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<!-- Add as first element within root element <SPConfig> of shibboleth2.xml -->
<OutOfProcess>
    <Extensions>
        <Library path="plugins.so" fatal="true"/>
    </Extensions>
</OutOfProcess>

<InProcess>
    <Extensions>
        <Library path="plugins-lite.so" fatal="true"/>
    </Extensions>
</InProcess>

[...]

<!-- Add at end of <Sessions> element -->
<Handler type="AttributeResolver" Location="/AttributeResolver"
         acl="127.0.0.1 ::1 130.59.0.0/16 2001:620:0:44::/64" />

Once the AttributeResolver handler is configured, it can for example be queried (e.g. from localhost) with:
$ curl -k 'https://localhost/Shibboleth.sso/AttributeResolver?format=urn:oasis:names:tc:SAML:2.0:nameid-format:persistent&entityID=https%3A%2F%2Faai-logon.switch.ch%2Fidp%2Fshibbolethhttps%3A%2F%2Fyour.idp.example.org%2Fidp%2Fshibboleth&nameId=123456789PfvsH8k4gvHoeq6QtM=

This will return a JSON data structure like:
{
    "displayName" : [

        "Lukas Hämmerle"
    ],
    "mail" : [

        "lukas.haemmerle@switch.ch"
    ],
    "schacHomeOrganization" : [

        "switch.ch"
    ],
    "persistent-id" : [

        "https://aai-logon.switch.chyour.idp.example.org/idp/shibboleth!https://test.eduidsp.example.chorg/shibboleth!1FmaGeMauPfvsH8k4gvHoeq6QtM="
    ],
    "idp" : [

        "https://aai-logon.switch.chyour.idp.example.org/idp/shibboleth"
    ]

}