Regular Expression for validating DNS label ( host name)

user_178435 0
Viewed 34830

I would like to validate a hostname using only regualr expression.

Host Names (or 'labels' in DNS jargon) were traditionally defined by RFC 952 and RFC 1123 and may be composed of the following valid characters.

List item

  • A to Z ; upper case characters
  • a to z ; lower case characters
  • 0 to 9 ; numeric characters 0 to 9
  • - ; dash

The rules say:

  • A host name (label) can start or end with a letter or a number
  • A host name (label) MUST NOT start or end with a '-' (dash)
  • A host name (label) MUST NOT consist of all numeric values
  • A host name (label) can be up to 63 characters

How would you write Regular Expression to validate hostname ?

dns validation forms expression regex
7 Answers
^(?![0-9]+$)(?!-)[a-zA-Z0-9-]{,63}(?<!-)$

I used the following testbed written in Python to verify that it works correctly:

tests = [
    ('01010', False),
    ('abc', True),
    ('A0c', True),
    ('A0c-', False),
    ('-A0c', False),
    ('A-0c', True),
    ('o123456701234567012345670123456701234567012345670123456701234567', False),
    ('o12345670123456701234567012345670123456701234567012345670123456', True),
    ('', True),
    ('a', True),
    ('0--0', True),
]

import re
regex = re.compile('^(?![0-9]+$)(?!-)[a-zA-Z0-9-]{,63}(?<!-)$')
for (s, expected) in tests:
    is_match = regex.match(s) is not None
    print is_match == expected
user_619740

It is worth noting that DNS labels and hostname components have slightly different rules. Most notably: '_' is not legal in any component of a hostname, but is a standard part of labels used for things like SRV records.

A more readable and portable approach is to require a string to match both of these POSIX ERE's:

^([[:alnum:]][[:alnum:]\-]{0,61}[[:alnum:]]|[[:alpha:]])$
^.*[[:^digit:]].*$

Those should be easy to use in any standard-compatible ERE implementation. Perl-style backtracking as in the Python example is widely available, but has the problem of not being exactly the same everywhere that it seems to work. Ouch.

It is possible in principle to make a single ERE of those two lines, but it would be long and unwieldy. The first line handles all of the rules other than the ban on all-digits, the second kills those.

user_12800050

The k8s API responds with the regex that it uses to validate e.g. an RFC 1123-compliant string:

(⎈ minikube:default)➜  cloud-app git:(mc/72-org-ns-names) ✗ k create ns not-valid1234234$%
The Namespace "not-valid1234234$%" is invalid: metadata.name: 
Invalid value: "not-valid1234234$%": a lowercase RFC 1123 label must consist of lower case 
alphanumeric characters or '-', and must start and end with an alphanumeric character 
(e.g. 'my-name',  or '123-abc', regex used for validation is
 '[a-z0-9]([-a-z0-9]*[a-z0-9])?')
user_25170330
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