2008-05-25 Finding open ports

When tightening up security on a Linux server, one of the first things the system administrator does, is find out which ports are open. In other words, which applications are listening on a port that is reachable from the network and/or internet.

We'll use netstat for this purpose. On a prompt, type:

  $ sudo netstat --tcp --listen -p

Overview of the options:

--tcp Show applications that use the TCP protocol (exclude UDP)
--listen Show only applications that listen, and exclude clients
-p Show the process ID and name of the application to which the port belongs

Netstat sometimes pauses during output. This is normal; it tries to resolve the addresses into human readable host names*. If you don't want this, use the -n option.

Example output from my laptop which runs Fedora 8 (I have removed the columns Foreign Address and State for the sake of brevity):

 Active Internet connections (only servers)
 Proto Recv-Q Send-Q Local Address                PID/Program name   
 tcp        0      0 telislt.sron.nl:irdmi        2381/nasd           
 tcp        0      0 *:55428                      1965/rpc.statd      
 tcp        0      0 telislt.sron.:commplex-main  6573/ssh            
 tcp        0      0 *:mysql                      2307/mysqld         
 tcp        0      0 *:sunrpc                     1945/rpcbind        
 tcp        0      0         2533/dnsmasq        
 tcp        0      0 telislt.sron.nl:privoxy      3581/ssh            
 tcp        0      0 telislt.sron.nl:ipp          2553/cupsd          
 tcp        0      0 telislt.sron.nl:smtp         2352/sendmail: acce 
 tcp        0      0 *:8730                       6030/skype          
 tcp        0      0 *:http                       2371/httpd          
 tcp        0      0 localhost6.localdom:privoxy  3581/ssh            
 tcp        0      0 *:ssh                        2205/sshd

Whenever there is an asterisk (star) instead of a host name, netstat tells us that the port is listened to on ALL interfaces, not only the local interface but also any present interfaces connected to the outside world. These are the ones we want to hunt down.

Now we know the program names, we can find out more about them. We'll take for instance the rpc.statd program. First we locate the complete path of this process:

 $ whereis rpc.statd
 rpc: /sbin/rpc.statd /usr/sbin/rpc.svcgssd /usr/sbin/rpc.idmapd 
 /usr/sbin/rpc.mountd /usr/sbin/rpc.rquotad /usr/sbin/rpc.gssd 
 /usr/sbin/rpc.nfsd /etc/rpc /usr/include/rpc

Whereis does a search and finds /sbin/rpc.statd. On RPM-based systems, we can request more information about the owning package:

 $ rpm -qif /sbin/rpc.statd
 The nfs-utils package provides a daemon for the kernel NFS server and
 related tools, which provides a much higher level of performance than the
 traditional Linux NFS server used by most users.

Now we know whether we want this package or not. If not, just remove it and the port will be closed. If we need the functionality, does it need to listen to the outside network? If not, we would typically Read The Fine Manual to see whether we can configure this package to listen locally.

Repeating this exercise for each line in the netstat output will tighten a server its security.