FQDN Validator — Fully Qualified Domain Name Check
Instant FQDN syntax check — label rules, length limits, and optional trailing root dot
How to Use This Tool
- Type or paste the hostname into the input field.
- Validation runs locally in your browser on each change.
- Pattern checks total length ≤ 253 and label structure rules.
- Labels allow a-z, 0-9, hyphens not at start or end.
- TLD segment requires at least two alphabetic characters.
- Optional trailing dot (root label) is accepted — www.example.com.
About This Tool
Fully qualified domain names (FQDNs) specify a host's exact position in the DNS tree — often with a trailing dot marking the root. Invalid labels break automation, TLS certificates, and API calls before any resolver query runs. VSPIC FQDN validator uses the MissingClientWidgets fqdn-validator kind: validation runs entirely in your browser with a regex pattern enforcing overall length up to 253 characters, dot-separated labels up to 63 characters, alphanumeric hyphen rules, and a TLD of at least two letters, with optional trailing dot.
Results show Valid FQDN or Invalid FQDN immediately as you type — no server round trip, no query logging. Use before pasting hostnames into DNS panels, Kubernetes ingress specs, or monitoring agent configuration files.
Common use cases
- •View all DNS records of a domain after migration
- •Confirm DNS records after domain changes
- •Test for DNS leaks when using a VPN
- •Debug email delivery with MX and TXT records
Why use VSPIC for ?
- Instant local validation — no network latency.
- Privacy-friendly — hostnames never sent to our servers.
- Clear valid/invalid badge for quick form QA.
- Accepts optional trailing root dot notation.
- Free with no account — works offline after page load.
- Matches operational FQDN rules used in DNS automation.
What makes a name fully qualified
An FQDN lists every label from host through TLD, optionally ending with a root dot. www.example.com. is fully qualified with explicit root; www.example.com is often used where context implies the root.
Resolvers and APIs treat malformed labels as hard errors. Validating syntax first saves failed API calls and ticket churn.
Label length and character rules
Each dot-separated label may be up to 63 octets. Total wire-format name is limited to 253 octets. Labels start and end with alphanumeric characters; hyphens are allowed internally.
Our browser widget enforces these constraints with a practical regex suited for common operational hostnames.
Trailing dot semantics
Trailing dot means empty root label — the name is absolute, not relative to a search list. Automation tools vary: some require trailing dot, others forbid it.
Validator accepts optional trailing dot so both www.example.com and www.example.com. pass when otherwise valid.
MissingClientWidgets browser execution
fqdn-validator kind renders MissingClientWidgets in the tool shell — no extended API call. Input never leaves your device for validation logic.
Ideal for security-sensitive hostname drafts on untrusted networks.
FQDN versus domain apex
example.com alone is a domain apex, not a host FQDN with leftmost host label. www.example.com includes host label www — validator expects at least one host label plus TLD structure.
Use domain validator for registrar-style apex validation with slightly different rules.
When syntax passes but DNS fails
Valid FQDN does not guarantee NXDOMAIN-free resolution. Follow with DNS lookup or A record lookup after syntax passes.
Typosquatting domains may validate syntactically while being wrong operationally — human review still required.
Underscores and SRV conventions
Underscores appear in SRV service names like _sip._tcp.example.com but are invalid in vanilla host labels per strict RFC host rules. This validator uses strict hostname pattern — underscore names may show invalid.
For SRV owner names, validate against your provider's documented exceptions separately.
IDN and punycode
Unicode domain labels must be punycode (xn--) for DNS wire format. Paste punycode for IDN validation; raw Unicode may fail regex until converted.
Use registrar IDN tools for full internationalized domain workflows.
Bulk and automation alternatives
For inventory CSV validation, replicate regex in CI scripts. Browser tool optimizes ad hoc single-host checks during incidents.
Pair validated hostnames with fqdn-correct configs in Terraform plans.
Important notes & limitations
- Syntax only — does not prove the name exists in DNS.
- Internationalized domain names (IDN) punycode not fully validated.
- Underscores in labels (SRV targets) may fail — intentional strictness.
- Single hostname per check — not bulk CSV validation.
- Does not validate IP literals or bracketed IPv6.
Frequently Asked Questions
Yes. VSPIC offers this FQDN validator at no cost with no account required. Results load in real time.
We do not permanently store your queries on our servers. Some tools run entirely in your browser; others fetch public data for the request only.
Yes. Open the page in any modern phone or tablet browser. Results work on Wi‑Fi and mobile data.
No. MissingClientWidgets runs regex validation locally in your browser. Syntax only — not existence.
Optional. Both www.example.com and www.example.com. validate when labels meet rules.
Underscore labels violate strict hostname character rules this validator enforces. SRV names may need provider-specific validation.
No for validation logic. Client-side widget only — hostnames stay in your browser.
Overall pattern enforces ≤ 253 characters with per-label ≤ 63 character structure.
FQDN validator expects host.example.tld structure with label rules. Domain validator targets registrar apex domains like example.com.
Next step for your check
Continue with domain validator on VSPIC.
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