netdev - saddr based blackhole/unreachable route

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Message-ID: <CAJO99Tn7oaGEhRKEBR83K_+JSJUaFXEqvMnbM3vOLVxDqF9wtw@mail.gmail.com>
Date:   Tue, 11 Oct 2016 20:39:49 +0200
From:   Bjørnar Ness <bjornar.ness@...il.com>
To:     netdev <netdev@...r.kernel.org>
Subject: saddr based blackhole/unreachable route

Hello, netdev

In a typical setup (eth0=internet, eth1=lan) i populate routing table
100 with saddrs I want
dropped, and: "ip ru a pref 100 lookup table 100"

What I would expect to see is packets with a saddr in table 100 coming
in eth0 will go out eth1,
with replies beeing dropped, but I do not see the packets going out eth1 at all.

Have tried searching and following the fib codepath, but have still
not managed to understand
what is really going on here.

Is the saddr looked up in the routing table?
Why dont I get icmp unreachable for unreachable routes?
Is tcpdump tricking me here?

I like the behavior, I just don't know if I can trust it.

Kernel 4.8.1

Regards,
-- 
Bj(/)rnar