Make cross compatible server

[techgaun]

The server can be made cross-compatible. Currently, only the netcat variants with -e are supported.
From netcat-openbsd package's manpage:
There is no -c or -e option in this netcat, but you still can execute a command after connection being established by redirecting file descriptors. Be cautious here because opening a port and let anyone connected execute arbitrary command on your site is DANGEROUS. If you really need to do this, here is an example:

On ‘server’ side:

       $ rm -f /tmp/f; mkfifo /tmp/f
       $ cat /tmp/f | /bin/sh -i 2>&1 | nc -l 127.0.0.1 1234 > /tmp/f

On ‘client’ side:

       $ nc host.example.com 1234
       $ (shell prompt from host.example.com)

By doing this, you create a fifo at /tmp/f and make nc listen at port 1234 of address 127.0.0.1 on ‘server’ side, when a ‘client’ establishes a connection successfully to that port, /bin/sh gets executed on ‘server’ side and the shell prompt is given to ‘client’ side.

When connection is terminated, nc quits as well. Use -k if you want it keep listening, but if the command quits this option won't restart it or keep nc running. Also don't forget to remove the file descriptor once you don't need it anymore:

       $ rm -f /tmp/f

[techgaun]

With some free time, I attempted to port to nc.openbsd but I believe its due to my limited knowledge that I've not been able to port it properly. Currently, support for nc.openbsd is not able to write the command output back to the client. Unfortunately, I didn't have enough time to dig more. PRs/suggestions/advices are welcome.