netdev - Re: Some ICMP-Reply messages don't arrive in User Space in recent Kernels

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Date:	Tue, 13 Oct 2015 15:18:12 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Jörg Pommnitz <j.pommnitz@...il.com>,
	netdev@...r.kernel.org
Subject: Re: Some ICMP-Reply messages don't arrive in User Space in recent
 Kernels

On Tue, Oct 13, 2015, at 08:47, Jörg Pommnitz wrote:
> Hello all,
> I'm moving an application from 2.6.23 (yes, it's ancient; that's why
> we are moving) to 3.18LTS. The application monitors multiple network
> links to the same target with ping packets. The different links are
> selected either by their next hop router (Ethernet) or the network
> interface (Point-to-Point links, aka cellular data).
> To force different routes to the same target, the outgoing packets are
> tagged with different firewall marks. Then I'm using routing rules to
> select different routing tables with different routes for the same
> target.
> The outgoing path works perfectly fine in both, 2.6.23 and 3.18.
> However, the same is not true for the incoming ICMP replies. They are
> incoming; I see them with tcpdump. But some packets do not get
> delivered to user space in 3.18. I'm not 100% sure, but I think this
> happens if there is no "normal" route to the ping target, e.g. the
> source address of the ICMP replies. This looks like some kind of
> misguided ingress filtering that keeps packets out if a normal routing
> lookup fails.
> 
> Am I on the right track? If so, is there a way to disable this
> filtering? If not, what could cause this changed behaviour?

Did you disable rp_filter? Sounds like it, but 2.6.23 also had it
activated by default, so maybe you missed the option in sysctl.conf.

Bye,
Hannes
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