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Displaying Connections
Netstat supports a set of options to display active or passive sockets. The options -t, -u, -w, and -x show active TCP, UDP, RAW, or UNIX socket connections. If you provide the -a flag in addition, sockets that are waiting for a connection (i.e. listening) are displayed as well. This will give you a list of all servers that are currently running on your system.Invoking netstat -ta on vlager produces:
$ netstat -ta
Active Internet connections
Proto Recv-Q Send-Q Local Address Foreign Address (State)
tcp 0 0 *:domain *:* LISTEN
tcp 0 0 *:time *:* LISTEN
tcp 0 0 *:smtp *:* LISTEN
tcp 0 0 vlager:smtp vstout:1040 ESTABLISHED
tcp 0 0 *:telnet *:* LISTEN
tcp 0 0 localhost:1046 vbardolino:telnet ESTABLISHED
tcp 0 0 *:chargen *:* LISTEN
tcp 0 0 *:daytime *:* LISTEN
tcp 0 0 *:discard *:* LISTEN
tcp 0 0 *:echo *:* LISTEN
tcp 0 0 *:shell *:* LISTEN
tcp 0 0 *:login *:* LISTEN
This shows most servers simply waiting for an incoming connection. However,
the fourth line shows an incoming SMTP connection from vstout, and
the sixth line tells you there is an outgoing telnet connection
to vbardolino.
Using the -a flag all by itself will display all sockets from all families.
Andrew Anderson
Thu Mar 7 23:22:06 EST 1996