Sanitize host string.
    https://tools.ietf.org/html/rfc1123#section-2.1

def sanitize_host(host):
    '''
    Sanitize host string.
    https://tools.ietf.org/html/rfc1123#section-2.1
    '''
    RFC952_characters = ascii_letters + digits + ".-"
    return "".join([c for c in host[0:255] if c in RFC952_characters])