This module matches the parameters in Authentication header of IPsec packets.
.TP
.BR "--ahspi " "[!] \fIspi\fP[:\fIspi\fP]"
Matches SPI.
.TP
.BR "--ahlen " "[!] \fIlength"
Total length of this header in octets.
.TP
.BI "--ahres"
Matches if the reserved field is filled with zero.