First published on TECHNET on Apr 13, 2018
Scenario: SFB Hybrid environment, SFB user is homed Online, ADFS is Configured, MA (Modern Auth) is Disabled ON premise but is Enabled in O365

NOTE:

I have tried my best to ensure the information below is accurate. Some of the terms I use to describe things like Modern Auth provider, O365 AD, Org ID etc. may not be standard terminology, I use them solely to make the understanding simpler. My intention here is to explain what happens in the background when a SFB client signs in so that it helps engineers and customers troubleshooting issues related to Sign in and Authentication.



How does it Work?

Detailed Explanation of SFB online Client Sign in process with LOG Snippets:



SIP URI of the user - test2@sfbisgreat.info

  1. SFB client Queries DNS for Lyncdiscover.domain.com. This should point to External web services URL (ON Premise Reverse Proxy) which in this case is webext.cloudsfb.com

  2. SFB Client then sends a unauthenticated GET request to Lyncdiscover.domain.com

  3. The Client is then redirected to Autodiscover

  4. SFB Client then sends a Request to Autodiscover to discover its pool for sign in.

  5. The Client is then challenged and is provided the URL for Webticket service where it can request a Webticket

  6. The Client then sends a POST request to Webticket Service

  7. Here the Client has to Authenticate (NTLM). The Client may receive a Password prompt (or previously saved password from credential manager is passed)

  8. Once the password is provided, Webticket service will issue a Webticket to the client.

  9. The client then submits this webticket to Autodiscover

  10. Since the SFB user is homed Online, In Response Autodiscover will provide the Online Autodiscover webservices URL's names ( https://webdir2a.online.lync.com/Autodiscover/AutodiscoverService.svc/root?originalDomain=sfbisgreat.info )

  11. SFB Client then sends a Request to Autodiscover to discover its pool for sign in.

  12. The Client is then challenged and is provided the URL for Webticket service where it can request a Webticket

  13. The Client then sends a POST request to Webticket Service

  14. Webticket Service Redirects the Client to Modern Auth Provider (login.windows.net)

  15. Now in order to authenticate the client reaches out to Login.windows.net and requests a Token, The intention here is to Get a Token from login.windows.net

  16. From this point onwards we will see that login.windows.net will redirect the client to login.microsoftonline.com

  17. Since the tenant is enabled for ADFS the client is then redirected to the ON Premise ADFS server

  18. SFB client will then send a request to ADFS server and request a token

  19. The Client may receive a Password prompt (or previously saved password from credential manager is passed) and once the correct password is provided, ADFS will issue a Token to the client

  20. The Client then submits this token to login.microsoftonline.com which in turn passes the client to Login.windows.net

  21. Login.windows.net will now issue the Modern Auth Token to the client

  22. The Client then submits this token that it received from Login.windows.net to Webticket Service

  23. Webticket service now will grant a Webticket to the Client

  24. The client then submits this webticket to Autodiscover

  25. In Response Autodiscover will provide the Pool names (sipfed2a.online.lync.com" port="443) where the client can send Register to Sign in

  26. The SFB client now sends a SIP register to the Online Edge pool (sipfed2a.online.lync.com" port="443)

  27. It is then challenged for authentication again, here the ONLY supported method of authentication is TLS-DSK, The client is provided a Cert provisioning URL ( https://webdir2a.online.lync.com:443/CertProv/CertProvisioningService.svc ) in the 401 unauthenticated response

  28. The SFB client then sends a request to Certprov

  29. Here again the Client is challenged for authentication and is redirected to webticket service to get Webticket

  30. The Client had already Obtained a webticket in step 23 above

  31. The client will submit the same webticket obtained in step 23 to the Cert provisioning service

  32. The Client then receives a certificate

  33. The SFB client can now send a Register again and use the certificate it downloaded for authentication




Below is a graphical representation of the SFB online Client Sign in process





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When a SFB client wants to Sign in, It needs to know where it can send its request to be able to Sign in. Whenever a user enters his SIP URI to sign in the SFB client forms an autodiscover URL using the domain name that it extracts from the users SIP URI to start the discovery process and then it sends an Unauthenticated Get request to the URL, lyncdiscover.domain.com. The response code for this request will be '200 ok' and in the response we should receive the external webservices URL for autodiscover.

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The SFB Client learns that it needs to Contact https://webext.sfbisgreat.info/ (This is the External webservices URL for autodiscover on the ON Premise SFB environment)

It then tries to Do a TCP handshake with webext.cloudsfb.com, Followed by a TLS handshake. (I haven't included the TCP and TLS handshake screen shots here, you can see those if you collect a Network trace while signing in)

The client then sends a request to the user URL. We are here trying to discover a specific users home pool, hence the request will go to the “User” URL.

In the response, the Client receives a Web ticket URL, which provides the location of the WebTicketService.

You can see the request and Response below

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The Client then needs to send a Request to the Web ticket service URL in order to obtain a Web ticket. The client will send this request in a POST message to the web ticket Service. In response it will receive a 401 and the authentication method supported is listed as NTLM, You can see that in the screen shot below

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The Next few exchanges between the Client and Webticket service will be for NTLM Challenge and response. You will see multiple POST requests and 401 between the Client and Webticket service

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Finally the client will provide the password (the user may get prompted to enter their password here or the existing password in Credential manager might be used)

Once the correct password and username is provided the Webticket service will issue a Web Ticket to the Client. You can see that below

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The Client will Then Submit this web ticket back to the AutoDiscover User URL - https://webext.sfbisgreat.info/Autodiscover/AutodiscoverService.svc/root/user?originalDomain=sfbisgreat.info&sipuri=test2@sfbisgreat.info

In response it will now receive the Internal and External addresses of the Pool names where the user is Homed.

You can see this in the trace below

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Now the Client will send a Unauthenticated Get request to Webdir2a.online.lync.com and in Response it receives the Autodiscover URL's specific to the users Tenant. You can see the request and Response below

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The client then sends a request to the user URL. We are here trying to discover a specific users home pool, hence the request will go to the “User” URL.

In the response, the Client receives a Web ticket URL, which provides the location of the WebTicketService.

You can see the request and Response below

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The Client then needs to send a Request to the Web ticket service URL in order to obtain a Web ticket. The client will send this request in a POST message to the web ticket Service. Now since Modern Authentication is enabled on the Tenant, in order to grant the webticket the client will first need to get a Token from the Modern Auth provider so the client is redirected to the Modern Auth provider URL - <af:OAuth af:authorizationUri="https://login.windows.net/common/oauth2/authorize" xmlns:af="urn:component:Microsoft.Rtc.WebAuthentication.2010" />

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The Client then sends a request to the MA/Oauth URL to request a Token, The intention here is to Get a Token from login.windows.net

From this point onwards we will see that login.windows.net will redirect the client to - login.microsoftonline.com.

Below is the Request that client sends to the MA/OAUTH URL and in response it is redirected to AD - login.microsoftonline.com

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We have to remember that "The intention here is to Get a Token from login.windows.net" we will see several exchanges happening between client to login.microsoftonline.com and login.windows.net. Below are screen shots showing these exchanges.

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Now, Since the customer has ADFS, the client will now be redirected to the ADFS Server. Below is the screen shot showing login.microsoftonline.com redirecting the client to ADFS

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The Client will then reach out to ADFS to get an ADFS Token. The Next Two Screen shots show that;

(This is where the user might get prompted to enter credentials or if his credentials are already stored in credential manager then those credentials will be passed in the background and the user may not see the prompt)

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The Client will then Submit this Token to Login.microsoftonline.com, where it will be redirected again to https://Login.windows.net and https://Login.windows.net will finally provide the client with the Modern Auth Token, This is shown in the two screen shots below

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Now the client will submit this token to the webticket URL, and the Webticket service will issue the webticket, Shown below

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The client will then submit this webticket to Autodiscover and in return it will receive the POOL names where it has to send the Register to Sign in.

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Once the Client receives the pool names it will then Send a SIP REGISTER message to the SFB pool in order to sign in. . You can see that in the Client UCCAPI log file. This is shown in the snippet below

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In response the Client will now receive a 401 Unauthorized message again and the server will again ask the client to authenticate itself. Here the ONLY method of authentication that is available is TLS-DSK (Cert based authentication)

The SFB online server will provide the Client a Cert provisioning URL in the 401 you can see that in the snippet below

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This means that the Client now needs to present a Certificate that can then be used to authenticate the client. Since this is the first time the client is signing in it will NOT have the certificate installed. This certificate is ideally downloaded after the client signs in for the first time and is valid for about 8 hours.

Since the client does not have a valid certificate it now has to Re-Authenticate to the Cert provisioning service.

The Process for this will again be the same, The client will send a request to the Cert Provisioning URL where it will be challenged to get a Webticket. The client has to first get a Web ticket from the webticket service URL, to get a web ticket it needs to get a Token from Modern Auth Provider, but we know that the client has already done these steps earlier. SO it already has a Web Ticket from the Web services URL. The Client needs to submit this same web ticket that it had obtained to the Cert provisioning Service and once it submits the web ticket it will serve as a proof of authentication.

The Client learns about this by first sending a Mex request to the Cert provisioning URL. You can see that in the Trace below

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The Client then submits the Web Ticket that it had received previously to the Cert provisioning URL it received above, after this it receives a 200 OK in which it receives the Certificate

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The clients will then submit this certificate back to the pool and will receive a 200 OK in response. The Sign in is then complete

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The Sign in is now Complete