Firewall failover

I just ran into a nice write up of stateful failover capable firewall running BSD pf.

• CARP (Common Address Redundancy Protocol) is used to switch identity during failover and CARP traffic is used as a measure of the availability node (the master advertises using CARP and if the backup doesn't hear from its master, it'll start advertising itself. Carp is IP protocol #112. CARP has a ARP balance feature that can be used to direct traffic to particular hosts and can be seen as similar to VRRP but presents the further advantages of being more secure and not encumbered by patents.
  

• pfsync implements the IP protocol #240 and performs connection state synchronization so that a stateful failover (i.e. not TCP connections are lost) can be supported. A node joining the firewall cluster will receive a bulk update of the existing connections and then will be updated periodically on a best effort basis.