Domain pattern ' ' has label ' ' exceeding RFC 1035 limit…

Error message

Domain pattern '%s' has label '%s' exceeding RFC 1035 limit of 63 characters (%d chars)

What it means

A constant (non-parameter) label in a domain pattern is longer than 63 bytes, violating the RFC 1035 per-label length limit enforced by fiber's domainMatcher. DNS resolvers truncate or reject overlong labels, so the pattern is rejected at compile time. The check fires in the else-branch that handles literal labels (not ':param' segments).

Solutions

Shorten or truncate the offending label to <= 63 bytes.
If the long value is data (not a constant route), move it into a ':param' segment and match it at request time instead of baking it into the pattern.
Add a length assertion on each generated label before constructing the pattern string.

Example fix

// before
label := strings.Repeat("a", 70)
app.Use(label + ".example.com", handler)
// after
if len(label) > 63 { label = label[:63] }
app.Use(label + ".example.com", handler)

Defensive patterns

Strategy: validation

Validate before calling

// validateDomainLabels checks RFC 1035 per-label length (63) for constant labels.
func validateDomainLabels(pattern string) error {
    for _, lbl := range strings.Split(strings.TrimSuffix(pattern, "."), ".") {
        if strings.HasPrefix(lbl, ":") {
            continue // param label, length-checked by matcher differently
        }
        if len(lbl) > 63 {
            return fmt.Errorf("label %q exceeds 63 bytes (%d)", lbl, len(lbl))
        }
    }
    return nil
}

if err := validateDomainLabels(domainPattern); err != nil { return err }

Try / catch

defer func() {
    if r := recover(); r != nil {
        log.Fatalf("domain pattern invalid: %v", r)
    }
}()
app.Use(domainPattern, handler)

Prevention

Truncate or hash long data-derived labels before baking them into a pattern.
Prefer ':param' segments over giant literal labels for variable data.
Assert label length in any pattern-generation helper.

When it happens

Trigger: Passing a domain pattern containing one literal label longer than 63 characters, e.g. a generated/auto-named tenant slug, a long service name, or a pasted hash token used as a hostname label: app.Use("aaaaaaaaaaaaa...64chars....com", handler).

Common situations: Programmatically building a domain pattern from a database column (tenant name, UUID, slug) without truncating; using a full URL or path segment as a label; copying a long subdomain from cloud infrastructure (AWS/GCP endpoints) into a route pattern.

Related errors

AI-assisted analysis of gofiber/fiber@a105acad6c (2026-08-11). Data as JSON: /api/errors/1c90db8fc96b251c. Report an issue: GitHub.

Appendix: source

Thrown at domain.go:112

				panic(fmt.Sprintf("Domain pattern '%s' contains empty parameter name at position %d", pattern, i))
			}
			paramName := part[1:]
			// Validate parameter name contains only ASCII-safe characters (a-z, A-Z, 0-9, underscore, hyphen).
			// Using explicit ASCII ranges rather than unicode.IsLetter/IsDigit to reject non-ASCII
			// characters that are invalid in DNS names.
			for _, ch := range paramName {
				if !isASCIIAlphanumeric(ch) && ch != '_' && ch != '-' {
					panic(fmt.Sprintf("Domain pattern '%s' contains invalid parameter name '%s' with character '%c'", pattern, paramName, ch))
				}
			}
			m.paramIdx = append(m.paramIdx, i)
			m.paramNames = append(m.paramNames, paramName) // preserve original case
			m.parts[i] = part                              // keep ":param" marker for matching
		} else {
			// Only lowercase constant labels (RFC 4343)
			// Enforce RFC 1035 per-label length limit (63 characters)
			if len(part) > 63 {
				panic(fmt.Sprintf("Domain pattern '%s' has label '%s' exceeding RFC 1035 limit of 63 characters (%d chars)",
					pattern, part, len(part)))
			}
			// Validate label contains only valid ASCII domain characters (a-z, 0-9, hyphen).
			normalized := utilsstrings.ToLower(part)
			for _, ch := range normalized {
				if !isASCIIAlphanumeric(ch) && ch != '-' {
					panic(fmt.Sprintf("Domain pattern '%s' contains invalid character '%c' in label '%s'", pattern, ch, part))
				}
			}
			m.parts[i] = normalized
		}
	}

	// Check if the domain pattern has too many parameters
	if len(m.paramNames) > maxParams {
		panic(fmt.Sprintf("Domain pattern '%s' has %d parameters, which exceeds the maximum of %d",
			pattern, len(m.paramNames), maxParams))
	}

View on GitHub (pinned to a105acad6c)