Nmap version scanning is actually rather
straightforward. It was designed to be as simple as possible while
still being scalable, fast, and accurate.
The truly nitty-gritty details are best discovered by downloading
and reviewing the source code, but a synopsis of the techniques used follows.
Nmap first does a port scan as per your instructions, and then
passes all the
TCP and/or UDP ports to the service scanning
module. Those ports are then interrogated in parallel, although a single port is described here for simplicity.
Nmap checks to see if the
port is one of the ports to be excluded, as specified by the
Exclude directive in
If it is, Nmap will not scan this port for reasons
mentioned in the section called “
nmap-service-probes File Format”.
If the port is TCP, Nmap starts by connecting to it. If the connection succeeds and the port had been in the
open|filtered state, it is changed to
open. This is rare (for TCP) since people trying to be so stealthy that they use a TCP scan type which produces
open|filtered ports (such as FIN scan) generally know better than to blow all of their stealth by performing version detection.
Once the TCP connection is made,
Nmap listens for roughly five seconds. Many
common services, including most FTP, SSH, SMTP, Telnet, POP3, and IMAP
servers, identify themselves in an initial
Nmap refers to this as the “NULL probe”
because Nmap just listens for responses
without sending any probe data. If any data is received,
Nmap compares it to about 3,000 NULL probe signatures
file (described in the section called “
nmap-service-probes File Format”).
If the service is fully identified, we are done with that port! The
regular expression in signatures can includes substring matches to pick
version numbers out of the response. In some cases,
Nmap gets a
on the service type, but no version info. In that case,
Nmap continues but only sends probes
that are known to recognize the soft-matched service
This point is where Nmap
starts for UDP probes, and TCP connections continue here if the NULL probe described above
fails or soft-matches. Since the reality is that most ports are used
by the service they are registered to in
nmap-services, every probe has a list of port
numbers that are considered to be most effective. For example, the probe
called GetRequest that recognizes web servers (among many other services)
lists 80-85, 8000-8010, and 8080-8085 among its
Nmap sequentially (in the order they appear in the file) executes the probe(s)
that match the port number being scanned.
Each probe includes a
(which can be arbitrary ASCII text or \xHH escaped binary),
which is sent to the port. Responses that come back are compared to a
signature regular expressions
of the same type as discussed in the NULL
probe description above. As with the NULL probe, these tests can
either result in a full match (ends processing for the remote
service), a soft match (limits future probes to those which match a
certain service), or no match at all. The exact list of regular expressions
that Nmap uses to test for a match depends
on the probe
For instance, the data returned from the X11Probe is
very unlikely to match any regular expressions crafted for the GetRequest
probe. On the other hand, it is likely that results returned from a
Probe such as RTSPRequest might match a regular expression crafted for
GetRequest because the two protocols being tested for are closely
related. So the RTSPRequest probe has a fallback to GetRequest matches.
For a more comprehensive explanation, see
the section called “Cheats and Fallbacks”.
If any response during version detection is ever received from a
UDP port which was in the
open|filtered state, that
state is changed to
open. This makes version
detection an excellent complement to UDP scan, which is forced to
label all scanned UDP ports as
some common firewall rules are in effect. While combining UDP
scanning with version detection can take many times as long as a plain
UDP scan, it is an effective and useful technique. This method is
described in the section called “Distinguishing Open from Filtered UDP Ports”.
In most cases, the NULL probe or the probable port
probe(s) (there is usually only one) described above match the service. Since the NULL
probe shares its connection with the probable port probe, this
allows service detection to be done with only one brief connection in
most cases. With UDP only one packet is usually required. But should
the NULL probe and probable port probe(s) fail, Nmap goes through
other existing probes sequentially. In the case of TCP, Nmap must
make a new connection for each probe to avoid having previous probes
corrupt the results. This worst-case scenario can take a bit of
time, especially since Nmap must wait
about five seconds for the results from each probe because of
slow network connections and otherwise slowly responding services.
Fortunately, Nmap utilizes several automatic techniques to speed up scans:
Nmap makes most probes
generic enough to match many services. For example, the GenericLines
probe sends two blank lines (“
\r\n\r\n”) to the service. This matches
daemons of many diverse service types, including FTP, ident, POP3, UUCP,
Postgres, and whois. The GetRequest probe matches even more service
types. Other examples include “
help\r\n” and generic RPC and MS SMB
If a service matches a
Nmap only needs to try probes that can potentially
match that service.
All probes were not created equal! Some match many more
services than others. Because of this, Nmap
metric to avoid trying probes that are extremely
unlikely to match. Experienced Nmap users
can force all probes to be tried regardless or limit probe attempts
even further than the default by using the
options discussed in the section called “Probe Selection and Rarity”.
One of the probes tests whether the target port is
If so (and if OpenSSL is available), Nmap connects back via SSL and
restarts the service scan to determine what is listening behind the
encryption. A special directive allows different probable ports for
normal and SSL tunneled
connections. For example, Nmap should start against port 443 (HTTPS)
with an SSL probe. But after SSL is detected and enabled, Nmap should
try the GetRequest probe against port 443 because that port usually
has a web server listening behind SSL encryption.
Another generic probe identifies RPC-based services.
When these are found, the Nmap RPC grinder (discussed later) is
initiated to brute force the RPC program number/name and supported
version numbers. Similarly, an SMB post-processor for fingerprinting Windows services is available as part of the
Chapter 9, Nmap Scripting Engine.
If at least one of the probes elicits some sort of response, yet
Nmap is unable to recognize the service, the response content is
printed to the user in the form of a
fingerprint. If users know
what services are actually listening, they are encouraged to submit
the fingerprint to Nmap developers for integration into Nmap, as
described in the section called “Submit Service Fingerprints”.
Even though Nmap waits a generous
amount of time for services to reply, sometimes an application is slow to respond
to the NULL probe. This can occur for a number of reasons, including
slow reverse DNS lookups performed by some services. Because of this,
Nmap can sometimes match the results from a subsequent
probe to a match line designed for the NULL probe.
For example, suppose we scan port 25 (SMTP) on a server to
determine what is listening. As soon as we connect, that service may
conduct a bunch of DNS blacklist lookups to determine whether we
should be treated as spammers and denied service. Before it finishes
that, Nmap gives up waiting for a NULL probe response and sends the
next probe with port 25 registered, which is “
HELP\r\n”. When the
service finally completes its anti-spam checks, it prints a greeting
banner, reads the Help probe, and responds as shown in Example 7.4.
Example 7.4. NULL probe cheat example output
220 hcsw.org ESMTP Sendmail 8.12.3/8.12.3/Debian-7.1; Tue, [cut]
214-2.0.0 This is sendmail version 8.12.3
214-2.0.0 HELO EHLO MAIL RCPT DATA
214-2.0.0 RSET NOOP QUIT HELP VRFY
214-2.0.0 EXPN VERB ETRN DSN AUTH
214-2.0.0 For more info use "HELP <topic>".
214-2.0.0 To report bugs in the implementation send email to
214-2.0.0 For local information send email to Postmaster at your site.
214 2.0.0 End of HELP info
Nmap reads this data from the socket
and finds that no regular expressions from the Help probe match the
data returned. This is because Nmap normally expects to receive the
ESMTP banner during the NULL probe and match it there.
Because this is a relatively common scenario, Nmap
“cheats” by trying to match responses to any of the NULL
Probe match lines if none of the probe-specific lines match. In this
case, a null match line exists which reports that the program is
Sendmail, the version is 8.12.3/8.12.3/Debian-7.1, and the hostname is
The NULL probe cheat is actually just a specific
example of a more general Nmap feature: fallbacks.
The fallback directive is described in detail in
the section called “
nmap-service-probes File Format”. Essentially, any probe that is
likely to encounter results that can be matched by regular expressions in other
probes has a fallback directive that specifies these other probes.
For example, in some configurations of the popular
Apache web server,
Apache won't respond to the GetRequest
GET / HTTP/1.0\r\n\r\n”) probe because no virtual host name has been
specified. Nmap is still able to correctly
identify these servers because those servers usually respond to the
HTTPOptions probe. That probe has a fallback to the GetRequest
regular expressions, which are sufficiently general to recognize
Apache's responses to the HTTPOptions
Probe Selection and Rarity
In determining what probes to use, Nmap
rarity. This is an indication of how likely the probe is to return useful data. If a probe has a high rarity, it is considered less common and
is less likely to be tried. Nmap users can specify
which probes are tried by changing the intensity level of the version scan, as described
below. The precise algorithm Nmap uses when determining which
probes to use follows:
For TCP, the NULL probe is always tried first.
All probes that have the port being scanned listed as a probable port
(see the section called “
nmap-service-probes File Format”) are tried
in the order they appear in
All other probes that have a rarity value less than or equal to the current
intensity value of the scan are tried, also in the order they
Once a probe is found to match, the algorithm terminates
and results are reported.
Because all of Nmap's probes (other than the NULL probe) have a rarity value
associated with them, it is relatively easy to control how many of them
are tried when performing a version scan. Simply choose
an intensity level appropriate for a scan. The higher an
intensity level, the more probes will be tried. So if
a very comprehensive scan is desired, a high intensity level is
appropriate—even though it may take longer than
a scan conducted at a lower intensity level. Nmap's
default intensity level is 7
but Nmap provides the following switches
for different scanning needs:
<intensity level between 0 and 9>
Sets the intensity level of a version scan to the specified value. If 0
is specified, only the NULL probe (for TCP) and probes that list the port
as a probable port are tried. Example:
nmap -sV --version-intensity 3 scanme.nmap.org
Sets the intensity level to 2. Example:
nmap -sV --version-light scanme.nmap.org
Sets the intensity level to 9. Since all probes have a rarity level
between 1 and 9, this tries all of the probes. Even if a soft match
occurs, all additional probes will be tried. Example:
nmap -sV --version-all scanme.nmap.org