Common port for DNS is UDP-53. This is widely used everywhere. The UDP requires no handshake or acknowledge between the two (2) machines.
Among reasons why UDP is used because successive DNS requests can go to different anycast root servers and routes can be unstable. UDP can switch quickly and cope with such changes. TCP would not cope well.
But TCP-53, on the other hand, is more reliable. But this is useful for zone transfer request. The normal UDP-53 is used for DNS queries.
So, you may use both TCP/UDP, or UDP only, depending on your requirement.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...