Performance Issues - PTR Records

Wed Nov 2 23:19:37 UTC 2011

On Nov 2, 2011, at 5:57 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?

You're returning NXDOMAIN, right? If they're doing a reverse lookup and you return NXDOMAIN it should fail quickly, or else the application is even more horribly broken than just doing reverse lookups would imply. On the other hand, if you're not responding to the PTR request then that could be causing the timeout.

-Ben