the autodiscover algorithm for locating the “source of information" | part 05#36

of 25 | The Autodiscover algorithm for locating the “source of information" | Part

05#36

Written by Eyal Doron | o365info.com | Copyright © 2012-2015

The Autodiscover algorithm for

locating the “source of information"

|Part 05#36

The Autodiscover algorithm that is used by the Autodiscover client in locating his

“source of information” (the Exchange CAS server), is based on a very sophisticated

algorithm was designed to overcome and bypass many types of “obstacles'” and

operate in different networking environments.

http://o365info.com/the-autodiscover-algorithm-for-locating-the-source-of-information-part-05-of-36


https://il.linkedin.com/in/eyaldoron1

http://o365info.com/


05#36


In the current article, we will review the Autodiscover algorithm that is used by the

Outlook client for locating the Autodiscover Endpoint (Exchange CAS server).

The source of information

The “information” that is required to Exchange clients such as Outlook is:

configuration settings needed for creating the Outlook mail profile and information

about the available Exchange web services.

The main question could be – who is the source that can provide this type of

information to the Exchange client?






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Technically, there two methods for Exchange client (such as Outlook) to get the

required information:

Local configuration file

An Autodiscover Endpoint






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1. Local configuration file

One method that Exchange client can use for “getting the required information” is –

by using a local file.

The local configuration file is a pre-configured file, which should be created and

placed in a specific path of the user desktop.

The method of using a Local configuration file is “removing” one of the most basic

characters of Autodiscover – the automation.

The method of local file could be described as “static method” because, the

information in the local file, is not automatically updated in case that the Exchange

environment changed in addition, there is no need for the Outlook mail client to

“discover” the element that will provide him the information (the Exchange CAS

server).

The method of using a local file is not recommended and most important doesn’t

support in Office 365 and Exchange Online environment.






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For this reason, we will not continue to review this option or the characters or this

method.

Note – the method of using a local file in an Exchange 2013 environment is

becoming redundant and useless because in an Exchange 2013 environment, we

don’t use a name of a specific Exchange CAS server instead, the Outlook client gets

a session ID that is dynamically created by one of the available Exchange CAS

servers.

2. Client\Server method

The Autodiscover method which can be described as “client\server method,” is

based on a mechanism, in which the client locates + address a specific server

(Autodiscover Endpoint) that will be described as the “information source” and will

be able to provide to the client the required information.

The Autodiscover Client\Server method is implemented by the Autodiscover client

in the following way:

Locate the source of information

Technically, the “source of information” doesn’t have to be an Exchange CAS

server, but most of the time, the Autodiscover information will be provided by

the Exchange CAS server.

The Autodiscover client has two ways of locating the “source of information” –

one way that is implemented by query the Active Directory and the other way, is

by query the DNS for an Autodiscover hostname provider (implemented in a non-

Active Directory environment).

Send the request for information

Given that the Autodiscover client manages to – locate + connect the

Autodiscover Endpoint, the Autodiscover sends a request for information.






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3. Server’s response

The “answer” or the Autodiscover response from the server, doesn’t have to be the

“final answer” that includes the configuration settings and the information about

the URL address of the available Exchange services.

In some scenario, the server response could include a redirection to other or

additional sources of information.

In that situation, the Autodiscover client starts the whole process over again.

When the Autodiscover process reaches to his end, the Autodiscover client will use

the configuration information for:

Creating a new Outlook mail profile

Save the information about the Exchange web service’s URL address for later

use






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How does the Exchange client locate the Exchange CAS

server?

As mentioned, the term “Autodiscover” has many meanings (the client side, the

server-side, etc.).

From the client perspective, the Autodiscover could be described as the answer for

the simple question – who is my Exchange CAS server?

The answer to the question of – who is my Exchange CAS server? Or, how to find

the required Exchange CAS server, depends on the network environment and the

configuration setting on the user desktop.






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Method 1: Autodiscover in an Active Directory environment

The Autodiscover method which described as – “Autodiscover in an Active Directory

environment”, could be only implemented in an Active Directory environment! (Yes,

I know that it’s a quite a stupid sentence).

As mention before, the Autodiscover client doesn’t know the name of the Exchange

CAS server, how many Exchange CAS servers are available and so on.

In case that the user desktop is a domain member and the Active Directory is

available and accessible, the Autodiscover client will address the Active Directory

asking for information about the names of existing Exchange CAS servers.

The Autodiscover client submits the query to the local Active Directory by using the

LDAP protocol.






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In case that the organization includes an Exchange infrastructure and the Exchange

CAS server was registered at the SCP partition at the Active Directory, the On-

Premise Active Directory, reply to the Autodiscover client query by providing him a

list of available Exchange CAS server\s.

If we want to be more accurate, the Active Directory response includes a URL

address of the local Exchange CAS server\s Autodiscover web service.

The Autodiscover client knows how to “extract” the Exchange CAS server name (the

FQDN) from the URL address and how to address the required host.

In the following diagram, we can see a simplified description of the Autodiscover

process in an Active Directory environment.

The Autodiscover connects the local Active Directory and gets a list of available

Exchange CAS server\s.

Using one of the names who appears in the list, the Autodiscover client tries to

connect the Exchange CAS server and completes the Autodiscover process.






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In our scenario, the Autodiscover process was completed successfully, and the

Exchange CAS server sends to the client the Autodiscover response, which include

the required information for the client (configuration setting needed for Outlook

mail profile and information about existing Exchange web services).

Method 2: Using Autodiscover in a non-Active Directory environment.

In case that Exchange client is not a domain member or, in case that the client

cannot access On-Premise Active Directory, the Exchange client will use a different

Autodiscover method.

In this scenario, the Autodiscover client will need to use a different method because






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he cannot use the Active Directory as a “pointer” or a source for information about

existing Exchange CAS server\s.

In a non-Active Directory environment, the Autodiscover client will need to “guess”

the name of the Exchange CAS server (the Autodiscover Endpoint).

Because the client (such as Outlook) doesn’t really know what is the name of the

Exchange CAS server, Outlook uses a method which as describe as “smart

guessing”.

The “smart guessing” process that outlook use is based on the E-mail address that

the user provides when using the outlook wizard for creating a new Outlook mail

profile.

The first step in creating a new Outlook mail profile, is the step in which we need to

provide the user E-mail address.

Outlook will take the “right part” of the recipient E-mail address, the part which

includes the SMTP domain name and use this name as the host name for the

Autodiscover Endpoint.

For example, in case that the recipient E-mail address is- [email protected]

Outlook will conclude that the Host name of the Autodiscover Endpoint is

– o365info.com






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In case that the Autodiscover client cannot find or connect the Autodiscover

Endpoint (the Exchange CAS server) using this domain name as a host name

(o365info.com in our scenario), the Autodiscover client will try to locate the “next

Autodiscover Endpoint host name”, by using the following naming convention

– Autodiscover + SMTP domain name.

In our scenario, Outlook will look for an Autodiscover Endpoint named –

autodiscover.o365info.com






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In the following diagram, we can see an example to the way that the Autodiscover

client use for “generating” or “guessing ” the name of the Autodiscover Endpoint.






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The logic of the passable Autodiscover methods

In the following section, we will review the “logic” of the Autodiscover algorithm that

is used by the Autodiscover client.

The Autodiscover journey is a quest for information.






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The “information”, could be the configuration setting and the information about the

Exchange web service’s URL or the “information” could be a “lead” or a “pointer” to

another passable source of information.






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The “Source of information” | The Autodiscover service

EndPoint.

In the next section, we will mention many times the term “source of information” or

the term: “Autodiscover Endpoint”.

This term was created for describing the “entity” or the node that the Autodiscover

client search and address for getting the required information.

Technically, the Autodiscover service endpoint, does not necessarily need to be an

Exchange CAS server.

For example, in Office 365 and Exchange Online environment, the Autodiscover

client never gets right away to “his Exchange CAS server”.

Instead, the Autodiscover client needs to “travels through” many nodes until he gets

to the destination.

This is the reason for another term that is used – Potential Autodiscover Endpoint.

We use the word – “Potential” because, the Autodiscover client can never be fully

sure that the “destination host” that he tries to connect is relaying an Autodiscover

Endpoint, or if the “destination host” is available and so on.






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Some of the “hosts” that the Autodiscover client will “meet” in his way, are just a

“logical router” that will point him to additional host and so on.

In the following section, we will review the “structure” or the logical of a couple of

passable Autodiscover scenarios, in which the Autodiscover client tries to find “his”

Exchange CAS server.

Method 1: Direct access to the information (information file)

One of the available options for Autodiscover client, is to find the required

information by accessing a local configuration file.

The configuration file format is based on the XML format.

To enable a mail client such as Outlook, to “find” the information file, we will need

to.

Create the information file.






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Save the information file locally on the user’s desktop hard drive.

Create and configure a register key that will “direct” the Outlook client to the

information file (by providing the file name and the path of the file).

The method of using a local configuration file, is used only in a rare scenario.

This method is not recommended because it’s based on a “static information” and

requires configuration setting for each of the user’s desktops.

The method of using a local configuration file is not supported in Office 365 and

Exchange Online environment.

Additional reading

You can read more information about the option of using a local configuration file

by using the following link:

Controlling Outlook Autodiscover behavior

Method 2: using the host name of the “information provider”





http://blogs.technet.com/b/kristinw/archive/2013/04/19/controlling-outlook-autodiscover-behavior.aspx


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The method which I describe as – “using the host name of the information provider”

is implemented in scenarios in which the mail client such as Outlook, has a

“preliminary knowledge” about the name of the Exchange CAS server.

This scenario made possible in case that the “standard Autodiscover process” was

implemented earlier and the Outlook client managed to get the name of the

Exchange CAS server, connect the Exchange CAS server and verify his identity.

There are two passable scenarios in which Outlook addresses the Exchange CAS

server using his name instead of starting a standard Autodiscover process:

Scenario 1: an existing Outlook mail profile

In case that we have already configured an Outlook mail profile, as long as the

Outlook client is active, Outlook will try to access the Exchange CAS server on a on

an hourly basis

(Every hour), to check with the Exchange server about passable updates or changes

in the information that was formerly provided by the Exchange CAS server.

In this scenario, Outlook will use the Exchange CAS server name or the session ID

(in a scenario that the Exchange environment is based on Exchange 2013

architecture) that was saved in a cache.

This method, is used by the client such as Outlook for “refreshing purpose.”

After the Outlook client finds the required server and get the required information,

Outlook saves the server name (Exchange CAS server name) and periodically

connects the Exchange server to ask for existing changes or updates to the

information.

Scenario 2: Outlook 2013 client and the feature of Cached URL in the Outlook

profile

The option of “Cached URL” in the Outlook profile is available only when using

Outlook 2013 version.

The interesting thing is that there is almost no information about this Outlook 2013

feature.

To the best of my knowledge, the father of “Cached URL in the Outlook profile”






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enable the Outlook client to “re use” a name of an Exchange CAS server in case that

Autodiscover process fails to find the required Exchange server name.

The option of “Cached URL” can be implemented only in a scenario in which the

past, Outlook succeeds to complete the Autodiscover process. The meaning is that

Outlook saves the name of the Exchange CAS server or the session ID in the

registry.

In case that Outlook client cannot locate the required Exchange CAS server, Outlook

2013 client will “fetch” the name of the Exchange CAS server from the registry.

Note – we can disable this option by using creating a new registry key named-

ExcludeLastKnownGoodURL

Additional reading

Outlook 2013 Autodiscover Policy Not Working?

How to turn off auto-discovery in outlook 2013 when use office 365 account?

Exchange & The Autodiscover Web Service

Method 3: address a trusted host, for getting a list of “information providers”

This Autodiscover method, is implemented in an Active Directory environment.

The concept of this Autodiscover method is to address a “trusted element”, that will





https://social.technet.microsoft.com/Forums/office/en-US/6fce3433-15a9-41dc-afdd-169ca678aa77/outlook-2013-autodiscover-policy-not-working?forum=outlook

http://community.office365.com/en-us/f/172/t/213703.aspx

http://blogs.technet.com/b/rmilne/archive/2011/10/21/exchange-amp-the-autodiscover-web-service.aspx


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guide us to our required destination, is very similar to the scenario in which we are

visiting an extremely big mall, which has many stores within.

We look for a famous jeans store, but, we don’t know how to get there.

In this case, we can use a directory or ask for instructions to the mail information

office for a direction for this jeans store.

Going back to our Exchange environment, this method is used by a client such as

Outlook, when the client workstation is a part of a domain and, the client can

connect the local Active Directory.

To be able to get the host names of available Exchange CAS server\s, the mail client

addresses an “element” which has information about the available Exchange CAS

server\s.

This “element,” is the local On-Premises Active directory.






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The mail client submits an LDAP query to the Active directory and request

information about the names of available Exchange CAS server’s.

Method 4: Generate the NAME of the “information Provider”

In a non-Active Directory environment, the Exchange mail client such as Outlook,

doesn’t have any “element” that can provide him information about available

Exchange CAS server\s (Autodiscover Endpoints).

For this reason, the Exchange client will need to use another method for locating

available Exchange CAS server\s.

The Autodiscover method that the Exchange client use, is based on a very

interesting concept.






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The Autodiscover method that Exchange client use, in non-Active Directory

environment is based on a method in which the client “guess” an optional name of

the Exchange CAS server (the Autodiscover Endpoint).

When a user provides his E-Mail address (when creating a new Outlook mail profile,

for example), the Outlook “takes” the SMTP domain name and creates a DNS query

looking for the IP address of the SMTP domain name.

In other words, the Autodiscover client, doesn’t know what the name of the

“information provider” is (Exchange server) until the user, provide his email

address.






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Method 5: Redirection or pointer to another “information Provider”

As mentioned before, the Autodiscover process doesn’t always implement as a “one

to one” process. In an enterprise environment or, in Office 365 and Exchange

Online environment, the structure of the Exchange CAS servers is quite

complicated.






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In these environments, the Autodiscover client will need to “jump” between a

couple of nodes, until he gets to his destination.

For example, in an Office 365 based environment, most of the Autodiscover Journey

is based on a concept in which the Autodiscover client locates + connect to an

Autodiscover Endpoint but the “answer” of this host will not include the required

configuration settings but instead, a name (FQDN or URL) of additional Potential

Autodiscover Endpoint.





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