DNS packet size -- what's the correct size

dnd dnd at po.cwru.edu
Sun Sep 30 18:28:22 UTC 2007


We recently dealt with the same problem after changing Bind versions
from 8.2.7 (ancient, I know) to 8.4.7
Turns out, since 8.3, the default EDNS size has been higher (can't
recall if it is 1024 or 2048).

In any event, the problem you describe is indeed with the Pix, but we
did a quick  fix by adding the following to our named.conf files. 

Add      `edns-udp-size 512;'      to your named.conf file as a work-around.

Before this fix, our name servers were unable to resolve certain
addresses (e.g. cluster1.us.messagelabs.com) which sent large packets. 
We have not had any further incidents after the named.conf modification.


Debbie Andrews

Rob Tanner wrote:
> Hi,
> It's my understanding that the max DNS packet size is 512 bytes and that 
> is apparently what Cisco thinks because our firewall is blocking DNS 
> packets over that size, calling them malformed.  The problem is that we 
> see numerous such packets and the real puzzler is that many of them are 
> originate with core servers.
> The issue is getting serious because there are some sites for which I 
> can't resolve addresses from on campus, but use an external name server 
> and those same sites resolve perfectly.  And, of course, I'm concerned 
> that this problem is related the dropping of over sized packets by the 
> firewall.
> Is Cisco's default limit too small?  Can someone explain to me what 
> might be going on. 
> Thanks,
> Rob

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