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

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Date:	Tue, 13 Oct 2015 15:22:35 +0200
From:	Jörg Pommnitz <j.pommnitz@...il.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	netdev@...r.kernel.org
Subject: Re: Some ICMP-Reply messages don't arrive in User Space in recent Kernels

Hannes,
thanks for your reply. It was indeed rp_filter (found it myself). What
changed were the default settings in Ubuntu (07.04 to 14.04).

Regards
  Joerg

2015-10-13 15:18 GMT+02:00 Hannes Frederic Sowa <hannes@...essinduktion.org>:
> 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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