Listen to other machine

Hi.

This one is little cloudy question...

There is a server that is connected to another machine (Linux based).
The other machine needs to transfer short data (one line of text) to the server.

I can choose how the other machine will transfer this data.
What should I prefer?
TCP / HTTP / other... ?

I know details are missing but by guidance, I'll try to provide more.

Thanks.

tcp 80 (99% of machines/switches/firewalls won't have this blocked unless the environment demands it)
any thought beyond that isn't required for sending a string...
or you could just generate and automatically send emails :D

How complex is it to listen to TCP, as you said, by C# program?

Thanks.