How to track which process is generating network traffic?

Question

After fresh system install I have observed strange activity on network monitor - some app is generating small network traffic (sending and receiving data)... how to track which process is causing this network traffic?

Answer 1

NetHogs can do the job:

You may want to decrease the refresh rate with the -d option.

Available options as of version 0.8.5-2:

usage: nethogs [-V] [-h] [-b] [-d seconds] [-v mode] [-c count] [-t] [-p] [-s] [device [device [device ...]]]
        -V : prints version.
        -h : prints this help.
        -b : bughunt mode - implies tracemode.
        -d : delay for update refresh rate in seconds. default is 1.
        -v : view mode (0 = KB/s, 1 = total KB, 2 = total B, 3 = total MB). default is 0.
        -c : number of updates. default is 0 (unlimited).
        -t : tracemode.
        -p : sniff in promiscious mode (not recommended).
        -s : sort output by sent column.
   -a : monitor all devices, even loopback/stopped ones.
        device : device(s) to monitor. default is all interfaces up and running excluding loopback

Answer 2

When you observe this traffic, note which port and protocol is used. Then check with

netstat -tulpen

First column is the protocol, the number after the : in the Local Address column is the port. The last column shows the PID/Program name