While Nmap does many things, its most fundamental feature is port
scanning. Point Nmap
at a remote machine, and it might tell you that ports
53/udp are open. Using its
nmap-services database of more than
2,200 well-known services, Nmap would report that those ports
probably correspond to a mail server (SMTP), web server (HTTP), and
name server (DNS) respectively. This lookup is usually accurate—the
vast majority of daemons listening on TCP port 25 are, in fact, mail
servers. However, you should not bet your security on this! People can and
do run services on strange ports. Perhaps their main web server was
already on port 80, so they picked a different port for a staging or test
server. Maybe they think hiding a vulnerable service on some obscure
port prevents “evil hackers” from finding it. Even more common
lately is that people choose ports based not on the service they
want to run, but on what gets through the firewall. When ISPs
blocked port 80 after major Microsoft IIS worms CodeRed and Nimda,
hordes of users responded by moving their personal web servers to
another port. When companies block Telnet access due to its
horrific security risks, I have seen users simply run telnetd on the
Secure Shell (SSH) port instead.
Even if Nmap is right, and the
hypothetical server above is running SMTP, HTTP, and DNS servers, that
is not a lot of information. When doing vulnerability assessments (or
even simple network inventories) of your companies or clients, you
really want to know which mail and DNS servers and versions are
running. Having an accurate version number
helps dramatically in determining which exploits a server is
vulnerable to. Do keep in mind that security fixes are often
back-ported to earlier versions of software, so you cannot rely solely
on the version number to prove a service is vulnerable. False negatives are rarer, but can happen when silly administrators spoof the version number of a vulnerable service to make it appear patched.
Another good reason for determining the service types and
version numbers is that many services share the same port number. For
258/tcp is used by both the Checkpoint Firewall-1 GUI
management interface and the yak Windows
chat client. This makes a guess based on the
nmap-services table even less accurate. Anyone
who has done much scanning knows that you also often find services
listening on unregistered ports—these are a complete mystery without
version detection. A final problem is that filtered UDP ports often
look the same to a simple port scanner as open ports (see the section called “UDP Scan (
-sU)”). But if they respond to the
service-specific probes sent by Nmap
version detection, you know for sure that they are open (and often
exactly what is running).
Service scans sometimes reveal information about a target
beyond the service type and version number. Miscellaneous
information discovered about a service is collected in the
“info” field. This is displayed in the
VERSION column inside parentheses
following the product name and version number. This field can include
protocol numbers, Apache modules, and much more.
Some services also report their configured hostnames, which
differ from machines'
hostnames surprisingly often. The
hostname field is reported on a
Service Info line
following the port table. It sounds like a minor information leak,
but can have consequences. One year at the CanSecWest security
conference, I was huddled up in my room with my laptop. Suddenly the
tcpdump window in the corner of my screen
went wild and I realized my machine was under attack. I scanned back
and found an unusual high port sitting open. Upon connecting, the
port spewed a bunch of binary characters, but one ASCII field in the
output gave a configured domain name. The domain was for a small
enough security company that I knew exactly who was responsible. I
had the front desk ring his hotel room, and boy was he surprised when
I asked him to stop probing my box.
Two more fields that version detection can discover are
operating system and device type. These are also reported on the
line. We use two techniques here. One
is application exclusivity. If we identify a service as Microsoft
Exchange, we know the operating system is Windows since Exchange
doesn't run on anything else. The other technique is to persuade more
portable applications to divulge the platform information. Many
servers (especially web servers) require very little coaxing. This
type of OS detection is intended to complement Nmap's OS detection
-O) and can sometimes report differing
results. For example, consider a Microsoft Exchange server hidden behind a
port-forwarding Unix firewall.
The Nmap version scanning subsystem
obtains all of this data by
connecting to open ports and interrogating them for further
information using probes that the specific services understand. This
allows Nmap to give a detailed assessment
of what is really running, rather than just what port numbers are
open. Example 7.1 shows the actual output.
Example 7.1. Simple usage of version detection
nmap -sV -T4 -F insecure.org
Starting Nmap ( http://nmap.org )
Nmap scan report for insecure.org (126.96.36.199)
Host is up (0.016s latency).
rDNS record for 188.8.131.52: web.insecure.org
Not shown: 95 filtered ports
PORT STATE SERVICE VERSION
22/tcp open ssh OpenSSH 4.3 (protocol 2.0)
25/tcp open smtp Postfix smtpd
80/tcp open http Apache httpd 2.2.3 ((CentOS))
113/tcp closed auth
443/tcp open ssl/http Apache httpd 2.2.3 ((CentOS))
Service Info: Host: web.insecure.org
Nmap done: 1 IP address (1 host up) scanned in 14.82 seconds
Nmap version detection offers the
following advanced features (fully described later):
High speed, parallel operation via non-blocking sockets and a
probe/match definition grammar designed for efficient yet powerful implementation.
Determines the application name and version number where available—not just the service protocol.
Supports both the TCP and UDP protocols, as well as both textual ASCII and packed binary services.
Multi-platform support, including Linux, Windows, Mac
OS X, FreeBSD/NetBSD/OpenBSD, Solaris, and all the other platforms on
which Nmap is known to work.
If SSL is detected, Nmap
connects using OpenSSL (if available) and tries to determine what
service is listening behind that encryption layer. This allows it to discover services like HTTPS, POP3S, IMAPS, etc. as well as providing version details.
If a SunRPC
service is discovered, Nmap launches its brute-force
to find the program number, name, and version number.
IPv6 is supported, including TCP, UDP, and SSL over TCP.
Common Platform Enumeration
output for interoperation with other software (some information is only
included in XML output). See
the section called “Common Platform Enumeration (CPE)” for more about CPE.
Community contributions: if Nmap gets data back from a service
that it does not recognize, a
is printed along with a
This system is patterned
after the extremely successful Nmap OS Detection
fingerprint submission process. New probes and corrections can also be
Nmap recognizes more than one thousand
service signatures, covering more than 180 unique service protocols
from ACAP, AFP, and AIM to XML-RPC, Zebedee, and Zebra.