def validate_root_domain(items, root_domains):
    """ Filters a list of potential FQDN's by cross checking the domain with IANA's list of valid root domains.

    :param items: List of FQDN strings
    :param root_domains: Path to the root domains file.
    :return: A filtered List of FQDN strings
    """

    valid = []
    with open(root_domains, 'r') as f:
        roots = [i.lower().strip('\n') for i in f]
        for hostname in items:
            root = hostname.split('.')[-1]
            if root in roots:
                valid.append(hostname)
        del roots
    del items
    return valid