Problem Description:

Does somebody knows how to close a TCP or UDP socket for a single connection via windows command line?

Googling about this, I saw some people asking the same thing. But the answers looked like a manual page of netstat or netsh commands focusing on how to monitor the ports. I don’t want answers on how to monitor them (I already do this). I want to close/kill them.

EDIT, for clarification: Let’s say that my server listens TCP port 80. A client makes a connection and port 56789 is allocated for it. Then, I discover that this connection is undesired (e.g. this user is doing bad things, we asked them to stop but the connection didn’t get dropped somewhere along the way). Normally, I would add a firewall to do the job, but this would take some time, and I was in an emergency situation. Killing the process that owns the connection is really a bad idea here because this would take down the server (all users would lose functionality when we just want to selectively and temporally drop this one connection).

In order to close the port you could identify the process that is listening on this port and kill this process.

You can’t close sockets without shutting down the process that owns those sockets. Sockets are owned by the process that opened them. So to find out the process ID (PID) for Unix/Linux. Use netstat like so:

That will print something like:

Where -a prints all sockets, -n shows the port number, -p shows the PID, -l shows only what’s listening (this is optional depending on what you’re after).

The real info you want is PID. Now we can shutdown that process by doing:

If you are shutting down a service it’s better to use:

Kill literally kills just that process and any children it owns. Using the service command runs the shutdown script registered in the init.d directory. If you use kill on a service it might not properly start back up because you didn’t shut it down properly. It just depends on the service.

Unfortunately, Mac is different from Linux/Unix in this respect. You can’t use netstat. Read this tutorial if you’re interested in Mac:

http://www.tech-recipes.com/rx/227/find-out-which-process-is-holding-which-socket-open/

And if you’re on Windows use TaskManager to kill processes, and services UI to shutdown services. You can use netstat on Windows just like Linux/Unix to identify the PID.

http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/netstat.mspx?mfr=true

You can’t close sockets on your server without owning those sockets hence you can’t actually close the socket down without having code running in the process that owns the server socket.

However, there is another option which is telling the client to close its socket. Sending a RST TCP packet to the port the client is connecting on will cause the client to drop their connection. You can do that with RST scanning using nmap.

http://nmap.org/

Try the tools TCPView (GUI) and Tcpvcon (command line) by Sysinternals/Microsoft. https://learn.microsoft.com/en-us/sysinternals/downloads/tcpview

Yes, this is possible. You don’t have to be the current process owning the socket to close it. Consider for a moment that the remote machine, the network card, the network cable, and your OS can all cause the socket to close.

Consider also that Fiddler and Desktop VPN software can insert themselves into the network stack and show you all your traffic or reroute all your traffic.

So all you really need is either for Windows to provide an API that allows this directly, or for someone to have written a program that operates somewhat like a VPN or Fiddler and gives you a way to close sockets that pass through it.

There is at least one program (CurrPorts) that does exactly this and I used it today for the purpose of closing specific sockets on a process that was started before CurrPorts was started. To do this you must run it as administrator, of course.

Note that it is probably not easily possible to cause a program to not listen on a port (well, it is possible but that capability is referred to as a firewall…), but I don’t think that was being asked here. I believe the question is “how do I selectively close one active connection (socket) to the port my program is listening on?”. The wording of the question is a bit off because a port number for the undesired inbound client connection is given and it was referred to as “port” but it’s pretty clear that it was a reference to that one socket and not the listening port.

open cmd

type in netstat -a -n -o

find TCP [the IP address]:[port number] .... #[target_PID]# (ditto for UDP)

(Btw, kill [target_PID] didn’t work for me)

CTRL+ALT+DELETE and choose “start task manager”

Click on “Processes” tab

Enable “PID” column by going to: View > Select Columns > Check the box for PID

Find the PID of interest and “END PROCESS”

Now you can rerun the server on [the IP address]:[port number] without a problem

I found the right answer to this one. Try TCPView from Sysinternals, now owned by Microsoft. You can find it at http://technet.microsoft.com/en-us/sysinternals/bb897437

instant/feasible/partial answer : https://stackoverflow.com/a/20130959/2584794

unlike from the previous answer where netstat -a -o -n was used incredibly long list was to be looked into without the name of application using those ports

you can use program like tcpview from sysinternal. I guess it can help you a lot on both monitoring and killing unwanted connection.

Use TCPView: http://technet.microsoft.com/en-us/sysinternals/bb897437.aspx or CurrPorts: https://www.nirsoft.net/utils/cports.html

Alternatively, if you don’t want to use EXTERNAL SOFTWARE (these tools don’t require an installation by the way), you can simply FIRST run the netstat command (preferably netstat -b ) & then setup Local Security Policy to block the IP address of the user’s machine in question, that’s what I have been doing with unwanted or even unknown connections – that allows you doing everything WITHOUT ANY EXTERNAL SOFTWARE (everything comes with Windows)…

wkillcx is a reliable windows command line tool for killing tcp connections from the command line that hasn’t been mentioned. It does have issues with servers with large number of connections sometimes though. I sometimes use tcpview for interactive kills but wkillcx can be used in scripts.

Use CurrPorts (it’s free and no-install): http://www.nirsoft.net/utils/cports.html

/close {Process Name}

Examples:

It also has a nice GUI with search and filter features.

Note: This answer is huntharo and JasonXA’s answer and comment put together and simplified to make it easier for readers. Examples come from CurrPorts’ web page.

For instance you want to free the port 8080 Then, follow these commands.

Done!

Yes there is possible to close TCP or UDP port there is a command in DOS

I hope this will work for You

If you’re runnning on Windows 8,`Windows Server 2012 or above with PowerShell v4 of above installed, you can use the below script. This finds the processes associated with the port & terminates them (i.e. kills the process along with its connection; not just the connection).

If you know the port that you want to free you can sort your netstat list by looking for the specif port like this:

Then the pid will appear at the rigth which you can kill with taskkill.

Also you may want to look at this question which is specifically for localhost, but I think it is relevant:

CurrPorts did not work for us and we could only access the server through ssh, so no TCPView either. We could not kill the process either, as to not drop other connections. What we ended up doing and was not suggested yet was to block the connection on Windows’ Firewall. Yes, this will block all connections that fit the rule, but in our case there was a single connection (the one we were interested in):

Replace the IP by the one you need and add other rules if needed.

If you know the particular port you want to kill, simply open Command Prompt as admin (on windows) and:

npx kill-port 1900

1900 above is the port number in my case. I use this most times when I want to close a port that React-Native developer tools (and Expo) is running on. Reason being that even after closing the developer window or stopping the server, the port still somehow remains in use.