Accessing CARP Backup Device over IPsec
We are having trouble passing traffic from clients on one side of an IPsec tunnel to a backup firewall on the other side. Traffic initiated at the backup firewall also will not traverse the IPsec tunnel. We have a two-site configuration where each site has a primary and backup firewall in a CARP HA configuration. The sites are connected with an IPsec tunnel.
We have found that it is impossible to access the backup firewalls across the IPsec tunnels because each backup firewall maintains SPDs for traffic directed to the other site even when the appropriate IPsec tunnel is not up. The SPDs grab the traffic before it hits the normal routing table and force it into a black hole since there is no active IPsec tunnel to service the traffic.
If I manually delete the SPDs on the backup firewall, traffic will pass from the backup firewall through the IPsec tunnel on the primary firewall to the other side of the tunnel; however, when we fail over to the backup firewall, the IPsec tunnel does not come up.
Has anyone else had experience with this? Is there a known workaround?
It's a known/expected issue.
You're probably already on manual outbound NAT. Add a rule there to NAT out LAN from the remote IPsec subnet source to a destination of the secondary's LAN IP address, translated to 'interface address'. Add another with a destination of the primary's LAN IP address (or use an alias and one rule).
That way when you try to reach the secondary it appears to originate from the primary and vice versa, avoiding the VPN knowledge issue on the secondary.