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I configured SPUEdit in ezproxy.cfg to use the SP's SessionInitiator. In our environment, as I suspect in most environments, a single ezproxy instance serves one institution, and so I hard-coded the Identity Provider's providerId into the SPUEdit directive. I tested with three different providerIds.
SPUEdit @^https@^https://biblos\.umd\.edu:4243/.*$@https://biblos.umd.edu:4243/Shibboleth.sso/Login?providerId=urn:usmai:cp:logintest.lib.umd.edu&target=$e0@irs
Then, I created simple ezproxy urls to reach my endpoints:
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