FreeBSD

On a FreeBSD Unix system, as with other BSD Unix systems, you have a number of utilities with a base system install that can be used for listing open files, running processes, and network connections. The netstat utility is maintained as a part of the FreeBSD base system by the FreeBSD core developers, and offers exactly the sort of functionality you need to list open ports on your system.

netstat

To list open network ports and the processes that own them on FreeBSD with netstat, you can use this command:

  netstat -a | egrep 'Proto|LISTEN'

The output for this on my laptop running FreeBSD is:

Proto Recv-Q Send-Q  Local Address      Foreign Address    (state)
tcp4       0      0  localhost.ipp      *.*                LISTEN
tcp6       0      0  localhost.ipp      *.*                LISTEN
tcp4       0      0  *.2200             *.*                LISTEN
tcp6       0      0  *.2200             *.*                LISTEN
tcp4       0      0  *.x11              *.*                LISTEN
tcp6       0      0  *.x11              *.*                LISTEN

The localhost.ipp entry refers to the Internet Printing Protocol, used by CUPS to talk to the network printer. The *.2200 entry refers to SSH, which I have set to a nonstandard port — so it’s not recognized by netstat’s port-to-service association capabilities. *.x11, meanwhile, refers to the X Window System protocol.

You can add the -n option to netstat to get port numbers instead of having the utility try to provide names for services:

  netstat -an | egrep 'Proto|LISTEN'

The output would then look somewhat different:

Proto Recv-Q Send-Q  Local Address      Foreign Address    (state)
tcp4       0      0  127.0.0.1.631      *.*                LISTEN
tcp6       0      0  ::1.631            *.*                LISTEN
tcp4       0      0  *.2200             *.*                LISTEN
tcp6       0      0  *.2200             *.*                LISTEN
tcp4       0      0  *.6000             *.*                LISTEN
tcp6       0      0  *.6000             *.*                LISTEN

This information can be used to determine what services are running, in cases where services are using standard ports. On a FreeBSD system, you can get a listing of standard port associations by searching through the contents of /etc/services. For instance, if you wanted to find out what was up with port 631, you might use this command:

  grep -w 631 /etc/services

The output:

ipp     631/tcp    #IPP (Internet Printing Protocol)
ipp     631/udp    #IPP (Internet Printing Protocol)

sockstat

In addition to netstat, the more limited command sockstat is effectively tailor-made for this kind of information gathering. To get a listing of listening ports and their associated processes, you can use this command:

  sockstat -4l

The output may even be more useful than that of netstat above:

USER     COMMAND    PID   FD PROTO  LOCAL ADDRESS         FOREIGN ADDRESS
root     cupsd      1701  4  tcp4   127.0.0.1:631         *:*
root     cupsd      1701  6  udp4   *:631                 *:*
root     sshd       1685  4  tcp4   *:2200                *:*
root     Xorg       1154  3  tcp4   *:6000                *:*
root     syslogd    907   7  udp4   *:514                 *:*

Linux distributions

As with FreeBSD, the obvious choice of tool to use for listing open ports is netstat. Most Linux distributions use a different version of the utility, however — maintained separately from the Linux distribution, as an independent software development project.

One consequence of that fact is that the command line options used to achieve the same results may be different with FreeBSD than with Debian, Ubuntu, or Fedora Core Linux systems. On a typical Linux system, this command will list open network ports and the processes that own them:

  netstat -lnptu

The output should look something like this:

Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address   Foreign Address  State   PID/Program name
tcp        0      0 127.0.0.1:631   0.0.0.0:*        LISTEN  2458/cupsd
tcp        0      0 127.0.0.1:5432  0.0.0.0:*        LISTEN  2353/postgres
tcp6       0      0 :::22           :::*             LISTEN  2335/sshd
udp        0      0 0.0.0.0:631     0.0.0.0:*                2458/cupsd

As you can see from this output, the Debian GNU/Linux system on which I ran that command has only four open ports — two for CUPS, so that the computer can communicate with the network printer; one for PostgreSQL so that it can be contacted by applications in development; SSH, so that I can access it remotely, from my laptop.

Microsoft Windows XP

Microsoft Windows also offers a netstat command that can be executed from the command line to get a list of open ports. The standard MS Windows version of netstat is slightly more limited than its Unix-like system counterparts, but still suffices to get a listing of listening services:

  netstat -a | find "LISTENING"

The output of this command should look something like this:

TCP    hostname:epmap           hostname:0               LISTENING
TCP    hostname:microsoft-ds    hostname:0               LISTENING
TCP    hostname:10110           hostname:0               LISTENING
TCP    hostname:netbios-ssn     hostname:0               LISTENING

. . . with “hostname” replaced by the system’s hostname, of course.

Commercial UNIX Systems

For most commercial UNIX systems, even if there is not a version of netstat or sockstat available, you should be able to install lsof — which is short for “list open files”. Most Linux distributions and BSD Unix systems will provide lsof with a default install or through their respective software management systems. Some commercial UNIX systems do so as well, and for many others you can download it. The following command will limit the output of the utility to network ports:

  lsof -i -n | egrep 'COMMAND|LISTEN'

The output should look something like this (as run on my laptop, again):

COMMAND   PID USER   FD   TYPE     DEVICE SIZE/OFF NODE NAME
Xorg     1154 root    1u  IPv6 0xc6042000      0t0  TCP *:x11 (LISTEN)
Xorg     1154 root    3u  IPv4 0xc6041cb0      0t0  TCP *:x11 (LISTEN)
sshd     1685 root    3u  IPv6 0xc6041ae0      0t0  TCP *:2200 (LISTEN)
sshd     1685 root    4u  IPv4 0xc6041910      0t0  TCP *:2200 (LISTEN)
cupsd    1701 root    3u  IPv6 0xc6041740      0t0  TCP [::1]:ipp (LISTEN)
cupsd    1701 root    4u  IPv4 0xc6041570      0t0  TCP 127.0.0.1:ipp (LISTEN)