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Could we configure DNS to use TCP ?

Posted on 2009-05-06
Medium Priority
Last Modified: 2012-06-27
I feel by default DNS uses UDP protocol on port 53. Could we configure it to let it use the TCP as well ? /etc/services shows it support both TCP and UDP.
bash-3.00# grep 53 /etc/services 
domain          53/udp
domain          53/tcp

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Question by:beer9
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LVL 48

Expert Comment

ID: 24312665
Depends what DNS server you are running.

Author Comment

ID: 24314160
What if I am using BIND, does is support both TCP and UDP by default? Thanks!
LVL 62

Expert Comment

ID: 24314213
DNS over TCP is almost never used.
Any server out there supports DNS over TCP but no clients use that by default.
Since it is that rarely used I suspect more problems in there.

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Accepted Solution

jarrell earned 1000 total points
ID: 24315868
Generally TCP uses UDP for speed, but if the answer doesn't fit into a single udp packet, it'll switch to using TCP.  Which I've seen cause problems with some clients; we broke some software when our SPF record (which is in a TXT on the domain names RR) got too big, and we were doing TCP all of a sudden.
LVL 62

Expert Comment

ID: 24318808
It depends on client. Some servers do send chain of UDP packets with huge record...

Author Closing Comment

ID: 31578360
Thank you :-)

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