libpcap

Libpcap provides a portable framework for low-level network
monitoring.  Libpcap can provide network statistics collection,
security monitoring and network debugging.  Since almost every system
vendor provides a different interface for packet capture, the libpcap
authors created this system-independent API to ease in porting and to
alleviate the need for several system-dependent packet capture modules
in each application.

Install libpcap if you need to do low-level network traffic monitoring
on your network.