OpenLDAP: How to use backend LDAP server only to authenticate user from frontend ldap?

I have users in OpenLDAP server. They are manually managed. dn: cn=alice,ou=contoso,dc=combined,dc=internal mail: alice@contoso.com dn: cn=bob,ou=fabrikam,dc=combined,dc=internal mail: bob@fabrikam.com It's DN says that user is from company fabrikam or contoso. When user makes simple bind to OpenLDAP server, I want OpenLDAP to * select backend server for this user based on his DN: * * ou=contoso,... => backend ldap server is 192.168.1.11 * * ou=fabrikam,... => backend ldap server is 192.168.1.12 * search for user within backend server by mail attribute: (&(mail={orig.mail})(objectClass=user)) to get DN of this user from backend server (back_dn) * try to bind as back_dn to backend server using password provided by user and return success or error I can't find any working example of this, or even something around it, please, help me...