Domain pattern '%s' has %d parts, which exceeds the maximum

Error message

Domain pattern '%s' has %d parts, which exceeds the maximum of %d

What it means

Panics from domain.go:76 when the pattern splits into more than maxDomainParts (16) dot-separated labels. The 16-label cap is a deliberate DoS guard below RFC 1035's 127-label theoretical max, preventing memory exhaustion from pathological patterns.

Source

Thrown at domain.go:76

	// consistent with Fiber's own host normalization in Subdomains().
	pattern = utils.TrimRight(pattern, '.')

	// Validate pattern is not empty after trimming
	if pattern == "" {
		panic("Domain pattern cannot be empty")
	}

	// Enforce RFC 1035 total length limit on patterns
	if len(pattern) > 253 {
		panic(fmt.Sprintf("Domain pattern '%s' exceeds RFC 1035 maximum of 253 characters (%d chars)",
			pattern, len(pattern)))
	}

	parts := strings.Split(pattern, ".")

	// Prevent DoS from patterns with excessive label counts
	if len(parts) > maxDomainParts {
		panic(fmt.Sprintf("Domain pattern '%s' has %d parts, which exceeds the maximum of %d",
			pattern, len(parts), maxDomainParts))
	}

	m := domainMatcher{
		parts:    make([]string, len(parts)),
		numParts: len(parts),
	}

	for i, part := range parts {
		// Validate no empty labels (e.g., "example..com" is invalid)
		if part == "" {
			panic(fmt.Sprintf("Domain pattern '%s' contains empty label at position %d", pattern, i))
		}

		if part[0] == ':' {
			// Validate parameter name is not empty
			if len(part) == 1 {
				panic(fmt.Sprintf("Domain pattern '%s' contains empty parameter name at position %d", pattern, i))

View on GitHub (pinned to 9a4c7e57fe)

Solutions

Reduce the pattern to 16 or fewer labels.
Validate len(strings.Split(pattern, ".")) <= 16 before calling Domain().
Rearchitect over-deep tenant schemes to use fewer subdomain levels or path-based routing.

Example fix

// before
app.Domain(genDeepDomain(n)).Get("/", h) // n > 16

// after
pat := genDeepDomain(n)
if len(strings.Split(pat, ".")) > 16 {
    return fmt.Errorf("domain pattern has too many labels")
}
app.Domain(pat).Get("/", h)

Defensive patterns

Strategy: validation

Validate before calling

if len(strings.Split(pattern, ".")) > 16 {
    return fmt.Errorf("domain pattern exceeds 16 labels")
}
app.Domain(pattern).Get("/", h)

Prevention

Bound the depth of generated subdomain patterns.
Reject over-deep tenant schemes at config load.
Prefer path-based routing for unbounded variability.

When it happens

Trigger: app.Domain("a.b.c.d....") with more than 16 labels; a pattern built by repeating ".x" in a loop; deeply nested generated subdomains.

Common situations: Generated wildcard/tenant patterns with unbounded depth; malformed input fed from a database column; test fixtures with artificially deep domains.

Related errors

AI-assisted analysis of gofiber/fiber@9a4c7e57fe (2026-08-04). Data as JSON: /data/errors/fc980bccbcebcf02.json. Report an issue: GitHub.