Find out which process is holding which socket open

Posted November 14, 2003 by acockrell in UNIX

You have run netstat -an. You see that port TCP/65237 is listening. What program is actually running and holding that port open? Here’s how to find out.

Sometimes you notice odd ports open and listening on your machine. In this day and age, that’s often a bad thing. But what do you kill to close off that port? Is that port supposed to be open (ie, something legitimately running).

You’ve probably heard of lsof. List Open Files. You usually use it to find out what files are open on a given mount point. Well, you can also use it to find out what open sockets your machine has:

# lsof -nl | egrep "TCP|UDP"

--snip long list--

wish 30766 500 4u IPv4 1051584 TCP *:65237 (LISTEN)
wish 30766 500 6u IPv4 1051588 TCP *:63251 (LISTEN)

That shows that my workstation, while typing this message, has wish running at PID 30766. It’s listening on two ports: TCP/65237 and TCP/63251.

You can then run ps to determine what that process really is (no worries… it’s aMSN on my workstation) and decide if you really want to kill it off.

If you’re running Solaris, you won’t be graced with lsof unless you install it from source or via SunFreeWare. It does come installed by default on many Linux distributions.

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