diff --git a/CHANGELOG b/CHANGELOG
index 0ee3aa0a4..d84c54718 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,5 +1,9 @@
 # Nmap Changelog ($Id$); -*-text-*-
 
+o [NSE] Added the firewalk script, which tries to find whether a
+  firewall blocks or forwards ports like the firewall tool does. [Henri
+  Doreau]
+
 o [NSE] Host tables now have a host.traceroute member when --traceroute
   is used. This array contains the IP address, reverse DNS name, and RTT
   for each traceroute hop. [Henri Doreau]
diff --git a/scripts/firewalk.nse b/scripts/firewalk.nse
new file mode 100644
index 000000000..341df093a
--- /dev/null
+++ b/scripts/firewalk.nse
@@ -0,0 +1,336 @@
+description = [[
+Try to discover firewall rules by using IP TTL expiration technique (method
+also known as firewalking").
+
+The scan requires a firewall (or "gateway") and a metric (or "target").
+
+For each filtered port on the target, send a probe with an IP TTL one greater
+than the number of hops to the gateway. The TTL can be supplied directly or
+retrieved by the script from traceroute results. In this second case, the
+script requires both the gateway IP address and the Nmap --traceroute flag.
+
+If the probe is forwarded by the gateway, then we can expect to receive an
+ICMP_TIME_EXCEEDED reply from the gateway next hop router, or eventually the
+target if it is directly connected to the gateway. Otherwise, the probe will
+timeout.
+
+As for UDP scans, this process can be quite slow if lots of ports are blocked
+by the gateway.
+
+From an original idea of M. Schiffman and D. Goldsmith, authors of the
+firewalk tool.
+]]
+
+
+---
+-- @usage
+-- nmap --script firewalk --script-args firewalk.gateway=a.b.c.d --traceroute target
+-- @usage
+-- nmap --script firewalk --script-args firewalk.ttl=7 target
+--
+-- @args firewalk.gateway IP address of the tested firewall. Must be present in the traceroute results.
+-- @args firewalk.ttl value of the TTL to use. Should be one greater than the
+-- number of hops to the gateway. In case both <code>firewalk.ttl</code> and
+-- <code>firewalk.gateway</code> IP address are
+-- supplied, <code>firewalk.gateway</code> is ignored.
+--
+-- @output
+-- |_firewalk:  forwarded ports (tcp): 21-80,443
+--
+
+
+-- 08/28/2010
+author = "Henri Doreau <henri.doreau[at]gmail.com>"
+
+license = "Same as Nmap--See http://nmap.org/book/man-legal.html"
+
+categories = {"safe", "discovery"}
+
+
+require('bin')
+require('packet')
+require('tab')
+
+
+local ICMP_TIME_EXCEEDED = 11
+
+
+-- number of retries for unanswered probes
+local MAX_RETRIES = 2
+
+
+--- ensure that the catched reply is a valid icmp time exceeded and return
+-- wether the reply appears to be valid or not
+local checkpkt = function(reply, orig)
+  local ip = packet.Packet:new(reply, reply:len())
+
+  if ip.ip_p ~= packet.IPPROTO_ICMP or ip.icmp_type ~= ICMP_TIME_EXCEEDED then
+    return false
+  end
+
+  local is = ip.buf:sub(ip.icmp_offset + 9)
+  local ip2 = packet.Packet:new(is, is:len(), true)
+
+  -- Check sent packet against ICMP payload
+  if ip2.ip_p ~= packet.IPPROTO_TCP or
+    ip2.ip_bin_src ~= orig.ip_bin_src or
+    ip2.ip_bin_dst ~= orig.ip_bin_dst or
+    ip2.tcp_sport ~= orig.tcp_sport or
+    ip2.tcp_dport ~= orig.tcp_dport then
+
+    return false
+  end
+
+  return true
+end
+
+--- pcap callback
+-- @return destination ip address, the ip protocol and icmp type
+local callback = function(size, layer2, layer3)
+  local ip = packet.Packet:new(layer3, layer3:len())
+  return bin.pack('ACC', ip.ip_bin_dst, ip.ip_p, ip.icmp_type)
+end
+
+--- set destination port and ip ttl to a generic tcp packet
+-- @param ip the ip object
+-- @param dport the layer 4 destination port
+-- @param ttl the ip ttl to set
+local updatepkt = function(ip, dport, ttl)
+  ip:ip_set_ttl(ttl)
+  ip:tcp_set_sport(math.random(0x401, 0xffff))
+  ip:tcp_set_dport(dport)
+  ip:tcp_set_seq(math.random(1, 0x7fffffff))
+  ip:tcp_count_checksum(ip.ip_len)
+  ip:ip_count_checksum()
+end
+
+--- create a generic tcp packet, with ip ttl and destination port set to zero
+-- @param host Host object that represents the destination
+-- @return the ip packet object
+local genericpkt = function(host)
+    local pkt = bin.pack("H",
+        "4500 002c 55d1 0000 8006 0000 0000 0000" ..
+        "0000 0000 0000 0000 0000 0000 0000 0000" ..
+        "6002 0c00 0000 0000 0204 05b4"
+    )
+
+    local tcp = packet.Packet:new(pkt, pkt:len())
+
+    tcp:ip_set_bin_src(host.bin_ip_src)
+    tcp:ip_set_bin_dst(host.bin_ip)
+
+    updatepkt(tcp, 0, 0)
+
+    return tcp
+end
+
+--- get the list of ports to probe
+-- @param host Host object that represents the targetted host
+-- @return list of ports to probe
+local getports = function(host)
+  local ports = {}
+  local port = nil
+
+  repeat
+    port = nmap.get_ports(host, port, "tcp", "filtered")
+    if port then
+      table.insert(ports, port.number)
+    end
+  until not port
+
+  return ports
+end
+
+--- store the firewalk ports into the registry
+-- @param host Host object that represents the targetted host
+-- @param ports list of ports to firewalk
+local setregs = function(host, ports)
+  if not nmap.registry[host.ip] then
+    nmap.registry[host.ip] = {}
+  end
+  nmap.registry[host.ip]['firewalk_ports'] = ports
+end
+
+--- host rule, check for requirements before to launch the script
+hostrule = function(host)
+  -- firewalk requires privileges to run
+  if not nmap.is_privileged() then
+    if not nmap.registry['firewalk'] then
+      nmap.registry['firewalk'] = {}
+    end
+    if nmap.registry['firewalk']['rootfail'] then
+      return false
+    end
+    nmap.registry['firewalk']['rootfail'] = true
+    if nmap.verbosity() > 0 then
+      nmap.log_write("stdout", "FIREWALK: not running for lack of privileges")
+    end
+    return false
+  end
+  if not host.interface then
+    return false
+  end
+  local ports = getports(host)
+  if #ports < 1 then
+    return false
+  end
+  setregs(host, ports)
+  return true
+end
+
+--- bind the scan to gateway(ttl) + 1
+-- @param host Host object that represents the targetted host
+-- @return the value of the ttl to use in our probes (or nil on error)
+local ttlmetric = function(host)
+  local ttl = stdnse.get_script_args("firewalk.ttl")
+  if ttl ~= nil then
+    return ttl
+  end
+
+  -- if no ttl is supplied, the script requires the gateway IP address and the
+  -- nmap traceroute resutls to find out the tt value to use
+  local gateway = stdnse.get_script_args("firewalk.gateway")
+  if not host.traceroute then
+    if not nmap.registry['firewalk'] then
+      nmap.registry['firewalk'] = {}
+    end
+    if nmap.registry['firewalk']['traceroutefail'] then
+      return nil
+    end
+    nmap.registry['firewalk']['traceroutefail'] = true
+    if nmap.verbosity() > 0 then
+      -- XXX maybe talk about the ttl option?
+      nmap.log_write("stdout", "FIREWALK: using the argument `firewalk.gateway' requires unavailable traceroute informations")
+    end
+    return nil
+  end
+  if gateway == host.ip then
+    if nmap.verbosity() > 0 then
+      nmap.log_write("stdout", "FIREWALK: metric and gateway cannot be the same host")
+      return nil
+    end
+  end
+
+  -- look for the ttl value to use according to traceroute results
+  for i, hop in pairs(host.traceroute) do
+    if hop.ip == gateway then
+      return i + 2
+    end
+  end
+
+  if nmap.verbosity() > 0 then
+    nmap.log_write("stdout", "FIREWALK: metric " .. gateway .. " doesn't appear in traceroute")
+  end
+
+  return nil
+end
+
+--- convert an array of ports into a port ranges string like "x,y-z"
+-- @param ports an array of numbers
+-- @return a string representing the ports as folded ranges
+local portrange = function(ports)
+  table.sort(ports)
+  local numranges = {}
+  for _, p in ipairs(ports) do
+    local stored = false
+
+    -- iterate over the ports list
+    for k, range in ipairs(numranges) do
+      -- increase an existing range by the left
+      if p == range["start"]-1 then
+        numranges[k]["start"] = p
+        stored = true
+      -- increase an existing range by the right
+      elseif p == range["stop"]+1 then
+        numranges[k]["stop"] = p
+        stored = true
+      -- port contained in an already existing range (catch doublons)
+      elseif p >= range["start"] and p <= range["stop"] then
+        stored = true
+      end
+    end
+    -- start a new range
+    if not stored then
+      local range = {}
+      range["start"] = p
+      range["stop"] = p
+      table.insert(numranges, range)
+    end
+  end
+
+  -- stringify the ranges
+  local strrange = {}
+  for i, val in ipairs(numranges) do
+    local start = tostring(val["start"])
+    local stop = tostring(val["stop"])
+    if start == stop then
+      table.insert(strrange, start)
+    else
+      -- contiguous ranges are represented as x-z
+      table.insert(strrange, start .. "-" .. stop)
+    end
+  end
+  -- ranges are delimited by `,'
+  return stdnse.strjoin(",", strrange)
+end
+
+-- main firewalking logic
+action = function(host)
+  local sock = nmap.new_dnet()
+  local pcap = nmap.new_socket()
+  local saddr = packet.toip(host.bin_ip_src)
+  local ports = nmap.registry[host.ip]['firewalk_ports']
+  local ttl = ttlmetric(host)
+  local try = nmap.new_try()
+  local fwdports = {}
+
+  -- abort if unable to bind the scan
+  if not ttl then
+    return nil
+  end
+
+  -- filter for incoming icmp time exceeded replies
+  pcap:pcap_open(host.interface, 104, 0, callback, "icmp and dst host " .. saddr)
+
+  try(sock:ip_open())
+
+  try = nmap.new_try(function() sock:ip_close() end)
+
+  pcap:set_timeout(3000)
+
+  local pkt = genericpkt(host)
+
+  -- iterate over the list of ports
+  for _, port in ipairs(ports) do
+    updatepkt(pkt, port, ttl)
+    local retry = 0
+
+    -- resend on timeout to increase reliability
+    while retry < MAX_RETRIES do
+      try(sock:ip_send(pkt.buf))
+
+      pcap:pcap_register(bin.pack('ACC', pkt.ip_bin_src, packet.IPPROTO_ICMP, ICMP_TIME_EXCEEDED))
+      local status, _, _, rep = pcap:pcap_receive()
+
+      if status then
+        if checkpkt(rep, pkt) then
+          stdnse.print_debug(1, "Firewalk: discovered fwd port " .. port)
+          table.insert(fwdports, port)
+          break
+        end
+      else
+        retry = retry + 1
+      end
+    end
+  end
+
+  sock:ip_close()
+  pcap:pcap_close()
+
+  if #fwdports < 1 then
+    return "\n no forwarded ports found"
+  else
+    return "\n forwarded ports (tcp): " .. portrange(fwdports)
+  end
+end
+
diff --git a/scripts/script.db b/scripts/script.db
index 7e9487dbb..f4acb7674 100644
--- a/scripts/script.db
+++ b/scripts/script.db
@@ -31,6 +31,7 @@ Entry { filename = "domino-enum-users.nse", categories = { "auth", "intrusive",
 Entry { filename = "drda-brute.nse", categories = { "auth", "intrusive", } }
 Entry { filename = "drda-info.nse", categories = { "discovery", "safe", "version", } }
 Entry { filename = "finger.nse", categories = { "default", "discovery", "safe", } }
+Entry { filename = "firewalk.nse", categories = { "discovery", "safe", } }
 Entry { filename = "ftp-anon.nse", categories = { "auth", "default", "safe", } }
 Entry { filename = "ftp-bounce.nse", categories = { "default", "intrusive", } }
 Entry { filename = "ftp-brute.nse", categories = { "auth", "intrusive", } }