dialer: Dial directly when only one interface is available

common/dialer/default_parallel_interface.go

@@ -19,6 +19,27 @@ func (d *DefaultDialer) dialParallelInterface(ctx context.Context, dialer net.Di
 		return nil, false, E.New("no available network interface")
 	}
 	defaultInterface := d.networkManager.InterfaceMonitor().DefaultInterface()
+	if len(primaryInterfaces)+len(fallbackInterfaces) == 1 {
+		var (
+			iif     adapter.NetworkInterface
+			primary bool
+		)
+		if len(primaryInterfaces) == 1 {
+			iif = primaryInterfaces[0]
+			primary = true
+		} else {
+			iif = fallbackInterfaces[0]
+		}
+		perNetDialer := dialer
+		if defaultInterface == nil || iif.Index != defaultInterface.Index {
+			perNetDialer.Control = control.Append(perNetDialer.Control, control.BindToInterface(nil, iif.Name, iif.Index))
+		}
+		conn, err := perNetDialer.DialContext(ctx, network, addr)
+		if err != nil {
+			return nil, false, E.Cause(err, "dial ", iif.Name, " (", iif.Index, ")")
+		}
+		return conn, primary, nil
+	}
 	if fallbackDelay == 0 {
 		fallbackDelay = N.DefaultFallbackDelay
 	}
@@ -93,6 +114,27 @@ func (d *DefaultDialer) dialParallelInterfaceFastFallback(ctx context.Context, d
 		return nil, false, E.New("no available network interface")
 	}
 	defaultInterface := d.networkManager.InterfaceMonitor().DefaultInterface()
+	if len(primaryInterfaces)+len(fallbackInterfaces) == 1 {
+		var (
+			iif     adapter.NetworkInterface
+			primary bool
+		)
+		if len(primaryInterfaces) == 1 {
+			iif = primaryInterfaces[0]
+			primary = true
+		} else {
+			iif = fallbackInterfaces[0]
+		}
+		perNetDialer := dialer
+		if defaultInterface == nil || iif.Index != defaultInterface.Index {
+			perNetDialer.Control = control.Append(perNetDialer.Control, control.BindToInterface(nil, iif.Name, iif.Index))
+		}
+		conn, err := perNetDialer.DialContext(ctx, network, addr)
+		if err != nil {
+			return nil, false, E.Cause(err, "dial ", iif.Name, " (", iif.Index, ")")
+		}
+		return conn, primary, nil
+	}
 	if fallbackDelay == 0 {
 		fallbackDelay = N.DefaultFallbackDelay
 	}