cmd/derper: permit port 80 in ACE targets

Updates tailscale/corp#32168
Updates tailscale/corp#32226

Change-Id: Iddc017b060c76e6eab8f6d0c989a775bcaae3518
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/derper/ace.go

@@ -35,8 +35,35 @@ func serveConnect(s *derp.Server, w http.ResponseWriter, r *http.Request) {
 			if err != nil {
 				return err
 			}
-			if port != "443" {
-				return fmt.Errorf("only port 443 is allowed")
+			if port != "443" && port != "80" {
+				// There are only two types of CONNECT requests the client makes
+				// via ACE: requests for /key (port 443) and requests to upgrade
+				// to the bidirectional ts2021 Noise protocol.
+				//
+				// The ts2021 layer can bootstrap over port 80 (http) or port
+				// 443 (https).
+				//
+				// Without ACE, we prefer port 80 to avoid unnecessary double
+				// encryption. But enough places require TLS+port 443 that we do
+				// support that double encryption path as a fallback.
+				//
+				// But ACE adds its own TLS layer (ACE is always CONNECT over
+				// https). If we don't permit port 80 here as a target, we'd
+				// have three layers of encryption (TLS + TLS + Noise) which is
+				// even more silly than two.
+				//
+				// So we permit port 80 such that we can only have two layers of
+				// encryption, varying by the request type:
+				//
+				//  1. TLS from client to ACE proxy (CONNECT)
+				//  2a. TLS from ACE proxy to https://controlplane.tailscale.com/key (port 443)
+				//  2b. ts2021 Noise from ACE proxy to http://controlplane.tailscale.com/ts2021 (port 80)
+				//
+				// But nothing's stopping the client from doing its ts2021
+				// upgrade over https anyway and having three layers of
+				// encryption. But we can at least permit the client to do a
+				// "CONNECT controlplane.tailscale.com:80 HTTP/1.1" if it wants.
+				return fmt.Errorf("only ports 443 and 80 are allowed")
 			}
 			// TODO(bradfitz): make policy configurable from flags and/or come
 			// from local tailscaled nodeAttrs