fqdn
Validates fully-qualified domain names against RFC 1123, so that they are acceptable to modern bowsers
Decision gist · record as of 2026-08-14
Yes, if you need RFC 1123 FQDN validation. The package is stable, low-friction to install, has no known vulnerabilities, and is widely used (top 1000 on PyPI). The MPL 2.0 copyleft license is acceptable for most projects. The main caveat: it has not been actively developed since 2021, so if you need support for newer Python versions or RFC updates, verify that its validation rules still match your requirements before relying on it.AI-flagged interpretation of the facts on this page — verify before relying
Before you install
- Low install friction with a single lightweight runtime dependency (cached-property).
- Package is in production/stable status but aging—last release was in 2021, though the repository remains active with a recent commit in 2025.
License · maintenance · safety
MPL 2.0 (copyleft) — Licensed under Mozilla Public License 2.0 (MPL 2.0), a copyleft license. You must disclose source modifications and distribute derivative works under the same license if distributed; suitable for internal use or open-source projects aligned with copyleft requirements.
last release 2021-03-11 (1982 days) · last repo commit 2025-03-05 · 37 stars
0 known vulnerabilities (OSV.dev, 2026-08-14) · 47,100,243 downloads/mo, #604 on PyPI
Alternatives
Verify before relying
from fqdn import FQDN
domain = FQDN('bbc.co.uk')
if domain.is_valid:
print(domain.absolute) # 'bbc.co.uk.'
print(domain.relative) # 'bbc.co.uk'- Whether the package's RFC 1123 validation aligns with your specific use case (e.g., CA issuance, browser DNS lookup, or general hostname acceptance).
- Performance characteristics when validating large batches of domain names.
- Compatibility with Python versions newer than 3.9 (classifiers list ends there; requires_python allows <4 but no explicit newer version testing documented).
What it is and what it does
fqdn is a Python library that validates fully-qualified domain names (FQDNs) according to IETF RFC 1123 standards. It determines whether a string would be traditionally acceptable as a public Internet hostname to RFC-conforming software—a stricter validation than what modern web browsers perform for DNS lookups. The package provides an FQDN class that checks validity, normalizes names to absolute and relative forms, and implements case-insensitive equality and hashing per IETF specification.
By default, the package enforces a minimum of two labels separated by periods (e.g., 'example.com' is valid, 'localhost' is not), but this constraint can be relaxed via configuration options. The library also supports relaxations for short hostnames without periods or names containing underscores, bringing validation closer to browser behavior. It has a single runtime dependency (cached-property) and supports Python 2.7 and 3.5+, making it lightweight and broadly compatible.
Use it for
- Validate user-supplied domain names in web forms or API inputs before DNS lookups or certificate issuance.
- Check hostnames in configuration files or deployment scripts to catch typos or invalid names early.
- Normalize and compare domain names case-insensitively in applications that store or index hostnames.
- Implement strict RFC 1123 compliance checks in DNS tools, mail servers, or network administration utilities.
- Relax validation rules for internal networks or legacy systems that use non-standard hostname formats.
Worth the install?
AI-flagged interpretation of the facts on this page. Verify before relying on it.
Yes, if you need RFC 1123 FQDN validation.
The package is stable, low-friction to install, has no known vulnerabilities, and is widely used (top 1000 on PyPI). The MPL 2.0 copyleft license is acceptable for most projects. The main caveat: it has not been actively developed since 2021, so if you need support for newer Python versions or RFC updates, verify that its validation rules still match your requirements before relying on it.
Install
fqdn on PyPI
Before you install
Low install friction with a single lightweight runtime dependency (cached-property). Package is in production/stable status but aging—last release was in 2021, though the repository remains active with a recent commit in 2025.
License in practice
Licensed under Mozilla Public License 2.0 (MPL 2.0), a copyleft license. You must disclose source modifications and distribute derivative works under the same license if distributed; suitable for internal use or open-source projects aligned with copyleft requirements.
Quickstart
from fqdn import FQDN
domain = FQDN('bbc.co.uk')
if domain.is_valid:
print(domain.absolute) # 'bbc.co.uk.'
print(domain.relative) # 'bbc.co.uk'Verify before relying
- Whether the package's RFC 1123 validation aligns with your specific use case (e.g., CA issuance, browser DNS lookup, or general hostname acceptance).
- Performance characteristics when validating large batches of domain names.
- Compatibility with Python versions newer than 3.9 (classifiers list ends there; requires_python allows <4 but no explicit newer version testing documented).
Package facts
| License | MPL 2.0 copyleft |
| Python support | Supports the current Python release >=2.7, !=3.0, !=3.1, !=3.2, !=3.3, !=3.4, <4 |
| Install friction | Low. Pure-Python wheel |
| Runtime dependencies | 1 packagecached-property |
| Maintenance | Aging 1,982 days since the last release |
| Last repo commit | |
| First released | |
| Downloads | 47,100,243 / month, #604 on PyPI 30-day window, as of 2026-08-14 |
| Known vulnerabilities | None known OSV.dev, checked 2026-08-14 |
| Classifiers | Development Status :: 5 - Production/StableEnvironment :: Web EnvironmentIntended Audience :: DevelopersIntended Audience :: System AdministratorsLicense :: OSI Approved :: Mozilla Public License 2.0 (MPL 2.0)Operating System :: OS IndependentProgramming Language :: Python :: 2.7Programming Language :: Python :: 3.5Programming Language :: Python :: 3.6Programming Language :: Python :: 3.7Programming Language :: Python :: 3.8Programming Language :: Python :: 3.9Programming Language :: Python :: Implementation :: CPythonProgramming Language :: Python :: Implementation :: PyPyTopic :: InternetTopic :: Internet :: Name Service (DNS)Topic :: System :: Systems AdministrationTopic :: Utilities |
Evidence: fqdn-1.5.1-py3-none-any.whl
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See also email-validator · certbot-dns-cloudflare · tldparse · certbot-dns-duckdns · PyFunceble-dev · pyIsEmail · certbot-dns-route53 · language-tags · backports.ssl_match_hostname · pyang