Authentication Header

The authentication header provides data authentication, strong integrity, and replay protection to IP datagrams. AH protects the greater part of the IP datagram. AH cannot protect fields that change nondeterministically between sender and receiver. For example, the IP TTL field is not a predictable field and, consequently, not protected by AH. AH is inserted between the IP header and the transport header. The transport header can be TCP, UDP, ICMP, or another IP header when tunnels are being used. See the tun(7M) man page for details on tunneling.

Authentication Algorithms and the AH Module

IPsec implements AH as a module that is automatically pushed on top of IP. The /dev/ipsecah entry tunes AH with the ndd command. Future authentication algorithms can be loaded on top of AH. Current authentication algorithms include HMAC-MD5 and HMAC-SHA-1. Each authentication algorithm has its own key size and key format properties. See the authmd5h(7M) and authsha1(7M) man pages for details. For tuning IP configuration parameters, see the ndd(1M) man page.

Security Considerations for AH

Replay attacks threaten an AH when an AH does not enable replay protection. An AH does not protect against eavesdropping. Adversaries can still see data that is protected with AH.