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I want to close an open port which is in listening mode between my client and server application.

Is there any manual command line option in Linux to close a port ??

NOTE: I came to know that "only the application which owns the connected socket should close it, which will happen when the application terminates."

I dont understand why it is only possible by the application which opens it ... But still eager to know if there is any another way to do it ??

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migrated from stackoverflow.com Apr 6 '10 at 5:47

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3  
No, opened ports belong to the process which opened them, there is no control possible from outside. Which is a good thing, or all applications would have to anticipate their open ports (and files) being messed with. However, you can block traffic to a port by firewalling (iptables), but that will not close and give up the port for other use. –  Jürgen Strobel Oct 6 '11 at 0:26
    
A lot of responders missed the point of this question. It is nonsense to declare that only the application that owns the port can disconnect it. I can disconnect it by walking up to the box and pulling the ethernet cable out of the socket, or by killing the application at the other end of the connection! The application must be written to handle this. So --- how do you test to be sure the application is written properly without requiring physical intervention and/or control of the other computer? –  Dale Wilson Sep 22 at 19:39

8 Answers 8

You're kind of asking the wrong question here. It isn't really possible to simply "close a port" from outside the application that opened the socket listening on it. The only way to do this is to completely kill the process that owns the port. Then, in about a minute or two, the port will become available again for use. Here's what's going on (if you don't care, skip to the end where I show you how to kill the process owning a particular port):

Ports are resources allocated by the OS to different processes. This is similar to asking the OS for a file pointer. However, unlike file pointers, only ONE process at a time may own a port. Through the BSD socket interface, processes can make a request to listen on a port, which the OS will then grant. The OS will also make sure no other process gets the same port. At any point, the process can release the port by closing the socket. The OS will then reclaim the port. Alternatively, if the process ends without releasing the port, the OS will eventually reclaim the port (though it won't happen immediately: it'll take a few minutes).

Now, what you want to do (simply close the port from the command-line), isn't possible for two reasons. First, if it were possible, it would mean one process could simply steal away another process's resource (the port). This would be bad policy, unless restricted to privileged processes. The second reason is it is unclear what would happen to the process that owned the port if we let it continue running. The process's code is written assuming that it owns this resource. If we simply took it away, it would end up crashing on it's own, so OS's don't let you do this, even if you're a privileged process. Instead, you must simply kill them.

Anyway, here's how to kill a process that owns a particular port:

sudo netstat -ap | grep :<port_number>

That will output the line corresponding to the process holding port . Then, look in the last column, you'll see /. Then execute this:

kill  <pid>

If that doesn't work (you can check by re-running the netstat command). Do this:

kill -9 <pid>

In general, it's better to avoid sending SIGKILL if you can. This is why I tell you to try kill before kill -9. Just using kill sends the gentler SIGTERM.

Like I said, it will still take a few minutes for the port to re-open if you do this. I don't know a way to speed this up. If someone else does, I'd love to hear it.

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@smehmood - Thanks for the detailed explanation .. A small doubt .. How does the kernel reclaim a open port by an abruptly killed process ?? .. the solution u provided seems to be killing the process holding the port ... –  codingfreak Apr 6 '10 at 4:17
1  
@codingfreak The kernel knows the process is gone. It knows it can reclaim the port. There are actually rules on the timing of closing the port, to make sure there aren't any stray packets floating on the 'net. How does it know it has these resources? that's what the kernel does, keeps track of things. –  Rich Homolka Nov 1 '13 at 2:05

I had same problem, the process must keep alive but the socket must close. Closing a socket in a running process is not impossible but difficult: 1) locate the process :

netstat -np

You get a source/destination ip:port portstate pid/processname map

2) locate the the socket's file descriptor in the process

lsof -np $pid

You get a list: process name, pid, user,fileDescriptor, ... a connection string.

Locate the matching fileDescriptor number for the connection.

Now connect the process:

gdb -p $pid

3) Now close the socket:

call close($fileDescritor)

//does not need ; at end.

Than detach:

quit

And the socket is closed.

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sudo lsof -np $pid gives me about 200 lines and I'm confused about how to find desired FD. In my case process is a Chrome tab and i'm trying to close opened websockets... –  SET Jun 10 at 9:25
    
A row typically looks like: firefox 14812 szupervigyor 97u IPv4 32814564 0t0 TCP 192.168.2.4:40385->173.194.39.65:https (ESTABLISHED) as: process_name pid user fd[opened_for] protocol device inode protocol_data_toString –  Dankó Dávid Jun 11 at 23:32
    
* A row typically looks like: firefox 14812 szupervigyor 97u IPv4 32814564 0t0 TCP 192.168.2.4:40385->173.194.39.65:https (ESTABLISHED) as: process_name pid user fd[opened_for] protocol device inode protocol_data_toString You need to know the remote ip address and find on the last col. In my example the 97 is the FileDescriptor. Searching is difficult if you opened multiple connection to the target host. –  Dankó Dávid Jun 11 at 23:38

Fuser can also be used

fuser -k -n *protocol portno*

Here protocol is tcp/udp and portno is the number you wish to close. E.g.

fuser -k -n tcp 37

More info at fuser man page

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Just killing owning process didn't work for me, but fuser did. Thx! –  webwurst May 13 '13 at 10:06
    
I got mixed results with it, even after using fuser, I got "socket already in use" when trying to reuse the port from killed app even doing so as root. On the other hand, the time to free the socket seemed to be shorter then before, so thanks anyway. –  Jan Vlcinsky Apr 14 at 8:16
    
@JanVlcinsky Maybe there is a "guardian" process which restarts the killed process moments after fuser is run? –  RedBaron Apr 15 at 5:12
    
@RedBaron: according to comment superuser.com/a/415236/153413 my problem originates in poorly written application, which does not clean up/close the socket under termination. So fuser finds the process using the port and kills it, but does not resolve the fact, that the socket is not closed. 60 seconds and kernel does it for me. –  Jan Vlcinsky Apr 15 at 16:25

You could alternatively use iptables:

iptables -I INPUT -p tcp --dport 80 -j DROP

It basically accomplishes what you want. This will drop all TCP traffic to port 80.

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netstat -anp | grep 80

It should tell you, if you're running apache, "httpd" (this is just an example, use the port your application is using instead of 80)

pkill -9 httpd 

or

killall -9 httpd
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2  
try a normal kill before resorting to -9 –  Thilo Apr 6 '10 at 3:49
    
@omfgroflmao - But it will kill the process which has opened the port ?? –  codingfreak Apr 6 '10 at 4:13
    
@codingfreak The process that's holding the port, yes it will kill it. –  Anonymous Apr 6 '10 at 4:48

You could write a script which modified the iptables and restarts them. One script for adding a rule dropping all packets on the port, another script for removing said rule.

The other answers have shown you how to kill the process bound to the port - this may not be what you want. If you want the server to keep running, but to prevent connections from clients then you want to block the port, not stop the process.

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@Michael Shimmins ... hmm sounds intresting as we can block the port on server side so that the client wont send any messages. –  codingfreak Apr 6 '10 at 4:18
    
Well the client can send messages all they want we've just closed the door so they can't get in. –  Michael Shimmins Apr 6 '10 at 4:30

You could probably just find out what process opened the socket that the port is associated with, and kill that process.

But, you would have to realize that unless that process has a handler that de-initializes all the stuff that it was using (open files, sockets, forks, stuff that can linger unless it's closed properly upon termination) then you would have created that drag on system performance. Plus, the socket will remain open until the kernel realizes that that the process has been killed. That usually just takes about a minute.

I suppose the better question would be: What port (belonging to what process) do you want to stop?

If you are trying to put an end to a backdoor or virus that you found, then you should at least learn what data is going back and forth before you terminate it. (wireshark is good for this) (And the process' executable name so you can delete it and prevent it from coming back on reboot) or, if it's something you installed (like HTTPD or FTPD or something) then you should already have access to the process itself.

Usually it will have a control program (HTTPD stop|start or something). Or, if it's a system thing, you probably should not mess with it. Anyway, i thought that since everyone else is giving you the "how-to" angle, i should give you the caveats.

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Very good comment. I have one program, which under termination does ot close the socket what results in described behaviour - the socket is unusable for about 60 seconds. When I stop and start that process, it is complaining for about a minute, that the address and port is already in use. The best solution is to correct badly behaving process to close properly, but sometime this is not an option. Is there a way to ask kernel checking that blocked socket sooner than in 60 seconds? –  Jan Vlcinsky Apr 15 at 16:21

One more issue: sometime the kernel owns ports themselves. I know NAT routing holds some ports open for NAT use. You can not kill a process for this, this is a kernel, and a reconfiguration and a reboot is required.

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