system interface for user-level packet capture

 libpcap (Packet CAPture) 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 there are several tools that require this
 functionality, 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.