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Drop Tunnel Interface

A drop tunnel interface prevents traffic from being sent out using an incorrect route when the configured route is down. Traffic sent to a drop tunnel interface does not leave the appliance, but is ostensibly dropped.

A drop tunnel interface should be used in conjunction with a VPN tunnel interface, although a drop tunnel interface can be used standalone. If a static route is bound to a tunnel interface, SonicWall recommends configuring a static route bound to a drop tunnel interface for the same network traffic. That way, if the tunnel interface goes down, the second static route is used and the traffic is effectively dropped. This prevents the data from being forwarded in the clear over another route.

When configuring a route over a VPN tunnel interface, if the tunnel is temporarily down, the corresponding route entry is disabled as well. SonicOS looks up a new route entry for the connections destined for the VPN protected network. In deployments that do not have a backup link for a remote VPN network, no other correct route entry is available. Traffic is sent to a wrong route entry, generally the default route, which causes security issues such as internal data sent without encryption.

For deployments without a backup link, consider configuring the route table as in this example:

route n:   local VPN network(source), remote VPN network(destination), VPN TI(egress_if)

route n+1: local VPN network(source), remote VPN network(destination), Drop If(egress_if)

When the VPN tunnel interface configured as in this example, the traffic matches the drop interface and is not sent out. When the VPN tunnel interface resumes, traffic resumes also.