Exchange Autodiscover – The Active Directory SCP

In a previous post I explained how you can use a SRV record to resolve certificate issues with Autodiscover when your Internal domain isn’t the same as your Email domain. This time, I’m going to explain how to fix things by making changes to Exchange and Active Directory that will allow things to function normally without having to use a SRV record or any DNS records at all, for that matter. But only if the computers that access Exchange are members of your Domain and you configure Outlook using user@domain.local. This is how Exchange hands out Autodiscover configuration URLs by default without any DNS or SRV records. However, if you have an Private Domain Name in your AD environment, which you should try to avoid when you’re building new environments now, you will always get a Certificate Error when you use Outlook because SSL certificates from third party CA providers won’t do private domains on SAN certificates anymore. To fix this little problem, I will first give you a little information on a lesser known feature in Active Directory called the Service Connection Point (SCP).

Service Connection Points

SCPs play an Important role in Active Directory. They are basically entries in the Active Directory Configuration Partition that define how domain based users and computers can connect to various services on the domain. Hence the name Service Connection Point. These will typically show up in one of the Active Directory tools that a lot of people overlook, but is *extremely* important in Exchange since 2007 was released, Active Directory Sites and Services (ADSS). ADSS is typically used to define replication boundaries and paths for Active Directory Domain Controllers, and Exchange uses the information in ADSS to direct users to the appropriate Exchange server in large environments with multiple AD Sites. But what you can also do is view and make changes to the SCPs that are set up in your AD environment. You do this with a feature that is overlooked even more than ADSS itself, the Services node in ADSS. This can be exposed by right clicking the Active Directory Sites and Services object when you have ADSS open, selecting view, then clicking “Show Services Node” like this:

ADSS - Services Node

Once you open the services node, you can see a lot of the stuff that AD uses in the back end to make things work in the domain. Our focus here, however, is Exchange, so go into the Microsoft Exchange node. You’ll see your Exchange Organization’s name there, and you can then expand it to view all of the Service Connection Points that are related to Exchange. I wouldn’t recommend making any changes in here unless you really know what you’re doing, since this view is very similar to ADSIEdit in that it allows you to examine stuff that can very rapidly break things in Active Directory.

Changing the Exchange Autodiscover SCP

If we look into the Microsoft Exchange services tree, you first see the Organization Name. Expand this, then navigate to the Administrative Group section. In any Exchange version that supports Autodiscover, this will show up as First Administrative Group (FYDIBOHF23SPDLT). If the long string of letters confuses you, don’t worry about it. That’s just a joke the developers of Exchange 2007 put into the system. It’s a +1 Caesar Cipher that means EXCHANGE12ROCKS when decoded. Programmers don’t get much humor in life, so we’ll just have to forgive them for that and move on. Once you expand the administrative group node, you’ll be able to see most of the configuration options for Exchange that are stored in AD. Most of these shouldn’t be touched. For now, expand the Servers node. This is the section that defines all of your Exchange servers and how client systems can connect to them. If you dig around in here. Mostly you just see folders, but if you right click on any of them and click Properties, you should be able to view an Attributes tab (in Windows 2008+, at least, prior to that you have to use ADSIEdit to expose the attributes involved in the Services for ADSS). There are lots of cool things you can do in here, like change the maximum size of your Transaction Log files, implement strict limits on number of databases per server, change how much the database grows when there isn’t enough space in the database to commit a transaction, and other fun things. What we’re focusing on here is Autodiscover, though, so expand the Protocols tree, then go to Autodiscover, as seen below.

autodiscover node

Now that we’re here, we see each one of the Exchange CAS servers in our environment. Mine is called Exchange2013 because I am an incredibly creative individual (Except when naming servers). Again, you can right click the server name and then select Properties, then go to the Attribute Editor tab to view all the stuff that you can control about Autodiscover here. It looks like a lot of stuff, right? Well, you’ll really only want to worry about two attributes here. The rest are defined and used by Exchange to do…Exchangey stuff (Technical term). And you’ll really only ever want to change one of them. The two attributes you should know the purpose of are “keywords” and “serviceBindingInformation”.

  • keywords: This attribute, as you may have noticed, defines the Active Directory Site that the CAS server is located in. This is filled in automatically by the Exchange subsystem in AD based on the IP address of the server. If you haven’t created subnets in ADSS and assigned them to the appropriate site, this value will always be the Default site. If you change this attribute, it will get written over in short order, and you’ll likely break client access until the re-write occurs. The *purpose* of this value is to allow the Autodiscover Service to assign a CAS server based on AD site. So, if you have 2 Exchange Servers, one in site A and another in site B, this value will ensure that clients in site A get configured to use the CAS server in that site, rather than crossing a replication boundary to view stuff in site B.
  • serviceBindingInformation: Here’s the value we are most concerned with in this post! This is the value that defines where Active Directory Domain joined computers will go for Autodiscover Information when you enter their email address as username@domain.local if you have a private domain name in your AD environment. By default, this value will be the full FQDN of the server, as it is seen in the Active Directory Domain’s DNS forward lookup zone. So, when domain joined computers configure Outlook using user@domain.local they will look this information up automatically regardless of any other Autodiscover, SRV, or other records that exist in DNS for the internal DNS zone. Note: If your email domain is different from your AD domain, you may need to use your AD domain as the email domain when configuring Outlook for the SCP lookup to occur. If you do not want to use the AD Domain to configure users, you will want to make sure there is an Autodiscover DNS record in the DNS zone you use for your EMail Domain.

Now, since we know that the serviceBindingInformation value sets the URL that Outlook will use for Autodiscover, we can change it directly through ADSS or ADSIEdit by replacing what’s there with https://servername.domain.com/Autodiscover/Autodiscover.xml . Once you do this, internal clients on the domain that use user@domain.local to configure Outlook will be properly directed to a value that is on the certificate and can be properly configured without certificate errors.

Now, if you’re a little nervous about making changes this way, you can actually change the value of the serviceBindingInformation attribute by using the Exchange Management Shell. You do this by running the following command:

get-clientaccessserver | set-clientaccessserver -autodiscoverserviceinternaluri “https://servername.domain.com/Autodiscover/Autodiscover.xml”

This will directly modify the Exchange AD SCP and allow your clients to use Autodiscover without getting certificate errors. Not too difficult and you don’t have to worry about split DNS or SRV records. Note, though, that like the SRV record you will be forcing your internal clients to go out of your network to the Internet to access your Exchange server. To keep this from happening, you will have to have an Internal version of your External DNS zone that has Internal IPs assigned in all the A records. There just is no way around that with private domain names any longer.

Final Note

Depending on your Outlook version and how your client machines connect, there is some additional configuration that will need to be completed to fully resolve any certificate errors you may have. Specifically, you will need to modify some of the Exchange Virtual Directory URLs to make sure they are returning the correct information to Autodiscover.

Avoiding Issues with Certificates in Exchange 2007+

For information, modern Active Directory Best Practices can help you avoid having trouble with certificate errors in Exchange. Go here to see some information about modern AD Domain Naming best practices. If you follow that best practice when creating your AD environment, you won’t have to worry so much about certificate errors in Exchange, as long as the Certificate you use has the Exchange Server(s) name listed. However, if you can’t build a new environment or aren’t already planning to migrate to a new AD environment in the near future, it isn’t worth the effort to do so when small configuration changes like the one above can fix certificate errors.

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Internal DNS and Exchange Autodiscover

 

The Issue

By now, anyone who has managed, deployed, or worked with an Exchange 2007 or later environment should be familiar with Autodiscover. If you aren’t yet, I’ll give a short Explanation of what it is and how it works.

Autodiscover is a feature that allows any Mail Client that supports Autodiscover to configure the appropriate server settings for communication so you don’t have to input everything manually. It’s very handy. Unfortunately, you can end up with a lot of headaches related to Autodiscover when you start having to deal with Certificates. The issues you may run into are specifically limited to Exchange Organizations that have a Domain Name that uses a non-public TLD like domain.local, or a public domain name that they don’t actually own and can’t use externally as well. On an unrelated note, this is one of the reasons that Microsoft has started recommending the use of Public domain names for Active Directory domains.

If you have a domain that isn’t publicly useable on your Exchange AD environment, you will run into certificate errors when mail clients use Autodiscover. This becomes particularly problematic when you use Exchange 2013 and try to use HTTPS for Outlook Anywhere. This is because Microsoft is now enforcing certificate validity with Exchange 2013’s Autodiscover features (Note, though, that Outlook Anywhere will be configured to use HTTP only when your Exchange Server certificate is determined to be invalid in Exchange 2013). With Exchange 2007 and 2010, you will get a Certificate error every time you open Outlook. Generally, this error will state that the name on the certificate is not valid.

The Cause

To solve the issue with certificates, you need to configure your environment so it enforces the appropriate action with Autodiscover. By default, Autodiscover will attempt to communicate with a number of URLs based on the Client’s email address (for external users) or domain name (for internal users). It will take the following pattern when checking for Autodiscover services:

1. Autodiscover will attempt to find the Autodiscover configuration XML file at the domain name of the SMTP address used in configuration (because internal domain computers configure themselves automatically by default, this matches the Internal Domain. For example, the first place autodiscover looks is https://domain.com/Autodiscover/autodiscover.xml for external addresses. Change domain.com with domain.local for what Exchange looks for on Internal clients.

2. If the autodiscover record is not found at domain.com/domain.local, the server will attempt to connect to https://autodiscover.domain.com/Autodiscover/Autodiscover.xml (replace domain.com with domain.local for internal). This is why the typical recommendation for having an A Record for Autodiscover in your DNS that points to the mail server exists. In addition, you would need to have autodiscover.domain.com as a SAN on the SSL certificate installed on the Exchange server for it to be valid when attempting to connect to autodiscover using this step.

3. If autodiscover information cannot be found in either of the first two steps, Exchange will attempt to use a Service Locator record in DNS to determine the appropriate location of the configuration files. This record points the Autodiscover service to a specific location for getting the configuration it needs.

Because of the way this works, there is some configuration necessary to get Autodiscover working correctly. This usually involves adding Subject Alternate Names to the SSL certificate you use for your Exchange Server to allow the many host names used to be authenticated with the certificate.

The problem lately, though, is that many Third Party Certificate Authorities that provide SSL certificates are beginning to deny requests for Subject Alternate Names that aren’t publicly available (There are valid security reasons for this that I won’t go in to in this post, but maybe later). As a result, you won’t be able to get a valid SSL certificate that allows domain.local as a SAN. This means that the automated steps Exchange uses for Autodiscover configuration will always fail on an Internal domain with a name that is not publicly accessible or not owned.

The Solution

There are actually two ways to solve the certificate issues, here. The first would be to prevent Outlook from automatically entering a user’s information when they create their profile. This will result in more work for you and your users, so I don’t recommend it. The other solution is to leverage that last step of the Autodiscover configuration search to force it to look at a host name that is listed on the certificate. This is actually fairly simple to do. Follow these steps to configure the Service Locator record in your internal domain.

  1. Open the DNS manager on one of your Domain Controllers.
  2. Expand out the management tree until you can see your Internal Domain’s Forward Lookup Zone. Click on it, and make sure there are no A records for autodiscover.domain.local in the zone.
  3. Once no autodiscover A records exist, right click the Zone name and select Other New Records.
  4. Select Service Location (SRV) from the list.
  5. Enter the settings as shown below:Image
  6. Hit OK to finish adding the record.

Once the SRV record is added to the internal DNS zone, Outlook and other autodiscover clients that attempt to configure themselves with a domain.local SMTP address will work properly without the Certificate errors on all versions of Exchange.

Other Nifty Stuff

There are some additional benefits to utilizing the Service Locator record for Autodiscover rather than an Autodiscover A record, even in your public domain. When you use a SRV record, you can also point public clients to communicate with mail.domain.com or outlook.domain.com, or whatever you have configured your external server name to be. This means you can get away with having a single host name on your SSL certificate, since you wouldn’t need autodiscover.domain.com to get autodiscover working. Since most Third Party CAs charge a bit more for SANs than they do for Single Name SSL certs, you can save a bit of money (for this to work, though, you may need to change your Internal and External Web Services URLs in Exchange to match the name you have configured).

Another Problem the SRV record Fixes

There are also some other issues you may run into that are easily fixed by adding a SRV record. One of the most common is the use of multiple Email Domains in a single Exchange Environment. If you have users that are not assigned a Primary or secondary SMTP address that matches the domain name listed on your SSL certificate, you’ll discover that those users and the rest of your users will not be able to share calendar data between their mailboxes. You can fix this by adding an Autodiscover SRV record to the DNS zone that manages the additional mail domains. For example, you have domain1.com and domain2.com on the same Exchange Server. user@domain1.com can’t see user@domain2.com’s calendar. The fix for this is to add the SRV record to the domain2.com DNS zone and point it to the public host name for domain1.com’s mail server. Once that’s done the services that operate the calendar sharing functions will be properly configured and both users will be able to share calendars.