Topic Last Modified: 2011-05-06
A trusted user is one whose credentials have been authenticated by a trusted server in Microsoft Lync Server 2010. This server is usually a Standard Edition server, Enterprise Edition Front End Server, or Director. Lync Server 2010 relies on Active Directory Domain Services as the single, trusted back-end repository of user credentials.
Authentication is the provision of user credentials to a trusted server. Lync Server 2010 uses the following authentication protocols, depending on the status and location of the user.
- MIT Kerberos version 5 security protocol for internal
users with Active Directory credentials. Kerberos requires client
connectivity to Active Directory Domain Services, which is why it
cannot be used for authenticating clients outside the corporate
- NTLM protocol for users with Active Directory
credentials who are connecting from an endpoint outside the
corporate firewall. The Access Edge service passes logon requests
to a Director, if present, or a Front End Server for
authentication. The Access Edge service itself performs no
Note: NTLM protocol offers weaker attack protection than Kerberos, so some organizations minimize usage of NTLM. As a result, access to Lync Server 2010 might be restricted to internal or clients connected through a VPN or DirectAccess connection.
- Digest protocol for so-called anonymous users. Anonymous
users are outside users who do not have recognized Active Directory
credentials but who have been invited to an on-premises conference
and possess a valid conference key. Digest authentication is not
used for other client interactions.
Lync Server 2010 authentication consists of two phases:
- A security association is established between the client and
- The client and server use the existing security association to
sign messages that they send and to verify the messages they
receive. Unauthenticated messages from a client are not accepted
when authentication is enabled on the server.
User trust is attached to each message that originates from a user, not to the user identity itself. The server checks each message for valid user credentials. If the user credentials are valid, the message is unchallenged not only by the first server to receive it but by all other servers in the trusted server cloud.
Users with valid credentials issued by a federated partner are trusted but optionally prevented by additional constraints from enjoying the full range of privileges accorded to internal users.
The ICE and TURN protocols also use the Digest challenge as described in the IETF TURN RFC. For details, see Media Traversal.
Client certificates provide an alternate way for users to be authenticated by Lync Server 2010. Instead of providing a user name and password, users have a certificate and the private key corresponding to the certificate that is required to resolve a cryptographic challenge. (This certificate must have a subject name or subject alternative name that identifies the user and must be issued by a Root CA that is trusted by servers running Lync Server 2010, be within the certificate’s validity period, and not have been revoked.) To be authenticated, users only need to type in a personal identification number (PIN). Certificates are particularly useful for smart-card access and for telephones and other devices running Microsoft Lync 2010 Phone Edition where it is difficult to enter a user name and/or password.