Re: [babel] Investigating ICMP behaviour of the Linux kernel in IPv6-only mode

Juliusz Chroboczek <jch@irif.fr> Tue, 15 June 2021 12:20 UTC

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Date: Tue, 15 Jun 2021 14:20:46 +0200
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From: Juliusz Chroboczek <jch@irif.fr>
To: Toke =?ISO-8859-1?Q?H=F8iland-J=F8rgensen?= <toke=40toke.dk@dmarc.ietf.org>
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Archived-At: <https://mailarchive.ietf.org/arch/msg/babel/pDIakcQ51RgLraVOnhEan1g22ts>
Subject: Re: [babel] Investigating ICMP behaviour of the Linux kernel in IPv6-only mode
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> I've submitted a patch[0] to make the kernel fall back to the dummy
> address specified in RFC7600.

> [0] https://lore.kernel.org/netdev/20210615110709.541499-1-toke@redhat.com/

Thanks a lot, Toke.  After spending a few hours going through the maze
that is the Linux ICMP code, I decided I needed help from somebody competent.

> What I found was that the kernel is perfectly happy to send out ICMP
> messages with an all-zero source address if no real address is
> configured on any interface.

The question is -- what is the behaviour of PMTUd implementations when
they receive such an ICMP packet?  Do they reduce the PMTU, or do they
drop the ICMP as obviously incorrect?

I'm going to merge the v4via6 code (I've reviewed it, and I think it needs
a few fixes), and going to enable it automatically if there's at least one
IPv4 address somewhere, including the loopback interface.  Thanks a lot
for the information above, that's very helpful.

-- Juliusz