Shell - Monitorar o tráfego da rede no Linux

sudo nethogs eth0

Retornará algo parecido com isso:

NetHogs version 0.8.0

  PID USER     PROGRAM                                   DEV    SENT      RECEIVED       
29461 marcos1  /usr/lib/firefox/firefox                  eth0   0.869       0.284 KB/sec
29063 marcos1  skype                                     eth0   0.092       0.074 KB/sec
?     root     192.168.1.4:4846-201.52.25.150:50472             0.000       0.000 KB/sec
?     root     192.168.1.4:4846-187.19.1.90:51548               0.000       0.000 KB/sec
?     root     unknown TCP                                      0.000       0.000 KB/sec

  TOTAL                                                         35.976      0.809 KB/sec

Para sair, pressione CTRL+C.

Fonte: http://techthrob.com/2010/07/26/how-to-monitor-network-traffic-in-linux/