My company uses an off-site LDAP server to handle authentication. I'm currently working on a local VPN for my branch that needs to use the off-site LDAP to check user's usernames and passwords, but I don't want every employee to have access to the VPN - I need to be able to control whether users can authenticate with the off-site LDAP based on whether they're allowed to use the VPN. My current solution involves having our own local LDAP server, which has a referral to the off-site server (I got most of my information from here: http://www.zytrax.com/books/ldap/ch7/referrals.html). This means that when local users try to check their credentials with the local server, it redirects them to the off-site server, which checks the credentials. This works for authentication, but not for authorization. It would be easiest to add a vpn_users group or is_vpn_user attribute on the off-site server, but, well, that's above my pay grade. Is there any way I can use the local server to control whether users have access to the VPN without needing to change the off-site server? If I could somehow use it to have a local vpn_users group without the users in it having to be located on the local server, that would probably work, but I have no idea how to set that up or if LDAP even supports such a configuration.

For reference, I'm using the openvpn-auth-ldap (https://code.google.com/p/openvpn-auth-ldap/) plugin.