This is libpcap, a system-independent interface for user-level packet
capture.  libpcap provides a portable framework for low-level network
monitoring.  Applications include network statistics collection,
security monitoring, network debugging, etc.  Since almost every
system vendor provides a different interface for packet capture, and
since we've developed several tools that require this functionality,
we've created this system-independent API to ease in porting and to
alleviate the need for several system-dependent packet capture modules
in each application.

Homepage:
https://www.tcpdump.org/
