IP_STATUS_FILE and routing improvements (#130)

- Obtains VPN public IP address from routing table
- Logs and writes VPN Public IP address to `/ip` as soon as VPN is up
- Obtain port forward, logs it and writes it as soon as VPN is up
- Routing fully refactored and tested
- Routing reads from `/proc/net/route`
- Routing mutates the routes using `ip route ...`

internal/routing/mutate.go

@@ -0,0 +1,34 @@
+package routing
+
+import (
+	"net"
+
+	"fmt"
+)
+
+func (r *routing) AddRoutesVia(subnets []net.IPNet, defaultGateway net.IP, defaultInterface string) error {
+	for _, subnet := range subnets {
+		exists, err := r.routeExists(subnet)
+		if err != nil {
+			return err
+		} else if exists { // thanks to @npawelek https://github.com/npawelek
+			if err := r.removeRoute(subnet); err != nil {
+				return err
+			}
+		}
+		r.logger.Info("adding %s as route via %s", subnet.String(), defaultInterface)
+		output, err := r.commander.Run("ip", "route", "add", subnet.String(), "via", defaultGateway.String(), "dev", defaultInterface)
+		if err != nil {
+			return fmt.Errorf("cannot add route for %s via %s %s %s: %s: %w", subnet.String(), defaultGateway.String(), "dev", defaultInterface, output, err)
+		}
+	}
+	return nil
+}
+
+func (r *routing) removeRoute(subnet net.IPNet) (err error) {
+	output, err := r.commander.Run("ip", "route", "del", subnet.String())
+	if err != nil {
+		return fmt.Errorf("cannot delete route for %s: %s: %w", subnet.String(), output, err)
+	}
+	return nil
+}