Validate a hostname string


Following up to Regular expression to match hostname or IP Address? and using Restrictions on valid host names as a reference, what is the most readable, concise way to match/validate a hostname/fqdn (fully qualified domain name) in Python? I've answered with my attempt below, improvements welcome.

5/23/2017 12:32:18 PM

Accepted Answer

import re
def is_valid_hostname(hostname):
    if len(hostname) > 255:
        return False
    if hostname[-1] == ".":
        hostname = hostname[:-1] # strip exactly one dot from the right, if present
    allowed = re.compile("(?!-)[A-Z\d-]{1,63}(?<!-)$", re.IGNORECASE)
    return all(allowed.match(x) for x in hostname.split("."))

ensures that each segment

  • contains at least one character and a maximum of 63 characters
  • consists only of allowed characters
  • doesn't begin or end with a hyphen.

It also avoids double negatives (not disallowed), and if hostname ends in a ., that's OK, too. It will (and should) fail if hostname ends in more than one dot.

10/28/2013 12:42:55 PM

Per The Old New Thing, the maximum length of a DNS name is 253 characters. (One is allowed up to 255 octets, but 2 of those are consumed by the encoding.)

import re

def validate_fqdn(dn):
    if dn.endswith('.'):
        dn = dn[:-1]
    if len(dn) < 1 or len(dn) > 253:
        return False
    ldh_re = re.compile('^[a-z0-9]([a-z0-9-]{0,61}[a-z0-9])?$',
    return all(ldh_re.match(x) for x in dn.split('.'))

One could argue for accepting empty domain names, or not, depending on one's purpose.

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