Skype for Business Desktop client sign in issue is one of the most common scenario for Helpdesk or Admins or Support Folks who are working in Messaging or Unified communication field. While there are lot of awesome blogs right from the OCS Days explaining about the client sign in call flow, troubleshooting, Log Analysis and etc. I always use to prefer my OneNote page created by taking bits and pieces from different places that covers all these details. I thought sharing the info here might help in getting all the details in one go.

Sign in with client certificate-based authentication Users can now sign into Skype for Business on Mac using client certificate-based authentication when required by their organization. Hide and show meeting content Users can choose to hide or show content that another user is sharing in a meeting. We're implementing Skype for Business and we've a mixture of Windows and Mac clients. We're able to easily add a GPO for the Windows clients to mark Lync traffic with DSCP tags for QoS, but I can't find any documentation or guidance on how to do the same for Skype for Business on MacOS.

Microsoft yesterday released the April update for Skype for Business on Mac. Skype for Business on Mac is the all new client that provides great communication experiences for Apple users with features like like one-click join, edge-to-edge video, and full screen sharing give you a superior Skype Meetings experience. Microsoft is committed to enhancing the Skype for Business on Mac client at a regular pace. To read more about these updates, see (and save as a favorite!) our What’s New Office article. We have made some changes to how Single/Tabbed Windows now work in the client – in some cases we allowed.

Before entering the troubleshooting phase, one should first understand the Skype for Business Client Sign in process flow to identity what’s expected and act accordingly. In this article, we will focus mainly on the Call flow when Skype for business Desktop Client login.

For simplicity, we could divide the entire Skype for Business Client Sign in process into below 5 steps:

Server Discovery
Connectivity Checks
Authentication
Optional Redirection
Retrieve Settings and Policies

1 : Server Discovery

Skype for Business Client is hardcoded to query certain DNS records to locate the Skype for business server information, which is required for Automatic Client sign in, below are the list of DNS records that client would query in order for Server discovery.

Lyncdiscover Records

Lyncdiscoverinternal.contoso.com

Lyncdiscover.contoso.com

SRV Records

_sipinternaltls._tcp.contoso.com

_sip._tls.contoso.com

DNS A Records

Sip.domain.com

Sipinternal.domain.com

Sipexternal.domain.com

At the End of this step, if we have DNS Records configured, skype for business client will get the FQDN/IP Address & Port combination of Skype for business server where it can reach to login.

2 : Connectivity Checks

Once Skype for Business client identified the Server Information, Client performs Network Connectivity checks to see if it can reach the server on identified IP address & Port combination and also it verifies if it can establish a TLS secure connection to the FQDN that it got in first step.

Port Connectivity Checks

Client attempts Network connectivity check to see if it can reach server on required port

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In Networking terms this is termed as TCP 3 Way Handshake [ACK–SYNC–ACK]

TLS connectivity Checks

Client attempts to check if it can establish a secure connection with the server

In this, basically client will check if the certificate presented by server is being trusted on client or not; and it also includes Cipher Selection.

In order for Client to be able to trust the presented certificate, client should have the Root CA Cert of the Certification authority that has issued the certificate to the server in its Certificate Trusted Root Store.

3 : Authentication

This is the actual step where client interacts with the Skype for business server using SIP protocol and authenticates itself. Overall process involves Client learning the set of supported Authentication mechanism on the Skype for Business Registrar Servers and Selecting appropriate Authentication methods and getting authenticated.

Firstly, Client sends an Unauthenticated REGISTER Request to the Skype for Business Server:

In response to this REGISTER request, Skype for Business Server would send the list of Authentication mechanisms available for Authentication in 401 Unauthorized:

Client would then select one of the authentication methods and gets authenticated (depending on whether signing in internally or externally, first time sign in or subsequent sign in). By default, Skype for Business Registrar Configurations has below 3 Authentications enabled:

If Client is signing in Internally, then all 3 above listed Authentication methods [Kerberos/NTLM/TLS-DSK] will be available.

If Client is signing in Externally, then only 2 authentication methods will be available [NTLM/TLS-DSK] will be available.

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If client uses Kerberos authentication

This is the default one that client uses internally during first time sign in I.e. when they don’t have user certificate to sign in using TLS DSK.

Client would reach out to AD Server and gets authentication ticket (Kerberos ticket) for accessing service on Skype for Business Server. Once it gets the Kerberos ticket, it submits that to Server in next REGISTER request, and server would authenticate the user and signs the user.(In Kerberos method, there is an interaction between clients and AD Servers, this is the primary reason why Kerberos Authentication isn't available for Remote Sign in)

When using Kerberos, in client side logs we will see 2 REGISTER Request/Responses between client and the skype server. https://ratingbrown.amebaownd.com/posts/15472715.

If client uses NTLM Authentication

This is the default one that client uses externally during first time sign in I.e. when they don’t have user certificate to sign in using TLS DSK

Client would send information/details required for authentication in the next REGISTER Requests to the skype for business server, skype for business server in turn talks to AD Server and validates the submitted information/details.

If the Validation succeeds then, skype for business server would consider user authentication as valid/genuine and signs the user. (In NTLM method, All the interaction is between client and the Skype for business server and Skype for business Server to Active directory, but no interaction from client directly with Active Directory)

Skype For Mac Client