DNS UDP or TCP?
Cricket Liu
cricket at menandmice.com
Wed Sep 26 23:56:19 UTC 2001
> > 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.
cricket
Men & Mice
DNS Software & Services
www.menandmice.com
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