All OpenID Providers publish their details at a publicly discoverable (and standard) domain: https://{hostname}/.well-known/openid-configuration.
For instance, you can see our OP meta data here [3].
This provides the foundation for using email as an identifier, i.e. in order to access protected resource at autonomous site, input email at a domain with an OP, and the RP can perform discovery to find where to send the user for authentication, and dynamic registration to register their client (app) with the OP to obtain user information ("claims").
[1] https://openid.net/specs/openid-connect-discovery-1_0.html
[2] https://openid.net/specs/openid-connect-registration-1_0.htm...
When you login to google, it prompts you for an email address first. If your email is associated with an organization that has configured Google Apps to use their OP for authentication, Google will redirect the user to their home domain based on the email.
Do you e-mails end with @idp.gluu.org? Or how would the RP discover that the domain is not "gluu.org" but "idp.gluu.org"?
https://accounts.google.com/.well-known/openid-configuration
No, no they don’t. Not by far. Google, for example, doesn’t - and even if they did, it wouldn’t be useful, as they don’t support dynamic client registration either, as they want lock-in. Being able to type in my email address into a generic widget and get the Google auth dialog is specifically what they don’t want.
Facebook has the same issue, as does Yahoo. I don’t think I know of a single implementer of OpenID Discovery and Dynamic Client Registration - the only purpose of OpenID Connect as deployed in the wild is to share development resources, not to create a system where people can type in their email address into a generic widget which works for every OpenID Connect supporting domain off the shelf with no RP-side configuration and get a login form.
See here:
https://accounts.google.com/.well-known/openid-configuration
And as far as I know, Facebook doesn't support OpenID Connect. They still roll their own custom OAuth2 implementation.