IETF RFC 5966 PDF

Introduction

Most DNS [RFC1034] transactions take place over UDP [RFC0768]. TCP [RFC0793] is always used for zone transfers and is often used for messages whose sizes exceed the DNS protocol's original 512-byte limit.

Section 6.1.3.2 of [RFC1123] states:

DNS resolvers and recursive servers MUST support UDP, and SHOULD support TCP, for sending (non-zone-transfer) queries.

However, some implementors have taken the text quoted above to mean that TCP support is an optional feature of the DNS protocol.

The majority of DNS server operators already support TCP and the default configuration for most software implementations is to support TCP. The primary audience for this document is those implementors whose failure to support TCP restricts interoperability and limits deployment of new DNS features.

This document therefore updates the core DNS protocol specifications such that support for TCP is henceforth a REQUIRED part of a full DNS protocol implementation.

Whilst this document makes no specific recommendations to operators of DNS servers, it should be noted that failure to support TCP (or the blocking of DNS over TCP at the network layer) may result in resolution failure and/or application-level timeouts.