# Copyright (c) 2017 Hubert Kario
#
# See the LICENSE file for legal information regarding use of this file.
"""Utilities for handling DNS hostnames"""
import re
[docs]
def is_valid_hostname(hostname):
"""
Check if the parameter is a valid hostname.
:type hostname: str or bytearray
:param hostname: string to check
:rtype: boolean
"""
try:
if not isinstance(hostname, str):
hostname = hostname.decode('ascii', 'strict')
except UnicodeDecodeError:
return False
if hostname[-1] == ".":
# strip exactly one dot from the right, if present
hostname = hostname[:-1]
# the maximum length of the domain name is 255 bytes, but because they
# are encoded as labels (which is a length byte and an up to 63 character
# ascii string), you change the dots to the length bytes, but the
# host element of the FQDN doesn't start with a dot and the name doesn't
# end with a dot (specification of a root label), we need to subtract 2
# bytes from the 255 byte maximum when looking at dot-deliminated FQDN
# with the trailing dot removed
# see RFC 1035
if len(hostname) > 253:
return False
# must not be all-numeric, so that it can't be confused with an ip-address
if re.match(r"[\d.]+$", hostname):
return False
allowed = re.compile(r"(?!-)[A-Z\d-]{1,63}(?<!-)$", re.IGNORECASE)
return all(allowed.match(x) for x in hostname.split("."))