Manually closing a port from commandline

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 don't understand why it is only possible by the application which opens it ... But I'm still eager to know if there is any other way to do it.

Solution 1:

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. It'll be something like "97u" which means "97".

3. Now connect the process:

   gdb -p $pid
   

4. Now close the socket:

   call close($fileDescriptor) //does not need ; at end.
   

   example:

   call close(97)
   

   Then detach gdb:

   quit
   

   And the socket is closed.

Solution 2:

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 , for example:

tcp  0  0 *:8000   *:* LISTEN  4683/procHoldingPort

In this case, procHoldingPort is the name of the process that opened the port, 4683 is its pid, and 8000 (note that it is TCP) is the port number it holds.

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.

Solution 3:

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

Solution 4:

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.