Visual RFC.
ALL RFCs / RFC 7766
Read the RFC ↗
RFC 7766 / DNS OVER TCP

TCP is a first-class DNS transport

01

DNS turns names into records. A resolver is a client that asks a DNS server for those records. Both ends need a working TCP path, not just UDP.

DIAGRAMRead RFC section 5 ↗
RFC 7766 · § 5EXAMPLE
4 / 4
TCP is a first-class DNS transportThe UDP truncated reply TC1 moves server to client. After TCP establishment, the client sends a DNS question on connection C1 and the server replies on the same C1. A separate direct-choice example sends TCP first over reused C1. Handshake is summarized; transport choice is not forced UDP-first. No answer data is fabricated.DNS transport choice · port 53ClientServerSeparate example · TCP firstC1 already open: reuse itTCP MAY precede any UDP queryTCP is a valid first choice, not only fallback
04
TCP can also be the first choice.

This is a separate exchange: a resolver selects TCP directly and reuses open C1. No prior UDP failure is required.

Source for this drawing: RFC 7766 · Read RFC section 5 ↗

Protocol participantThreat or exceptionProtocol controlSimplified drawings · precise section references
New to this? Start hereA quick picture + helpful background

A message needs an address.

Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.

Helpful first, if these ideas are unfamiliar

RFC 1035 DNS wire RFC 9293 TCP

These pictures are analogies to help you learn. The linked RFC explains the actual rules.

Related RFCs and changes

“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.

KEY POINT

Support both transports; keep the reply on its request path.

  • All general-purpose DNS implementations MUST support both UDP and TCP. This includes stub resolvers, recursive servers or forwarders, and authoritative servers.
  • Stub and recursive resolvers MAY choose TCP before any UDP query. If an open TCP connection to the server exists, the resolver SHOULD reuse it.
  • Recursive and authoritative servers MUST reply using the query’s transport; for TCP, they MUST also use the same connection.
DETAILS

TC is the truncated-response flag: the UDP response is incomplete. Section 4 describes retrying over TCP after TC=1; the old 512-byte UDP limit applies without EDNS. EDNS can advertise larger UDP capacity, but does not remove the need for TCP. The TCP handshake is summarized, not animated as DNS data. These are implementation requirements, not a promise every network permits port 53.

Read the RFC section ↗

All chapters in RFC 7766

Drawing view

Scroll the drawing to see every part. Playback controls stay above it.

RFC 9700 / CHAPTERS

Chapters

For full details, tables, algorithms, and exceptions, read the original RFC.

RFC status and corrections ↗