FQDN Validation Library

1.5.1 · active · verified Sat Mar 28

The `fqdn` Python library, currently at version 1.5.1, provides RFC-compliant validation and manipulation of Fully Qualified Domain Names (FQDNs). Its primary purpose is to ensure that domain names adhere to Internet Engineering Task Force specifications like RFC 1123, making them suitable for traditional internet hostname usage. This is often a stricter subset of what modern web browsers accept. The library typically sees updates in response to specification clarifications or bug fixes, though major releases are infrequent.

Warnings

gotcha The `fqdn` library validates against strict RFC standards (e.g., RFC 1123, 1035), which are often more restrictive than what modern web browsers or certain Certificate Authorities (like Let's Encrypt) might accept as a valid domain name. This can lead to domains that work in browsers being rejected by `fqdn`.
Fix: Understand that `fqdn` prioritizes strict RFC compliance. If browser-like permissiveness is needed, custom relaxation of rules or an alternative library may be required.
gotcha By default, the `fqdn` library requires FQDNs to have a minimum of two labels (e.g., 'example.com' is valid, but 'example' is not). This is an explicit default constraint enabled to prevent breaking backwards compatibility with earlier versions.
Fix: If single-label hostnames or other non-standard formats are expected, consult the library's source or documentation for configuration options to relax specific constraints.
gotcha Do not confuse this library (`fqdn`) with Python's built-in `socket.getfqdn()`. The `fqdn` library is designed for *validating arbitrary string inputs* as FQDNs, whereas `socket.getfqdn()` is used for *retrieving the Fully Qualified Domain Name of the local machine*.
Fix: Use `from fqdn import FQDN` for external string validation. Use `import socket; socket.getfqdn()` only when inquiring about the local system's hostname resolution.
gotcha The `fqdn` library primarily validates the *syntactic structure* of an FQDN according to RFCs. It does not perform live DNS lookups or validate against dynamic lists like IANA TLDs or the Public Suffix List. Therefore, a syntactically valid FQDN might not correspond to a *registrable* or *existing* domain on the internet.
Fix: For validation against actual registrable domains or live existence, combine `fqdn` with other libraries that perform DNS lookups (e.g., `dnspython`) or consult public suffix lists (e.g., `fqdn-parser`).

Install

pip install fqdn Install stable version

Imports

FQDN
```
from fqdn import FQDN
```
The primary class for FQDN validation is `FQDN`.

Quickstart

Instantiate the `FQDN` class with a domain string and use its `is_valid` property to check for RFC compliance. Other properties like `absolute` and `relative` provide different representations.

from fqdn import FQDN

domain_name_valid = 'example.com'
domain_name_invalid = 'localhost'

fqdn_obj_valid = FQDN(domain_name_valid)
fqdn_obj_invalid = FQDN(domain_name_invalid)

print(f"'{domain_name_valid}' is valid: {fqdn_obj_valid.is_valid}")
print(f"Absolute form: {fqdn_obj_valid.absolute}")
print(f"'{domain_name_invalid}' is valid: {fqdn_obj_invalid.is_valid}")

# Example with relaxed constraints (though specific params aren't directly exposed in quickstart)
# fqdn_obj_relaxed = FQDN('example', allow_single_label=True) # Illustrative, actual param might differ or be via a different method
# print(f"'example' is valid (relaxed): {fqdn_obj_relaxed.is_valid}")

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