Two of the fundamental aspects of Linux system security and troubleshooting are knowing what services are running, and what connections and services are available. We're all familiar with ps for viewing active services. netstat goes a couple of steps further, and displays all available connections, services, and their status. It shows one type of service that ps does not: services run from inetd or xinetd, because inetd/xinetd start them up on demand. If the service is available but not active, such as telnet, all you see in ps is either inetd or xinetd:

$ ps ax | grep -E 'telnet|inetd'
  520 ?            Ss         0:00 /usr/sbin/inetd

But netstat shows telnet sitting idly, waiting for a connection:

$ netstat --inet -a | grep telnet
tcp      0     0     *:telnet      *:*    LISTEN

This netstat invocation shows all activity:

$ netstat -a
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address  Foreign Address State
tcp     0      0      *:telnet       *:*           LISTEN
tcp     0      0      *:ipp          *:*           LISTEN
tcp     0      0      *:smtp         *:*           LISTEN
tcp     0      0      192.168.1.5:32851 nest.anthill.echid:ircd     ESTABLISHED
udp     0      0      *:ipp          *:*
Active UNIX domain sockets (servers and established)
Proto RefCnt Flags    Type     State       I-Node Path
unix  2      [ ACC ]  STREAM   LISTENING   1065   /tmp/ksocket-carla/klaunchertDCh2b.slave-socket
unix  2      [ ACC ]  STREAM   LISTENING   1002   /tmp/ssh-OoMGfFm666/agent.666
unix  2      [ ACC ]  STREAM   LISTENING   819    private/smtp

Your total output will probably run to a couple hundred lines. (A fun and quick way to count lines of output is netstat -a | wc -l.) You can ignore everything under "Active UNIX domain sockets." Those are local inter-process communications, not network connections. To avoid displaying them at all, do this:

$ netstat --inet -a

This will display only network connections, both listening and established. Already netstat has earned its keep--both the telnet and smtp services are running. This is bad, because I don't want to have either a telnet or smtp server running on this machine. So now I know I need to turn them off, and re-configure my startup files so they won't start at boot.

How do you know what services you want running? That is a mondo subject for another day, and an important one. For example, if your system has been compromised, this is one place to find evidence of a Trojan horse or other malware phoning home. In this example, ipp is Internet Printing Protocol, which belongs to CUPS (Common Unix Printing System.) If you want your printer to work, this needs to be here. The connection on 192.168.1.5:32851 is my active IRC (Internet Relay Chat) connection. Refer to your /etc/services file to learn more about TCP and UDP ports, and the services assigned to them.

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