Switch transport when a response is truncated
04Original DNS uses UDP and TCP on server port 53. TCP messages have a two-octet length prefix.
Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
DIAGRAM§ 4.2 ↗
RFC 1035 · § 4.2EXAMPLE
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03
A TCP exchange carries a length-prefixed complete message.
A TCP exchange carries a length-prefixed complete message.
Protocol participantThreat or exceptionProtocol controlSimplified drawings · precise section references
RFC 1035 · 512-byte UDP baselineRFC 6891 § 4.3
EDNS allows a requester to advertise a larger UDP payload size; it is not a universal fixed larger limit.
Read the updated RFC section ↗The original UDP message limit is 512 octets; TC indicates truncation.
- The original UDP message limit is 512 octets; TC indicates truncation.
- Later EDNS signaling can support larger UDP responses; that does not remove path-MTU constraints.
The drawing is a simplified worked example. Follow the linked RFC and its updates for implementation requirements.
Read the RFC section ↗Diagrams simplify the RFC. Read the linked sections for complete requirements.
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