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Example Nmap output

Nmap Network Scanning

Custom Scan Types with --scanflags

Truly advanced Nmap users need not limit themselves to the canned scanned types. The --scanflags option allows you to design your own scan by specifying arbitrary TCP flags. Let your creative juices flow, while evading intrusion detection systems whose vendors simply paged through the Nmap man page adding specific rules!

The --scanflags argument can be a numerical flag value such as 9 (PSH and FIN), but using symbolic names is easier. Just mash together any combination of URG, ACK, PSH, RST, SYN, and FIN. For example, --scanflags URGACKPSHRSTSYNFIN sets everything, though it's not very useful for scanning. The order these are specified in is irrelevant.

In addition to specifying the desired flags, you can specify a TCP scan type (such as -sA or -sF). That base type tells Nmap how to interpret responses. For example, a SYN scan considers no-response indicative of a filtered port, while a FIN scan treats the same as open|filtered. Nmap will behave the same way it does for the base scan type, except that it will use the TCP flags you specify instead. If you don't specify a base type, SYN scan is used.

Custom SYN/FIN Scan

One interesting custom scan type is SYN/FIN. Sometimes a firewall administrator or device manufacturer will attempt to block incoming connections with a rule such as drop any incoming packets with only the SYN flag set. They limit it to only the SYN flag because they don't want to block the SYN/ACK packets which are returned as the second step of an outgoing connection.

The problem with this approach is that most end systems will accept initial SYN packets which contain other (non-ACK) flags as well. For example, the Nmap OS fingerprinting system sends a SYN/FIN/URG/PSH packet to an open port. More than half of the fingerprints in the database respond with a SYN/ACK. Thus they allow port scanning with this packet and generally allow making a full TCP connection too. Some systems have even been known to respond with SYN/ACK to a SYN/RST packet! The TCP RFC is ambiguous as to which flags are acceptable in an initial SYN packet, though SYN/RST certainly seems bogus.

Example 5.13 shows Ereet conducting a successful SYN/FIN scan of Google. He is apparently getting bored with scanme.nmap.org.

Example 5.13. A SYN/FIN scan of Google

krad# nmap -sS --scanflags SYNFIN -T4 www.google.com

Starting Nmap ( http://nmap.org )
Warning: Hostname www.google.com resolves to 4 IPs. Using 74.125.19.99.
Nmap scan report for cf-in-f99.google.com (74.125.19.99)
Not shown: 996 filtered ports
PORT    STATE  SERVICE
80/tcp  open   http
113/tcp closed auth
179/tcp closed bgp
443/tcp open   https

Nmap done: 1 IP address (1 host up) scanned in 7.58 seconds

Similar scan types, such as SYN/URG or SYN/PSH/URG/FIN will generally work as well. If you aren't getting through, don't forget the already mentioned SYN/RST option.

PSH Scan

the section called “TCP FIN, NULL, and Xmas Scans (-sF, -sN, -sX)” noted that RFC-compliant systems allow one to scan ports using any combination of the FIN, PSH, and URG flags. While there are eight possible permutations, Nmap only offers three canned modes (NULL, FIN, and Xmas). Show some personal flair by trying a PSH/URG or FIN/PSH scan instead. Results rarely differ from the three canned modes, but there is a small chance of evading scan detection systems.

To perform such a scan, just specify your desired flags with --scanflags and specify FIN scan (-sF) as the base type (choosing NULL or Xmas would make no difference). Example 5.14 demonstrates a PSH scan against a Linux machine on a local network.

Example 5.14. A custom PSH scan

krad# nmap -sF --scanflags PSH  para

Starting Nmap ( http://nmap.org )
Nmap scan report for para (192.168.10.191)
(The 995 ports scanned but not shown below are in state: closed)
PORT     STATE         SERVICE
22/tcp   open|filtered ssh
53/tcp   open|filtered domain
111/tcp  open|filtered rpcbind
515/tcp  open|filtered printer
6000/tcp open|filtered X11
MAC Address: 00:60:1D:38:32:90 (Lucent Technologies)

Nmap done: 1 IP address (1 host up) scanned in 5.95 seconds

Because these scans all work the same way, I could keep just one of -sF, -sN, and -sX options, letting users emulate the others with --scanflags. There are no plans to do this because the shortcut options are easier to remember and use. You can still try the emulated approach to show off your Nmap skills. Execute nmap -sF --scanflags FINPSHURG target rather than the more mundane nmap -sX target.

[Warning]Warning

In my experience, needlessly complex Nmap command-lines don't impress girls. They usually respond with a condescending sneer, presumably recognizing that the command is redundant.

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