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+18 votes
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Which one of the following uses UDP as the transport protocol?

  1. HTTP
  2. Telnet
  3. DNS
  4. SMTP
asked in Computer Networks by Veteran (59.6k points) | 1.4k views
+3

DNS uses both TCP and UDP for valid reasons. UDP messages are not larger than 512 Bytes and are trucncted when greater than this size. So DNS uses TCP for Zone transfer and UDP for name queries either regular (primary) or reverse. UDP can be used to exchange small information whereas TCP must be used to exchange information larger than 512 bytes. If a client doesn’t get response from DNS it must retransmit the data using TCP after 3-5 seconds of interval.

We all know that there shouldn’t be any inconsistency in DNS zones – to make this happen DNS always transfer Zone data using TCP because TCP is reliable and make sure zone data is consistent by transfering the full zone to other DNS servers who has requested the data.

Source: http://blogs.msmvps.com/systmprog/2006/12/22/dns-works-on-both-tcp-and-udp/

5 Answers

+25 votes
Best answer
The answer is C.

Where quick response is needed, there UDP is preferred.
answered by Boss (19.7k points)
selected by
+2

https://support.microsoft.com/en-us/help/556000 article says DNS uses TCP for zone data transfer and UDP for name queries. So is DNS, the correct answer to this question?

+2
Yes. DNS is correct. Here the context is name queries hence DNS would use UDP. If explicitly mentioned about zone transfer then one can go for TCP.
+1
DNS uses UDP when the size of the response if less than 512 bytes. If it's more than that, then TCP connection is used.
+7 votes
DNS uses the UDP at the transport layer with port number 53.
answered by Loyal (9k points)
+1
it uses tcp also .....
+6 votes

Refer Table: 

Option C

answered by Boss (17.6k points)
edited by
+1 vote

Options a,b & d never use UDP.

DNS isn't limited to any one (TCP or UDP)
It does use TCP for inter DNS server communication because security is the priority.
For answering DNS queries, UDP is used because speed is the priority.

Source : http://techgenix.com/whydnsworksonbothtcpandudp/

Answer : (c)

answered by Active (2.1k points)
–4 votes
answer is c
answered by Junior (609 points)
Answer:

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