Editing Domain Name System (section)

===Conventional: DNS over UDP and TCP ports 53 (Do53)===

# UDP reserves port number 53 for servers listening to queries.<ref name="rfc1035" /> Such a query consists of a clear-text request sent in a single UDP packet from the client, responded to with a clear-text reply sent in a single UDP packet from the server. When the length of the answer exceeds 512 bytes and both client and server support [[Extension Mechanisms for DNS]] (EDNS), larger UDP packets may be used.<ref>{{IETF RFC|2671}}, ''Extension Mechanisms for DNS (EDNS0)'', P. Vixie (August 1999)</ref> Use of DNS over UDP is limited by, among other things, its lack of transport-layer encryption, authentication, reliable delivery, and message length. In 1989, RFC 1123 specified optional [[Transmission Control Protocol]] (TCP) transport for DNS queries, replies and, particularly, [[DNS zone transfer|zone transfers]]. Via fragmentation of long replies, TCP allows longer responses, reliable delivery, and re-use of long-lived connections between clients and servers. For larger responses, the server refers the client to TCP transport.