Validate a hostname string

kostmo picture kostmo · Mar 28, 2010 · Viewed 47.4k times · Source

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.

Answer

Tim Pietzcker picture Tim Pietzcker · Mar 28, 2010
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.