I recently performed a Microsoft Lync Sever 2013 migration and following this process I noted that Lync 2013 clients connecting from external networks were continually prompted for Outlook authentication in order to retrieve data from Exchange Web Services (EWS). After investigating the issue, it appears this occurs when utilising forms based authentication for the /Autodiscover/* and /EWS/* virtual directories when utilising an SSL web listener in Forefront TMG 2010. In order to resolve the issue a separate global IP address was obtained and assigned to a web listener that does not perform any pre-authentication and simply passes the authentication request directly to Exchange Server 2010. The reason a separate global IP address and web listener was utilised, is that should you be using a single web listener for all Exchange services you will need to disable forms based authentication for OWA, Outlook Anywhere and Exchange ActiveSync, in most environment this would not be a desirable approach, however using a separate listener purely for autodiscover and EWS satisfies most security requirements. The following steps were performed in Forefront TMG 2010 to resolve the issue.
1. In Forefront TMG 2010 right click “Firewall Policy” -> “New” -> “Exchange Web Client Access Publishing Rule”.
2. When the wizard invokes enter a name for the publishing rule such as “Exchange Web Services” and click Next.
3. On the select services page click the drop down item and select the appropriate version of Exchange Server for your environment, and then check to select “Outlook Anywhere (RPC\HTTP(s))” and then also select “Publish additional folders on the Exchange Server for Outlook 2007 clients” and then click Next.
4. On the Publishing Type page ensure that “Publish a single website or load balancer” is checked and click Next, on the following Server Connection Security page select “Use SSL to connect to the published web server for server farm” and click Next.
5. On the Internal Publishing Details page in the Internal site name dialogue box enter the fully qualified domain name of your Exchange Client Access Server and then click Next.
6. On the Public Names page enter the the fully qualified domain name used for external autodiscover, for example autodiscover.domain.com and then click Next.
7. On the Select Web Listener page click “New” and then enter and appropriate name for the listener and click next, on the following page select “Require SSL secured connections with client” and then click Next.
8. On the Web Listener IP address page, click External and then “Select IP Addresses” and continue to select the new global IP address that is to be used for Exchange Web Services and then click Next.
9. On the Listener SSL Certificate page click “Select Certificate” and then choose your third party SSL certificate for Exchange Services, this certificate must include the subject alternative name of autodiscover.domain.com, once selected click Next.
10. On the Authentication Settings page click the drop down item and select “No Authentication” and thenclick next and then next again past the Single Sign On page and the click Finish.
11. Back in the main publishing rule wizard, ensure the newly created listener is selected and then click Next. On the Authentication Delegation page click the drop down item and select “No delegation but client may authenticate directly” and then click Next, on the following User Sets page click Next and then Finish to create the publishing rule.
12. Once the rule has been created right click it and select properties and select the paths tab and remove the /OAB/* and /rpc/* entries and click OK. Following this change click Apply on the Firewall Policy page and wait for the TMG configuration store to update accordingly.
13. The Exchange Web Services rule is now created, and should look like the following as detailed below, please click to enlarge.
14. If the publishing of the rule has applied correctly, when connecting with your Lync 2013 client externally you should now longer be continually prompted for Outlook credentials and additionally under the configuration information section of the client, which can be accessed by holding down the control (Ctrl) key and then left clicking the Lync 2013 task tray icon and selecting “Configuration Information”, the EWS status should now say “EWS Status OK”.
That’s it, hopefully your EWS external access now works as intended.