Performance Issues - PTR Records

Thu Nov 3 17:47:32 UTC 2011

On 11/02/2011 05:57 PM, Matt Chung wrote:
> I work for a regional ISP and very recently there has been an influx of
> calls reporting "slowness" when accessing certain websites (i.e
> google.com/voice/b) via HTTP.  After performing a tcpdump and analyzing the
> session, I have been able to pinpoint the latency at the application
> layer.  After the tcp session has been established, it takes up to 15-20
> seconds before the application begins sending data.   The root of the
> problem was that the PTR record for our customer(s) address does not
> exist.  As soon as the record is created, latency from the application is
> eliminated.  This is analogous to latency when accessing a server over SSH
> when no PTR is available.
>
> A seperate packet capture from another customer exhibiting similar
> performance behavior showed many TCP retransmissions.  At first glance, I
> assumed this was network related however this correlates with the
> application not responding and inducing retransmissions at the TCP layer
> (different symptoms, same problem).
>
> Historically, there was no compelling reason to create PTR records for our
> CPE however more and more applications seem to be dependent on it.
> Although we will be assigning a record for each address, my question is why
> is the application (specifically HTTP) dependent on a reverse record ?
> What is the purpose?
>
> Hope this is helpful as well
>

   We recently encountered a similar issue with a customer.
The sites that had slowness issues were configured to
use the traditional Google Analytics javascript instead
of the newer asynchronous code.
   The problem was not the lack of a PTR record, but rather
the in-addr delegation was pointing to lame servers that were
no longer responding (hence the long timeouts as the
Google servers attempted to perform reverse lookups
on the client IP's).  As others have pointed out, as long
as there's a valid nameserver responding, a lack of PTR
record would not cause issues as an NXDOMAIN record would
be sent back immediately and the Google Analytics server
will move on.

  -Larry Blunk
   Merit