End Reverse DNS Lookups

Posted On 2005-11-1 by FortyPoundHead
Keywords: Exchange DNS Reverse XADM
Tags: Tip Exchange Networking 
Views: 1640

One common complaint about Exchange is how slow it is, especially when dealing with Internet mail. The reality is, at least as far as Internet mail goes, much of the slowness may not be due to Exchange itself but with the way Exchange deals with DNS.

Whenever a piece of Internet mail is received or sent, Exchange performs a reverse DNS lookup on the recipient or sender (depending on the direction the mail is going in) and appends the results to the mail's "Received from:" header. The headers in question look something like this:

Received: from server3.com by server4.com; 27 Oct 96 15:27:39 PST
Received: from server2.com by server3.com; 27 May 02 15:15:13 PST
Received: from server1.com by server2.com; 27 May 02 15:01:59 PST
Date: 27 May 02 15:01:01 PST
From: henry@server1.com
Subject: Example headers
To: jerry@server2.com

Each one of these "Received:" headers is added at a different step during the mail delivery process, and each one involves a separate DNS lookup by a separate machine.

What is often overlooked is that DNS lookups are sometimes notoriously slow, especially if you are using a public DNS server or one inside your firewall that gets hammered a great deal. In truth, performing a DNS lookup on the addresses in the "Received:" line is not essential - all it does it provide a fully-qualified domain name rather than an IP address in that line. If you are experiencing slowness because of DNS lookups that take a long time to resolve, you may want to consider disabling reverse resolution lookup in Exchange. The main disadvantage of this is that your domain name will not appear in "Received:" headers; only the IP address of your mail server(s) will appear.

To disable reverse DNS in Exchange, open REGEDIT and locate the key:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\ MSExchangeIMC\Parameters

Add a new DWORD value named DisableReverseResolve and set it to 1. You'll need to restart Exchange before this takes effect.

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