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I would like to run a bash command when a remote connection is successfully established to a specified local open port. I have looked into netcat and socat, but I'm not sure if either have the functionality that I want. I do not care about the content of the communication, only that a connection has been established (the connection can be immediately dropped after starting).

This is because, I have a local resource (a Minecraft server) that I want to start only when requested. In theory, a client should attempt a connection (which will of course fail, initially), the server will then start, and after a few moments the client can connect once more to a running server.

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    portknocking works like that: a service is started (or some firewall rules are changed) only after a specific packet is sent to a specific port (and sometimes with specific content). You may get ideas from such features. – Patrick Mevzek Jun 11 at 23:00
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There's the -F option to nc:

-F Pass the first connected socket using sendmsg(2) to stdout and exit. This is useful in conjunction with -X to have nc perform connection setup with a proxy but then leave the rest of the connection to another program (e.g. ssh(1) using the ssh_config(5) ProxyUseFdpass option).

I guess you could use it along this line:

nc -Fl 22222 | true && echo connection arrived

Of course you'd start the server instead of echoing. 22222 is the port and l stands for listening.

  • I'm not sure what's going on, but I actually don't have that functionality on my netcat version. On the man-page, the flags of my version go straight from -d to -h! From a quick Google search, it looks like the NetBSD version of netcat has that flag, do you have any idea how I can get it onto my system? I'm currently running Ubuntu 16.04.6 LTS – sgbrown Jun 11 at 23:25
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    @sgbrown It looks like I have a BSD version on Debian. Depending on your OS, you could just install it, or probably download the source code, compile it and then install. – Tomasz Jun 11 at 23:30

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