In certain cases it may be desirable to have some users authenticate in the standard Salesforce manner, whilst another user population authenticates via single sign-on (SSO). This mixed authentication scheme scenario may result from rollouts to new departments etc. What are the considerations?
The first concern is whether Delegated or Federated Authentication will be used for the SSO population.
With Delegated Authentication (DA), a mixed scheme is straightforward as the SSO configuration is applied through a User Profile setting, enabling a subset of users to have their authentication request routed to the DA on-premise authentication web service. The remaining users will simply login in the standard Salesforce manner.
With Federated Authentication (FA) via SAML 2.0, a mixed scheme is not so trivial. There is no User Profile setting or indeed any user-level setting controlling whether a user is SSO-enabled or not. Instead SSO is inferred from the manner by which the authentication is approached. If the user accesses https://login.salesforce.com and attempts to sign-in they will be authenticated as a Salesforce user, no SSO. With the identity provider initiated SAML flow, the user is handed-across from the IdP in a pseudo-authenticated state, if the service provider (i.e. Salesforce) can validate the SAML assertion and map the user to an active Salesforce user (via username or Federation Id) then authentication takes place. So the IdP initiated flow poses no problems in a mixed scheme, standard Salesforce users authenticate as normal, SSO users authenticate via the IdP (intranet link etc.).
So far so good. But what if the client also needs to support the service provider initiated SAML flow in a mixed scheme? Perhaps Chatter Desktop or Mobile is being rolled out.
The service provider initiated flow requires a My Domain to be configured in the Salesforce org e.g. https://force365.my.salesforce.com, this enables anonymous authentication attempts using the My Domain to be mapped to the correct SSO configuration and routed to the IdP. Which works perfectly for SAML enabled clients and deep-links. For the standard users in the org we now have a problem – all unauthenticated references to resources based on the My Domain will take the user to the IdP to authenticate. So sharing of links via email etc. will require the standard user to have pre-existing session, otherwise they will be presented with an unfamiliar login challenge (Active Directory Forms Authentication perhaps) or worse a login failure in the integrated case. In theory this could be resolved by ensuring the standard users are correctly configured for authentication both at the IdP and Salesforce, enabling SSO authentication to occur where required.