FQDN Checker

The FQDN Checker validates the syntax of fully qualified domain names and can optionally verify DNS reachability. It is intended for developers, system administrators, network engineers, and SEO specialists who must confirm that a candidate hostname is correctly formed before certificate issuance, DNS configuration, or deployment. The tool supports common RFC-driven rules and provides explicit feedback on where a string fails validation, enabling quick remediation and reliable downstream processing in automation pipelines.
Conceptually, validation proceeds by splitting the input on dots into labels, ensuring each label length lies between 1 and 63 characters, and restricting characters to ASCII letters, digits, and hyphens (with strict positioning of hyphens). The overall length must not exceed 255 characters, and trailing dots may or may not be accepted depending on the mode. If DNS checking is enabled, the system queries the configured resolver and collects A/AAAA/CNAME records along with TTL, providing a real-world occupancy signal for the hostname.
Simplified normalization is performed to produce a canonical ASCII form, and IDN inputs are accepted via punycode handling to ensure compatibility with legacy systems. The tool returns a clear validity flag, a reason code if invalid, and a normalized name when valid. Advanced users may supply a custom DNS resolver or timeout, enabling integration with private networks or restricted environments.
Use cases include pre-flight DNS validation in CI pipelines, ensuring domain names used in certificates, mail exchangers, and hosting configurations conform to standards. Security and SEO teams benefit from consistent hostname formatting to prevent misconfigurations and canonicalization issues. The tool provides deterministic outputs for automation, reduces operational risk, and supports both global domains and IDN scenarios.