Add custom TLS options

Signed-off-by: Kevin Allen kallen@bostondynamics.com

net/tlsdial/tlsdial.go

@@ -41,6 +41,17 @@ var debug = envknob.RegisterBool("TS_DEBUG_TLS_DIAL")
 // Headscale, etc.
 var tlsdialWarningPrinted sync.Map // map[string]bool
 
+var (
+	// rootCAOverride creates environment variable config TS_TLS_DIAL_ROOT_CA which
+	// will override the certificate authority used to verify the server instead
+	// of the system default
+	rootCAOverride = envknob.RegisterString("TS_TLS_DIAL_ROOT_CA")
+	// serverHostOverride creates environment variable TS_TLS_DIAL_CONNECT_TO which
+	// will override the server name the certificate is validated against AND the SNI
+	// name presented to the server, which may affect virtual hosts
+	serverHostOverride = envknob.RegisterString("TS_TLS_DIAL_CONNECT_TO")
+)
+
 // Config returns a tls.Config for connecting to a server.
 // If base is non-nil, it's cloned as the base config before
 // being configured and returned.
@@ -52,6 +63,9 @@ func Config(host string, base *tls.Config) *tls.Config {
 		conf = base.Clone()
 	}
 	conf.ServerName = host
+	if len(serverHostOverride()) != 0 {
+		conf.ServerName = serverHostOverride()
+	}
 
 	if n := sslKeyLogFile; n != "" {
 		f, err := os.OpenFile(n, os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0600)
@@ -93,6 +107,21 @@ func Config(host string, base *tls.Config) *tls.Config {
 		for _, cert := range cs.PeerCertificates[1:] {
 			opts.Intermediates.AddCert(cert)
 		}
+
+		// Check against user overriden root CA if provided
+		if overrideRoots() != nil {
+			opts.Roots = overrideRoots()
+			_, err := cs.PeerCertificates[0].Verify(opts)
+			if debug() {
+				log.Printf("tlsdial(override %q): %v", host, err)
+			}
+			if err == nil {
+				atomic.AddInt32(&counterFallbackOK, 1)
+				return nil
+			}
+			return err
+		}
+
 		_, errSys := cs.PeerCertificates[0].Verify(opts)
 		if debug() {
 			log.Printf("tlsdial(sys %q): %v", host, errSys)
@@ -272,3 +301,27 @@ func bakedInRoots() *x509.CertPool {
 	})
 	return bakedInRootsOnce.p
 }
+
+var overrideRootsOnce struct {
+	sync.Once
+	p *x509.CertPool
+}
+
+func overrideRoots() *x509.CertPool {
+	if len(rootCAOverride()) == 0 {
+		return nil
+	}
+	overrideRootsOnce.Do(func() {
+		pem, err := os.ReadFile(rootCAOverride())
+		if err != nil {
+			panic(fmt.Sprintf("Error loading custom root CA %s: %v", rootCAOverride(), err))
+		}
+
+		p := x509.NewCertPool()
+		if !p.AppendCertsFromPEM(pem) {
+			panic(fmt.Sprintf("Invalid PEM in custom root CA %s", rootCAOverride()))
+		}
+		overrideRootsOnce.p = p
+	})
+	return overrideRootsOnce.p
+}