Maint: refactor internal/routing

- Split Go files better
- Reduce public API for exported errors

internal/routing/routes.go

@@ -0,0 +1,70 @@
+package routing
+
+import (
+	"errors"
+	"fmt"
+	"net"
+	"strconv"
+
+	"github.com/qdm12/gluetun/internal/netlink"
+)
+
+var (
+	errLinkByName = errors.New("cannot obtain link by name")
+)
+
+func (r *Routing) addRouteVia(destination net.IPNet, gateway net.IP,
+	iface string, table int) error {
+	destinationStr := destination.String()
+	r.logger.Info("adding route for " + destinationStr)
+	r.logger.Debug("ip route replace " + destinationStr +
+		" via " + gateway.String() +
+		" dev " + iface +
+		" table " + strconv.Itoa(table))
+
+	link, err := r.netLinker.LinkByName(iface)
+	if err != nil {
+		return fmt.Errorf("%w: interface %s: %s", errLinkByName, iface, err)
+	}
+
+	route := netlink.Route{
+		Dst:       &destination,
+		Gw:        gateway,
+		LinkIndex: link.Attrs().Index,
+		Table:     table,
+	}
+	if err := r.netLinker.RouteReplace(&route); err != nil {
+		return fmt.Errorf("%w: for subnet %s at interface %s",
+			err, destinationStr, iface)
+	}
+
+	return nil
+}
+
+func (r *Routing) deleteRouteVia(destination net.IPNet, gateway net.IP,
+	iface string, table int) (err error) {
+	destinationStr := destination.String()
+	r.logger.Info("deleting route for " + destinationStr)
+	r.logger.Debug("ip route delete " + destinationStr +
+		" via " + gateway.String() +
+		" dev " + iface +
+		" table " + strconv.Itoa(table))
+
+	link, err := r.netLinker.LinkByName(iface)
+	if err != nil {
+		return fmt.Errorf("%w: for interface %s: %s", errLinkByName, iface, err)
+	}
+
+	route := netlink.Route{
+		Dst:       &destination,
+		Gw:        gateway,
+		LinkIndex: link.Attrs().Index,
+		Table:     table,
+	}
+	if err := r.netLinker.RouteDel(&route); err != nil {
+		return fmt.Errorf("%w: for subnet %s at interface %s",
+			err, destinationStr, iface)
+	}
+
+	return nil
+}