Yes, but doesn't IPv6 also increase the "maximum safe UDP packet size" from 512 bytes to 1280?

> Existing deployments of DNSSEC [RFC4033] have shown that truncation at the 512-byte boundary is now commonplace. For example, a Non-Existent Domain (NXDOMAIN) (RCODE == 3) response from a DNSSEC-signed zone using NextSECure 3 (NSEC3) [RFC5155] is almost invariably larger than 512 bytes.

This has been a flagged issue in DNSSEC since it was originally considered. This was a massive oversight on their part and was only added because DNSSEC originally made it quite easy to probe entire DNS trees and expose obscured RRs.

> The MTU most commonly found in the core of the Internet is around 1500 bytes, and even that limit is routinely exceeded by DNSSEC-signed responses.

> Stub resolver implementations (e.g., an operating system's DNS resolution library) MUST support TCP since to do otherwise would limit the interoperability between their own clients and upstream servers.

Fair enough but are network clients actually meant to use DNSSEC? Isn't this just an issue for authoritative and recursive DNSSEC resolvers to and down the roots?

    s/6677/7766/

If this were the default on DNS servers, then DNS amplification attacks would be nearly impossible. They rely on spoofing a DNS request from the victim, and amplify because the response may be many times larger than the request. If TCP were required to be used before UDP responses can be received, then the victim would have to be first tricked into making a DNS request over TCP to each public DNS server.

The DNS Cookies standard (RFC 7873) doesn't do much to stop this, since it is impractical to fail queries from non-cookie clients.

DNS over TCP is supposed to be supported, so implementing this will push firewall admins in the right direction (allow both TCP/UDP outbound on 53).

Spoofing/amplification attacks are already a problem, no?