How should the network be designed in order to tolerate a firewall failure?
It’s best to have one or more DNS servers that are configured as a replacement firewall, in case of failure. In a typical colocated network, the DNS servers are the only other severs connected directly to the Internet, and which are not behind the firewall – these servers are then configured with redundant network cards, and are on the internal LAN as well as the Internet – thus a simple reconfiguration can be done remotely in the case of a firewall failure.
