1. Username and password input fields in ranger should be the ldap user and password authenticating through knox.
2. Configure the admin topology to meet your configuration needs by default it points to the demo ldap server on knox server.
3. To test manually, login to the URL directly using ldap credentials from the ranger server. https://<knoxhost>:8443/gateway/admin/api/v1/topologies
4. Make sure the ranger server trusts the knox certificate being presented; if it doesn't; you will need to add the knox gateway trust certificate to the ranger/java truststore.
5. Currently knox service lookups don't work in ranger and a bug has been filed to fix this. Currently, as of HDP 2.4, only topology lookup in ranger works.