Routing improvements (#268)

- Fixes #82 
- Remove `EXTRA_SUBNETS`
- Remove no longer needed iptables rules
- Reduce routing interface arity
- Routing setup is done in main.go instead of in the firewall
- Routing setup gets reverted at shutdown

internal/routing/enable.go

@@ -0,0 +1,62 @@
+package routing
+
+import (
+	"fmt"
+	"net"
+)
+
+var (
+	ErrSetup    = fmt.Errorf("cannot setup routing")
+	ErrTeardown = fmt.Errorf("cannot teardown routing")
+)
+
+const (
+	table    = 200
+	priority = 100
+)
+
+func (r *routing) Setup() (err error) {
+	defaultIP, err := r.defaultIP()
+	if err != nil {
+		return fmt.Errorf("%s: %w", ErrSetup, err)
+	}
+	defaultInterfaceName, defaultGateway, err := r.DefaultRoute()
+	if err != nil {
+		return fmt.Errorf("%s: %w", ErrSetup, err)
+	}
+
+	defer func() {
+		if err == nil {
+			return
+		}
+		if err := r.TearDown(); err != nil {
+			r.logger.Error(err)
+		}
+	}()
+	if err := r.addIPRule(defaultIP, table, priority); err != nil {
+		return fmt.Errorf("%s: %w", ErrSetup, err)
+	}
+	if err := r.addRouteVia(net.IPNet{}, defaultGateway, defaultInterfaceName, table); err != nil {
+		return fmt.Errorf("%s: %w", ErrSetup, err)
+	}
+	return nil
+}
+
+func (r *routing) TearDown() error {
+	defaultIP, err := r.defaultIP()
+	if err != nil {
+		return fmt.Errorf("%s: %w", ErrTeardown, err)
+	}
+	defaultInterfaceName, defaultGateway, err := r.DefaultRoute()
+	if err != nil {
+		return fmt.Errorf("%s: %w", ErrTeardown, err)
+	}
+
+	if err := r.deleteRouteVia(net.IPNet{}, defaultGateway, defaultInterfaceName, table); err != nil {
+		return fmt.Errorf("%s: %w", ErrTeardown, err)
+	}
+	if err := r.deleteIPRule(defaultIP, table, priority); err != nil {
+		return fmt.Errorf("%s: %w", ErrTeardown, err)
+	}
+	return nil
+}