Pfsync

Pfsync

Repository	bxr.su/o/sys/net/if_pfsync.c

pfsync is a used to synchronise firewall states between machines running Packet Filter (PF) for high availability. It is used along with to make sure a backup firewall has the same information as the main firewall. When the main machine in the firewall cluster dies, the backup machine is able to accept current connections without loss.

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Wikipedia