Книга: Iptables Tutorial 1.2.2

Nmap

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Nmap

Nmap is an excellent tool for looking at the pure firewall perspective, and to find out which ports are open and more low level information. It has support for OS fingerprinting, several different port scanning methods, IPv6 and IPv4 support and network scanning.

The basic form of scanning is done with a very simple commandline syntax. Don't forget to specify which ports to scan through with the -p option, for example -p 1-1024. As an example, take a look below.

blueflux@work3:~$ nmap -p 1-1024 192.168.0.1

Starting nmap 3.50 ( http://www.insecure.org/nmap/ ) at 2004-03-18 17:19 CET

Interesting ports on firewall (192.168.0.1):

(The 1021 ports scanned but not shown below are in state: closed)

PORT STATE SERVICE

22/tcp open ssh

25/tcp open smtp

587/tcp open submission

Nmap run completed -- 1 IP address (1 host up) scanned in 3.877 seconds

It is also able to automatically guess the operating system of the scanned host by doing OS fingerprinting. Fingerprinting requires root privileges though, but it may also be very interesting to use to find out what most people will think of the host. Using OS fingerprinting may look something like the example listing below.

work3:/home/blueflux# nmap -O -p 1-1024 192.168.0.1

Starting nmap 3.50 ( http://www.insecure.org/nmap/ ) at 2004-03-18 17:38 CET

Interesting ports on firewall (192.168.0.1):

(The 1021 ports scanned but not shown below are in state: closed)

PORT STATE SERVICE

22/tcp open ssh

25/tcp open smtp

587/tcp open submission

Device type: general purpose

Running: Linux 2.4.X|2.5.X

OS details: Linux Kernel 2.4.0 - 2.5.20

Uptime 6.201 days (since Fri Mar 12 12:49:18 2004)

Nmap run completed -- 1 IP address (1 host up) scanned in 14.303 seconds

OS fingerprinting isn't perfect, as you can see, but it will help narrow it down, both for you, and for the attacker. Hence, it is interesting for you to know as well. The best thing to do, is to give as little material as possible for the attacker to get a proper fingerprint on, and with this information you will know fairly well what the attacker knows about your OS as well.

Nmap also comes with a graphical user interface that can be used, called the nmapfe (Nmap Front End). It is an excellent frontend of the nmap program, and if you know that you will need a little bit more complicated searches, you may wish to use it. For an example screenshot, take a look below.

Of course, the nmap tool has more usages than this, which you can find out more about on the nmap homepage. For more information, take a look at the Nmap resources.

As you may understand, this is an excellent tool to test your host with, and to find out which ports are actually open and which are not. For example, after finishing your setup, use nmap to see if you have actually succeeded in doing what you wanted to do. Do you get the correct responses from the correct ports, and so on.

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