caddyserver/caddy · error

loading fallback selection policy: %s

Error message

loading fallback selection policy: %s

What it means

Error "loading fallback selection policy: %s" thrown in caddyserver/caddy.

Source

Thrown at modules/caddyhttp/reverseproxy/selectionpolicies.go:506

// CaddyModule returns the Caddy module information.
func (QueryHashSelection) CaddyModule() caddy.ModuleInfo {
	return caddy.ModuleInfo{
		ID:  "http.reverse_proxy.selection_policies.query",
		New: func() caddy.Module { return new(QueryHashSelection) },
	}
}

// Provision sets up the module.
func (s *QueryHashSelection) Provision(ctx caddy.Context) error {
	if s.Key == "" {
		return fmt.Errorf("query key is required")
	}
	if s.FallbackRaw == nil {
		s.FallbackRaw = caddyconfig.JSONModuleObject(RandomSelection{}, "policy", "random", nil)
	}
	mod, err := ctx.LoadModule(s, "FallbackRaw")
	if err != nil {
		return fmt.Errorf("loading fallback selection policy: %s", err)
	}
	s.fallback = mod.(Selector)
	return nil
}

// Select returns an available host, if any.
func (s QueryHashSelection) Select(pool UpstreamPool, req *http.Request, _ http.ResponseWriter) *Upstream {
	// Since the query may have multiple values for the same key,
	// we'll join them to avoid a problem where the user can control
	// the upstream that the request goes to by sending multiple values
	// for the same key, when the upstream only considers the first value.
	// Keep in mind that a client changing the order of the values may
	// affect which upstream is selected, but this is a semantically
	// different request, because the order of the values is significant.
	vals := strings.Join(req.URL.Query()[s.Key], ",")
	if vals == "" {
		return s.fallback.Select(pool, req, nil)
	}

View on GitHub (pinned to 50e54ee279)

Solutions

1. Check the fallback selection policy module name and its configuration; the wrapped error identifies the problem.

When it happens

Trigger: Thrown at modules/caddyhttp/reverseproxy/selectionpolicies.go:506 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of caddyserver/caddy@50e54ee279 (2026-08-15). Data as JSON: /api/errors/a905868e0e1d49bb. Report an issue: GitHub.