DNS UDP or TCP?

[Kevin Darcy]

Chris Parker wrote:

> Are DNS queries UDP or TCP?

Yes :-)

Seriously, they can be either UDP or TCP, although in practice most
queries are UDP. You should be prepared to deal with both protocols.

- Kevin

[Brad Knowles]

At 4:32 PM -0700 9/26/01, Chris Parker wrote:

> Are DNS queries UDP or TCP?

Depends. By default, DNS queries use UDP, but if there is a
truncation in the response (i.e., it won't fit into a 512-byte UDP
packet), then the server is responsible for retrying the query using
TCP. TCP is also used for zone transfers. And for any particular
query, you can specify if you want to use TCP instead of UDP (for
whatever reason).

--
Brad Knowles, <brad.k...@skynet.be>

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[Cricket Liu]

> Are DNS queries UDP or TCP?

Mostly UDP, but occasionally TCP.

cricket

Men & Mice
DNS Software & Services
www.menandmice.com

[Cricket Liu]

> > Are DNS queries UDP or TCP?
>

> Depends. By default, DNS queries use UDP, but if there is a
> truncation in the response (i.e., it won't fit into a 512-byte UDP
> packet), then the server is responsible for retrying the query using
> TCP. TCP is also used for zone transfers. And for any particular
> query, you can specify if you want to use TCP instead of UDP (for
> whatever reason).

Actually, there are (or have been) some resolvers that use
TCP by default. I think an older AIX resolver fell into this
category, as did the resolver on the HP1000.

But it's certainly true that the vast majority of DNS
queries are UDP-based, and as Brad explained, fall back
to TCP only if the answer is truncated.