fix mtu detection

cmd/cni-kindnet/netdev.go

@@ -7,6 +7,7 @@ import (
 	"net"
 	"os"
 	"runtime"
+	"sort"
 	"strconv"
 
 	"github.com/vishvananda/netlink"
@@ -290,38 +291,25 @@ func deletePodInterface(ifName string, netNS string) error {
 }
 
 func getDefaultGwInterfaceMTU() int {
-	routes, err := netlink.RouteList(nil, netlink.FAMILY_ALL)
+	routes, err := netlink.RouteListFiltered(netlink.FAMILY_ALL, &netlink.Route{Dst: nil}, netlink.RT_FILTER_DST)
 	if err != nil {
 		return 0
 	}
-
+	if len(routes) == 0 {
+		return 0
+	}
+	// use the route with higher priority
+	sort.Slice(routes, func(i, j int) bool {
+		return routes[i].Priority < routes[j].Priority
+	})
+	// use the mtu of the first interface
 	for _, r := range routes {
-		// no multipath
-		if len(r.MultiPath) == 0 {
-			if r.Gw == nil {
-				continue
-			}
-			intfLink, err := netlink.LinkByIndex(r.LinkIndex)
-			if err != nil {
-				logger.Printf("Failed to get interface link for route %v : %v", r, err)
-				continue
-			}
-			return intfLink.Attrs().MTU
-		}
-
-		// multipath, use the first valid entry
-		// xref: https://github.com/vishvananda/netlink/blob/6ffafa9fc19b848776f4fd608c4ad09509aaacb4/route.go#L137-L145
-		for _, nh := range r.MultiPath {
-			if nh.Gw == nil {
-				continue
-			}
-			intfLink, err := netlink.LinkByIndex(r.LinkIndex)
-			if err != nil {
-				logger.Printf("Failed to get interface link for route %v : %v", r, err)
-				continue
-			}
-			return intfLink.Attrs().MTU
+		intfLink, err := netlink.LinkByIndex(r.LinkIndex)
+		if err != nil {
+			logger.Printf("Failed to get interface link for route %v : %v", r, err)
+			continue
 		}
+		return intfLink.Attrs().MTU
 	}
 	return 0
 }