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