UPD size in DNS

Barry Margolin barmar at genuity.net
Tue Jan 15 22:29:45 UTC 2002


In article <a21ugv$s4 at pub3.rc.vix.com>,
Type your name here <Type.your.username.here at fiol.brock.dk> wrote:
>Hello,
>
>I have read that DNS can only handle UPD-packets of 512-bytes or less.
>Eventhrough the theoretical size is 65.535 bytes - 20 bytes IP-header - 8
>bytes UDP-header.
>Does the DNS-protocol have this limit? If yes, why?

Yes.  It was done to make it unlikely that DNS packets would require
fragmentation.

If a response overflows this size, the server will put as much as it can
fit in 500 bytes and set the Truncated flag.  The client is then supposed
to retry using TCP.

-- 
Barry Margolin, barmar at genuity.net
Genuity, Woburn, MA
*** DON'T SEND TECHNICAL QUESTIONS DIRECTLY TO ME, post them to newsgroups.
Please DON'T copy followups to me -- I'll assume it wasn't posted to the group.


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