New option --defeat-icmp-ratelimit. Closes #353, Fixes #216

CHANGELOG

@@ -1,5 +1,10 @@
 # Nmap Changelog ($Id$); -*-text-*-
 
+o [GH#353] New option --defeat-icmp-ratelimit dramatically reduces UDP scan
+  times in exchange for labeling unresponsive (and possibly open) ports as
+  "closed|filtered". Ports which give a UDP protocol response to one of Nmap's
+  scanning payloads will be marked "open". [Sergey Khegay]
+
 o [NSE] Script cics-enum enumerates CICS transaction IDs, mapping to screens in
   TN3270 services. [Soldier of Fortran]
 

NmapOps.cc

@@ -323,6 +323,7 @@ void NmapOps::Initialize() {
   open_only = false;
   scanflags = -1;
   defeat_rst_ratelimit = 0;
+  defeat_icmp_ratelimit = 0;
   resume_ip.s_addr = 0;
   osscan_limit = 0;
   osscan_guess = 0;
@@ -554,7 +555,11 @@ dialog where you can start NPF if you have administrator privileges.";
     servicescan = 0;
 
   if (defeat_rst_ratelimit && !synscan) {
-      fatal("Option --defeat-rst-ratelimit works only with a SYN scan (-sS)");
+    fatal("Option --defeat-rst-ratelimit works only with a SYN scan (-sS)");
+  }
+
+  if (defeat_icmp_ratelimit && !udpscan) {
+    fatal("Option --defeat-icmp-ratelimit works only with a UDP scan (-sU)");
   }
 
   if (resume_ip.s_addr && generate_random_ips)

NmapOps.h

@@ -304,6 +304,11 @@ class NmapOps {
             slow against it. If we don't distinguish between closed and filtered ports,
             we can get the list of open ports very fast */
 
+  int defeat_icmp_ratelimit; /* If a host rate-limits ICMP responses, then scanning
+            is very slow against it. This option prevents Nmap to adjust timing
+            when it changes the port's state because of ICMP response, as the latter
+            might be rate-limited. Doing so we can get scan results faster. */
+
   struct in_addr resume_ip; /* The last IP in the log file if user
                                requested --restore .  Otherwise
                                restore_ip.s_addr == 0.  Also

docs/refguide.xml

@@ -2940,6 +2940,25 @@ worth the extra time.</para>
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><option>--defeat-icmp-ratelimit</option>
+        <indexterm><primary><option>--defeat-icmp-ratelimit</option></primary></indexterm></term>
+        <listitem>
+
+<para>Similar to <option>--defeat-rst-ratelimit</option>, the
+  <option>--defeat-icmp-ratelimit</option> option trades accuracy for speed,
+  increasing UDP scanning speed against hosts that rate-limit ICMP error
+  messages.  Because this option causes Nmap to not delay in order to receive
+  the port unreachable messages, a non-responsive port will be labeled
+  <literal>closed|filtered</literal> instead of the default
+  <literal>open|filtered</literal>. This has the effect of only treating ports
+  which actually respond via UDP as <literal>open</literal>. Since many UDP
+  services do not respond in this way, the chance for inaccuracy is greater
+  with this option than with <option>--defeat-rst-ratelimit</option>.</para>
+
+        </listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><option>--nsock-engine
         epoll|kqueue|poll|select</option>

nmap.cc

@@ -585,6 +585,8 @@ void parse_options(int argc, char **argv) {
     {"scanflags", required_argument, 0, 0},
     {"defeat_rst_ratelimit", no_argument, 0, 0},
     {"defeat-rst-ratelimit", no_argument, 0, 0},
+    {"defeat_icmp_ratelimit", no_argument, 0, 0},
+    {"defeat-icmp-ratelimit", no_argument, 0, 0},
     {"host_timeout", required_argument, 0, 0},
     {"host-timeout", required_argument, 0, 0},
     {"scan_delay", required_argument, 0, 0},
@@ -835,6 +837,8 @@ void parse_options(int argc, char **argv) {
           delayed_options.pre_scan_delay = l;
         } else if (optcmp(long_options[option_index].name, "defeat-rst-ratelimit") == 0) {
           o.defeat_rst_ratelimit = 1;
+        } else if (optcmp(long_options[option_index].name, "defeat-icmp-ratelimit") == 0) {
+          o.defeat_icmp_ratelimit = 1;
         } else if (optcmp(long_options[option_index].name, "max-scan-delay") == 0) {
           l = tval2msecs(optarg);
           if (l < 0)

output.cc

@@ -2543,6 +2543,9 @@ void printfinaloutput() {
       log_write(LOG_PLAIN, "OS detection performed. Please report any incorrect results at https://nmap.org/submit/ .\n");
     else if (o.servicescan)
       log_write(LOG_PLAIN, "Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .\n");
+    else if (o.udpscan && o.defeat_icmp_ratelimit)
+      log_write(LOG_PLAIN, "WARNING: Some ports marked closed|filtered may actually be open. For more accurate results, do not use --defeat-icmp-ratelimit .\n");
+
   }
 
   log_write(LOG_STDOUT | LOG_SKID,

scan_engine.cc

@@ -865,7 +865,8 @@ static void set_default_port_state(std::vector<Target *> &targets, stype scantyp
       (*target)->ports.setDefaultPortState(IPPROTO_TCP, PORT_OPENFILTERED);
       break;
     case UDP_SCAN:
-      (*target)->ports.setDefaultPortState(IPPROTO_UDP, PORT_OPENFILTERED);
+      (*target)->ports.setDefaultPortState(IPPROTO_UDP,
+        o.defeat_icmp_ratelimit ? PORT_CLOSEDFILTERED : PORT_OPENFILTERED);
       break;
     case IPPROT_SCAN:
       (*target)->ports.setDefaultPortState(IPPROTO_IP, PORT_OPENFILTERED);
@@ -2125,6 +2126,25 @@ void ultrascan_port_probe_update(UltraScanInfo *USI, HostScanStats *hss,
       adjust_timing = false;
     adjust_ping = false;
   }
+  /* Do not slow down if
+     1)  we are in --defeat-icmp-ratelimit mode
+     2)  the new state is closed or filtered
+     3)  this is a UDP scan
+     We don't want to adjust timing when we get ICMP response, as the host might
+     be ratelimiting them. E.g. the port is actually closed, but the host ratelimiting
+     ICMP responses so we had to retransmit the probe several times in order to
+     match the (slow) rate limit that the target is using for responses. We
+     do not want to waste time on such ports.
+     On the other hand if the port is detected to be open it is a good idea to
+     adjust timing as we could have done retransmissions due to conjested network */
+  if (rcvdtime != NULL
+      && o.defeat_icmp_ratelimit
+      && (newstate == PORT_CLOSED || newstate == PORT_FILTERED)
+      && USI->udp_scan) {
+    if (probe->tryno > 0)
+      adjust_timing = false;
+    adjust_ping = false;
+  }
 
   if (adjust_timing) {
     ultrascan_adjust_timing(USI, hss, probe, rcvdtime);