Re: ifupdown behaviour with IPv6 DAD failure (Was: proposal: Hybrid network stack for Trixie)

From	Philipp Kern <pkern@debian.org>
Newsgroups	linux.debian.devel
Subject	Re: ifupdown behaviour with IPv6 DAD failure (Was: proposal: Hybrid network stack for Trixie)
Date	2024-09-23 17:50 +0200
Message-ID	<JpTu2-ejdS-1@gated-at.bofh.it> (permalink)
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On 23.09.24 13:39, Daniel Gröber wrote:
> On Mon, Sep 23, 2024 at 12:25:15PM +0200, Chris Hofstaedtler wrote:
>> * Pierre-Elliott Bécue <peb@debian.org> [240923 11:34]:
>>> I like ifupdown. It's simple and just works.
>>
>> I find this quite funny, given a recent discussion about IPv6 dad
>> issues with ifupdown on #debian-admin.
> 
> The "discussion" was about ifup@eth0 being in a failed state on a
> particular server due to a DAD failure and someone having to manually
> intervene.

I find my ghost being invoked here.

> Chris, what behaviour do you expect here? Below I'm going to assume what
> you're getting at is that we should continue to retry DAD.
> 
> To me going to a stable failure state seems desirable. Continuing to re-try
> for IPs could cause instability in the face of legitimate address
> conflicts: when the owning machine reboots the conflicting machine would
> now win the IP due to continous retrying. The change in owner would cause
> disruption to services entirely unrelated to the machine that was just
> rebooted.

DAD did not fail, it timed out after 60 sleeps of 0.1, aka 6s. The 
kernel subsequently succeeded to configure the network. The script in 
question was added in response to [1] and [2] to have a pause during 
boot to give the kernel time to resolve the situation before continuing 
the bootup. So it left the race around because there's not that much it 
can do better as a script-based setup without much state.

Unfortunately there's zero information from ifup@eth0 in the process as 
to when that happened. Which adds to the frustrating debugging stories 
when you can't get enough intel about what happened after the fact. 
(Which to be fair, also probably needs env vars to be set with 
systemd-networkd to increase the debug level.) As far as I can see 
processes started listening on the IP in question (that... again... 
wasn't logged because it's eaten by the script) a second afterwards.

So no, it did not enter a stable state. It let the kernel do its thing, 
which was to actually enable the address. I don't know why it takes 
Linux to run DAD for that long and what the assumptions around that are. 
But if you listen on netlink you learn when that happens and don't need 
to poll and could send events once that happens.

To be ultimately fair to ifupdown: There was probably not much of a 
winning move here. The annoying bit was the systemd service that was 
still in a failed state even though the failure condition resolved 
itself <1s later.

Kind regards
Philipp Kern

[1] https://www.agwa.name/blog/post/beware_the_ipv6_dad_race_condition
[2] http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=705996

Thread

proposal: Hybrid network stack for Trixie Lukas Märdian <slyon@debian.org> - 2024-09-20 13:20 +0200
  Re: proposal: Hybrid network stack for Trixie Marco d'Itri <md@Linux.IT> - 2024-09-20 15:50 +0200
    Re: proposal: Hybrid network stack for Trixie Bill MacAllister <bill@ca-zephyr.org> - 2024-09-20 22:20 +0200
  Re: proposal: Hybrid network stack for Trixie Chris Hofstaedtler <zeha@debian.org> - 2024-09-22 12:30 +0200
    Re: proposal: Hybrid network stack for Trixie Marc Haber <mh+debian-devel@zugschlus.de> - 2024-09-22 13:10 +0200
      Re: proposal: Hybrid network stack for Trixie Chris Hofstaedtler <zeha@debian.org> - 2024-09-22 13:50 +0200
        Re: proposal: Hybrid network stack for Trixie "Jonathan Dowland" <jmtd@debian.org> - 2024-09-22 16:50 +0200
          Re: proposal: Hybrid network stack for Trixie "Andrew M.A. Cater" <amacater@einval.com> - 2024-09-22 17:20 +0200
        Re: proposal: Hybrid network stack for Trixie Hakan Bayındır <hakan@bayindir.org> - 2024-09-22 23:30 +0200
    Re: proposal: Hybrid network stack for Trixie Simon McVittie <smcv@debian.org> - 2024-09-22 13:10 +0200
      Re: proposal: Hybrid network stack for Trixie Josh Triplett <josh@joshtriplett.org> - 2024-09-22 20:10 +0200
        Re: proposal: Hybrid network stack for Trixie "Andrea Pappacoda" <andrea@pappacoda.it> - 2024-09-22 22:40 +0200
          Re: proposal: Hybrid network stack for Trixie Josh Triplett <josh@joshtriplett.org> - 2024-09-23 00:10 +0200
            Re: proposal: Hybrid network stack for Trixie Lukas Märdian <slyon@debian.org> - 2024-09-23 12:40 +0200
    Re: proposal: Hybrid network stack for Trixie Lukas Märdian <slyon@debian.org> - 2024-09-23 13:10 +0200
      Re: proposal: Hybrid network stack for Trixie Marco d'Itri <md@Linux.IT> - 2024-09-23 14:10 +0200
      Re: proposal: Hybrid network stack for Trixie Marvin Renich <mrvn@renich.org> - 2024-09-23 14:50 +0200
      Re: proposal: Hybrid network stack for Trixie Didier 'OdyX' Raboud <odyx@debian.org> - 2024-09-23 15:00 +0200
      Re: proposal: Hybrid network stack for Trixie Daniel Baumann <daniel@debian.org> - 2024-09-24 08:10 +0200
    Re: proposal: Hybrid network stack for Trixie Simon Richter <sjr@debian.org> - 2024-09-24 14:20 +0200
  Re: proposal: Hybrid network stack for Trixie Ansgar 🙀 <ansgar@debian.org> - 2024-09-22 16:30 +0200
    Re: proposal: Hybrid network stack for Trixie Lukas Märdian <slyon@debian.org> - 2024-09-23 12:30 +0200
      Re: proposal: Hybrid network stack for Trixie Ansgar 🙀 <ansgar@debian.org> - 2024-09-23 12:50 +0200
        Re: proposal: Hybrid network stack for Trixie Lukas Märdian <slyon@debian.org> - 2024-09-23 13:20 +0200
          Re: proposal: Hybrid network stack for Trixie Richard Lewis <richard.lewis.debian@googlemail.com> - 2024-09-23 13:40 +0200
            Re: proposal: Hybrid network stack for Trixie Lukas Märdian <slyon@debian.org> - 2024-09-24 15:40 +0200
              Re: proposal: Hybrid network stack for Trixie Ansgar 🙀 <ansgar@debian.org> - 2024-09-27 10:10 +0200
          Re: proposal: Hybrid network stack for Trixie Bjørn Mork <bjorn@mork.no> - 2024-09-23 14:20 +0200
        Re: proposal: Hybrid network stack for Trixie Steve Langasek <vorlon@debian.org> - 2024-09-27 20:20 +0200
          Re: proposal: Hybrid network stack for Trixie Ansgar 🙀 <ansgar@debian.org> - 2024-09-27 21:10 +0200
  Re: proposal: Hybrid network stack for Trixie Sirius <sirius@trudheim.com> - 2024-09-22 17:50 +0200
    Re: proposal: Hybrid network stack for Trixie Jonas Smedegaard <jonas@jones.dk> - 2024-09-22 23:50 +0200
      Re: proposal: Hybrid network stack for Trixie Sirius <sirius@trudheim.com> - 2024-09-23 08:30 +0200
        ifupdown-ng source stanza (Was: proposal: Hybrid network stack for  Trixie) Daniel Gröber <dxld@darkboxed.org> - 2024-09-23 11:00 +0200
          Re: ifupdown-ng source stanza (Was: proposal: Hybrid network stack  for Trixie) Sirius <sirius@trudheim.com> - 2024-09-24 13:40 +0200
  Re: proposal: Hybrid network stack for Trixie Holger Levsen <holger@layer-acht.org> - 2024-09-23 10:10 +0200
    Re: proposal: Hybrid network stack for Trixie Chris Hofstaedtler <zeha@debian.org> - 2024-09-23 10:20 +0200
      Re: proposal: Hybrid network stack for Trixie Holger Levsen <holger@layer-acht.org> - 2024-09-23 10:30 +0200
        Re: proposal: Hybrid network stack for Trixie Marco d'Itri <md@Linux.IT> - 2024-09-23 11:10 +0200
          Re: proposal: Hybrid network stack for Trixie Lukas Märdian <slyon@debian.org> - 2024-09-23 12:10 +0200
          Re: proposal: Hybrid network stack for Trixie Holger Levsen <holger@layer-acht.org> - 2024-09-23 12:10 +0200
            Re: proposal: Hybrid network stack for Trixie Chris Hofstaedtler <zeha@debian.org> - 2024-09-23 12:30 +0200
  Re: proposal: Hybrid network stack for Trixie Pierre-Elliott Bécue <peb@debian.org> - 2024-09-23 11:40 +0200
    Re: proposal: Hybrid network stack for Trixie Chris Hofstaedtler <zeha@debian.org> - 2024-09-23 12:30 +0200
      ifupdown behaviour with IPv6 DAD failure (Was: proposal: Hybrid  network stack for Trixie) Daniel Gröber <dxld@darkboxed.org> - 2024-09-23 13:50 +0200
        Re: ifupdown behaviour with IPv6 DAD failure (Was: proposal: Hybrid  network stack for Trixie) Philipp Kern <pkern@debian.org> - 2024-09-23 17:50 +0200
          Re: ifupdown behaviour with IPv6 DAD failure (Was: proposal: Hybrid  network stack for Trixie) Noah Meyerhans <noahm@debian.org> - 2024-09-24 00:20 +0200
      Re: proposal: Hybrid network stack for Trixie Pierre-Elliott Bécue <peb@debian.org> - 2024-09-24 10:50 +0200
  Re: proposal: Hybrid network stack for Trixie Antoine Beaupré <anarcat@debian.org> - 2024-10-07 18:30 +0200

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