lib/connections: Only announce punchable nats (fixes #4519)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4523

lib/connections/kcp_listen.go

@@ -24,6 +24,8 @@ import (
 	"github.com/syncthing/syncthing/lib/nat"
 )
 
+const stunRetryInterval = 5 * time.Minute
+
 func init() {
 	factory := &kcpListenerFactory{}
 	for _, scheme := range []string{"kcp", "kcp4", "kcp6"} {
@@ -246,6 +248,13 @@ func (t *kcpListener) stunRenewal(listener net.PacketConn) {
 			if oldType != natType {
 				l.Infof("%s detected NAT type: %s", t.uri, natType)
 				t.nat.Store(natType)
+				oldType = natType
+			}
+
+			// We can't punch through this one, so no point doing keepalives
+			// and such, just try again in a minute and hope that the NAT type changes.
+			if !isPunchable(natType) {
+				break
 			}
 
 			for {
@@ -285,12 +294,11 @@ func (t *kcpListener) stunRenewal(listener net.PacketConn) {
 					break
 				}
 			}
-
-			oldType = natType
 		}
 
-		// We failed to contact all provided stun servers, chillout for a while.
-		time.Sleep(time.Minute)
+		// We failed to contact all provided stun servers or the nat is not punchable.
+		// Chillout for a while.
+		time.Sleep(stunRetryInterval)
 	}
 }
 
@@ -312,3 +320,7 @@ func (f *kcpListenerFactory) New(uri *url.URL, cfg *config.Wrapper, tlsCfg *tls.
 func (kcpListenerFactory) Enabled(cfg config.Configuration) bool {
 	return true
 }
+
+func isPunchable(natType stun.NATType) bool {
+	return natType == stun.NATNone || natType == stun.NATPortRestricted || natType == stun.NATRestricted || natType == stun.NATFull
+}