Expired session not posting saml after idp redirect

[Peter Schober]

* JamesP <jcparsons at gmail.com> [2018-10-16 06:06]:
> Our realm on the IdP does not have an application location/URL set
> as we are hosting multiple apps on the same apache proxy host.  so
> the vhost servername that redirects to the IdP, is what the IdP uses
> for the redirect back
> 
> any ideas on why it's not posting back to the
> /Shibboleth.sso/SAML2/POST?

I don't understand most of what you wrote but did you trace all these
messages from the resource/SP to the IDP and back in detail,
e.g. using some browser function or the SAML tracer extension? I.e.,
are you sure that's what happening?

If the SP sends a SAML 2.0 authentication request to the IDP and the
IDP literally replies with a redirect to the SP's resource then the
IDP would be broken: An IDP only deals with SAML protocol messages, as
far as any SP is concerned. In reply to an authn request it would only
send a SAML response to the requested (or defaulted) Assertion
Consumer Service URL of the SP. It's the SP that would make any final
redirects to the resource.

What implementation is that SAML IDP?

-peter