tcptrack

A quite handy tool for monitoring tcp traffic is tcptrack. Get it with:

and start it with

the interface parameter -i eth0 must be according to your needs, you can check your interfaces with ifconfig.

There is also tcpdump, tcpflow and other nice tools

Netstat

Sometimes one like to figure out what programs listen at a port. Here we can get an overview with:

Where t=tcp, u=udp, p=show program name, l=show listening ports, n= numeric (not resolve machine names)
The Output could look something like this:

 

lsof

With lsof you can determine easily which program listens on a port and under which user that program runs: