I've just configured Shibboleth IdP3.2 with my web application that authenticates the users against an LDAP server at the backend.
I could test this authentication process at the local machine. But, while deploying the code on CI server, I realized that the authentication process could not be completed successfully.
The reason behind this failure is that the Service Provider (SP) cannot access the (IdP). From our initial investigation, We chose SAML as the authentication protocol over other protocols like CAS because it did not need a back channel communication. As long as the user has access to both SP and IdP,the authentication process would still work.(SP and IdP need not interact with each other)
On testing we found that the attribute resolution is successful, but the subsequent artifact resolution is failing. In artifact resolution, IdP directly contacts the SP and expects a response. SP cannot send a response to IdP as it is inaccessible. Hence, the authentication fails. (Tomcat logs show: unknownHostException)
Some SAML flows in Web Browser SSO do not require direct communication between SP and IdP as seen from flow diagram in this link.
Does Shibboleth IdP make provisions for such implementations? Is there a work around for implementing Shibboleth IdP without any back channel communication?
SOLUTION:
As Stefan mentioned, there are alternative bindings like HTTP-Redirect and HTTP-POST that do not use back channel communication. You can read more about these bindings here
I changed the SP metadata to make HTTP-POST as the default binding, referring this link.
I did not have to make any changes to Shibboleth IdP configuration as these alternative bindings were already being supported, as substantiated by the metadata file.
