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5. Transport Protocol Selection

Section 6.1.3.2 of [RFC1123] is updated: All general-purpose DNS implementations MUST support both UDP and TCP transport.

  • Authoritative server implementations MUST support TCP so that they do not limit the size of responses to what fits in a single UDP packet.

  • Recursive server (or forwarder) implementations MUST support TCP so that they do not prevent large responses from a TCP-capable server from reaching its TCP-capable clients.

  • 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.

Regarding the choice of when to use UDP or TCP, Section 6.1.3.2 of RFC 1123 also says:

... a DNS resolver or server that is sending a non-zone-transfer query MUST send a UDP query first.

This requirement is hereby relaxed. Stub resolvers and recursive resolvers MAY elect to send either TCP or UDP queries depending on local operational reasons. TCP MAY be used before sending any UDP queries. If the resolver already has an open TCP connection to the server, it SHOULD reuse this connection. In essence, TCP ought to be considered a valid alternative transport to UDP, not purely a retry option.

In addition, it is noted that all recursive and authoritative servers MUST send responses using the same transport as the query arrived on. In the case of TCP, this MUST also be the same connection.

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