On 26/07/2012 02:40, David McGuffey wrote:
> On Jul 25, 2012, at 21:27, "Joseph L. Casale" <jcasale@xxxxxxxxxxxxxxxxx> wrote:
>
>>> DNS lookups default to using 53/udp, and only use 53/tcp for zone
>>> transfers. could it be 53/udp is being lost/blocked between this host
>>> and your ns1 ?
>>
>> Unfortunately that is a common misconception.
>>
>> Tcp is used far more often than "only" as stated such as for size of request
>> exceeding udp response size etc...
>>
>> Bottom line is both ports are needed, not just for zone xfers.
>>
> Except that the malware guys have figured out how to abuse port 53. Security recommendation is to block TCP unless you're running a DNS server. And also block oversize port 53 UDP packets.
Blocking oversize UDP packets is a very bad idea. EDNS is used for a lot
of look ups these days due to DNSSEC, and so blocking oversize UDP
packets will force you to use TCP to get many of your DNS requests.
>
> Dave M
Tris
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