Reverse Path Filtering check in ip_input.c

[Łukasz Bromirski]

Yann Berthier wrote:

>    If this yet to be found wiser guy would not forget the loose check
>    too (verrevpath in ipfw speaking), where packets matching the default
>    route are ok ... :)

Actually it does that and will until we'll have option to have two
or more default routes.

Presently, if packets comes via interface and reply for it should be
sent on the same interface (because default route points to it and
there are no other routes pointing for the same destination to
another interface) it will work.

Check fails if there's either interface mismatch, or source is present
in routing table but marked as RTF_REJECT/BLACKHOLE one.

OpenBSD imported KAME mroute extension that enables them to have
more than one route for given destination simultaneously in routing
table. I'm looking into it now, as it's very attractive thing,
however as Andre is doing rework of network code I'm sure we'll have
it sooner or later and then maybe someone will revise old checks
already marked as 'XXX' in the code ;)

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