control/controlhttp: add health warning for macOS filtering blocking Tailscale (#12546)

Updates #9658
Updates #12545

Change-Id: I6612b9b65eb193a1a651e219b5198c7c20ed94e1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Co-authored-by: Andrea Gottardo <andrea@tailscale.com>

control/controlhttp/client.go

@@ -32,12 +32,14 @@ import (
 	"net/http/httptrace"
 	"net/netip"
 	"net/url"
+	"runtime"
 	"sort"
 	"sync/atomic"
 	"time"
 
 	"tailscale.com/control/controlbase"
 	"tailscale.com/envknob"
+	"tailscale.com/health"
 	"tailscale.com/net/dnscache"
 	"tailscale.com/net/dnsfallback"
 	"tailscale.com/net/netutil"
@@ -396,12 +398,28 @@ func (a *Dialer) resolver() *dnscache.Resolver {
 	}
 }
 
+func isLoopback(a net.Addr) bool {
+	if ta, ok := a.(*net.TCPAddr); ok {
+		return ta.IP.IsLoopback()
+	}
+	return false
+}
+
+var macOSScreenTime = health.Register(&health.Warnable{
+	Code:  "macos-screen-time",
+	Title: "Tailscale blocked by Screen Time",
+	Text: func(args health.Args) string {
+		return "macOS Screen Time seems to be blocking Tailscale. Try disabling Screen Time in System Settings > Screen Time > Content & Privacy > Access to Web Content."
+	},
+	ImpactsConnectivity: true,
+})
+
 // tryURLUpgrade connects to u, and tries to upgrade it to a net.Conn. If addr
 // is valid, then no DNS is used and the connection will be made to the
 // provided address.
 //
 // Only the provided ctx is used, not a.ctx.
-func (a *Dialer) tryURLUpgrade(ctx context.Context, u *url.URL, addr netip.Addr, init []byte) (net.Conn, error) {
+func (a *Dialer) tryURLUpgrade(ctx context.Context, u *url.URL, addr netip.Addr, init []byte) (_ net.Conn, retErr error) {
 	var dns *dnscache.Resolver
 
 	// If we were provided an address to dial, then create a resolver that just
@@ -423,6 +441,30 @@ func (a *Dialer) tryURLUpgrade(ctx context.Context, u *url.URL, addr netip.Addr,
 		dialer = stdDialer.DialContext
 	}
 
+	// On macOS, see if Screen Time is blocking things.
+	if runtime.GOOS == "darwin" {
+		var proxydIntercepted atomic.Bool // intercepted by macOS webfilterproxyd
+		origDialer := dialer
+		dialer = func(ctx context.Context, network, address string) (net.Conn, error) {
+			c, err := origDialer(ctx, network, address)
+			if err != nil {
+				return nil, err
+			}
+			if isLoopback(c.LocalAddr()) && isLoopback(c.RemoteAddr()) {
+				proxydIntercepted.Store(true)
+			}
+			return c, nil
+		}
+		defer func() {
+			if retErr != nil && proxydIntercepted.Load() {
+				a.HealthTracker.SetUnhealthy(macOSScreenTime, nil)
+				retErr = fmt.Errorf("macOS Screen Time is blocking network access: %w", retErr)
+			} else {
+				a.HealthTracker.SetHealthy(macOSScreenTime)
+			}
+		}()
+	}
+
 	tr := http.DefaultTransport.(*http.Transport).Clone()
 	defer tr.CloseIdleConnections()
 	tr.Proxy = a.getProxyFunc()