Exchange Server 2007 CAS-CAS Proxying Guidelines..

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Hello Folks,

I’ve seen a lot of posts about CAS-CAS Proxying in Exchange Server 2007. Then, I will go over some key points that we must not forget when we are deploying this kind of solution. These points can be seen below:

  • 1 CAS Server in each site that has a MAILBOX server
  • Only in the CAS which will be published on the internet you must set up the externalURL attribute with your external name
  • All other “child” CAS servers must be set up with Integrated Authentication insted of forms authentication
  • ExternalURL in the child sites must be empty
    Set-OwaVirtualDirectory <owa> -ExternalURL:$null
    Ps.: If you enableExternalURL in child CAS Servers, then you will have Redirection feature not Proxy.
  • Make sure that you internalURL is appropriate. By default is the name of server, make sure that it makes sense (I wrote this one, because I saw a customer using external address into the internal address [:D]) 
  • make sure that RedirectToOptimalOWAServer is set to $True, you can use the following cmdlet: set-owavirtualdirectory “owa (default web site)” -RedirectToOptimalOWAServer $true
  • The same for Exchange Server 2003 back-end servers you must not enable forms authentication
  • If you are using ISA Server to publish the CAS Server (recommended) you must disable Forms in the “parent” CAS server, ISA server will take care of the Forms
  • Give some time for Active Directory Replication

Built in tools to help you in this kind of scenario:

  • IIS logs are your friend, you can validate by them if everything is going well
  • You can also validated using OWA session on the About section you can see if CAS Proxy-to-Proxy is working as well

I hope that it can help you in your CAS-CAS Proxying deployment.

Best Regards,
Anderson Patricio

Written by Anderson Patricio

Anderson Patricio

Anderson Patricio is a Canadian MVP in Cloud and Datacenter Management, and Office Server and Services, besides the Microsoft Award he also holds a Solutions Master (MCSM) in Exchange and several other certifications. Anderson has been contributing to the Microsoft Community with articles, tutorials, blog posts, twitter, forums and book reviews. He is a regular contributor here at,, and Anderson (Portuguese).