Re: [PATCH nft] doc: extend description of fib expression

[Florian Westphal <fw@strlen.de>]

Pablo Neira Ayuso <pablo@netfilter.org> wrote> > since _OIF and _OIFNAME was restricted to prerouting, nf hookfn has NULL
> > output interface, so there is nothing we could compare against.
> > 
> > Now its available in forward too so it could be selectively relaxed for
> > this, but, what is the use case?
> > 
> > Do a RPF in forward, then we need to compare vs. incoming interface.
> 
> This is for an esoteric scenario: Policy-based routing using input
> interface as key. The fib rule for RPF does not work from prerouting
> because iif cannot be inferred, there is no way to know if route in
> the reverse direction exists until the route lookup for this direction
> is done.

Yes, that internally sets fibs iif to the oif.

> > But for outgoing interface, we'd do a normal route lookup, but the stack
> > already did that for us (as packet is already being forwarded).
> >
> > So what would be the desired outcome for a 'fib daddr . oif' check?
> 
> Hm, this always evaluates true from forward and any later hook.
> 
> I missing now, what is the point of . oif in general?

Its for use with the 'type' output, i.e. consult fib to determine
the type of the daddr (multicast, broadcast etc).

I don't see an application for the fib case, with exception
of the 'rpf lookup in forward' case.