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Windows 2003: The EDNS0 enigma

Troubleshooting the EDNS0 enigma in Windows 2003.

During a migration to Windows 2003 Server, we upgraded our root domain name server (DNS). Although everything appeared fine, we started receiving complaints about getting to certain sites. Areas of Yahoo, such as and, seemed to be the biggest issue. At first, it looked like Yahoo was unresponsive to queries. However, we found host records to other sites were resolving properly, but their MX records were not. This meant that e-mail was not routing!

As a means of troubleshooting, we double-checked all our DNS configurations. Everything looked fine. As a second step, we gathered network traces to find out what was going on. The traces showed packets leaving the root DNS server, destined for Yahoo, but showed no replies returning.

The problem here is that Windows 2003 enables Extension Mechanisms for DNS (EDNS0 as defined in RFC 2671), a standard introduced in 1999, by default. EDNSO allows requestors to advertise their EDNS0 capabilities, hence receiving UDP packets larger than 512 bytes.

While this in itself is not problematic, some firewalls do not allow UDP packets larger than 512 bytes. This explains why the network traces showed nothing returning! Our DNS servers were sending out packets advertising themselves as capable of EDNS0, and our firewalls were dropping the responses. Turning off EDNS0 support allowed all queries to work as expected.

If you're experiencing the same issue or planning an upgrade of your own, this command will disable this enabled-by-default feature:

dnscmd ServerName /Config /EnableEDnsProbes 0

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ABOUT THE AUTHOR: Marcus Oh works for Cox Communications, Inc. in Alpharetta, GA., deploying MOM for 250+ servers, rolling out SMS 2003 and Windows 2003, and supporting the company's directory services infrastructure.

This article first appeared in myITforum, the premier online destination for IT professionals responsible for managing their corporations' Microsoft Windows systems. The centerpiece of is a collection of member forums where IT professionals actively exchange technical tips, share their expertise, and download utilities that help them better manage their Windows environments, specifically Microsoft Systems Management Server (SMS). It is part of the TechTarget network of Web sites. To register for the site and sign up for the myITforum daily newsletter, click here:

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