pfSense

Here is the setup:

Two cable modems from the same provider, one is used for VPN connections, the other one for all the rest. Because they are from the same provider, both have the same gateway IP, which makes monitoring with normal settings (the IP of the Gateway) impossible. Configuration is done through haveing one gateway as the default and the other handling allowing only incoming VPN connections.

To enable monitoring, I set up Googles DNS servers ips (one to 8.8.8.8, the other to 8.8.4.4) and everything seemed to be working just fine. But then, from time to time, the firewall send out emails, that the connection of one of the gateways was down. Checking for the status on the firewall gave "all up and running".

And here comes the "strange problems": The internal LAN was not able to send any packages to the internet EXCEPT to these two monitored DNS Servers.

I think I read somewhere, that special routes are added, wenn a monitoring IP is setup for a gateway. That could explain, why these addresses are still reachable.

I would need some advice in how to investigate the problem further, as I guess these vague informations I can provide at this point are not sufficient.