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In OSX if I type netstat I can see certain things that have an established connection. I don't want to change any settings to close these, I just want to be able to close whatever I ports I choose in the terminal. How do you do that?

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Do you want to close extant connections, prevent further ones on the same port, or both? –  Darael Apr 3 '12 at 13:18
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2 Answers

You can't close an open socket just like this. Ideally, you would just kill the process that has established the connection.

Check your connections with lsof (netstat won't show the process), filtering the output with whatever connection status you want:

lsof -i
lsof -i | grep LISTEN
lsof -i | grep ESTABLISHED

Or, to get the port, e.g. 17500:

lsof -i:17500

Then, just kill the process. For example:

$ lsof -i | grep "Skype"
Skype     438 werner    9u  IPv4 0xffffff801dd0c640      0t0  UDP localhost:52218
Skype     438 werner   42u  IPv4 0xffffff80231a7a08      0t0  TCP *:29429 (LISTEN)
Skype     438 werner   43u  IPv4 0xffffff8022e18a40      0t0  UDP *:29429

Kill Skype:

killall Skype

Note though that this won't prevent the connections from being made – something you have to specify in your Firewall preferences.

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Can also use the use the fuser or netstat commands.

fuser syntax is

fuser -k -n protocol portno

Example:

$ fuser -k -n udp 7777

7777/udp:            11774

The number 11774 is the pid.

netstat example:

$ sudo netstat -ap | grep :9050

tcp        0      0 localhost:9050          *:*       LISTEN      1613/tor

The number 1613 is the pid and "tor" is the process name.

Once you have the pid just end it using the kill or killall command

kill pid

or

killall -9 command_name

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Your instructions are for showing the perceived symptoms described in the question, open ports belonging to processes. You left out how to accomplish the desired task, to close the port. –  Eroen Sep 15 '12 at 10:20
    
@Eroen Thanks for pointing it out, added it. –  Silver Moon Sep 15 '12 at 11:15
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