We have outgrown the use of a single server for monitoring our network so we are looking to add another monitoring server. Unfortunately, we were shortsighted when we initially configured the hosts being monitored. We configured the hosts on our network to only allow monitoring from a single IP. To add another IP to all the hosts firewall rules is not logistically possible. I won't go into why but suffice it to say the servers have a mix of owners and cannot be updated at will.
So initially we were thinking of just having an OpenBSD box doing NAT with PF in front of the two servers. We're not thrilled about the additional complexity or, most especially, the single point of failure.
So, can we do some magic to allow both servers to use the same internal IP address without adding another host? Also, the system needs to continue working if either one of these hosts go down. So doing the NAT on one of the two hosts and routing the second server through the first isn't really enough. We basically need failover source NAT in front of the shared IP...without adding two additional boxes.