Multiple IdPs without discovery

[Ted O'Connor]

I am currently running a 2.5.3 SP on Apache that handles requests from
multiple subdomains (a.example.com, b.example.com and c.example.com).  All
the session initiation is started by sending the users to the "/login" path
like this:

<Location /login>
  AuthType shibboleth
  ShibRequestSetting requireSession On
  require valid-user
</Location>

I have multiple IdPs but they are each only valid for one of the
subdomains.  Is there some way to force an IdP based on the domain without
prompting the user?

I tried setting the entityID query string parameter but that doesn't seem
to propagate through the Apache location.  e.g.
https://a.example.com/login?entityID=https%3A%2F%2Ffoo

Calling the Shibboleth.sso/Login path directly with the query string takes
me to the correct IdP but then doesn't return me to my /login location.

Thanks.
Ted
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