Fourth fallback, use DROP temporarily instead of REJECT

internal/firewall/flush.go

@@ -10,33 +10,52 @@ import (
 	"github.com/qdm12/gluetun/internal/netlink"
 )
 
-// Note remove is a no-op if conntrack netlink is supported by the kernel.
 func (c *Config) flushExistingConnections(ctx context.Context) error {
-	err := c.netlinker.FlushConntrack()
-	switch {
-	case err == nil:
-		return nil
-	case errors.Is(err, netlink.ErrConntrackNetlinkNotSupported):
-		c.logger.Debugf("falling back to marking and filtering unmarked packets because flush conntrack failed: %s", err)
-		err = c.impl.AcceptOutputPublicOnlyNewTraffic(ctx)
-		if err != nil {
-			if errors.Is(err, iptables.ErrKernelModuleMissing) {
-				c.logger.Debugf("falling back to killing connections for one second because marking packets failed: %s", err)
-				return c.rejectOutputTrafficTemporarily(ctx)
-			}
-			return fmt.Errorf("accepting only new output public traffic: %w", err)
-		}
-		return nil
-	default:
-		return fmt.Errorf("flushing conntrack: %w", err)
+	tries := []struct {
+		name string
+		f    func(ctx context.Context) error
+	}{
+		{name: "flushing conntrack", f: func(_ context.Context) error {
+			return c.netlinker.FlushConntrack()
+		}},
+		{name: "marking and filtering unmarked packets", f: c.impl.AcceptOutputPublicOnlyNewTraffic},
+		{name: "rejecting connections for one second", f: c.rejectOutputTrafficTemporarily},
+		{name: "dropping connections for one second", f: c.dropOutputTrafficTemporarily},
 	}
+	errs := make([]error, 0, len(tries))
+	for i, try := range tries {
+		if i > 0 {
+			c.logger.Debugf("falling back to %s because %s failed: %s", try.name, tries[i-1].name, errs[i-1])
+		}
+		err := try.f(ctx)
+		if err == nil {
+			return nil
+		}
+		err = fmt.Errorf("%s: %w", try.name, err)
+		if !errors.Is(err, iptables.ErrKernelModuleMissing) && !errors.Is(err, netlink.ErrConntrackNetlinkNotSupported) {
+			return err
+		}
+		errs = append(errs, err)
+	}
+	return fmt.Errorf("all tries failed: %v", errs) //nolint:err113
 }
 
 func (c *Config) rejectOutputTrafficTemporarily(ctx context.Context) error {
+	return setupThenRevert(ctx, c.impl.RejectOutputPublicTraffic)
+}
+
+func (c *Config) dropOutputTrafficTemporarily(ctx context.Context) error {
+	return setupThenRevert(ctx, c.impl.DropOutputPublicTraffic)
+}
+
+// setupThenRevert is a helper function to run a setup function that takes a remove boolean argument,
+// and then run the same function with remove set to true after one second or when the context is canceled,
+// whichever comes first.
+func setupThenRevert(ctx context.Context, f func(ctx context.Context, remove bool) error) error {
 	remove := false
-	err := c.impl.RejectOutputPublicTraffic(ctx, remove)
+	err := f(ctx, remove)
 	if err != nil {
-		return fmt.Errorf("rejecting only new output public traffic: %w", err)
+		return fmt.Errorf("setting up: %w", err)
 	}
 	timer := time.NewTimer(time.Second)
 	select {
@@ -47,9 +66,9 @@ func (c *Config) rejectOutputTrafficTemporarily(ctx context.Context) error {
 	remove = true
 	// Use [context.Background] to make sure this is removed, even if the context
 	// passed to this function is canceled.
-	err = c.impl.RejectOutputPublicTraffic(context.Background(), remove)
+	err = f(context.Background(), remove)
 	if err != nil {
-		return fmt.Errorf("reverting rejecting only new output public traffic: %w", err)
+		return fmt.Errorf("reverting: %w", err)
 	}
 	return nil
 }

internal/firewall/interfaces.go

@@ -28,6 +28,7 @@ type firewallImpl interface { //nolint:interfacebloat
 	SaveAndRestore(ctx context.Context) (restore func(context.Context), err error)
 	AcceptOutputPublicOnlyNewTraffic(ctx context.Context) error
 	RejectOutputPublicTraffic(ctx context.Context, remove bool) error
+	DropOutputPublicTraffic(ctx context.Context, remove bool) error
 	AcceptInputThroughInterface(ctx context.Context, intf string) error
 	AcceptEstablishedRelatedTraffic(ctx context.Context) error
 	AcceptInputToPort(ctx context.Context, intf string, port uint16, remove bool) error

internal/firewall/iptables/iptables.go

@@ -208,6 +208,14 @@ func (c *Config) AcceptOutputPublicOnlyNewTraffic(ctx context.Context) error {
 }
 
 func (c *Config) RejectOutputPublicTraffic(ctx context.Context, remove bool) error {
+	return c.targetOutputPublicTraffic(ctx, "REJECT", remove)
+}
+
+func (c *Config) DropOutputPublicTraffic(ctx context.Context, remove bool) error {
+	return c.targetOutputPublicTraffic(ctx, "DROP", remove)
+}
+
+func (c *Config) targetOutputPublicTraffic(ctx context.Context, target string, remove bool) error {
 	removeInstructions := []string{
 		"-D OUTPUT -j PUBLIC_ONLY",
 		"-F PUBLIC_ONLY",
@@ -223,11 +231,15 @@ func (c *Config) RejectOutputPublicTraffic(ctx context.Context, remove bool) err
 		ipv6Instructions = append(ipv6Instructions, instruction)
 	}
 
-	// Block UDP and ICMP, sending back ICMP port unreachable.
-	appendToBoth("-A PUBLIC_ONLY -m conntrack --ctstate RELATED,ESTABLISHED -j REJECT")
-	// Block TCP by sending back TCP RST packets.
-	appendToBoth("-A PUBLIC_ONLY -p tcp -m conntrack --ctstate RELATED,ESTABLISHED " +
-		"-j REJECT --reject-with tcp-reset")
+	if target == "REJECT" {
+		// Block TCP by sending back TCP RST packets.
+		appendToBoth("-A PUBLIC_ONLY -p tcp -m conntrack --ctstate RELATED,ESTABLISHED " +
+			"-j REJECT --reject-with tcp-reset")
+		// Block UDP and ICMP, sending back ICMP port unreachable.
+		appendToBoth("-A PUBLIC_ONLY -m conntrack --ctstate RELATED,ESTABLISHED -j REJECT")
+	} else {
+		appendToBoth("-A PUBLIC_ONLY -m conntrack --ctstate RELATED,ESTABLISHED -j " + target)
+	}
 	appendToBoth("-I OUTPUT -j PUBLIC_ONLY")
 
 	kernelErr := checkKernelModulesAreOK(c.modules.nfConntrack,