[dns-operations] Announcement - DNS flag day on 2019-02-01

[Florian Weimer]

* Mark Andrews:

> No, a you just fragment at network MTU. The IETF even specified a
> setsockopt in the advanced socket API to tell the kernel to do that.

As specified, IPv6 does not have a network MTU, just like IPv4.

| In response to an IPv6 packet that is sent to an IPv4 destination
| (i.e., a packet that undergoes translation from IPv6 to IPv4), the
| originating IPv6 node may receive an ICMP Packet Too Big message
| reporting a Next-Hop MTU less than 1280.  In that case, the IPv6 node
| is not required to reduce the size of subsequent packets to less than
| 1280, but must include a Fragment header in those packets so that the
| IPv6-to-IPv4 translating router can obtain a suitable Identification
| value to use in resulting IPv4 fragments.

<https://tools.ietf.org/html/rfc2460#section-5>

RFC 6946 affirms this bizarre behavior.

Therefore, if you want to avoid state, you need to send atomic
fragments unconditionally, but that causes interoperability problems,
so you cannot do this in practice.