A blackhole route is a route that drops all traffic sent to it. It is very much like /dev/null in Linux programming.

Blackhole routes are used to dispose of packets instead of responding to suspicious inquiries. This provides added security since the originator will not discover any information from the target network.

Blackhole routes can also limit traffic on a subnet. If some subnet addresses are not in use, traffic to those addresses (traffic which may be valid or malicious) can be directed to a blackhole for added security and to reduce traffic on the subnet.

The loopback interface, a virtual interface that does not forward traffic, was added to enable easier configuration of blackhole routing. Similar to a normal interface, this loopback interface has fewer parameters to configure, and all traffic sent to it stops there. Since it cannot have hardware connection or link status problems, it is always available, making it useful for other dynamic routing roles. Once configured, you can use a loopback interface in security policies, routing, and other places that refer to interfaces. You configure this feature only from the CLI. For more information, see the system chapter of the FortiGate CLI Reference.