# Somehow lookup USERID given SSL_CLIENT_S_DN
RequestHeader set X-User-ID %{USERID}

SetEnvIf SSL_CLIENT_S_DN [User 1's SSL_CLIENT_S_DN] USERID=12
SetEnvIf SSL_CLIENT_S_DN [User 2's SSL_CLIENT_S_DN] USERID=34
SetEnvIf SSL_CLIENT_S_DN [User 3's SSL_CLIENT_S_DN] USERID=56
...