tsnet: add FunnelTLSConfig FunnelOption type

And also validate opts for unknown types, before other side effects.

Fixes #15833

Change-Id: I4cabe16c49c5b7566dcafbec59f2cd1e0c8b4b3c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tsnet/tsnet.go

@@ -1101,13 +1101,33 @@ type FunnelOption interface {
 	funnelOption()
 }
 
-type funnelOnly int
+type funnelOnly struct{}
 
 func (funnelOnly) funnelOption() {}
 
 // FunnelOnly configures the listener to only respond to connections from Tailscale Funnel.
 // The local tailnet will not be able to connect to the listener.
-func FunnelOnly() FunnelOption { return funnelOnly(1) }
+func FunnelOnly() FunnelOption { return funnelOnly{} }
+
+type funnelTLSConfig struct{ conf *tls.Config }
+
+func (f funnelTLSConfig) funnelOption() {}
+
+// FunnelTLSConfig configures the TLS configuration for [Server.ListenFunnel]
+//
+// This is rarely needed but can permit requiring client certificates, specific
+// ciphers suites, etc.
+//
+// The provided conf should at least be able to get a certificate, setting
+// GetCertificate, Certificates or GetConfigForClient appropriately.
+// The most common configuration is to set GetCertificate to
+// Server.LocalClient's GetCertificate method.
+//
+// Unless [FunnelOnly] is also used, the configuration is also used for
+// in-tailnet connections that don't arrive over Funnel.
+func FunnelTLSConfig(conf *tls.Config) FunnelOption {
+	return funnelTLSConfig{conf: conf}
+}
 
 // ListenFunnel announces on the public internet using Tailscale Funnel.
 //
@@ -1140,6 +1160,26 @@ func (s *Server) ListenFunnel(network, addr string, opts ...FunnelOption) (net.L
 		return nil, err
 	}
 
+	// Process, validate opts.
+	lnOn := listenOnBoth
+	var tlsConfig *tls.Config
+	for _, opt := range opts {
+		switch v := opt.(type) {
+		case funnelTLSConfig:
+			if v.conf == nil {
+				return nil, errors.New("invalid nil FunnelTLSConfig")
+			}
+			tlsConfig = v.conf
+		case funnelOnly:
+			lnOn = listenOnFunnel
+		default:
+			return nil, fmt.Errorf("unknown opts FunnelOption type %T", v)
+		}
+	}
+	if tlsConfig == nil {
+		tlsConfig = &tls.Config{GetCertificate: s.getCert}
+	}
+
 	ctx := context.Background()
 	st, err := s.Up(ctx)
 	if err != nil {
@@ -1177,19 +1217,11 @@ func (s *Server) ListenFunnel(network, addr string, opts ...FunnelOption) (net.L
 	}
 
 	// Start a funnel listener.
-	lnOn := listenOnBoth
-	for _, opt := range opts {
-		if _, ok := opt.(funnelOnly); ok {
-			lnOn = listenOnFunnel
-		}
-	}
 	ln, err := s.listen(network, addr, lnOn)
 	if err != nil {
 		return nil, err
 	}
-	return tls.NewListener(ln, &tls.Config{
-		GetCertificate: s.getCert,
-	}), nil
+	return tls.NewListener(ln, tlsConfig), nil
 }
 
 type listenOn string