Maint: refactor internal/routing

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

internal/routing/vpn.go

@@ -0,0 +1,71 @@
+package routing
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"net"
+
+	"github.com/qdm12/gluetun/internal/netlink"
+)
+
+var (
+	ErrVPNDestinationIPNotFound  = errors.New("VPN destination IP address not found")
+	ErrVPNLocalGatewayIPNotFound = errors.New("VPN local gateway IP address not found")
+)
+
+type VPNDestinationIPGetter interface {
+	VPNDestinationIP() (ip net.IP, err error)
+}
+
+func (r *Routing) VPNDestinationIP() (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)
+	}
+
+	defaultLinkIndex := -1
+	for _, route := range routes {
+		if route.Dst == nil {
+			defaultLinkIndex = route.LinkIndex
+			break
+		}
+	}
+	if defaultLinkIndex == -1 {
+		return nil, fmt.Errorf("%w: in %d route(s)", ErrLinkDefaultNotFound, len(routes))
+	}
+
+	for _, route := range routes {
+		if route.LinkIndex == defaultLinkIndex &&
+			route.Dst != nil &&
+			!IPIsPrivate(route.Dst.IP) &&
+			bytes.Equal(route.Dst.Mask, net.IPMask{255, 255, 255, 255}) {
+			return route.Dst.IP, nil
+		}
+	}
+	return nil, fmt.Errorf("%w: in %d routes", ErrVPNDestinationIPNotFound, len(routes))
+}
+
+type VPNLocalGatewayIPGetter interface {
+	VPNLocalGatewayIP(vpnIntf string) (ip net.IP, err error)
+}
+
+func (r *Routing) VPNLocalGatewayIP(vpnIntf string) (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)
+	}
+	for _, route := range routes {
+		link, err := r.netLinker.LinkByIndex(route.LinkIndex)
+		if err != nil {
+			return nil, fmt.Errorf("%w: %s", ErrLinkByIndex, err)
+		}
+		interfaceName := link.Attrs().Name
+		if interfaceName == vpnIntf &&
+			route.Dst != nil &&
+			route.Dst.IP.Equal(net.IP{0, 0, 0, 0}) {
+			return route.Gw, nil
+		}
+	}
+	return nil, fmt.Errorf("%w: in %d routes", ErrVPNLocalGatewayIPNotFound, len(routes))
+}