Name Compression Tool
Check domain name length compliance for DNS. Enter any domain to validate each label against the 63-character RFC 1035 limit and the full FQDN against the 253-character limit. Colour-coded progress bars and per-label analysis make it easy to spot compliance issues - completely free, no signup required.
Check domain name length compliance for DNS. Enter a domain to validate each label and the full FQDN against RFC 1035 limits (max 63 characters per label, 253 per FQDN).
Enter a domain name above to analyse its DNS length compliance.
Try clicking one of the example domains above.
Features
DNS Length Compliance Check
Validates domain names against RFC 1035 specifications: maximum 253 characters for the full FQDN and 63 characters per label. Colour-coded progress bars and status indicators make compliance easy to assess at a glance.
Per-Label Analysis
Each label (subdomain, domain, TLD) is analysed individually with its own progress bar and validation status. Hover and see exact character counts, invalid characters, hyphen placement issues, and numeric TLD detection.
Comprehensive Error Detection
Detects empty labels, labels exceeding 63 characters, invalid characters (underscores, spaces, special chars), hyphens at the start or end of a label, purely numeric TLDs, and FQDNs exceeding the 253-character limit.
Live Analysis with Examples
Results update in real-time as you type. Pre-built example domains demonstrate edge cases like hyphen-start errors, long labels, and maximum-length FQDNs so you can learn what valid and invalid look like.
Common Use Cases
Domain Registration Validation
Before registering a domain name, verify that all labels comply with DNS length limits. Avoid registering domains that are too long to be practical or that may cause issues with DNS resolution and email delivery.
DNS Configuration Debugging
When a DNS zone file fails to load or a domain stops resolving, check if a recently added subdomain or CNAME target exceeds label length limits. Many DNS errors trace back to a single overlong label.
Subdomain Planning
Plan your subdomain hierarchy with confidence. Ensure that deeply nested subdomains like very-deep-subdomain.region.environment.service.example.com stay within the 253-character FQDN limit.
SSL Certificate Validation Prep
Certificate Authorities enforce strict domain name length limits. Verify that all SANs (Subject Alternative Names) in your certificate request comply with DNS name constraints before submitting your CSR.
Email Infrastructure Setup
SPF, DKIM, and DMARC records often reference long DNS names. Validate that your email authentication records use label-compliant selectors and domains to prevent delivery failures caused by DNS truncation.
DNS Education & Learning
Use the tool to learn about DNS name structure. See how labels are parsed, understand the difference between a FQDN and a label, and discover what makes a DNS name valid or invalid according to RFC standards.
About DNS Name Length Compliance
What Are DNS Name Length Limits?
The Domain Name System (DNS) has strict length limits defined in RFC 1035. Each label (the part between dots) can be at most 63 characters, and the full Fully Qualified Domain Name (FQDN) including dots can be at most 253 characters. These limits ensure compatibility across all DNS software and network equipment. Exceeding them can cause resolution failures, email delivery issues, and SSL certificate validation errors.
How Domains Are Parsed into Labels
A domain name like 'subdomain.example.com' is split into three labels: 'subdomain' (9 chars), 'example' (7 chars), and 'com' (3 chars). Each label is validated independently for character rules (letters, digits, hyphens) and length. The total FQDN length includes the dots between labels. Our tool parses and validates each label automatically, showing progress bars for visual length comparison against the 63-character limit.
Common DNS Name Violations
The most common violations include: labels exceeding 63 characters (often from long descriptive words), hyphens at the start or end of a label (invalid per RFC 1035), underscores in labels (common but only allowed for specific purposes like SRV records), empty labels (consecutive dots), and purely numeric TLDs. Email systems are particularly sensitive to these issues, as some mail servers will reject messages from domains with invalid label syntax.
Practical Implications of Length Limits
Beyond basic DNS resolution, domain name length affects: SSL/TLS certificate requests (CAs validate domain lengths), email deliverability (SPF/DKIM lookups may fail with long names), cookie domain attributes (browsers enforce limits), and URL usability (long domains are harder to share and remember). Always check domain length compliance before deploying DNS changes, especially for deeply nested subdomains used in cloud infrastructure.
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