Detecting Intrusions

When the system detects a possible network intrusion, it logs an entry to the security log.

The Enable Network Intrusion Detection System (NIDS) setting controls the logging behavior. The security log prefix identifies the type of packet detected, as shown in the following table:

Prefix Packet Type
SECURITY: NIDS/unknown_tcp Packet that attempts to connect or probe a closed TCP port
SECURITY: NIDS/unknown_udp Packet that probes a closed UDP port
SECURITY: NIDS/invalid_tcp TCP packet in an invalid state
SECURITY: NIDS/invalid_icmp ICMP or ICMPv6 packet in an invalid state
SECURITY: NIDS/unknown Packet with an unknown protocol number in the IP header
SECURITY: NIDS/flood Stream of ICMP or ICMPv6 ping requests or TCP connections to an opened TCP port

Following the message prefix, the security log entry includes the time stamp and the IP, TCP, UDP, ICMP, or ICMPv6 headers. For example, the following security log entry shows an unknown_udp intrusion:

2009-05-08 21:32:52 WARNING kernel: SECURITY: NIDS/unknown_udp IN=eth0 OUT= MAC=00:e0:db:08:9a:ff:00:19:aa:da:11:c3:08:00 SRC= DST= LEN=28 TOS=0x00 PREC=0x00 TTL=63 ID=22458 PROTO=UDP SPT=1450 DPT=7788 LEN=8