Messages sent using UDP user server port 53 (decimal).
Messages carried by UDP are restricted to 512 bytes (not counting the IP or UDP headers). Longer messages are truncated and the TC bit is set in the header.
UDP is not acceptable for zone transfers, but is the recommended method for standard queries in the Internet. Queries sent using UDP may be lost, and hence a retransmission strategy is required. Queries or their responses may be reordered by the network, or by processing in name servers, so resolvers should not depend on them being returned in order.
The optimal UDP retransmission policy will vary with performance of the Internet and the needs of the client, but the following are recommended:
More suggestions on server selection and retransmission policy can be found in the resolver section of this memo.