lib/connections, lib/model, gui: Specify allowed networks per device (fixes #219)

This adds a new config AllowedNetworks per device, which when set should
contain a list of network prefixes (192.168.0.0/126 etc) that are
allowed for the given device. The connection service will not attempt
connections to addresses outside of the given networks and incoming
connections will be rejected as well.

I've added the config to the normal device editor and shown it (when
set) in the device summary on the main screen.

There's a unit test for the IsAllowedNetwork method, I've done some
manual sanity testing on top of that.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4073

lib/model/model.go

@@ -119,6 +119,7 @@ var (
 	errNotRelative         = errors.New("not a relative path")
 	errFolderPaused        = errors.New("folder is paused")
 	errFolderMissing       = errors.New("no such folder")
+	errNetworkNotAllowed   = errors.New("network not allowed")
 )
 
 // NewModel creates and starts a new model. The model starts in read-only mode,
@@ -1321,21 +1322,27 @@ func (m *Model) OnHello(remoteID protocol.DeviceID, addr net.Addr, hello protoco
 		return errDeviceIgnored
 	}
 
-	if cfg, ok := m.cfg.Device(remoteID); ok {
-		// The device exists
-		if cfg.Paused {
-			return errDevicePaused
-		}
-		return nil
+	cfg, ok := m.cfg.Device(remoteID)
+	if !ok {
+		events.Default.Log(events.DeviceRejected, map[string]string{
+			"name":    hello.DeviceName,
+			"device":  remoteID.String(),
+			"address": addr.String(),
+		})
+		return errDeviceUnknown
 	}
 
-	events.Default.Log(events.DeviceRejected, map[string]string{
-		"name":    hello.DeviceName,
-		"device":  remoteID.String(),
-		"address": addr.String(),
-	})
+	if cfg.Paused {
+		return errDevicePaused
+	}
 
-	return errDeviceUnknown
+	if len(cfg.AllowedNetworks) > 0 {
+		if !connections.IsAllowedNetwork(addr.String(), cfg.AllowedNetworks) {
+			return errNetworkNotAllowed
+		}
+	}
+
+	return nil
 }
 
 // GetHello is called when we are about to connect to some remote device.