Windows – Utility to open TCP port to listen state

firewallportwindows

Is there some basic utility to open a specific network TCP port on my machine?

I need to test how my program deals with ports in the listening state.

Best Answer

netcat should do what you want. Have it listen on you machine and echo stuff to STDOUT:

nc -4 -k -l -v localhost 1026

when you want it to close when the connection ends, don't use -k

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Windows Networking – How to Find EXE Listening on Port

Try netstat -o to get the process id (PID) and then use tasklist |findstr <pid> to see the process name and type. Task Manager also shows PID and process name.

You can combine your other switches with -o like so: netstat -bona -p tcp

How to make `nc` listen on a port without setting `SO_REUSEPORT`

Looking to one of the netcat source code netcat.c in the method local_listen :

        ret = setsockopt(s, SOL_SOCKET, SO_REUSEADDR, &x, sizeof(x));
        if (ret == -1)
            err(1, NULL);

# if defined(SO_REUSEPORT)
        ret = setsockopt(s, SOL_SOCKET, SO_REUSEPORT, &x, sizeof(x));
        if (ret == -1)
            err(1, NULL);
# endif

Then in order to modify the behaviour, you need to modify the code.

However I made a test with netcat-openbsd 1.105-7 that is part of Ubuntu Xenial and it seems built without defining SO_REUSEPORT. As it set SO_REUSEADDR but not the SO_REUSEPORT (under a kernel > 3.9) it works like you expect refusing to run a second intance on same port.

Best Answer

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Windows Networking – How to Find EXE Listening on Port

How to make `nc` listen on a port without setting `SO_REUSEPORT`

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