Microsoft has released the following hotfixes: Update for Windows Server 2003 (KB2669182), Update for Windows Server 2003 for Itanium-based Systems (KB2669182), Update for Windows Server 2003 x64 Edition (KB2669182), and Update for Windows XP x64 Edition (KB2669182)

Issue 1
Consider the following scenario:

You have Microsoft Internet Security and Acceleration (ISA) Server 2006 installed on a remote access server (RAS) that is running Windows Server 2003.
You configure VPN remote client access by using ISA Server 2006.
You install the following update on the server.
2509553 MS11-030: Vulnerability in DNS Resolution could allow remote code execution: April 12, 2011
A user enters their username by using the user principal name (UPN) format to establish a VPN connection to the RAS. For example, the user enters username@example.microsoft.com as their username.

In this situation, the RAS encounters a deadlock situation, and the user cannot establish the VPN connection. To recover from this issue, you have to restart the server.

Issue 2
After you install the update that is described in Microsoft Knowledge Base (KB) article 2509553 on a Windows Server 2003-based server, users experience slow network performance. This issue is caused by slow DNS name resolution.

Update for Windows Server 2003 (KB2669182)
Update for Windows Server 2003 for Itanium-based Systems (KB2669182)
Update for Windows Server 2003 x64 Edition (KB2669182)
Update for Windows XP x64 Edition (KB2669182)