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Date:   Tue, 13 Sep 2016 04:42:38 +0200
From:   Hannes Frederic Sowa <>
To:     Sowmini Varadhan <>
Cc:     Andreas Hübner <>,
        netdev <>,
        "d. caratti" <>,
        Sowmini Varadhan <>
Subject: Re: icmpv6: issue with routing table entries from link local

On 13.09.2016 04:03, Sowmini Varadhan wrote:
> On Mon, Sep 12, 2016 at 1:26 PM, Hannes Frederic Sowa
> <> wrote:
>> Hello,
>> On 12.09.2016 16:27, Andreas Hübner wrote:
>>> I have the following setup:
>>>   - 2 directly connected hosts (A+B), both have only link local addresses
>>>     configured (interface on both hosts is eth0)
>            :
>>>       fe80::/64 dev eth1  proto kernel  metric 256
>>>       fe80::/64 dev eth0  proto kernel  metric 256
>            :
>>> The issue I currently have is, that the echo reply that host B should
>>> generate is never sent back to host A. If I change the order of the
>>> routing table entries on host B, everything works fine.
>>> (host A is connected on eth0)
>    :
>> This shouldn't be the case. We certainly carry over the ifindex of the
>> received packet into the routing lookup of the outgoing packet, thus the
>> appropriate rule, with outgoing ifindex should be selected.
> Like Hannes,  I too would first check "is B sending out the echo-resp? on
> which interface?".
> But a couple of unexpected things I noticed in linux: the link-local
> prefix should have a prefixlen of /10 according to
> but "ip -6 route show"  lists this as a /64..

The link local subnet is still specified to be a /64 as the other parts
of the address must be 0. Legally we probably could blackhole them.

> moreover, even though I cannot use "ip [-6] route add.." to add the
> same prefix multiple times (with different nexthop and/or interface)
> unless I explicitly mark them as ECMP with /sbin/ip, it seems like you
> can create the same onlink prefix on multiple interfaces, but the
> kernel will not treat this as an ECMP group (and sometimes this
> can produce inconsistent results depending on the order of
> route addition, e.g., for ipv4 rp_filter checks). I dont know if some
> variant of  this (latter observation) may be the reason for the behavior
> that  Andreas reports.

iproute sets the NLM_F_EXCL flag. Use ip route prepend ...

We don't have urpf checks for ipv6, those are implemented in netfilter
only. This could very well be a firewall issue or something like that.


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