ipv6: Add flag IFA_F_NODAD for ipv6 address.

This is done so that interfaces with ipv6 addresses can immediately
be used once the network is up.
A similar flag is set by libnetwork as well.
It should be the responsibility of the upper stack to
make sure ipv6 addresses dont clash.

Depends-on: github.com/#2420

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

network.go

@@ -129,6 +129,17 @@ func updateLink(netHandle *netlink.Handle, link netlink.Link, iface *types.Inter
 		netlinkAddrStr := fmt.Sprintf("%s/%s", addr.Address, addr.Mask)
 		netlinkAddr, err := netlink.ParseAddr(netlinkAddrStr)
 
+		// With ipv6 addresses, there is a brief period during which the address is marked as "tentative"
+		// making it unavailable. A process called duplicate address detection(DAD) is performed during this period.
+		// Disble DAD so that networking is available once the container is up. The assumption is
+		// that it is the reponsibility of the upper stack to make sure the addresses assigned to containers
+		// do not conflict. A similar operation is performed by libnetwork:
+		// https://github.com/moby/moby/issues/18871
+
+		if addr.GetFamily() == types.IPFamily_v6 {
+			netlinkAddr.Flags = netlinkAddr.Flags | syscall.IFA_F_NODAD
+		}
+
 		if err != nil {
 			return grpcStatus.Errorf(codes.Internal, "Could not parse %q: %v", netlinkAddrStr, err)
 		}

network_test.go

@@ -13,6 +13,7 @@ import (
 	"reflect"
 	"runtime"
 	"strings"
+	"syscall"
 	"testing"
 
 	"github.com/kata-containers/agent/pkg/types"
@@ -301,6 +302,16 @@ func TestListInterfaces(t *testing.T) {
 		ip := net.ParseIP(ipAddr.Address)
 		assert.True(ip.IsLinkLocalUnicast())
 	}
+
+	// Check IFA_F_NODAD flag is set on ipv6 address
+	l, err := netlink.LinkByName(ifc.Name)
+	assert.Nil(err)
+	addrList, err := netlink.AddrList(l, netlink.FAMILY_V6)
+	assert.Nil(err)
+
+	if addrList[0].Flags&syscall.IFA_F_NODAD == 0 {
+		t.Fatalf("Unexpected interface flags for addr %+v: 0x%x. Expected to contain 0x%x", addrList[0], addrList[0].Flags, syscall.IFA_F_NODAD)
+	}
 }
 
 func TestListRoutes(t *testing.T) {