Always query DNS records over TCP instead of UDP

[kanashimia]

TXT records can be quite large, with is a problem for DNS challenges, they don't fit in the 512 buffer especially if there are multiple. DNS servers can increase the buffer size through EDNS0 extension, and retry over TCP if that wasn't enough.
DNS server should truncate response if it doesn't fit in the buffer, and set TC (Truncate) bit, then DNS client should retry over TCP.

1220 value for UDP buffer size was chosen based on this recommendation: https://www.isc.org/blogs/dns-flag-day-2020-2/

This changes only TXT request, I left other methods untouched. But it would make sense to do the same logic for all requests, by building some small abstraction around miekg/dns for requests.

Also see docs for Exchange function:
https://pkg.go.dev/github.com/miekg/dns#Client.Exchange

Was originally found in
#536 (comment)

[kanashimia]

References letsencrypt/challtestsrv#35
challtestsrv would also need to be updated, although there isn't a strict requirement on it here,
if TC bit isn't set pebble just wont retry.

[jsha]

Thanks for the patch! I'd say let's simplify and always query over TCP for all query types rather than have a fallback. We know what the backend will be in the default config and we know it supports TCP. Also RFC 9210:

recursive servers MUST support and service both TCP and UDP queries

[kanashimia]

I'd say let's simplify and always query over TCP for all query types rather than have a fallback

Yeah you are right, that would be ok here too, and is a much simpler change, updated the PR.

recursive servers MUST support and service both TCP and UDP queries

obligatory mention of http://dnsneedsfuckingtcp.info heheh 😄

[kanashimia]

oh, I committed a binary, sorry