For a project as part of the European Grid Infrastructure (EGI) we need SSL client certificate verification for a service running on nginx. As there are several root CAs allowed within EGI, we need nginx to check them all during client certificate validation. In the documentation of nginx I could only find the parameter ssl_client_certificate which allows to specify just one file containing a root certificate.

Is there a way to specify more than one root CA for client certificate verification in nginx or do I have to use Apache for this?