Maint: refactor internal/routing

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

internal/routing/default.go

@@ -0,0 +1,68 @@
+package routing
+
+import (
+	"errors"
+	"fmt"
+	"net"
+
+	"github.com/qdm12/gluetun/internal/netlink"
+)
+
+var (
+	ErrRouteDefaultNotFound = errors.New("default route not found")
+)
+
+type DefaultRouteGetter interface {
+	DefaultRoute() (defaultInterface string, defaultGateway net.IP, err error)
+}
+
+func (r *Routing) DefaultRoute() (defaultInterface string, defaultGateway net.IP, err error) {
+	routes, err := r.netLinker.RouteList(nil, netlink.FAMILY_ALL)
+	if err != nil {
+		return "", nil, fmt.Errorf("%w: %s", ErrRoutesList, err)
+	}
+	for _, route := range routes {
+		if route.Dst == nil {
+			defaultGateway = route.Gw
+			linkIndex := route.LinkIndex
+			link, err := r.netLinker.LinkByIndex(linkIndex)
+			if err != nil {
+				return "", nil, fmt.Errorf("%w: for default route at index %d: %s", ErrLinkByIndex, linkIndex, err)
+			}
+			attributes := link.Attrs()
+			defaultInterface = attributes.Name
+			r.logger.Info("default route found: interface " + defaultInterface +
+				", gateway " + defaultGateway.String())
+			return defaultInterface, defaultGateway, nil
+		}
+	}
+	return "", nil, fmt.Errorf("%w: in %d route(s)", ErrRouteDefaultNotFound, len(routes))
+}
+
+type DefaultIPGetter interface {
+	DefaultIP() (defaultIP net.IP, err error)
+}
+
+func (r *Routing) DefaultIP() (ip net.IP, err error) {
+	routes, err := r.netLinker.RouteList(nil, netlink.FAMILY_ALL)
+	if err != nil {
+		return nil, fmt.Errorf("%w: %s", ErrRoutesList, err)
+	}
+
+	defaultLinkName := ""
+	for _, route := range routes {
+		if route.Dst == nil {
+			linkIndex := route.LinkIndex
+			link, err := r.netLinker.LinkByIndex(linkIndex)
+			if err != nil {
+				return nil, fmt.Errorf("%w: for default route at index %d: %s", ErrLinkByIndex, linkIndex, err)
+			}
+			defaultLinkName = link.Attrs().Name
+		}
+	}
+	if defaultLinkName == "" {
+		return nil, fmt.Errorf("%w: in %d route(s)", ErrLinkDefaultNotFound, len(routes))
+	}
+
+	return r.assignedIP(defaultLinkName)
+}