I'm creating an OpenId Connect host using IdentityServer v3, backed by ASP.NET Identity.
In cases where the user is already logged in to the host application using ASP.NET Identity, how can I use the existing session, rather than prompting the user to log in again?
An example is if you are already logged in to your Google account, and you navigate to https://jabbr.net/, you will find that Google does not prompt you to enter your username and password in order to authenticate through Google.
IdentityServer will not prompt your for credentials when there is an existing authentication session - IOW a cookie. Check if this cookie is present or maybe lost somehow. By default the cookie is only valid as long as the browser window is not closed.
IdSrv3 beta 2 also has a persistent cookie feature now -
https://identityserver.github.io/Documentation/docs/configuration/authenticationOptions.html