OSDev.org

hi is there some sort of server that is installed in linux by default and writes to standard output what it receives over the net?
or is there some simple one-line implementation in some popular language?

What do you mean by "receives over the net?" Pick an OSI layer. Maybe you want http://www.wireshark.org/

Well, the discard service.. almost always unused these days, sends all received data to "/dev/null".

Are you incapable of writing a program that can do something similar?

http://www.beej.us/guide/bgnet/

A server that accepts data over TCP/IP and writes it on screen and the discard service doesn't do that. There are a lot of strange programs installed by default on linuces so it'd be interesting to know it, if there is none I'll have to write one but now another question arises- perhaps there is some clever implementation in some particularly fitted language, or maybe a way to do that as a bash command with use of pipes or something like that.

tcpdump

tcpdump neither, I mean a server to which the client has to connect through TCP, it seems there's no such thing installed by default so I'll make my own

netcat, but seriously its a 10 line program