Nmap is usually finished working on a port once it has deduced
the service and version information as demonstrated above. However,
there are certain services for which Nmap performs additional work.
The post-processors presently available are Nmap
Scripting Engine integration, RPC grinding, and SSL tunneling. Windows
SMB interrogation is under consideration.
Nmap Scripting Engine Integration
The regular-expression based approach of version detection is
powerful, but it cannot recognize everything. Some services cannot be recognized by simply sending a standard probe and matching a pattern to the response. Some services require custom probe strings or a complex multi-step handshaking process. Others require more advanced processing than a regular expression to recognize a response. For example, the Skype v2 service was designed to be difficult to detect due to the risk that incumbent carriers (such as phone companies providing DSL lines) would consider them a competitor and degrade or block the service from their subscribers. The only way we could find to detect this service involved analyzing responses to two different probes. Similarly, we could recognize more SNMP services if we
tried a few hundred different community names by brute force. Neither
of these tasks are well suited to traditional Nmap version detection,
but both were accomplished with the
Chapter 9, Nmap Scripting Engine.
For these reasons, version detection now calls NSE by default to handle some tricky services, as described in the section called “Version Detection Using NSE”.
SunRPC (Sun Remote Procedure Call) is a common Unix protocol
used to implement many services including NFS.
Nmap ships with an
database of almost 600 RPC programs.
Many RPC services use high-numbered ports and/or the UDP transport
protocol, making them available through many poorly configured
firewalls. RPC programs (and the infrastructure libraries themselves)
also have a long history of serious remotely exploitable security holes.
So network administrators and security auditors often wish to learn more
about any RPC programs on their networks.
If the portmapper (rpcbind) service (UDP or TCP port 111) is
available, RPC services can be enumerated with the Unix
rpcinfo command. Example 7.5 demonstrates this against a default Solaris 9 server.
Example 7.5. Enumerating RPC services with rpcinfo
rpcinfo -p ultra
program vers proto port
100000 4 tcp 111 rpcbind
100000 4 udp 111 rpcbind
100232 10 udp 32777 sadmind
100083 1 tcp 32775 ttdbserverd
100221 1 tcp 32777 kcms_server
100068 5 udp 32778 cmsd
100229 1 tcp 32779 metad
100230 1 tcp 32781 metamhd
100242 1 tcp 32783 rpc.metamedd
100001 4 udp 32780 rstatd
100002 3 udp 32782 rusersd
100002 3 tcp 32785 rusersd
100008 1 udp 32784 walld
100012 1 udp 32786 sprayd
100011 1 udp 32788 rquotad
100024 1 udp 32790 status
100024 1 tcp 32787 status
100133 1 udp 32790 nsm_addrand
100133 1 tcp 32787 nsm_addrand
[ Dozens of lines cut for brevity ]
This example shows that hosts frequently offer many RPC
services, which increases the probability that one is exploitable.
You should also notice that most of the services are on strange
high-numbered ports (which may change for any number of reasons) and
split between UDP and TCP transport protocols.
Because the RPC information is so sensitive, many administrators
try to obscure this information by blocking the portmapper port
(111). Unfortunately, this does not close the hole. Nmap can
determine all of the same information by directly communicating with open RPC
ports through the following three-step process.
The TCP and/or UDP port scan finds all of the open ports.
Version detection determines which of the open ports use the SunRPC protocol.
The RPC brute force engine determines the program
identity of each RPC port by trying a null command against each of
the 600 programs numbers in
nmap-rpc. Most of
the time Nmap guesses wrong and receives an error message
stating that the requested program number is not listening on the
port. Nmap continues trying each
number in its list until success is returned for one of them.
Nmap gives up in the unlikely event
that it exhausts all of its known program numbers or if the port sends
malformed responses that suggest it is not really
The RPC program identification probes are done in parallel, and
retransmissions are handled for UDP ports. This feature is
automatically activated whenever version detection finds any RPC
Example 7.6 demonstrates direct RPC scanning done as part of version detection.
Example 7.6. Nmap direct RPC scan
nmap -F -A -sSU ultra
Starting Nmap ( http://nmap.org )
Nmap scan report for ultra.nmap.org (192.168.0.50)
(The 2171 ports scanned but not shown below are in state: closed)
PORT STATE SERVICE VERSION
[A whole bunch of ports cut for brevity]
32776/tcp open kcms_server 1 (rpc #100221)
32776/udp open sadmind 10 (rpc #100232)
32777/tcp open kcms_server 1 (rpc #100221)
32777/udp open sadmind 10 (rpc #100232)
32778/tcp open metad 1 (rpc #100229)
32778/udp open cmsd 2-5 (rpc #100068)
32779/tcp open metad 1 (rpc #100229)
32779/udp open rstatd 2-4 (rpc #100001)
32780/tcp open metamhd 1 (rpc #100230)
32780/udp open rstatd 2-4 (rpc #100001)
32786/tcp open status 1 (rpc #100024)
32786/udp open sprayd 1 (rpc #100012)
32787/tcp open status 1 (rpc #100024)
32787/udp open rquotad 1 (rpc #100011)
Device type: general purpose
Running: Sun Solaris 9
OS details: Sun Solaris 9
Nmap finished: 1 IP address (1 host up) scanned in 252.701 seconds
As discussed in the technique section,
Nmap has the ability to detect the SSL
encryption protocol and then launch an encrypted session through which
it executes normal version detection. As with the RPC grinder
discussed previously, the SSL post-processor is automatically executed
whenever an appropriate (SSL) port is detected. This is demonstrated by Example 7.7.
Example 7.7. Version scanning through SSL
nmap -Pn -sSV -T4 -F www.amazon.com
Starting Nmap ( http://nmap.org )
Nmap scan report for 207-171-184-16.amazon.com (188.8.131.52)
(The 1214 ports scanned but not shown below are in state: filtered)
PORT STATE SERVICE VERSION
80/tcp open http Apache Stronghold httpd 2.4.2 (based on Apache 1.3.6)
443/tcp open ssl/http Apache Stronghold httpd 2.4.2 (based on Apache 1.3.6)
Nmap finished: 1 IP address (1 host up) scanned in 35.038 seconds
Note that the version information is the same for each of the
two open ports, but the service is
port 80 and
ssl/http on port 443. The
common case of HTTPS on port 443 is not hard-coded—Nmap
should be able to detect SSL on any
port and determine the underlying protocol for any service that Nmap can
detect in clear-text. If Nmap had not detected the server listening
behind SSL, the service listed would be
ssl/unknown. If Nmap
had not been built with SSL support, the service listed would have
ssl. The version field would be blank in both
of these cases.
The SSL support for Nmap depends on
the free OpenSSL library. It is not included in the Linux
RPM binaries, to avoid breaking systems which lack these libraries.
The Nmap source code distribution attempts to detect OpenSSL on
a system and link to it when available. See Chapter 2, Obtaining, Compiling, Installing, and Removing Nmap for details on customizing the build process to include or exclude OpenSSL.