[Click] Click as a router between real interface and loopback

[Christian Kreibich]

I have a pretty standard Click setup in which a user-space Click running
on Linux realizes a basic router between two interfaces (eth0 and eth1),
grabbing and sending packets via FromDevice and ToDevice. When testing
this setup with a client process located on machine A, a server process
located on machine B, and the machine running Click sitting in between,
everything works as expected.

For various reasons it would be handy to run the server process and the
Click instance on the same host, with the server listening on loopback.
When I try this, packets appear on lo in tcpdump, but the server process
doesn't see them. I seem to recall that packets sent via ToDevice will
not be processed by the local kernel. Is that in fact the case, and if
so, what's a good way to make this work? Do I need to go the KernelTun
or ToHost route?

Thanks,
-C.