Fix default route replacement

Multus was failing to detect existing Default routes in container and
remove them before replacement, which causes later adding of default
route to fail with EEXIST, which is ignored and not detected as a error.

This leads to container keeping existing default route in case of
chained CNIs instead of getting new default route from Multus.

Fixes regression introduced by f18d96b

There was a breaking change in that bump:
vishvananda/netlink@acdc658
route.Dst is a zero IPNet instead of nil

Check for default route using isIPNetZero adopted from
github.com/containernetworking/plugins/pkg/ip.IsIPNetZero. Avoid
importing ip.IsIPNetZero to avoid brining more dependencies.

Signed-off-by: Viktor Oreshkin <imselfish@stek29.rocks>

Author: stek29

pkg/netutils/netutils.go

@@ -30,6 +30,16 @@ import (
 	"gopkg.in/k8snetworkplumbingwg/multus-cni.v4/pkg/logging"
 )
 
+func isIPNetZero(ipnet *net.IPNet) bool {
+	if ipnet == nil {
+		return true
+	}
+	if ones, _ := ipnet.Mask.Size(); ones != 0 {
+		return false
+	}
+	return ipnet.IP.Equal(net.IPv4zero) || ipnet.IP.Equal(net.IPv6zero)
+}
+
 // DeleteDefaultGW removes the default gateway from marked interfaces.
 func DeleteDefaultGW(netnsPath string, ifName string) error {
 	netns, err := ns.GetNS(netnsPath)
@@ -43,7 +53,7 @@ func DeleteDefaultGW(netnsPath string, ifName string) error {
 		link, _ := netlink.LinkByName(ifName)
 		routes, _ := netlink.RouteList(link, netlink.FAMILY_ALL)
 		for _, nlroute := range routes {
-			if nlroute.Dst == nil {
+			if isIPNetZero(nlroute.Dst) {
 				err = netlink.RouteDel(&nlroute)
 			}
 		}

pkg/netutils/netutils_test.go

@@ -40,6 +40,17 @@ func TestNetutils(t *testing.T) {
 	RunSpecs(t, "netutils")
 }
 
+var _ = Describe("default route detection", func() {
+	DescribeTable("detects default routes",
+		func(dst *net.IPNet) {
+			Expect(isIPNetZero(dst)).To(BeTrue())
+		},
+		Entry("nil destination", nil),
+		Entry("IPv4 zero prefix", &net.IPNet{IP: net.IPv4zero, Mask: net.CIDRMask(0, 32)}),
+		Entry("IPv6 zero prefix", &net.IPNet{IP: net.IPv6zero, Mask: net.CIDRMask(0, 128)}),
+	)
+})
+
 // helper function
 func testAddRoute(link netlink.Link, ip net.IP, mask net.IPMask, gw net.IP) error {
 	dst := &net.IPNet{
@@ -234,6 +245,18 @@ var _ = Describe("netutil netlink function testing", func() {
 				Expect(DeleteDefaultGW(args.Netns, IFNAME)).Should(Succeed())
 				return nil
 			})).Should(Succeed())
+
+			Expect(targetNS.Do(func(ns.NetNS) error {
+				defer GinkgoRecover()
+				link, err := netlink.LinkByName(IFNAME)
+				Expect(err).NotTo(HaveOccurred())
+				routes, err := netlink.RouteList(link, netlink.FAMILY_ALL)
+				Expect(err).NotTo(HaveOccurred())
+				for _, route := range routes {
+					Expect(isIPNetZero(route.Dst)).To(BeFalse())
+				}
+				return nil
+			})).Should(Succeed())
 		})
 	})