def load_host_keys(filename):
    """
    Read a file of known SSH host keys, in the format used by openssh, and
    return a compound dict of ``hostname -> keytype ->`` `PKey
    <paramiko.pkey.PKey>`. The hostname may be an IP address or DNS name.  The
    keytype will be either ``"ssh-rsa"`` or ``"ssh-dss"``.

    This type of file unfortunately doesn't exist on Windows, but on posix,
    it will usually be stored in ``os.path.expanduser("~/.ssh/known_hosts")``.

    Since 1.5.3, this is just a wrapper around `.HostKeys`.

    :param str filename: name of the file to read host keys from
    :return:
        nested dict of `.PKey` objects, indexed by hostname and then keytype
    """
    from paramiko.hostkeys import HostKeys
    return HostKeys(filename)
