DNS Transport Protocols: UDP (with TCP as a fallback)
The Domain Name System (DNS) does not run on Telnet or HTTP. DNS primarily uses the transport protocol UDP for queries (port 53) and can also use TCP (port 53) when needed (e.g., large responses or zone transfers). This means, among the given options, the correct choice is (ii) UDP.
Key terms you’ll use while reasoning about this:
- DNS
- UDP
- TCP
- Port 53
DNS Explained: UDP vs TCP (Transport Layer) — Overview
Why DNS usually uses UDP
DNS is designed to be efficient for frequent small request/response interactions. UDP provides low overhead because it is connectionless. Standard DNS operation therefore commonly uses UDP for queries and answers.
Why DNS also uses TCP
Some DNS responses can exceed what fits comfortably in a UDP datagram. When DNS responses are truncated, the server indicates this and the client may retry using TCP to obtain the full response. TCP is also required for specific DNS operations such as zone transfers.
Pro tip: If you see “DNS over UDP,” remember it’s the common case—TCP is the fallback for reliability/size constraints rather than the primary path for most queries.
Exam mapping to your question:
- (i) Telnet → incorrect (Telnet is a remote terminal protocol)
- (iii) HTTP → incorrect (HTTP is for web traffic)
- (iv) None of the above → incorrect
- (ii) UDP → correct
Answering the MCQ: Which transport protocol does DNS use?
- 1Step 1
DNS messages are carried over transport protocols—most commonly UDP, with TCP used when UDP is insufficient.
- 2Step 2
Telnet and HTTP are application protocols unrelated to the standard DNS transport mechanism.
- 3Step 3
Choose UDP because DNS queries run primarily over UDP (port 53).
- 4Step 4
If the DNS response is truncated or requires reliability, TCP can be used—this does not change the MCQ’s correct choice of UDP.
Pro Tip
If your MCQ asks “the protocol on which DNS application run,” the expected single answer is typically UDP, even though TCP is also supported for special cases.
Common Confusion
DNS is often mentioned alongside “port 53,” but port numbers alone aren’t the protocol choice. DNS uses UDP and TCP on port 53; the MCQ is asking which one is the primary transport.
Where UDP and TCP fit in typical DNS behavior
Typical query
Step 1Client sends DNS query over UDP/53."
Normal response
Step 2Server replies over UDP when response fits."
Need for a larger or reliable response
Step 3Client/server switches to TCP/53 as required (e.g., truncation)."
UDP vs TCP for DNS (conceptual emphasis)
UDP is the common/default transport; TCP is used when UDP is insufficient.
DNS protocol essentials for MCQs
Knowledge Check
The protocol on which DNS application runs is: