DNS packet size -- what's the correct size

[Rob Tanner]

Thanks to all who replied. It's odd that my O'Reilly DNS book still 
lists 512 bytes as the max size.  From the comments I got, I've asked 
our network manager to either turn that check off entirely or set the 
limit to 2048.
Again thanks.

-- Rob

On 09/30/2007 11:31 AM, dnd wrote:
> Rob:
>
> 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.
>
> Regards,
>
> 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
>>
>>
>>   
>>     
>
>