadbeefcafe</literal>, <literal>0020F2</literal>, and <literal>Cisco</literal>. This option only affects raw packet scans such as SYN scan or OS detection, not connection-oriented features such as version detection or the Nmap Scripting Engine.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--proxies <replaceable>Comma-separated list of proxy
URLs</replaceable></option> (Relay TCP connections through a chain of
proxies)
<indexterm significance="preferred"><primary><option>--proxies</option></primary></indexterm>
<indexterm><primary>proxy</primary></indexterm>
<indexterm><primary>proxies</primary></indexterm>
</term>
<listitem>
<para>Asks Nmap to establish TCP connections with a final
target through supplied chain of one or more HTTP or SOCKS4
<indexterm><primary>proxies</primary></indexterm>
proxies. Proxies
can help hide the true source of a scan or evade certain
firewall restrictions, but they can hamper scan performance
by increasing latency. Users may need to adjust Nmap
timeouts and other scan parameters accordingly. In
particular, a lower <option>--max-parallelism</option> may
help because some proxies refuse to handle as many
concurrent connections as Nmap opens by default.</para>
<para>This option takes a list of proxies as argument, expressed as
URLs in the format <literal>proto://host:port</literal>. Use commas to separate
node URLs in a chain. No authentication is supported yet. Valid
protocols are <literal>HTTP</literal> and <literal>SOCKS4</literal>.
</para>
<para>Warning: this feature is still under development and has
limitations. It is implemented within the nsock library and thus has
no effect on the ping, port scanning and OS discovery phases
of a scan. Only NSE and version scan benefit from this
option so far&mdash;other features may disclose your true address. SSL
connections are not yet supported, nor is proxy-side DNS
resolution (hostnames are always resolved by Nmap).</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--badsum</option> (Send packets with bogus TCP/UDP checksums)
<indexterm><primary><option>--badsum</option></primary></indexterm>
<indexterm><primary>TCP checksum</primary></indexterm>
<indexterm><primary>checksums</primary></indexterm>
</term>
<listitem>
<para>Asks Nmap to use an invalid TCP, UDP or SCTP checksum for
packets sent to target hosts. Since virtually all
host IP stacks properly drop these packets, any responses received
are likely coming from a firewall or IDS that didn't bother to
verify the checksum. For more details on this technique, see <ulink
url="https://nmap.org/p60-12.html"/></para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--adler32</option> (Use deprecated Adler32 instead of CRC32C for SCTP checksums)
<indexterm><primary><option>--adler32</option></primary></indexterm>
<indexterm><primary>CRC32C checksum</primary></indexterm>
<indexterm><primary>Adler32 checksum</primary></indexterm>
<indexterm><primary>SCTP checksum</primary></indexterm>
</term>
<listitem>
<para>Asks Nmap to use the deprecated Adler32 algorithm
for calculating the SCTP checksum. If <option>--adler32</option>
is not given, CRC-32C (Castagnoli) is used.
<ulink role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc2960.txt">RFC 2960</ulink>
originally defined Adler32 as checksum algorithm for SCTP;
<ulink role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc4960.txt">RFC 4960</ulink>
later redefined the SCTP checksums to use CRC-32C. Current SCTP
implementations should be using CRC-32C, but in order to elicit
responses from old, legacy SCTP implementations, it may be
preferable to use Adler32.</para>
</listitem>
</varlistentry>
</variablelist>
<indexterm class="endofrange" startref="man-bypass-ids-indexterm"/>
<indexterm class="endofrange" startref="man-bypass-filewalls-indexterm"/>
</refsect1>
<refsect1 id='man-output'>
<title>Output</title>
<indexterm class="startofrange" id="man-output-indexterm"><primary>output formats</primary></indexterm>
<para>Any security tool is only as useful as the output it
generates. Complex tests and algorithms are of little value if
they aren't presented in an organized and comprehensible fashion.
Given the number of ways Nmap is used by people and other
software, no single format can please everyone. So Nmap offers
several formats, including the interactive mode for humans to read
directly and XML for easy parsing by software.</para>
<para>In addition to offering different output formats, Nmap provides
options for controlling the verbosity of output as well as debugging
messages. Output types may be sent to standard output or to named
files, which Nmap can append to or clobber. Output files may also be
used to resume aborted scans.</para>
<para>Nmap makes output available in five different formats.
The default is called
<firstterm>interactive output</firstterm>,<indexterm><primary>interactive output</primary></indexterm>
and it is sent to
standard output (stdout).<indexterm><primary>standard output</primary></indexterm>
There is also
<firstterm>normal output</firstterm>,<indexterm><primary>normal output</primary></indexterm>
which is similar to interactive except that it
displays less runtime information and warnings since it is expected to
be analyzed after the scan completes rather than interactively.</para>
<para><firstterm>XML output</firstterm><indexterm><primary>XML output</primary></indexterm>
is one of the most important output types, as it can
be converted to HTML, easily parsed by programs such as Nmap graphical
user interfaces, or imported into databases.</para>
<para>The two remaining output types are the simple
<firstterm>grepable output</firstterm><indexterm><primary>grepable output</primary></indexterm>
which includes most information for a target host on
a single line, and
<firstterm>sCRiPt KiDDi3 0utPUt</firstterm><indexterm><primary sortas="script kiddie output">scR1pT kIddI3 output</primary></indexterm>
for users
who consider themselves |&lt;-r4d.</para>
<para>While interactive output is the default and has no associated
command-line options, the other four format options use the same
syntax. They take one argument, which is the filename that results
should be stored in. Multiple formats may be specified, but each
format may only be specified once. For example, you may wish to save
normal output for your own review while saving XML of the same scan
for programmatic analysis. You might do this with the options
<option>-oX myscan.xml -oN myscan.nmap</option>. While this chapter
uses the simple names like <literal>myscan.xml</literal> for brevity,
more descriptive names are generally recommended. The names chosen
are a matter of personal preference, though I use long ones that
incorporate the scan date and a word or two describing the scan, placed
in a directory named after the company I'm scanning.</para>
<para>While these options save results to files, Nmap still prints
interactive output to stdout as usual. For example, the command
<command>nmap -oX myscan.xml target</command> prints XML to
<filename>myscan.xml</filename> and fills standard output with the same interactive results it would have printed if <option>-oX</option>
wasn't specified at all. You can change this by passing a hyphen
character as the argument to one of the format types. This causes
Nmap to deactivate interactive output, and instead print
results in the format you specified to the standard output stream. So the
command <command>nmap -oX - target</command> will send only XML output to
stdout.<indexterm><primary>output</primary><secondary>to stdout with <literal>-</literal></secondary></indexterm>
Serious errors may still be printed to the normal error
stream, stderr.<indexterm><primary>standard error</primary></indexterm>
</para>
<para>Unlike some Nmap arguments, the space between the logfile option
flag (such as <option>-oX</option>) and the filename or hyphen is
mandatory. If you omit the flags and give arguments such as
<option>-oG-</option> or <option>-oXscan.xml</option>, a backwards
compatibility feature of Nmap will cause the creation of
<emphasis>normal format</emphasis> output files named
<filename>G-</filename> and <filename>Xscan.xml</filename>
respectively.</para>
<para>All of these arguments support
<function>strftime</function>-like<indexterm><primary><function>strftime</function> conversions in filenames</primary></indexterm>
conversions in the filename. <literal>%H</literal>, <literal>%M</literal>,
<literal>%S</literal>, <literal>%m</literal>, <literal>%d</literal>,
<literal>%y</literal>, and <literal>%Y</literal> are all exactly the same
as in <function>strftime</function>. <literal>%T</literal> is the same
as <literal>%H%M%S</literal>, <literal>%R</literal> is the same as
<literal>%H%M</literal>, and <literal>%D</literal> is the same as
<literal>%m%d%y</literal>. A <literal>%</literal> followed by any other
character just yields that character (<literal>%%</literal> gives you a
percent symbol). So <option>-oX 'scan-%T-%D.xml'</option> will use an XML
file with a name in the form of <filename>scan-144840-121307.xml</filename>.</para>
<para>Nmap also offers options to control scan verbosity and to append
to output files rather than clobbering them. All of these options are
described below.</para>
<variablelist><title>Nmap Output Formats</title>
<varlistentry>
<term>
<option>-oN <replaceable>filespec</replaceable></option> (normal output)
<indexterm><primary><option>-oN</option></primary></indexterm>
<indexterm><primary>normal output</primary></indexterm></term>
<listitem>
<para>Requests that normal output be
directed to the given filename. As discussed above, this
differs slightly from <literal>interactive output</literal>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-oX <replaceable>filespec</replaceable></option> (XML output)
<indexterm><primary><option>-oX</option></primary></indexterm>
<indexterm><primary>XML output</primary></indexterm></term>
<listitem>
<para>Requests that XML output be
directed to the given filename. Nmap includes a document
type definition (DTD) which allows XML parsers to validate
Nmap XML output. While it is primarily intended for
programmatic use, it can also help humans interpret Nmap XML
output. The DTD defines the legal elements of the format,
and often enumerates the attributes and values they can take
on. The latest version is always available from <ulink
url="https://svn.nmap.org/nmap/docs/nmap.dtd"/>.</para>
<para>XML offers a stable format that is easily parsed by
software. Free XML parsers are available for all major
computer languages, including C/C++, Perl, Python, and
Java. People have even written bindings for most of these
languages to handle Nmap output and execution specifically.
Examples are
<ulink url="http://sourceforge.net/projects/nmap-scanner/">Nmap::Scanner</ulink><indexterm><primary>Nmap::Scanner</primary></indexterm>
and
<ulink url="http://nmapparser.wordpress.com/">Nmap::Parser</ulink><indexterm><primary>Nmap::Parser</primary></indexterm>
in Perl
CPAN. In almost all cases that a non-trivial application
interfaces with Nmap, XML is the preferred format.</para>
<para>The XML output references an XSL stylesheet which can
be used to format the results as HTML. The easiest way to
use this is simply to load the XML output in a web browser
such as Firefox or IE. By default, this will only work on
the machine you ran Nmap on (or a similarly configured one)
due to the hard-coded <filename>nmap.xsl</filename>
filesystem path. Use the <option>--webxml</option> or
<option>--stylesheet</option> options to create portable XML
files that render as HTML on any web-connected
machine.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-oS <replaceable>filespec</replaceable></option> (ScRipT KIdd|3 oUTpuT)
<indexterm><primary><option>-oS</option></primary></indexterm>
<indexterm><primary sortas="script kiddie output">scR1pT kIddI3 output</primary></indexterm></term>
<listitem>
<para>Script kiddie output is like interactive output, except that
it is post-processed to better suit the l33t HaXXorZ who
previously looked down on Nmap due to its consistent capitalization
and spelling. Humor impaired people should note that this option
is making fun of the script kiddies before flaming me for
supposedly <quote>helping them</quote>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-oG <replaceable>filespec</replaceable></option> (grepable output)
<indexterm><primary><option>-oG</option></primary></indexterm>
<indexterm><primary>grepable output</primary></indexterm></term>
<listitem>
<para>This output format is covered last because it is deprecated.
The XML output format is far more powerful, and is nearly as
convenient for experienced users. XML is a standard for which dozens
of excellent parsers are available, while grepable output is my own
simple hack. XML is extensible to support new Nmap features as they
are released, while I often must omit those features from grepable
output for lack of a place to put them.</para>
<para>Nevertheless, grepable output is still quite popular. It is a
simple format that lists each host on one line and can be trivially
searched and parsed with standard Unix tools such as grep, awk, cut,
sed, diff, and Perl. Even I usually use it for one-off tests done at the
command line. Finding all the hosts with the SSH port open or that
are running Solaris takes only a simple grep to identify the hosts,
piped to an awk or cut command to print the desired fields.</para>
<para>Grepable output consists of comments (lines starting with a
pound (#))<indexterm><primary>grepable output</primary><secondary>comments in</secondary></indexterm>
and target lines. A target line includes a combination
of six labeled fields, separated by tabs and followed with a colon.
The fields are <literal>Host</literal>, <literal>Ports</literal>,
<literal>Protocols</literal>, <literal>Ignored State</literal>,
<literal>OS</literal>, <literal>Seq Index</literal>,
<literal>IP ID</literal>, and <literal>Status</literal>.</para>
<para>The most important of these fields is generally
<literal>Ports</literal>, which gives details on each interesting
port. It is a comma separated list of port entries. Each port entry
represents one interesting port, and takes the form of seven slash
(/) separated subfields. Those subfields are: <literal>Port
number</literal>, <literal>State</literal>, <literal>Protocol</literal>,
<literal>Owner</literal>, <literal>Service</literal>, <literal>SunRPC
info</literal>, and <literal>Version info</literal>.</para>
<para>As with XML output, this man page does not allow for documenting
the entire format. A more detailed look at the Nmap grepable output
format is available
<man>from <ulink url="https://nmap.org/book/output-formats-grepable-output.html" />.</man>
<notman>in <xref linkend="output-formats-grepable-output"/>.</notman>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-oA <replaceable>basename</replaceable></option> (Output to all formats)
<indexterm significance="preferred"><primary><option>-oA</option></primary></indexterm></term>
<listitem><para>
As a convenience, you may specify <option>-oA
<replaceable>basename</replaceable></option> to store scan
results in normal, XML, and grepable formats at once. They
are stored in
<filename><replaceable>basename</replaceable>.nmap</filename>,
<filename><replaceable>basename</replaceable>.xml</filename>, and
<filename><replaceable>basename</replaceable>.gnmap</filename>,
respectively. As with most programs, you can prefix the filenames
with a directory path, such as
<filename>~/nmaplogs/foocorp/</filename> on Unix or
<filename>c:\hacking\sco</filename> on Windows.</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist><title>Verbosity and debugging options</title>
<varlistentry>
<term>
<option>-v</option> (Increase verbosity level)
<indexterm><primary><option>-v</option></primary></indexterm>
<indexterm><primary>verbosity</primary></indexterm>
</term>
<term>
<option>-v<replaceable>level</replaceable></option> (Set verbosity level)
</term>
<listitem>
<para>Increases the verbosity level, causing Nmap to
print more information about the scan in progress. Open
ports are shown as they are found and completion time
estimates are provided when Nmap thinks a scan will take
more than a few minutes. Use it twice or more for even greater
verbosity: <option>-vv</option>, or give a verbosity level
directly, for example <option>-v3</option>.<indexterm><primary><option>-v</option></primary><secondary>giving more than once</secondary></indexterm>
</para>
<para>Most changes only affect interactive output, and some
also affect normal and script kiddie output. The other
output types are meant to be processed by machines, so Nmap
can give substantial detail by default in those formats
without fatiguing a human user. However, there are a few
changes in other modes where output size can be reduced
substantially by omitting some detail. For example, a
comment line in the grepable output that provides a list of
all ports scanned is only printed in verbose mode because it
can be quite long.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-d</option> (Increase debugging level)
<indexterm><primary><option>-d</option></primary></indexterm>
<indexterm><primary>debugging</primary></indexterm>
</term>
<term>
<option>-d<replaceable>level</replaceable></option> (Set debugging level)
</term>
<listitem>
<para>When even verbose mode doesn't provide sufficient data for you,
debugging is available to flood you with much more! As with the
verbosity option (<option>-v</option>), debugging is enabled with a
command-line flag (<option>-d</option>) and the debug level can be
increased by specifying it
multiple times,<indexterm><primary><option>-d</option></primary><secondary>giving more than once</secondary></indexterm>
as in <option>-dd</option>, or by setting a level directly. For
example, <option>-d9</option> sets level nine. That is the highest
effective level and will produce thousands of lines unless you run a
very simple scan with very few ports and targets.</para>
<para>Debugging output is useful when a bug is suspected in Nmap,
or if you are simply confused as to what Nmap is doing and why. As this
feature is mostly intended for developers, debug lines aren't always
self-explanatory. You may get something like: <computeroutput>Timeout
vals: srtt: -1 rttvar: -1 to: 1000000 delta 14987 ==> srtt: 14987
rttvar: 14987 to: 100000</computeroutput>. If you don't understand a line, your only recourses
are to ignore it, look it up in the source code, or request help from
the development list
(<citetitle>nmap-dev</citetitle>).<indexterm><primary><citetitle>nmap-dev</citetitle> mailing list</primary></indexterm>
Some lines are self explanatory, but
the messages become more obscure as the debug level is
increased.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--reason</option> (Host and port state reasons)
<indexterm><primary><option>--reason</option></primary></indexterm>
<indexterm><primary>reason reporting</primary><see><option>--reason</option></see></indexterm>
</term>
<listitem>
<para>
Shows the reason each port is set to a specific state and the reason
each host is up or down. This option displays the type of the packet
that determined a port or hosts state. For example, A <literal>RST</literal> packet from
a closed port or an echo reply from an alive host. The information
Nmap can provide is determined by the type of scan or ping. The SYN
scan and SYN ping (<option>-sS</option> and <option>-PS</option>) are very detailed, but the
TCP connect scan (<option>-sT</option>) is limited by the
implementation of the <function>connect</function> system call. This feature is automatically enabled by
the debug option
(<option>-d</option>)<indexterm><primary><option>--reason</option></primary><secondary>implied by <option>-d</option></secondary></indexterm>
and the results are stored in XML log files
even if this option is not specified.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--stats-every <replaceable>time</replaceable></option> (Print periodic timing stats)
<indexterm><primary><option>--stats-every</option></primary></indexterm>
</term>
<listitem>
<para>
Periodically prints a timing status message after each
interval of <replaceable>time</replaceable>. The time is a
specification of the kind described in
<xref linkend="man-performance"/>; so for example, use
<option>--stats-every 10s</option> to get a status update
every 10 seconds. Updates are printed to interactive output
(the screen) and XML output.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--packet-trace</option> (Trace packets and data sent and received)
<indexterm><primary><option>--packet-trace</option></primary></indexterm>
</term>
<listitem>
<para>Causes Nmap to print a summary of every packet sent
or received. This is often used for debugging, but is
also a valuable way for new users to understand exactly
what Nmap is doing under the covers. To avoid printing
thousands of lines, you may want to specify a limited
number of ports to scan, such as <option>-p20-30</option>. If you only care
about the goings on of the version detection subsystem, use
<option>--version-trace</option> instead. If you only care about script tracing,
specify <option>--script-trace</option>. With <option>--packet-trace</option>, you get
all of the above.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--open</option> (Show only open (or possibly open) ports)
<indexterm><primary><option>--open</option></primary></indexterm>
</term>
<listitem>
<para>Sometimes you only care about ports you can actually connect to
(<literal>open</literal> ones), and don't want results cluttered with
<literal>closed</literal>, <literal>filtered</literal>, and
<literal>closed|filtered</literal> ports. Output customization is
normally done after the scan using tools such as
<application>grep</application>, <application>awk</application>, and
<application>Perl</application>, but this feature was added due to
overwhelming requests. Specify <option>--open</option> to only see
hosts with at least one
<literal>open</literal>, <literal>open|filtered</literal>, or
<literal>unfiltered</literal> port, and only see ports in those states. These three states are treated just as they normally are, which means that <literal>open|filtered</literal> and <literal>unfiltered</literal> may be condensed into counts if there are an overwhelming number of them.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--iflist</option> (List interfaces and routes)
<indexterm><primary><option>--iflist</option></primary></indexterm>
</term><listitem>
<para>Prints the interface list and system routes as detected
by Nmap. This is useful for debugging routing problems or
device mischaracterization (such as Nmap treating a PPP
connection as ethernet).</para> </listitem> </varlistentry>
</variablelist>
<variablelist><title>Miscellaneous output options</title>
<varlistentry>
<term>
<option>--append-output</option> (Append to rather than clobber output files)
<indexterm><primary><option>--append-output</option></primary></indexterm>
</term>
<listitem>
<para>When you specify a filename to an output format flag
such as <option>-oX</option> or <option>-oN</option>, that
file is overwritten by default. If you prefer to keep the
existing content of the file and append the new results,
specify the <option>--append-output</option> option. All
output filenames specified in that Nmap execution will then
be appended to rather than clobbered. This doesn't work
well for XML (<option>-oX</option>) scan data as the
resultant file generally won't parse properly until you fix
it up by hand.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--resume <replaceable>filename</replaceable></option> (Resume aborted scan)
<indexterm><primary><option>--resume</option></primary></indexterm>
<indexterm><primary>resuming scans</primary></indexterm>
</term>
<listitem>
<para>Some extensive Nmap runs take a very long time&mdash;on
the order of days. Such scans don't always run to
completion. Restrictions may prevent Nmap from being run
during working hours, the network could go down, the machine
Nmap is running on might suffer a planned or unplanned
reboot, or Nmap itself could crash. The administrator running Nmap
could cancel it for any other reason as well, by pressing
<keycap>ctrl-C</keycap>. Restarting the whole scan from the
beginning may be undesirable. Fortunately, if normal
(<option>-oN</option>) or grepable (<option>-oG</option>)
logs were kept, the user can ask Nmap to resume scanning
with the target it was working on when execution ceased.
Simply specify the <option>--resume</option> option and pass
the normal/grepable output file as its argument. No other
arguments are permitted, as Nmap parses the output file to
use the same ones specified previously. Simply call Nmap as
<command>nmap --resume
<replaceable>logfilename</replaceable></command>. Nmap will
append new results to the data files specified in the
previous execution. Resumption does not support the XML
output format because combining the two runs into one valid
XML file would be difficult.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--stylesheet <replaceable>path or URL</replaceable></option> (Set XSL stylesheet to transform XML output)
<indexterm significance="preferred"><primary><option>--stylesheet</option></primary></indexterm>
</term>
<listitem>
<para>Nmap ships with an XSL<indexterm><primary>XSL</primary></indexterm>
stylesheet<indexterm><primary>stylesheet</primary></indexterm>
named <filename>nmap.xsl</filename><indexterm><primary><filename>nmap.xsl</filename></primary></indexterm>
for viewing or translating XML output to HTML.<indexterm><primary>HTML from XML output</primary></indexterm>
The XML output includes an <literal>xml-stylesheet</literal>
directive which points to <filename>nmap.xml</filename>
where it was initially installed by Nmap. Run the XML file
through an XSLT processor such as
<ulink url="http://xmlsoft.org/XSLT/">xsltproc</ulink><indexterm><primary>xsltproc</primary></indexterm>
to produce an HTML file. Directly opening the XML file in a
browser no longer works well because modern browsers limit the
locations a stylesheet may be loaded from.
If you wish to use a different
stylesheet, specify it as the argument to
<option>--stylesheet</option>. You must pass the full
pathname or URL. One common invocation is
<option>--stylesheet
https://nmap.org/svn/docs/nmap.xsl</option>. This
tells an XSLT processor to load the latest version of the stylesheet
from Nmap.Org. The <option>--webxml</option> option
does the same thing with less typing and memorization.
Loading the XSL from Nmap.Org makes it easier to view results on
a machine that doesn't have Nmap (and thus
<filename>nmap.xsl</filename>) installed. So the URL is
often more useful, but the local filesystem location of
<filename>nmap.xsl</filename> is used by default for privacy reasons.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--webxml</option> (Load stylesheet from Nmap.Org)
<indexterm significance="preferred"><primary><option>--webxml</option></primary></indexterm>
</term>
<listitem>
<para>This is a convenience option, nothing more than an alias for
<option significance="preferred">--stylesheet https://nmap.org/svn/docs/nmap.xsl</option>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option significance="preferred">--no-stylesheet</option> (Omit XSL stylesheet declaration from XML)
<indexterm significance="preferred"><primary><option>--no-stylesheet</option></primary></indexterm>
</term>
<listitem>
<para>Specify this option to prevent Nmap from associating any XSL
stylesheet with its XML output. The <literal>xml-stylesheet</literal> directive
is omitted.</para>
</listitem>
</varlistentry>
</variablelist>
<indexterm class="endofrange" startref="man-output-indexterm"/>
</refsect1>
<refsect1 id='man-misc-options'>
<title>Miscellaneous Options</title>
<para>This section describes some important (and not-so-important)
options that don't really fit anywhere else.</para>
<variablelist>
<varlistentry>
<term>
<option>-6</option> (Enable IPv6 scanning)
<indexterm significance="preferred"><primary><option>-6</option></primary></indexterm>
<indexterm><primary>IPv6</primary></indexterm>
</term>
<listitem>
<para>Nmap has IPv6 support for its most
popular features. Ping scanning,
port scanning, version detection, and the Nmap Scripting
Engine all support IPv6. The command syntax is the same as
usual except that you also add the <option>-6</option>
option. Of course, you must use IPv6 syntax if you specify
an address rather than a hostname. An address might look
like
<literal>3ffe:7501:4819:2000:210:f3ff:fe03:14d0</literal>,
so hostnames are recommended. The output looks the same as
usual, with the IPv6 address on the <quote>interesting
ports</quote> line being the only IPv6 giveaway.</para>
<para>While IPv6 hasn't exactly taken the world by storm, it
gets significant use in some (usually Asian) countries and
most modern operating systems support it. To use Nmap with
IPv6, both the source and target of your scan must be
configured for IPv6. If your ISP (like most of them) does
not allocate IPv6 addresses to you, free tunnel brokers are
widely available and work fine with Nmap. I use the free
IPv6 tunnel broker<indexterm><primary>IPv6 tunnel broker</primary></indexterm>
service at <ulink url="http://www.tunnelbroker.net"/>.
Other tunnel brokers are
<ulink url="http://en.wikipedia.org/wiki/List_of_IPv6_tunnel_brokers">listed
at Wikipedia</ulink>. 6to4 tunnels are another popular,
free approach.</para>
<para>On Windows, raw-socket IPv6 scans are supported only on
ethernet devices (not tunnels), and only on Windows
Vista<indexterm><primary>Windows Vista</primary></indexterm>
and later. Use the
<option>--unprivileged</option><indexterm><primary><option>--unprivileged</option></primary></indexterm>
option in other situations.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-A</option> (Aggressive scan options)
<indexterm significance="preferred"><primary><option>-A</option></primary></indexterm>
</term>
<listitem>
<para>This option enables additional advanced and aggressive
options. Presently this enables OS detection
(<option>-O</option>), version scanning
(<option>-sV</option>), script scanning
(<option>-sC</option>) and traceroute
(<option>--traceroute</option>).<indexterm><primary><option>-A</option></primary><secondary>features
enabled by</secondary></indexterm> More features may be
added in the future. The point is to enable a comprehensive
set of scan options without people having to remember a
large set of flags. However, because script scanning with
the default set is considered intrusive, you should not use
<option>-A</option> against target networks without
permission. This option only enables features, and not
timing options (such as <option>-T4</option>) or verbosity
options (<option>-v</option>) that you might want as
well. Options which require privileges (e.g. root access)
such as OS detection and traceroute will only be enabled
if those privileges are available.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--datadir <replaceable>directoryname</replaceable></option> (Specify custom Nmap data file location)
<indexterm significance="preferred"><primary><option>--datadir</option></primary></indexterm>
</term>
<listitem>
<para>Nmap obtains some special data at runtime in files
named <filename>nmap-service-probes</filename>,
<filename>nmap-services</filename>,
<filename>nmap-protocols</filename>,
<filename>nmap-rpc</filename>,
<filename>nmap-mac-prefixes</filename>, and
<filename>nmap-os-db</filename>. If the
location of any of these files has been specified (using the
<option>--servicedb</option> or <option>--versiondb</option> options),
that location is used for that file. After that, Nmap
searches these files in the directory specified with the
<option>--datadir</option> option (if any). Any files not
found there, are searched for in the directory specified by
the
<envar>NMAPDIR</envar><indexterm><primary><envar>NMAPDIR</envar> environment variable</primary></indexterm>
environment variable.
Next comes
<filename>~/.nmap</filename><indexterm><primary sortas="nmap directory"><filename>.nmap</filename> directory</primary></indexterm>
for real and effective UIDs; or on Windows,
<filename><replaceable>HOME</replaceable>\AppData\Roaming\nmap</filename>
(where <replaceable>HOME</replaceable> is the user's home directory,
like <filename>C:\Users\user</filename>). This is
followed by the location of the <filename>nmap</filename>
executable and the same location with
<code>../share/nmap</code> appended. Then a compiled-in
location such as <filename>/usr/local/share/nmap</filename> or <filename>/usr/share/nmap</filename>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--servicedb <replaceable>services file</replaceable></option> (Specify custom services file)
<indexterm significance="preferred"><primary><option>--servicedb</option></primary></indexterm>
</term>
<listitem>
<para>Asks Nmap to use the specified services file rather than
the <filename>nmap-services</filename> data file that
comes with Nmap. Using this option also causes a fast scan
(<option>-F</option>) to be used. See the description for
<option>--datadir</option> for more information on Nmap's data
files.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--versiondb <replaceable>service probes file</replaceable></option> (Specify custom service probes file)
<indexterm significance="preferred"><primary><option>--versiondb</option></primary></indexterm>
</term>
<listitem>
<para>Asks Nmap to use the specified service probes file rather than
the <filename>nmap-service-probes</filename> data file
that comes with Nmap. See the description for
<option>--datadir</option> for more information on Nmap's data
files.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--send-eth</option> (Use raw ethernet sending)
<indexterm significance="preferred"><primary><option>--send-eth</option></primary></indexterm>
</term>
<listitem>
<para>Asks Nmap to send packets at the raw ethernet (data
link) layer rather than the higher IP (network) layer. By
default, Nmap chooses the one which is generally best for
the platform it is running on.
Raw sockets (IP layer)<indexterm><primary>raw sockets</primary></indexterm>
are
generally most efficient for Unix machines, while ethernet
frames are required for Windows operation since Microsoft
disabled raw socket support. Nmap still uses raw IP packets
on Unix despite this option when there is no other choice
(such as non-ethernet connections).</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--send-ip</option> (Send at raw IP level)
<indexterm significance="preferred"><primary><option>--send-ip</option></primary></indexterm>
</term>
<listitem>
<para>Asks Nmap to send packets via raw IP sockets rather
than sending lower level ethernet frames. It is the
complement to the <option>--send-eth</option> option discussed
previously.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--privileged</option> (Assume that the user is fully privileged)
<indexterm significance="preferred"><primary><option>--privileged</option></primary></indexterm>
</term>
<listitem>
<para>Tells Nmap to simply assume that it is privileged
enough to perform raw socket sends, packet sniffing, and
similar operations that usually require
root privileges<indexterm><primary>privileged users</primary></indexterm><indexterm><primary>authorized users</primary><see>privileged users</see></indexterm>
on Unix systems. By default Nmap quits if such operations are
requested but <function>geteuid</function> is not
zero. <option>--privileged</option> is useful with Linux
kernel capabilities and similar systems that may be
configured to allow unprivileged users to perform raw-packet
scans. Be sure to provide this option flag before any flags
for options that require privileges (SYN scan, OS detection,
etc.). The
<envar>NMAP_PRIVILEGED</envar><indexterm><primary><envar>NMAP_PRIVILEGED</envar> environment variable</primary></indexterm>
environment variable
may be set as an equivalent alternative to
<option>--privileged</option>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--unprivileged</option> (Assume that the user lacks raw socket privileges)
<indexterm significance="preferred"><primary><option>--unprivileged</option></primary></indexterm>
<indexterm><primary>unprivileged users</primary></indexterm>
</term>
<listitem>
<para>This option is the opposite of
<option>--privileged</option>. It tells Nmap to treat the
user as lacking network raw socket and sniffing privileges.
This is useful for testing, debugging, or when the raw
network functionality of your operating system is somehow
broken. The
<envar>NMAP_UNPRIVILEGED</envar><indexterm><primary><envar>NMAP_UNPRIVILEGED</envar> environment variable</primary></indexterm>
environment variable
may be set as an equivalent alternative to
<option>--unprivileged</option>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>--release-memory</option> (Release memory before quitting)
<indexterm><primary><option>--release-memory</option></primary></indexterm>
</term>
<listitem>
<para>This option is only useful for memory-leak debugging.
It causes Nmap to release allocated memory just before it
quits so that actual memory leaks are easier to spot.
Normally Nmap skips this as the OS does this anyway upon
process termination.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-V</option>; <option>--version</option> (Print version number)
<indexterm><primary><option>-V</option></primary></indexterm>
<indexterm><primary><option>--version</option></primary></indexterm>
</term>
<listitem>
<para>Prints the Nmap version number and exits.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-h</option>; <option>--help</option> (Print help summary page)
<indexterm><primary><option>-h</option></primary></indexterm>
<indexterm><primary><option>--help</option></primary></indexterm>
</term>
<listitem>
<para>Prints a short help screen with the most common
command flags. Running Nmap without any arguments does the
same thing.</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1 id='man-runtime-interaction'>
<title>Runtime Interaction<indexterm><primary>runtime interaction</primary></indexterm></title>
<para>During the execution of Nmap, all key presses are
captured. This allows you to interact with the program
without aborting and restarting it. Certain special
keys will change options, while any other keys will print out a
status message telling you about the scan. The convention is
that
<emphasis>lowercase letters increase</emphasis> the
amount of printing, and
<emphasis>uppercase letters decrease</emphasis> the
printing. You may also press &lsquo;<emphasis>?</emphasis>&rsquo; for help.</para>
<variablelist>
<varlistentry>
<term>
<option>v</option> / <option>V</option>
</term>
<listitem>
<para>Increase / decrease the verbosity level</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>d</option> / <option>D</option>
</term>
<listitem>
<para>Increase / decrease the debugging Level</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>p</option> / <option>P</option>
</term>
<listitem>
<para>Turn on / off packet tracing</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>?</option>
</term>
<listitem>
<para>Print a runtime interaction help screen</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
Anything else
</term>
<listitem>
<para>Print out a status message like this:</para>
<screen>
Stats: 0:00:07 elapsed; 20 hosts completed (1 up), 1 undergoing Service Scan
Service scan Timing: About 33.33% done; ETC: 20:57 (0:00:12 remaining)
</screen>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1 id='man-examples'>
<title>Examples</title>
<para>Here are some Nmap usage examples, from the simple and
routine to a little more complex and esoteric. Some actual
IP addresses and domain names are used to make things
more concrete. In their place you should substitute
addresses/names from
<emphasis>your own network</emphasis>. While I don't think
port scanning other networks is or should be illegal, some network
administrators don't appreciate unsolicited scanning of their networks and may
complain. Getting permission first is the best approach.</para>
<para>For testing purposes, you have permission to scan the host
scanme.nmap.org.<indexterm><primary>scanme.nmap.org</primary></indexterm>
This permission only includes
scanning via Nmap and not testing exploits or denial of service
attacks. To conserve bandwidth, please do not initiate more than
a dozen scans against that host per day. If this free scanning
target service is abused, it will be taken down and Nmap will
report <computeroutput>Failed to resolve given hostname/IP:
scanme.nmap.org</computeroutput>. These permissions also apply to
the hosts scanme2.nmap.org,
scanme3.nmap.org, and so on, though those hosts
do not currently exist.</para>
<para>
<command>nmap -v scanme.nmap.org</command>
<indexterm><primary><option>-v</option></primary><secondary>example of</secondary></indexterm>
</para>
<para>This option scans all reserved TCP ports on the machine
<literal>scanme.nmap.org</literal> . The <option>-v</option>
option enables verbose mode.</para>
<para>
<command>nmap -sS -O scanme.nmap.org/24</command>
<indexterm><primary><option>-sS</option></primary><secondary>example of</secondary></indexterm>
<indexterm><primary><option>-O</option></primary><secondary>example of</secondary></indexterm>
</para>
<para>Launches a stealth SYN scan against each machine that is
up out of the 256 IPs on the class C sized network where
Scanme resides. It also tries to determine what
operating system is running on each host that is up and
running. This requires root privileges because of the SYN scan
and OS detection.</para>
<para>
<command>nmap -sV -p 22,53,110,143,4564
198.116.0-255.1-127</command>
<indexterm><primary><option>-p</option></primary><secondary>example of</secondary></indexterm>
</para>
<para>Launches host enumeration and a TCP scan at the first half
of each of the 255 possible eight-bit subnets in the 198.116 class B
address space. This tests whether the systems run SSH, DNS, POP3,
or IMAP on their standard ports, or anything on port 4564. For any
of these ports found open, version detection is used to determine
what application is running.</para>
<para>
<command>nmap -v -iR 100000 -Pn -p 80</command>
<indexterm><primary><option>-iR</option></primary><secondary>example of</secondary></indexterm>
<indexterm><primary><option>-Pn</option></primary><secondary>example of</secondary></indexterm>
</para>
<para>Asks Nmap to choose 100,000 hosts at random and scan them
for web servers (port 80). Host enumeration is disabled with
<option>-Pn</option> since first sending a couple probes to
determine whether a host is up is wasteful when you are only
probing one port on each target host anyway.</para>
<para>
<command>nmap -Pn -p80 -oX logs/pb-port80scan.xml -oG
logs/pb-port80scan.gnmap 216.163.128.20/20</command>
<indexterm><primary><option>-oX</option></primary><secondary>example of</secondary></indexterm>
<indexterm><primary><option>-oG</option></primary><secondary>example of</secondary></indexterm>
</para>
<para>This scans 4096 IPs for any web servers (without pinging
them) and saves the output in grepable and XML formats.</para>
</refsect1>
<notbook>
<refsect1 id='man-book'>
<title>Nmap Book</title>
<!-- This paragraph is a bit jumbled together for man page rendering purposes, where extra newlines were causing problems -->
<para>While this reference guide details all material Nmap
options, it can't fully demonstrate how to apply those features to
quickly solve real-world tasks. For that, we
released <notweb><citetitle>Nmap Network Scanning: The Official
Nmap Project Guide to Network Discovery and Security
Scanning</citetitle>.</notweb><web><ulink url="https://nmap.org/book/"><citetitle>Nmap
Network Scanning: The Official Nmap Project Guide to Network
Discovery and Security Scanning</citetitle></ulink>.</web> Topics
include subverting firewalls and intrusion detection systems,
optimizing Nmap performance, and automating common networking
tasks with the Nmap Scripting Engine. Hints and instructions are
provided for common Nmap tasks such as taking network inventory,
penetration testing, detecting rogue wireless access points, and
quashing network worm outbreaks. Examples and diagrams show
actual communication on the wire. More than half of the book is
available free online. See <ulink url="https://nmap.org/book"/>
for more information.</para>
</refsect1>
</notbook>
<refsect1 id='man-bugs'>
<title>Bugs</title>
<indexterm><primary>bugs, reporting</primary></indexterm>
<para>Like its author, Nmap isn't perfect. But you can help make
it better by sending bug reports or even writing patches. If Nmap
doesn't behave the way you expect, first upgrade to the latest
version available from <ulink
url="https://nmap.org"/>. If the problem persists,
do some research to determine whether it has already been
discovered and addressed. Try searching for the error message on our search page at <ulink url="http://insecure.org/search.html"/> or at Google. Also try
browsing the <citetitle>nmap-dev</citetitle> archives at <ulink
url="http://seclists.org/" />.<indexterm><primary><citetitle>nmap-dev</citetitle> mailing list</primary></indexterm>
Read this full manual page as
well. If nothing comes of this, mail a bug report to
<email>dev@nmap.org</email>. Please include everything
you have learned about the problem, as well as what version of
Nmap you are running and what operating system version it is
running on. Problem reports and Nmap usage questions sent to
<email>dev@nmap.org</email> are far more likely to be answered than
those sent to Fyodor directly. If you subscribe to the nmap-dev
list before posting, your message will bypass moderation and get
through more quickly. Subscribe at <ulink url="https://nmap.org/mailman/listinfo/dev"/>.</para>
<para>Code patches to fix bugs are even better than bug reports.
Basic instructions for creating patch files with your changes are
available at <ulink
url="https://svn.nmap.org/nmap/HACKING" />. Patches may
be sent to <citetitle>nmap-dev</citetitle> (recommended) or to Fyodor directly.</para>
</refsect1>
<refsect1 id='man-author'>
<title>Author</title>
<para>Gordon <quote>Fyodor</quote> Lyon
<email>fyodor@nmap.org</email>
(<ulink url="http://insecure.org" />)
</para>
<para>Hundreds of people have made valuable contributions to Nmap
over the years. These are detailed in the
<filename>CHANGELOG</filename><indexterm><primary>changelog</primary></indexterm>
file which is distributed with Nmap
and also available from <ulink
url="https://nmap.org/changelog.html" />.</para>
</refsect1>
<refsect1 id='man-legal'>
&legal-notices;
</refsect1>
</refentry>
<indexterm class="endofrange" startref="man-nmap1-indexterm"/>
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