fix: Scope default gateway detection to main routing table

The `getDefaultGwInterfaces` function previously filtered routes from all
routing tables. This caused interfaces to be incorrectly identified as
default gateways and excluded from the ResourceSlice if they had a
default route in ANY table, even non-main ones.

Author: gauravkghildiyal

pkg/inventory/db.go

@@ -152,6 +152,7 @@ func (db *DB) Run(ctx context.Context) error {
 	// Obtain data that will not change after the startup
 	db.instance = getInstanceProperties(ctx)
 	db.gwInterfaces = getDefaultGwInterfaces()
+	klog.V(2).Infof("Default gateway interfaces: %v", db.gwInterfaces.UnsortedList())
 
 	for {
 		err := db.rateLimiter.Wait(ctx)

pkg/inventory/net.go

@@ -21,14 +21,21 @@ import (
 	"github.com/cilium/ebpf/link"
 	"github.com/google/dranet/internal/nlwrap"
 	"github.com/vishvananda/netlink"
+	"golang.org/x/sys/unix"
 
 	"k8s.io/apimachinery/pkg/util/sets"
 	"k8s.io/klog/v2"
 )
 
+// getDefaultGwInterfaces returns a set of interface names that are configured
+// as default gateways in the main routing table. It identifies these by querying
+// the main routing table for routes with an unspecified destination (0.0.0.0/0
+// for IPv4 or ::/0 for IPv6).
 func getDefaultGwInterfaces() sets.Set[string] {
 	interfaces := sets.Set[string]{}
-	filter := &netlink.Route{}
+	filter := &netlink.Route{
+		Table: unix.RT_TABLE_MAIN,
+	}
 	routes, err := nlwrap.RouteListFiltered(netlink.FAMILY_ALL, filter, netlink.RT_FILTER_TABLE)
 	if err != nil {
 		return interfaces
@@ -68,7 +75,6 @@ func getDefaultGwInterfaces() sets.Set[string] {
 			interfaces.Insert(intfLink.Attrs().Name)
 		}
 	}
-	klog.V(4).Infof("Found following interfaces for the default gateway: %v", interfaces.UnsortedList())
 	return interfaces
 }