shibboleth SP - how to customize HTTP redirect request?

[Peter Schober]

* Paolo Veronesi <paolo.veronesi at cup2000.it> [2016-07-29 14:20]:
> Is it possible to change the HTTP redirect request created by the SP in
> order to append a custom attribute (customAttr=value) with the value
> selected in a form by a user?

The "HTTP redirect request" is specified in the SAML standard, and I
don't think it's allowed to add arbitrary other things to standardised
protocol messages.
You can extend the SAML Authentication Request, though, that's the
payload of that redirect.

> This attribute should be used by the gateway to skip the default
> idp's chooser page and redirect the user to the appropriate IdP's
> login page.

SAML provides a standard mechanism for that, AFAIR,
cf. SimpleSAMLphp's documentation on that:
https://simplesamlphp.org/docs/stable/simplesamlphp-scoping

I don't know whether or how the Shibboleth SP supports that, but if
you're not signing authentiaction requests from your SP you can
generate the authnrequest yourself any way you like, of course.
-peter