pf(4) is an Internet Protocol packet filter originally written for OpenBSD. In addition to filtering packets, it also has packet normalization capabilities. States in pf(4) let ICMP and ICMP6 packets pass if they have a packet in their payload matching an existing condition. pf(4) does not check if the outer ICMP or ICMP6 packet has the same destination IP as the source IP of the inner protocol packet. A maliciously crafted ICMP/ICMP6 packet could bypass the packet filter rules and be passed to a host that would otherwise be unavailable.