The default value for the attribute is uid and if you want to change that then you should use the user_search.filter as stated in the documentation, instead of user_search.attribute (which I mistakenly suggested above) since it is deprecated.
I will not be able to help you much further if I don't have a view of how your user objects are stored in your LDAP directory. Can you share an example of a user located under DC=rd,DC=net so that I can guide you to change your realm settings ?