network: Handle default route where gateway is empty

In case of ipvlan plugin, the default route has an empty gateway.
Set this to default gateway value before adding this route with
netlink.

Fixes kata-containers#403

Signed-off-by: Archana Shinde <archana.m.shinde@intel.com>

network.go

@@ -31,6 +31,18 @@ var (
 	errNoRoutes = grpcStatus.Errorf(codes.InvalidArgument, "Need network routes")
 )
 
+const (
+	// ipvlan plugin adds a route of the format "default dev eth0 scope link"
+	// Here since source, dest and gateway are empty, netlink will complain.
+	// Set the gateway address explcitly to handle this, setting this address
+	// is equivalent to the above route.
+
+	defaultV4RouteIP = "0.0.0.0"
+
+	// Use the below address for ipv6 gateway once ipv6 support is added
+	// defaultV6RouteIP = "::"
+)
+
 // Network fully describes a sandbox network with its interfaces, routes and dns
 // related information.
 type network struct {
@@ -536,6 +548,13 @@ func (s *sandbox) updateRoute(netHandle *netlink.Handle, route *types.Route, add
 		return grpcStatus.Errorf(codes.Internal, "Could not get link's attributes for device %s", route.Device)
 	}
 
+	// We do not modify the gateway in the list of routes provided,
+	// since we loop through the routes checking for the gateway again.
+	gateway := route.Gateway
+	if route.Dest == "" && route.Gateway == "" && route.Source == "" {
+		gateway = defaultV4RouteIP
+	}
+
 	var dst *net.IPNet
 	if route.Dest == "default" || route.Dest == "" {
 		dst = nil
@@ -550,7 +569,7 @@ func (s *sandbox) updateRoute(netHandle *netlink.Handle, route *types.Route, add
 		LinkIndex: linkAttrs.Index,
 		Dst:       dst,
 		Src:       net.ParseIP(route.Source),
-		Gw:        net.ParseIP(route.Gateway),
+		Gw:        net.ParseIP(gateway),
 		Scope:     netlink.Scope(route.Scope),
 	}