[skuhn]

It's non-trivial to determine if your response will fit into the UDP packet size, other than erring heavily on the side of caution.

Part of the request query is included in the response, IPv6 AAAA records are considerably larger than IPv4 A records, DNSSEC adds size, RFC 1035 message compression comes into play, EDNS0 comes into play, etc.

Which is all to say that you're right that for maximum compatibility you should endeavor to fit all responses into a UDP packet, but most places lack the sophistication to ensure that is always the case.

Supporting TCP DNS on the client side should not be considered optional if you want to successfully resolve records that you don't control.