If you don’t have lsof already you can download and install it by becoming root and running:

root@mystery:~# apt-get install lsof

This will download and install the package for you, along with any dependencies which might be required:

Reading package lists... Done
Building dependency tree... Done
The following NEW packages will be installed:
  lsof
0 upgraded, 1 newly installed, 0 to remove and 16 not upgraded.
Need to get 339kB of archives.
After unpacking 549kB of additional disk space will be used.
Get:1 http://http.us.debian.org unstable/main lsof 4.75.dfsg.1-1 [339kB]
Fetched 339kB in 3s (90.8kB/s)
Selecting previously deselected package lsof.
(Reading database ... 69882 files and directories currently installed.)
Unpacking lsof (from .../lsof_4.75.dfsg.1-1_i386.deb) ...
Setting up lsof (4.75.dfsg.1-1) ...

Once you have the package installed you can now discover precisely which processes are bound upon particular ports.

If you have the Apache webserver running on port 80 that will provide a suitable test candidate. If not you can choose another port you know is in use.

To discover the process name, ID (pid), and other details you need to run:

lsof -i :port

So to see which process is listening upon port 80 we can run:

root@mystery:~# lsof -i :80

This gives us the following output:

COMMAND   PID     USER   FD   TYPE   DEVICE SIZE NODE NAME
apache2 10437     root    3u  IPv6 22890556       TCP *:www (LISTEN)
apache2 10438 www-data    3u  IPv6 22890556       TCP *:www (LISTEN)
apache2 10439 www-data    3u  IPv6 22890556       TCP *:www (LISTEN)
apache2 10440 www-data    3u  IPv6 22890556       TCP *:www (LISTEN)
apache2 10441 www-data    3u  IPv6 22890556       TCP *:www (LISTEN)
apache2 10442 www-data    3u  IPv6 22890556       TCP *:www (LISTEN)
apache2 25966 www-data    3u  IPv6 22890556       TCP *:www (LISTEN)
apache2 25968 www-data    3u  IPv6 22890556       TCP *:www (LISTEN)

Here you can see the command running (apache2), the username it is running as www-data, and some other details.

Similarly we can see which process is bound to port 22:

root@mystery:~# lsof -i :22
COMMAND   PID USER   FD   TYPE   DEVICE SIZE NODE NAME
sshd     8936 root    3u  IPv6 12161280       TCP *:ssh (LISTEN)

To see all the ports open for listening upon the current host you can use another command netstat (contained in the net-tools package):

root@mystery:~# netstat -a |grep LISTEN |grep -v unix
tcp        0      0 *:2049                  *:*                     LISTEN     
tcp        0      0 *:743                   *:*                     LISTEN     
tcp        0      0 localhost.localdo:mysql *:*                     LISTEN     
tcp        0      0 *:5900                  *:*                     LISTEN     
tcp        0      0 localhost.locald:sunrpc *:*                     LISTEN     
tcp        0      0 *:8888                  *:*                     LISTEN     
tcp        0      0 localhost.localdom:smtp *:*                     LISTEN     
tcp6       0      0 *:www                   *:*                     LISTEN     
tcp6       0      0 *:distcc                *:*                     LISTEN     
tcp6       0      0 *:ssh                   *:*                     LISTEN

Here you can see that there are processes listening upon ports 20497435900, and several others.

(The second grep we used above was to ignore Unix domain sockets).

If you’re curious to see which programs and services are used in those sockets you can look them up as we’ve already shown:

root@mystery:~# lsof -i :8888
COMMAND   PID    USER   FD   TYPE   DEVICE SIZE NODE NAME
gnump3d 25834 gnump3d    3u  IPv4 61035200       TCP *:8888 (LISTEN)

This tells us that the process bound to port 8888 is the gnump3d MP3 streamer.

Port 2049 and 743 are both associated with NFS. The rest can be tracked down in a similar manner. (You’ll notice that some ports actually have their service names printed next to them, such as the smtp entry for port 25).

lsof is a very powerful tool which can be used for lots of jobs. If you’re unfamiliar with it I recommend reading the manpage via:

man lsof

If you do so you’ll discover that the -i flag can take multiple different types of arguments, to allow you to check more than one port at a time, and use IPv6 addresses too.

It’s often used to see which files are open upon mounted devices, so you can kill the processes and unmount them cleanly.