How Does Exchange Autodiscover Work?

Autodiscover is one of the more annoying features of Exchange since Microsoft reworked the way their Email solution worked in Exchange 2007. All versions since have implemented it and Microsoft may eventually require its use in versions following Exchange 2016. So what is Autodiscover and how does it work?

Some Background

Prior to Exchange 2007, Outlook clients had to be configured manually. In order to do that, you had to know the name of the Exchange server and use it to configure Outlook. Further, if you wanted to use some of the features introduced in Exchange 2003 SP2 and Outlook 2003 (and newer), you had to manually configure a lot of settings that didn’t really make sense. In particular, Outlook Anywhere requires configuration settings that might be a little confusing to the uninitiated. This got even more complicated in larger environments that had numerous Exchange servers but could not yet afford the expense of a load balancer.

The need to manually configure email clients resulted in a lot of administrative overhead, since Exchange admins and Help Desk staff were often required to configure Outlook for users or provide a detailed list of instructions for people to do it themselves. As most IT people are well aware, even the best set of instructions can be broken by some people, and an IT guy was almost always required to spend a lot of time configuring Outlook to talk to Exchange.

Microsoft was not deaf to the cries of the overworked IT people out there, and with Exchange 2007 and Outlook 2007 introduced Autodiscover.

Automation Salvation!

Autodiscover greatly simplifies the process of configuring Outlook to communicate with an Exchange server by automatically determining which Exchange server the user’s Mailbox is on and configuring Outlook to communicate with that server. This makes it much easier for end users to configure Outlook, since the only things they need to know are their email address, AD user name, and password.

Not Complete Salvation, Though

Unfortunately, Autodiscover didn’t completely dispense with the need to get things configured properly. It really only shifted the configuration burden from Users over to the Exchange administrator, since the Exchange environment has to be properly configured to work with Autodiscover. If things aren’t set up properly, Autodiscover will fail annoyingly.

How it Works

In order to make Autodiscover work without user interaction, Microsoft developed a method for telling Outlook where it needed to look for the configuration info it needed. They decided this was most easily accomplished with a few DNS lookups based on the one piece of information that everyone had to put in regardless of their technical know how, the email address. Since they could only rely on getting an email address from users, they knew they’d need to have a default pattern for the lookups, otherwise the client machines would need at least a little configuration before working right. Here’s the pattern they decided on:

  1. Look in Active Directory to see if there is information about Exchange
  2. Look at the root domain of the user’s email Address for configuration info
  3. Look at autodiscover.emaildomain.com for configuration info
  4. Look at the domain’s root DNS to see if any SRV records exist that point to a host that holds configuration info.

Note here that Outlook will only move from one step to the next if it doesn’t find configuration information.

For each step above, Outlook is looking for a specific file or a URL that points it to that file. The file in question is autodiscover.xml. By default, this is kept at https://<exchangeservername>/autodiscover/autodiscover.xml. Each step in the check process will try to find that file and if it’s not there, it moves on. If, by the end of step 4, Outlook finds nothing, you’ll get an error saying that an Encrypted Connection was unavailable, and you’ll probably start tearing your hair out in frustration.

What’s in the File?

Autodiscover.xml is a dynamically generated file written in XML that contains the information Outlook needs to access the mailbox that was entered in the configuration wizard. When Outlook makes a request to Exchange Autodiscover, the following things will happen:

  1. Exchange requests credentials to access the mailbox.
  2. If the credentials are valid, Exchange checks the AD attributes on the mailbox that has the requested Email address.
  3. Exchange determines which server the Mailbox is located on. This information is usually stored in the msExchangeHomeServer attribute on the associated AD account.
  4. Exchange examines its Topology data to determine the best Client Access Server (CAS) to use for access to the mailbox. The Best CAS is determined using the following checks:
    1. Determine AD Site the Mailbox’s Server is located in
    2. Determine if there is a CAS assigned to that AD site
    3. If no CAS is in the site, use Site Topology to determine next closest AD Site.
    4. Step 3 is repeated until a CAS is found.
  5. Exchange returns all necessary configuration data stored in AD for the specific server. The configuration data returned is:
    1. CAS server name
    2. Exchange Web Services URL
    3. Outlook Anywhere Configuration Data, if enabled.
    4. Unified Communications Server info
    5. Mapi over HTTPS Proxy server address (if that is enabled)
  6. Outlook will take the returned information and punch it into the necessary spots in the user’s profile information.

Necessary Configuration

Because all of this is done automatically, it is imperative that the Exchange server is configured to return the right information. If the information returned to Autodiscover is incorrect, either the mailbox connection will fail or you’ll get a certificate error. To get Autodiscover configured right, parts 5.1, 5.2, 5.3, and 5.5 of the above process must be set. This can be done with a script, in the Exchange Management Shell, and in the Exchange Management UI (EMC for 2007 and 2010, ECP/EAP for 2013/2016).

Importance of Autodiscover

With the release of Outlook 2016, it is no longer possible to configure server settings manually in Outlook. You must use Autodiscover. Earlier versions can avoid using it by manually configuring each outlook client. However, before doing that, consider the cost of having to touch each and every computer to properly configure Outlook. It can take 5 minutes or more to configure Outlook on one computer using the manual method, and with Exchange 2013 it can take longer as you also are required to input Outlook Anywhere configuration settings, which are more complex than just entering a server name, username, and password. If you multiply that by the number of computers you might have in your environment and add in the time it takes to actually get to the computers, boot them up, and get to the Outlook settings, the time spent configuring Outlook manually starts to add up very quickly. Imagine how much work you’d be stuck with configuring 100 systems!

In contrast, it usually only takes 10 to 20 minutes to configure Autodiscover. When Autodiscover is working properly, all you have to do is tell your users what their email address is and Outlook will do all the work for you. With a little more configuration or some GPO work, you don’t even have to tell them that!

When you start to look at the vast differences in the amount of time you have to spend configuring Outlook, whether or not to use Autodiscover stops being a question of preference and starts being an absolutely necessary part of any efficient Exchange-based IT environment. Learning to configure it properly is, therefore, one of the most important jobs of an Exchange administrator.

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2 responses to “How Does Exchange Autodiscover Work?

  1. AutoDiscover is nice for simple profile creation, but what about when you want to control other aspects of Outlook? For that, the Outlook Profiler tool comes in handy.

    • Outlook Profiler doesn’t negate the need for Autodiscover, nor does Autodiscover prevent the use of Outlook Profiler. They’re meant to manage different things.

      Specifically, Autodiscover is meant to ensure the Exchange server can be discovered and configured automatically.

      Outlook Profiler, on the other hand, is meant to generate PRF files, which can be used to configure Outlook with more specific settings, like opening additional mailboxes, define whether cached mode is on or off, and a few other things.

      Setting the server names with the Outlook Profiler can cause some issues, particularly in a High Availability environment where there are multiple Exchange servers, so the best idea for setting Outlook up if you need more customized Outlook Profiles is to use Outlook Profiler to create PRF files that utilize Autodiscover.

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