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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Configuring Exchange Autodiscover

As of the release of Outlook 2016, Microsoft has chosen to begin requiring the use of Autodiscover for setting up Outlook clients to communicate with the server. This means that, moving forward, Autodiscover will need to be properly configured.

This page contains some information and some links to other posts I’ve written on the subject of Autodiscover. This page is currently under construction as I write additional posts to assist in configuring and troubleshooting Autodiscover.

Initial Configuration

The initial configuration of Autodiscover requires that you have a Digital Certificate properly installed on your Exchange Server. If you use a Multi-Role configuration (No longer recommended by MS for Exchange versions after 2010), the Certificate should be installed on the CAS server.

Certificate Requirements

The certificate should have a Common Name that matches the name your users will be using to access Exchange. If you want users to use mail.domain.com to access the Exchange server, make sure that is the Common Name when creating the certificate.

The optimal configuration for Exchange also requires that you include autodiscover.domain.com as a Subject Alternate Name (SAN). You should also make sure that there is also an A or CNAME record in DNS to point users to autodiscover.domain.com. SAN certificates can cost significantly more money than a normal certificate, but there are ways to bypass the need for a SAN certificate (See the next section below for more info).

A Wildcard certificate is usable with Exchange, and can serve as a less expensive way to provide support for a large number of URLs. A Wildcard can also be used on other servers that use the same DNS domain as the Exchange server. However, wildcards are technically not as secure as a SAN cert, since they can be used with any URL in the domain. In addition, they do not support Sub-domains.

The certificate you install on Exchange should also be obtained from a reputable Third Party Certificate Authority. The following Certificate Authorities can generate Certificates that are trusted by the majority of web browsers and operating systems:

Comodo PositiveSSL
DigiCert
Entrust
Godaddy
Network Solutions

Also note, when generating your Certificate Signing Request (CSR), you should generate the CSR with a sufficient bit length. Currently, the recommended minimum for CSR generation is 2048 bits. 1024 and lower bit lengths may not be supported by Certificate Authorities.

Exchange Server Configuration

Autodiscover will determine the settings to apply to client machines by reading the Exchange Server configuration. This means the Exchange Service URLs must be properly configured. If they are not configured to use a name that exists on the Certificate in use, Outlook will generate a Certificate Error.

I will write a post on this subject in the future. For now, you can get this information easily from a Google Search.

DNS configuration

There are 2 different URLs Autodiscover will use when searching for configuration information. These URLs are based on the user’s Email Domain (The portion of the email address after the @). For bob@acbrownit.com, the Email Domain is acbrownit.com. The URLs checked automatically are:

domain.com
autodiscover.domain.com

As long as one of the above URLs exists on the Certificate and has an A record or CNAME record in DNS pointing to a CAS server, Autodiscover will work properly. The instructions for this can vary depending on the DNS provider you use.

Other Configurations

There are some situations that may cause autodiscover to fail if the above requirements are all met. The following situations require additional setup and configuration.

Domain Joined Computers

Computers that are part of the same Active Directory Domain as the Exchange server will attempt to reach the Active Directory Service Connection Point (SCP) for Autodiscover before attempting to find autodiscover at the normal URLs listed above. In this situation, you will typically need to configure the SCP to point to one of the URLs on your certificate.

Go to this post to find instructions for configuring the SCP:

Exchange Autodiscover Part 2 – The Active Directory SCP

Single Name Certificates

If you do not want to spend the additional money required to obtain a SAN or Wildcard certificate for Exchange, you can use a Service Locator (SRV) Record in DNS to define the location of autodiscover. A Service Locator Record allows you to define any URL you want for the Autodiscover service, so you can create one to bypass the need for having a SAN or Wildcard certificate.

Go to this post to find instructions for configuring a SRV record:

Internal DNS and Exchange Autodiscover