Domain pattern '%s' contains invalid character '%c' in label

Error message

Domain pattern '%s' contains invalid character '%c' in label '%s'

What it means

Panics from domain.go:119 when a constant label (after lowercasing per RFC 4343) contains a character other than ASCII a-z, 0-9, or hyphen. These are the only characters legal in DNS hostnames, so any other byte (underscore, wildcard '*', slash, unicode, etc.) is rejected.

Source

Thrown at domain.go:119

				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))
	}

	return m
}

// match checks if a hostname matches the domain pattern.
// It returns true if matched and a slice of parameter values (parallel to paramNames).
// Uses a stack-allocated buffer to avoid heap allocation for typical domain names.

View on GitHub (pinned to 9a4c7e57fe)

Solutions

Use only letters, digits, and hyphens in constant labels.
Use a parameter label (":sub.example.com") instead of '*' to capture arbitrary subdomains.
Punycode-encode IDN labels before registering them.

Example fix

// before
app.Domain("*.example.com").Get("/", h) // '*' invalid

// after - capture any single subdomain label
app.Domain(":sub.example.com").Get("/", h)
// then read fiber.DomainParam(c, "sub")

Defensive patterns

Strategy: validation

Validate before calling

for _, label := range strings.Split(pattern, ".") {
    if strings.HasPrefix(label, ":") {
        continue
    }
    lower := strings.ToLower(label)
    for _, ch := range lower {
        if !((ch >= 'a' && ch <= 'z') || (ch >= '0' && ch <= '9') || ch == '-') {
            return fmt.Errorf("invalid char %q in label %s", ch, label)
        }
    }
}
app.Domain(pattern).Get("/", h)

Prevention

Use only [a-z0-9-] in constant labels.
Use ':param' labels instead of '*' for wildcard subdomains.
Punycode-encode IDN labels before registering.

When it happens

Trigger: app.Domain("sub_domain.example.com") panics on '_'; app.Domain("*.example.com") panics on '*'; app.Domain("cafe.example.com") with a non-ASCII 'e' panics. Wildcards are not supported as constant labels.

Common situations: Trying to use wildcard subdomains via '*' (fiber's domain router is pattern-based on labels, not glob); underscores from service-discovery names; pasting internationalized names without IDNA punycode encoding.

Related errors

AI-assisted analysis of gofiber/fiber@9a4c7e57fe (2026-08-04). Data as JSON: /data/errors/a4bbfe77b1664a2e.json. Report an issue: GitHub.