Free Domain Syntax & RFC Validator
Verify domain syntax conformance against RFC 1035, check max label lengths (63 chars), total length (253 chars), illegal characters, and IDN/Punycode encoding.
Input Domain String
Type the complete domain name or subdomain string.
RFC Rule Execution
Checks RFC 1035 length, regex character sets, and hyphen rules.
Comprehensive Report
Review pass/fail status and identify formatting errors.
Audits against official Internet Engineering Task Force standards.
Ensures no segment exceeds the 63-character maximum.
Identifies internationalized domains starting with xn--.
- Avoid using underscores (_) in web domain names; while allowed in TXT or SRV records, they are invalid in hostnames.
- Keep domains under 20 characters for optimal user memorability, word-of-mouth branding, and click-through rates.
Frequently Asked Questions about Domain Validation
Helpful answers to common questions about checking username availability, domain extensions, and registration.
What are the RFC rules for a valid domain name?
Under RFC 1035 and RFC 1123, domain names must consist of labels between 1 and 63 characters long, contain only alphanumeric ASCII characters and hyphens (-), cannot start or end with a hyphen, and cannot exceed 253 total characters in length.
What is an Internationalized Domain Name (IDN) or Punycode?
Punycode (starting with 'xn--') is an encoding syntax defined in RFC 3492 that translates non-ASCII characters (such as accented letters, Cyrillic, Chinese, or Arabic) into standard ASCII character sequences compatible with DNS.
Why should I validate domain strings before submitting them to APIs?
Validating domain syntax prevents invalid registry queries, protects against command injection vulnerabilities in network tools, and avoids costly DNS lookup failures.