either ServerName or InsecureSkipVerify must be specified…

Error message

either ServerName or InsecureSkipVerify must be specified in the tls.Config

What it means

At the end of CreateTunnelConfig, the resulting *tls.Config is validated: TLS server-name verification requires either a ServerName (the hostname to validate the server certificate against) or InsecureSkipVerify=true. If both ServerName is empty and InsecureSkipVerify is false, crypto/tls could never verify the peer, so cloudflared rejects the config up front with this error instead of failing obscurely at handshake time.

Solutions

Pass the correct hostname (e.g. the edge or origin SNI, such as your tunnel's Cloudflare hostname) as the serverName argument.
If verification must be disabled (testing only), construct the config then set tlsConfig.InsecureSkipVerify = true before use — or better, keep ServerName set and avoid disabling verification.
Fix the upstream source of the hostname (flag/config/env) so it is non-empty by the time CreateTunnelConfig is called; validate it earlier in startup.

Example fix

// before
hostname := os.Getenv("TUNNEL_HOSTNAME") // may be ""
tlsCfg, err := tlsconfig.CreateTunnelConfig(caPath, hostname)
// after
hostname := os.Getenv("TUNNEL_HOSTNAME")
if hostname == "" {
	log.Fatal().Msg("TUNNEL_HOSTNAME must be set")
}
tlsCfg, err := tlsconfig.CreateTunnelConfig(caPath, hostname)

Defensive patterns

Strategy: validation

Validate before calling

func validateTunnelTLSInput(caCert, serverName string) error {
	if serverName == "" {
		return errors.New("serverName must be non-empty (or set InsecureSkipVerify on the returned config)")
	}
	return nil
}

Try / catch

tlsCfg, err := tlsconfig.CreateTunnelConfig(caCert, serverName)
if err != nil {
	if strings.Contains(err.Error(), "ServerName or InsecureSkipVerify") {
		log.Fatal().Msg("hostname/SNI missing in config; set the tunnel hostname")
	}
	log.Fatal().Err(err).Msg("failed to build tunnel TLS config")
}

Prevention

Fail fast on empty hostname flags/env vars before building TLS config.
Prefer setting ServerName over InsecureSkipVerify; never ship skip-verify in production.
Derive serverName from the tunnel hostname already validated at startup.

When it happens

Trigger: Calling CreateTunnelConfig(caCert, "") — i.e. with an empty serverName — and not setting InsecureSkipVerify on the returned config before use. Any caller (prepareTunnelConfig, probeTLSConfig, tests) that passes an empty server name without disabling verification.

Common situations: A config file omits the hostname/SNI field; hostname is derived from an env var or flag that is unset and yields ""; code builds the tunnel config before the target hostname is known and forgets to set InsecureSkipVerify for that case.

Understand the failure class

Background: "is required", "must be set", "missing required field": configuration validation errors across open-source libraries — this error's family across 36 libraries.

SSL/TLS and certificate errors — how TLS handshakes and certificate validation fail.

Related errors

AI-assisted analysis of cloudflare/cloudflared@2253eeeb25 (2026-09-06). Data as JSON: /api/errors/f804931246973f30. Report an issue: GitHub.

Appendix: source

Thrown at tlsconfig/origin_ca.go:107

	}

	if tlsConfig.RootCAs == nil {
		rootCAPool, err := x509.SystemCertPool()
		if err != nil {
			return nil, errors.Wrap(err, "unable to get x509 system cert pool")
		}
		cfRootCA, err := GetCloudflareRootCA()
		if err != nil {
			return nil, errors.Wrap(err, "could not append Cloudflare Root CAs to cloudflared certificate pool")
		}
		for _, cert := range cfRootCA {
			rootCAPool.AddCert(cert)
		}
		tlsConfig.RootCAs = rootCAPool
	}

	if tlsConfig.ServerName == "" && !tlsConfig.InsecureSkipVerify {
		return nil, fmt.Errorf("either ServerName or InsecureSkipVerify must be specified in the tls.Config")
	}
	return tlsConfig, nil
}

func loadOriginCertPool(originCAPoolPEM []byte, log *zerolog.Logger) (*x509.CertPool, error) {
	// Get the global pool
	certPool, err := loadGlobalCertPool(log)
	if err != nil {
		return nil, err
	}

	// Then, add any custom origin CA pool the user may have passed
	if originCAPoolPEM != nil {
		if !certPool.AppendCertsFromPEM(originCAPoolPEM) {
			log.Info().Msg("could not append the provided origin CA to the cloudflared certificate pool")
		}
	}

View on GitHub (pinned to 2253eeeb25)