This authentication method operates similarly to
    password except that it uses BSD Authentication
    to verify the password. BSD Authentication is used only
    to validate user name/password pairs. Therefore the user's role must
    already exist in the database before BSD Authentication can be used
    for authentication. The BSD Authentication framework is currently
    only available on OpenBSD.
   
    BSD Authentication in PostgreSQL uses
    the auth-postgresql login type and authenticates with
    the postgresql login class if that's defined
    in login.conf. By default that login class does not
    exist, and PostgreSQL will use the default login class.
   
     To use BSD Authentication, the PostgreSQL user account (that is, the
     operating system user running the server) must first be added to
     the auth group.  The auth group
     exists by default on OpenBSD systems.