Ultrix, libpcap library and, 13
unauthorized access, 5
UNIX, libdnet library and, 39
unreachable headers (ICMP), 58-59
urgent data, libnids library and, 92
user data, libpcap library and, 16
user datagram protocol (UDP), 72, 77-86, 95, 199-215, 217, 223-224, 228, 239-256, 327-412
user interface, 4