Hack 42 Keep an Inventory of Your Network

Use Nmap to keep track of the devices and services on your network.

As we saw in [Hack #40] , Nmap (http://www.insecure.org/nmap/) is free a tool that can be used to conduct various sorts of scans on networks. Normally when people think of using Nmap, they assume it's used to conduct some sort of nefarious network reconnaissance in preparation for an attack. But as with all powerful tools, Nmap can be used for far more than breaking into networks.

For example, simple TCP connect scans can be conducted without needing root privileges:

$ nmap rigel



Starting nmap 3.48 ( http://www.insecure.org/nmap/ ) at 2003-12-15 17:42 MST

Interesting ports on rigel (192.168.0.61):

(The 1595 ports scanned but not shown below are in state: filtered)

PORT      STATE  SERVICE

7/tcp     open   echo

9/tcp     open   discard

13/tcp    open   daytime

19/tcp    open   chargen

21/tcp    open   ftp

22/tcp    open   ssh

23/tcp    open   telnet

25/tcp    open   smtp

37/tcp    open   time

79/tcp    open   finger

111/tcp   open   rpcbind

512/tcp   open   exec

513/tcp   open   login

514/tcp   open   shell

587/tcp   open   submission

4045/tcp  open   lockd

7100/tcp  open   font-service

32771/tcp open   sometimes-rpc5

32772/tcp open   sometimes-rpc7

32773/tcp open   sometimes-rpc9

32774/tcp open   sometimes-rpc11

32775/tcp open   sometimes-rpc13

32776/tcp open   sometimes-rpc15

32777/tcp open   sometimes-rpc17



Nmap run completed -- 1 IP address (1 host up) scanned in 75.992 seconds

This is tremendously useful for checking on the state of your own machines. You could probably guess that this scan was performed on a Solaris machine, and one that needs to have some services disabled at that.

Nmap can also scan ranges of IP addresses by specifying the range or using CIDR notation:

nmap 192.168.0.1-254

nmap 192.168.0.0/24

Nmap can provide much more information if it is run as root. When run as root, it can use special packets to determine the operating system of the remote machine by using the -O flag. Additionally, you can do half-open TCP scanning by using the -sS flag. When doing a half-open scan, Nmap will send a SYN packet to the remote host and wait to receive the ACK from it; if it receives an ACK, it knows that the port is open. This is different from a normal three-way TCP handshake, where the client will send a SYN packet and then send an ACK back to the server once it has received the initial server ACK. Attackers typically use this option to avoid having their scans logged on the remote machine.

Try it out for yourself:

# nmap -sS -O rigel



Starting nmap V. 3.00 ( www.insecure.org/nmap/ )

Interesting ports on rigel.nnc (192.168.0.61):

(The 1578 ports scanned but not shown below are in state: filtered)

Port       State       Service

7/tcp      open        echo                    

9/tcp      open        discard                 

13/tcp     open        daytime                 

19/tcp     open        chargen                 

21/tcp     open        ftp                     

22/tcp     open        ssh                     

23/tcp     open        telnet                  

25/tcp     open        smtp                    

37/tcp     open        time                    

79/tcp     open        finger                  

111/tcp    open        sunrpc                  

512/tcp    open        exec                    

513/tcp    open        login                   

514/tcp    open        shell                   

587/tcp    open        submission              

7100/tcp   open        font-service            

32771/tcp  open        sometimes-rpc5          

32772/tcp  open        sometimes-rpc7          

32773/tcp  open        sometimes-rpc9          

32774/tcp  open        sometimes-rpc11         

32775/tcp  open        sometimes-rpc13         

32776/tcp  open        sometimes-rpc15         

32777/tcp  open        sometimes-rpc17         

Remote operating system guess: Solaris 9 Beta through Release on SPARC

Uptime 44.051 days (since Sat Nov  1 16:41:50 2003)



Nmap run completed -- 1 IP address (1 host up) scanned in 166 seconds

With OS detection enabled, Nmap has confirmed that the operating system is Solaris, but now you also know that it's probably Version 9 running on a SPARC processor.

One powerful feature that can be used to help keep track of your network is Nmap's XML output capabilities. This is activated by using the -oX command-line switch:

# nmap -sS -O -oX scandata.xml rigel

This is especially useful when scanning a range of IP addresses or your whole network, because you can put all the information gathered from the scan into a single XML file that can be parsed and inserted into a database. Here's what an XML entry for an open port looks like:

<port protocol="tcp" portid="22">

<state state="open" />

<service name="ssh" method="table" conf="3"  />

</port>

Nmap is a powerful tool. By using its XML output capabilities, a little bit of scripting, and a database, you can create an even more powerful tool that can monitor your network for unauthorized services and machines.