Improve routing decision logic

client/firewall/uspfilter/uspfilter.go

@@ -1,6 +1,7 @@
 package uspfilter
 
 import (
+	"errors"
 	"fmt"
 	"net"
 	"net/netip"
@@ -31,11 +32,6 @@ const (
 	// EnvDisableUserspaceRouting disables userspace routing, to-be-routed packets will be dropped.
 	EnvDisableUserspaceRouting = "NB_DISABLE_USERSPACE_ROUTING"
 
-	// EnvForceNativeRouter forces forwarding to the native stack (even if doesn't support routing).
-	// This is useful when routing/firewall setup is done manually instead of by netbird.
-	// This setting always disables userspace routing and filtering of routed traffic.
-	EnvForceNativeRouter = "NB_FORCE_NATIVE_ROUTER"
-
 	// EnvForceUserspaceRouter forces userspace routing even if native routing is available.
 	EnvForceUserspaceRouter = "NB_FORCE_USERSPACE_ROUTER"
 )
@@ -88,49 +84,23 @@ type decoder struct {
 
 // Create userspace firewall manager constructor
 func Create(iface common.IFaceMapper, disableServerRoutes bool) (*Manager, error) {
-	return create(iface, disableServerRoutes)
+	return create(iface, nil, disableServerRoutes)
 }
 
 func CreateWithNativeFirewall(iface common.IFaceMapper, nativeFirewall firewall.Manager, disableServerRoutes bool) (*Manager, error) {
-	mgr, err := create(iface, disableServerRoutes)
+	if nativeFirewall == nil {
+		return nil, errors.New("native firewall is nil")
+	}
+
+	mgr, err := create(iface, nativeFirewall, disableServerRoutes)
 	if err != nil {
 		return nil, err
 	}
 
-	mgr.nativeFirewall = nativeFirewall
-
-	if disableServerRoutes {
-		// skip native vs userspace router logic altogether
 	return mgr, nil
 }
 
-	if forceUserspaceRouter, _ := strconv.ParseBool(os.Getenv(EnvForceUserspaceRouter)); forceUserspaceRouter {
+func create(iface common.IFaceMapper, nativeFirewall firewall.Manager, disableServerRoutes bool) (*Manager, error) {
-		log.Info("userspace routing is forced")
-		return mgr, nil
-	}
-
-	forceNativeRouter, _ := strconv.ParseBool(EnvForceNativeRouter)
-
-	// if the OS supports routing natively, or it is explicitly requested, then we don't need to filter/route ourselves
-	// netstack mode won't support native routing as there is no interface
-	if forceNativeRouter ||
-		!netstack.IsEnabled() && mgr.nativeFirewall != nil && mgr.nativeFirewall.IsServerRouteSupported() {
-
-		mgr.nativeRouter = true
-		mgr.routingEnabled = true
-		if mgr.forwarder != nil {
-			mgr.forwarder.Stop()
-		}
-
-		log.Info("native routing is enabled")
-		return mgr, nil
-	}
-
-	log.Info("userspace routing is enabled")
-	return mgr, nil
-}
-
-func create(iface common.IFaceMapper, disableServerRoutes bool) (*Manager, error) {
 	disableConntrack, _ := strconv.ParseBool(os.Getenv(EnvDisableConntrack))
 
 	m := &Manager{
@@ -147,6 +117,7 @@ func create(iface common.IFaceMapper, disableServerRoutes bool) (*Manager, error
 				return d
 			},
 		},
+		nativeFirewall: nativeFirewall,
 		outgoingRules:  make(map[string]RuleSet),
 		incomingRules:  make(map[string]RuleSet),
 		routeRules:     make(map[string]RouteRule),
@@ -154,7 +125,6 @@ func create(iface common.IFaceMapper, disableServerRoutes bool) (*Manager, error
 		localipmanager: newLocalIPManager(),
 		routingEnabled: false,
 		stateful:       !disableConntrack,
-		// TODO: support changing log level from logrus
 		logger:         nblog.NewFromLogrus(log.StandardLogger()),
 		netstack:       netstack.IsEnabled(),
 	}
@@ -172,17 +142,7 @@ func create(iface common.IFaceMapper, disableServerRoutes bool) (*Manager, error
 		m.tcpTracker = conntrack.NewTCPTracker(conntrack.DefaultTCPTimeout, m.logger)
 	}
 
-	disableUspRouting, _ := strconv.ParseBool(os.Getenv(EnvDisableUserspaceRouting))
+	m.determineRouting(iface, disableServerRoutes)
-	if disableUspRouting || disableServerRoutes {
-		log.Info("userspace routing is disabled")
-	} else {
-		m.routingEnabled = true
-	}
-
-	// netstack needs the forwarder for local traffic
-	if m.netstack || m.routingEnabled {
-		m.initForwarder(iface)
-	}
 
 	if err := iface.SetFilter(m); err != nil {
 		return nil, fmt.Errorf("set filter: %w", err)
@@ -190,6 +150,54 @@ func create(iface common.IFaceMapper, disableServerRoutes bool) (*Manager, error
 	return m, nil
 }
 
+func (m *Manager) determineRouting(iface common.IFaceMapper, disableServerRoutes bool) {
+	disableUspRouting, _ := strconv.ParseBool(os.Getenv(EnvDisableUserspaceRouting))
+	forceUserspaceRouter, _ := strconv.ParseBool(os.Getenv(EnvForceUserspaceRouter))
+
+	switch {
+	case disableUspRouting:
+		m.routingEnabled = false
+		m.nativeRouter = false
+		log.Info("userspace routing is disabled")
+
+	case disableServerRoutes:
+		//  if server routes are disabled we will let packets pass to the native stack
+		m.routingEnabled = true
+		m.nativeRouter = true
+
+		log.Info("server routes are disabled")
+
+	case forceUserspaceRouter:
+		m.routingEnabled = true
+		m.nativeRouter = false
+
+		log.Info("userspace routing is forced")
+
+	case !m.netstack && m.nativeFirewall != nil && m.nativeFirewall.IsServerRouteSupported():
+		// if the OS supports routing natively, then we don't need to filter/route ourselves
+		// netstack mode won't support native routing as there is no interface
+
+		m.routingEnabled = true
+		m.nativeRouter = true
+
+		log.Info("native routing is enabled")
+
+	default:
+		m.routingEnabled = true
+		m.nativeRouter = false
+
+		log.Info("userspace routing enabled by default")
+	}
+
+	// netstack needs the forwarder for local traffic
+	if m.netstack ||
+		m.routingEnabled && !m.nativeRouter {
+
+		m.initForwarder(iface)
+	}
+}
+
+// initForwarder initializes the forwarder, it disables routing on errors
 func (m *Manager) initForwarder(iface common.IFaceMapper) {
 	// Only supported in userspace mode as we need to inject packets back into wireguard directly
 	intf := iface.GetWGDevice()
@@ -218,7 +226,7 @@ func (m *Manager) IsServerRouteSupported() bool {
 }
 
 func (m *Manager) AddNatRule(pair firewall.RouterPair) error {
-	if m.nativeRouter {
+	if m.nativeRouter && m.nativeFirewall != nil {
 		return m.nativeFirewall.AddNatRule(pair)
 	}
 
@@ -229,7 +237,7 @@ func (m *Manager) AddNatRule(pair firewall.RouterPair) error {
 
 // RemoveNatRule removes a routing firewall rule
 func (m *Manager) RemoveNatRule(pair firewall.RouterPair) error {
-	if m.nativeRouter {
+	if m.nativeRouter && m.nativeFirewall != nil {
 		return m.nativeFirewall.RemoveNatRule(pair)
 	}
 	return nil
@@ -313,7 +321,7 @@ func (m *Manager) AddRouteFiltering(
 	dPort *firewall.Port,
 	action firewall.Action,
 ) (firewall.Rule, error) {
-	if m.nativeRouter {
+	if m.nativeRouter && m.nativeFirewall != nil {
 		return m.nativeFirewall.AddRouteFiltering(sources, destination, proto, sPort, dPort, action)
 	}
 
@@ -337,7 +345,7 @@ func (m *Manager) AddRouteFiltering(
 }
 
 func (m *Manager) DeleteRouteRule(rule firewall.Rule) error {
-	if m.nativeRouter {
+	if m.nativeRouter && m.nativeFirewall != nil {
 		return m.nativeFirewall.DeleteRouteRule(rule)
 	}