ipn/ipnlocal: also accept service IP IPv6 literal in brackets for quad100

The fix in 4fc8538e2 was sufficient for IPv6. Browsers (can?) send the
IPv6 literal, even without a port number, in brackets.

Updates tailscale/corp#7948

Change-Id: I0e429d3de4df8429152c12f251ab140b0c8f6b77
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -3925,6 +3925,17 @@ func (b *LocalBackend) HandleQuad100Port80Conn(c net.Conn) {
 	s.Serve(netutil.NewOneConnListener(c, nil))
 }
 
+func validQuad100Host(h string) bool {
+	switch h {
+	case "",
+		tsaddr.TailscaleServiceIPString,
+		tsaddr.TailscaleServiceIPv6String,
+		"[" + tsaddr.TailscaleServiceIPv6String + "]":
+		return true
+	}
+	return false
+}
+
 func (b *LocalBackend) handleQuad100Port80Conn(w http.ResponseWriter, r *http.Request) {
 	w.Header().Set("X-Frame-Options", "DENY")
 	w.Header().Set("Content-Security-Policy", "default-src 'self';")
@@ -3932,9 +3943,7 @@ func (b *LocalBackend) handleQuad100Port80Conn(w http.ResponseWriter, r *http.Re
 		http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
 		return
 	}
-	switch r.Host {
-	case "", tsaddr.TailscaleServiceIP().String(), tsaddr.TailscaleServiceIPv6().String():
-	default:
+	if !validQuad100Host(r.Host) {
 		http.Error(w, "bad request", http.StatusBadRequest)
 		return
 	}