Speeding up NMAP service scanning 16x
Speeding up nmap service scanning 16x

  In my previous post post, I began writing about how I was designing a port and service scanner for large-scale networks by combining port-scanning tools like masscan/zmap and service scanning tools like nmap. In this post, I’m going to dive into some of the details of nmap’s service scanning, and outline how I was able to speed up nmap’s service scanning by 16-times.

In order to determine the service running on a specific port, nmap uses a so-called “service detection probe list” which is located in a file named “nmap-service-probes” (on my system, it is located in /usr/share/nmap/nmap-service-probes).

A probe looks like the following:

Probe TCP GetRequest q|GET / HTTP/1.0\r\n\r\n|

The syntax for the probe is the following:

Probe <protocol> <probename> <probestring> [no-payload]

The format is quite simple:

  protocol: TCP or UDP
  probename: An arbitrary name of the probe such as “GenericLines”, “RPCCheck”, or “X11Probe”.
  probestring: The data sent to the server when it is probed. Note: q|[characters]| is perl’s “quote operator” which allows you to create strings without needing to escape special characters.
  [no-payload]: Used for UDP scanning so we ignore it for now.


A series of matching rules follow each probe which match on the response to each probe. An example is the following:

match compuware-lm m|^Hello, I don't understand your request\.  Good bye\.\.\.\. $| p/Compuware Distributed License Management/

The syntax for the probe is the following:

match <service> <pattern> [<versioninfo>]

The format is also quite simple:

  service: The service name such as “http”, “ssh”, “mysql”, and so on.
  pattern: a perl-form regex pattern to match the response received from the probe.
  [<versioninfo>]: Various optional flags for extracting/displaying extrra information about the match (read more here)


It is extremely noteworthy that when nmap sends a probe, it deliberately waits for a pre-defined amount of time. That is to say, there is a minimum amount of time each probe takes. Probes are not parallelized per port. The first probe will be sent and a 6 seconds wait-time (assuming the connection is not closed) will happen. After 6 seconds, an attempt at matching the response happens. If a match is not received, the next probe is sent; up to a certain ‘rarity’ of probe – “Nmap uses the rarity metric to avoid trying probes that are extremely unlikely to match”. By default, probes are sent up to the rarity of 7. For probe rarity 1-7, each probe waits at least 6-seconds (but most wait 7.5-seconds):
# Wait for at least 6 seconds for data.  It used to be 5, but some
# smtp services have lately been instituting an artificial pause (see
# FEATURE('greet_pause') in Sendmail, for example)
totalwaitms 6000

In reality, scanning a host for a service which is completely unidentifiable, will keep you waiting around 160-seconds:
# time nmap -sV localhost -p2223
Starting Nmap 7.80 ( https://nmap.org ) at 2023-08-12 14:11 UTC
Nmap scan report for localhost (127.0.0.1)
Host is up (0.000098s latency).

[..]

Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 160.66 seconds

real    2m40.676s
user    0m0.417s
sys     0m0.065s


2 minutes and 40 seconds is an unacceptable time for service scanning a single host. How can we improve this?

The most obvious solution is to simply lower the totalwaitms value to something more reasonable. This will sacrifice niche results such as of the mail servers which employ these anti-spam techniques, however this is a sacrifice I believe most are willing to make. totalwaitms can be changed the nmap-service-probes file.
# grep 'totalwaitms' /usr/share/nmap/nmap-service-probes
totalwaitms 6000
totalwaitms 7500
totalwaitms 7500
totalwaitms 7500
totalwaitms 7500
totalwaitms 11000

tcpwrappedms also must be lowered, since it should be lower than totalwaitms:
# If the service closes the connection before 3 seconds, it's probably
# tcpwrapped. Adjust up or down depending on your false-positive rate.
tcpwrappedms 300


After replacing these values with totalwaitms 300 and tcpwrappedms 200, it is expected that the scan will now take just a few seconds. However…

# time nmap -sV localhost -p2223
[..]

real    2m17.561s
user    0m0.401s
sys     0m0.084s

That isn’t much of an improvement at all.

As it turns out, probes which do not have a specifically defined servicewaitms use a default value of 5000. Therefore, we can either add values to each of the probes, or we can compile nmap ourselves and change the default value in service_scan.h. I went for the second option, and changed the default value in nmap’s source code and then compiled my own version of nmap.

I didn’t want to change the values of totalwaitms and tcpwrappedms at all in the /usr/share/nmap/nmap-service-probes file, so I also edited the parsing code such that these values in the nmap-service-probes file are completely ignored:

I once again tried scanning, and..
# time nmap localhost -p9999 -sV
[..]

real    0m10.481s
user    0m0.400s
sys     0m0.037s


Success! We’ve just turned our 160-second scan into just 10-seconds. Can we do anything to make it even faster?
Using nmap’s debugging flag, we can check to see the timeout and delays for various of its actions:
# nmap localhost -p9999 -sV -d2
[...]
NSOCK INFO [9.1460s] nsock_write(): Write request for 48 bytes to IOD #29 EID 699 [127.0.0.1:9999]
NSOCK INFO [9.1460s] nsock_read(): Read request from IOD #29 [127.0.0.1:9999] (timeout: 300ms) EID 706
NSOCK INFO [9.1460s] nsock_trace_handler_callback(): Callback: WRITE SUCCESS for EID 699 [127.0.0.1:9999]
NSOCK INFO [9.4460s] nsock_trace_handler_callback(): Callback: READ TIMEOUT for EID 706 [127.0.0.1:9999]
NSOCK INFO [9.4460s] nsock_iod_delete(): nsock_iod_delete (IOD #29)
Completed Service scan at 15:02, 9.01s elapsed (1 service on 1 host)
NSE: Script scanning 127.0.0.1.
NSE: Starting runlevel 1 (of 2) scan.
Initiating NSE at 15:02
Completed NSE at 15:02, 0.00s elapsed
NSE: Starting runlevel 2 (of 2) scan.
Initiating NSE at 15:02
[..]
NSOCK INFO [9.4620s] nsock_read(): Read request from IOD #1 [127.0.0.1:9999] (timeout: 7000ms) EID 26


We can see that the timeout is correctly being set to 300ms for the service scanning. However, NSE scripts, which nmap uses for version detection (among other things), use a different system for setting the timeout – in this case, there is a maximum timeout of 7000ms. Diving into nselib/comm.lua reveals how this timeout is set by the scripts:
-- This timeout value (in ms) is added to the connect timeout and represents
-- the amount of processing time allowed for the host before it sends a packet.
-- For justification of this value, see totalwaitms in nmap-service-probes
local REQUEST_TIMEOUT = 6000

-- Function used to get a connect and request timeout based on specified options
local function get_timeouts(host, opts)
  local connect_timeout, request_timeout
  -- connect_timeout based on options or stdnse.get_timeout()
  if opts and opts.connect_timeout then
    connect_timeout = opts.connect_timeout
  elseif opts and opts.timeout then
    connect_timeout = opts.timeout
  else
    connect_timeout = stdnse.get_timeout(host)
  end

  -- request_timeout based on options or REQUEST_TIMEOUT + connect_timeout
  if opts and opts.request_timeout then
    request_timeout = opts.request_timeout
  elseif opts and opts.timeout then
    request_timeout = opts.timeout
  else
    request_timeout = REQUEST_TIMEOUT
  end
  request_timeout = request_timeout + connect_timeout

  return connect_timeout, request_timeout
end


Basically, a connection timeout and a request timeout is set based on the options defined in the NSE files. Not all default NSE scripts implicitly set a timeout, so a minimum 6-second request_timeout is often used. Even from the scripts that do set a timeout, the timeout is usually 5-seconds or more.

Instead of editing every NSE file to implicitly set a low timeout or nselib/comm.lua, we can edit the l_set_timeout function in nse_nsock.cc to set a maximum of a 500ms timeout.

The exact speed-up from doing this depends on the scripts which run during service and version scanning.

In this post, we’ve explored how nmap has a very high forced-delay for its service scanning, as well as high timeouts for its NSE scripts. By making a few simple changes, we can speed up service scanning by up to 16x. Note that the values of 500, 300, and 200ms are all arbitrary, however they are what I decided on based on my use-case and the network configuration of my environment.

A patch is provided for nmap.
diff --git a/nse_nsock.cc b/nse_nsock.cc
index 18a75a7bb..2e88c0fed 100644
--- a/nse_nsock.cc
+++ b/nse_nsock.cc
@@ -769,6 +769,8 @@ static int l_set_timeout (lua_State *L)
   int timeout = nseU_checkinteger(L, 2);
   if (timeout < -1) /* -1 is no timeout */
     return luaL_error(L, "Negative timeout: %f", timeout);
+  if (timeout > 500)
+    timeout = 500;
   nu->timeout = timeout;
   return nseU_success(L);
 }
diff --git a/service_scan.cc b/service_scan.cc
index f7de2ea8c..9b6d3af1f 100644
--- a/service_scan.cc
+++ b/service_scan.cc
@@ -1342,10 +1342,11 @@ void parse_nmap_service_probe_file(AllProbes *AP, const char *filename) {
       } else if (strncmp(line, "fallback ", 9) == 0) {
         newProbe->fallbackStr = strdup(line + 9);
       } else if (strncmp(line, "totalwaitms ", 12) == 0) {
-        long waitms = strtol(line + 12, NULL, 10);
+/*        long waitms = strtol(line + 12, NULL, 10);
         if (waitms < 100 || waitms > 300000)
           fatal("Error on line %d of nmap-service-probes file (%s): bad totalwaitms value.  Must be between 100 and 300000 milliseconds", lineno, filename);
         newProbe->totalwaitms = waitms;
+*/
       } else if (strncmp(line, "tcpwrappedms ", 13) == 0) {
         long waitms = strtol(line + 13, NULL, 10);
         if (waitms < 100 || waitms > 300000)
diff --git a/service_scan.h b/service_scan.h
index b17e3d242..807faa77a 100644
--- a/service_scan.h
+++ b/service_scan.h
@@ -84,8 +84,8 @@
 #include <assert.h>
 
 /**********************  DEFINES/ENUMS ***********************************/
-#define DEFAULT_SERVICEWAITMS 5000
-#define DEFAULT_TCPWRAPPEDMS 2000   // connections closed after this timeout are not considered "tcpwrapped"
+#define DEFAULT_SERVICEWAITMS 300
+#define DEFAULT_TCPWRAPPEDMS 200   // connections closed after this timeout are not considered "tcpwrapped"
 #define DEFAULT_CONNECT_TIMEOUT 5000
 #define DEFAULT_CONNECT_SSL_TIMEOUT 8000  // includes connect() + ssl negotiation
 #define MAXFALLBACKS 20 /* How many comma separated fallbacks are allowed in the service-probes file? */


