IPTraf is a console-based network monitoring program for Linux that
displays information about IP traffic. It returns such information as:

Current TCP connections UDP, ICMP, OSPF, and other types of IP packets
Packet and byte counts on TCP connections IP, TCP, UDP, ICMP, non-IP,
and other packet and byte counts TCP/UDP counts by ports Interface
activity Flag statuses on TCP packets LAN station statistics

This program can be used to determine the type of traffic on your network,
and what kind of service is the most heavily used on what machines, among
others.

IPTraf works on Ethernet, FDDI, ISDN, PLIP, and SLIP/PPP interfaces.