health: Configure sysctl when IPv6 is disabled

When IPv6 is disabled, ensure that the health endpoint's device is
configured to disable IPv6 so that it doesn't emit any IPv6 autoconf
frames or similar.

Signed-off-by: Joe Stringer <joe@cilium.io>

cilium-health/launch/endpoint.go

@@ -41,6 +41,7 @@ import (
 	"github.com/cilium/cilium/pkg/node"
 	"github.com/cilium/cilium/pkg/option"
 	"github.com/cilium/cilium/pkg/pidfile"
+	"github.com/cilium/cilium/pkg/sysctl"
 
 	"github.com/containernetworking/plugins/pkg/ns"
 	"github.com/vishvananda/netlink"
@@ -120,7 +121,12 @@ func configureHealthInterface(netNS ns.NetNS, ifName string, ip4Addr, ip6Addr *n
 			return err
 		}
 
-		if ip6Addr != nil {
+		if ip6Addr == nil {
+			name := fmt.Sprintf("net.ipv6.conf.%s.disable_ipv6", ifName)
+			// Ignore the error; if IPv6 is completely disabled
+			// then it's okay if we can't write the sysctl.
+			_ = sysctl.Write(name, "1")
+		} else {
 			if err = netlink.AddrAdd(link, &netlink.Addr{IPNet: ip6Addr}); err != nil {
 				return err
 			}