potentially disallowing IP fragmentation on wg packets, and handling routing loops better

Vasili Pupkin diggest at gmail.com
Sun Jun 6 10:39:21 UTC 2021


I've dig into the subject two years ago and only vague remember details. 
As far as I can recall there was a time when WireGuard set DF flag by 
default and there were two issues:

1) for security reasons WireGuard doesn't issue ICMP fragmentation 
required response in the unencrypted channel if an encrypted packed 
didn't fit and was dropped
2) there is no way client can tell the server of MTU limitation it has 
on its side

Combining the two we have a situation in a chained wireguard VPN setup 
when MTU size is misconfigured on the server and the remote host 
wouldn't get any icmp to help with its PMTUD algorithm. The client can 
still set MSS in its TCP connection though.

Again sorry if I missed or messed something, it was long ago and I don't 
remember details.

On 06.06.2021 12:13, Jason A. Donenfeld wrote:
> Hi,
> WireGuard is an encrypted point-to-multipoint tunnel, where onion
> layering of packets via a single interface or multiple is a useful
> feature. This makes handling routing loops very hard to manage and
> detect. I'm considering changing and simplifying loop mitigation to a
> different strategy, but not without some discussion of its
> implications.
> Specifically the change would be to not allow IP fragmentation of the
> encrypted UDP packets. This way, in the case of a loop, eventually the
> packet size exceeds MTU, and it gets dropped: dumb and effective.
> Depending on how this discussion goes, a compromise would be to not
> allow fragmentation, but only for forwarded and kernel-generated
> packets, not not for locally generated userspace packets. That's more
> complex and I don't like it as much as just disallowing IP
> fragmentation all together.
> Pros:
> - It solves the routing loop problem very simply.
> - Usually when people are fragmenting packets like that, things become
> very, very slow anyway, and it'd be better to just stop working
> entirely, so that people adjust their MTU.
> - Is anybody actually relying on this?
> Cons:
> - Maybe people are running
> wireguard-over-gre-over-vxlan-over-l2tp-over-pppoe-over-god-knows-what-else,
> and this reduces the MTU to below 1280, yet they still want to put
> IPv6 through wireguard, and are willing to accept the performance
> implications.
> - Some people don't know how to fix their MTUs, and breaking rather
> than just becoming really slow isn't the best outcome there, maybe.
> - Maybe people are relying on this?
> Before anybody asks: we're not going to add a knob for this, simply by
> virtue of this being a decision with pros and cons. Please don't bring
> that up.
> I'd be very interested in opinions about this. Are there additional
> pros and cons? I know the matter has come up a few times on the list,
> mostly with people _wanting_ fragmentation (I've CCd a few people from
> those threads - Roman, I expect you to vigorously argue the
> pro-fragmentation stance ;-). but I'm not convinced the outcome of
> those threads was correct, other than, "yea, that's easy enough to
> enable." But on the other hand, maybe the cons are real enough we
> should rethink this.
> Please let me know thoughts and ideas.
> Thanks,
> Jason

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