[CentOS] DNS lookup delay with centos & postfix

Thu Jul 26 12:21:30 UTC 2012
Tris Hoar <trishoar at bgfl.org>

On 26/07/2012 02:40, David McGuffey wrote:
> On Jul 25, 2012, at 21:27, "Joseph L. Casale" <jcasale at activenetwerx.com> 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


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