ipsecah, AH - IPsec Authentication Header
The ipsecah module (AH) provides strong integrity, authentication, and
partial sequence integrity (replay protection) to IP datagrams. AH
protects the parts of the IP datagram that can be predicted by the
sender as it will be received by the receiver. For example, the IP TTL
field is not a predictable field, and is 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, if tun‐
nels are being used.
AH is implemented as a module that is auto-pushed on top of IP. The
entry /dev/ipsecah is used for tuning AH with ndd(1M).
Current authentication algorithms supported include HMAC-MD5 and HMAC-
SHA-1. Each authentication algorithm has its own key size and key for‐
mat properties. You can obtain a list of authentication algorithms and
their properties by using the ipsecalgs(1M) command. You can also use
the functions described in the getipsecalgbyname(3NSL) man page to
retrieve the properties of algorithms.
Without replay protection enabled, AH is vulnerable to replay attacks.
AH does not protect against eavesdropping. Data protected with AH can
still be seen by an adversary.
See attributes(5) for descriptions of the following attributes:
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
│Interface Stability │ Committed │
SEE ALSOipsecalgs(1M), ipsecconf(1M), ndd(1M), attributes(5), getipsecalgby‐
name(3NSL), ip(7P), ipsec(7P), ipsecesp(7P)
Kent, S. and Atkinson, R.RFC 2402, IP Authentication Header, The Inter‐
net Society, 1998.
Sep 25, 2009 IPSECAH(7P)