fix(kube-proxy): fix IPv6 support check for Linux

The current Linux implementation checks whether a node supports
IPv6 by testing for the presence of '/proc/net/if_inet6'. However,
that check does not confirm that IPv6 support is actually enabled.
As a result, kube-proxy can incorrectly enable dual‑stack support
on nodes that do not have IPv6 enabled.

The possibility to disable IPv6 support has existed since
Linux kernel v2.6.27.

Update the IPv6 support check to also consider the contents of
'/proc/sys/net/ipv6/conf/all/disable_ipv6'.

Signed-off-by: Tero Kauppinen <tero.kauppinen@est.tech>

cmd/kube-proxy/app/server_linux.go

@@ -25,6 +25,7 @@ import (
 	"errors"
 	"fmt"
 	"os"
+	"strings"
 
 	v1 "k8s.io/api/core/v1"
 	utilsysctl "k8s.io/component-helpers/node/util/sysctl"
@@ -106,7 +107,14 @@ func (s *ProxyServer) platformCheckSupported(ctx context.Context) (ipv4Supported
 
 	// Check if the OS has IPv6 enabled, by verifying if the IPv6 interfaces are available
 	_, errIPv6 := os.Stat("/proc/net/if_inet6")
-	if errIPv6 != nil {
+	if errIPv6 == nil {
+		// Also check that IPv6 hasn't been disabled
+		content, err := os.ReadFile("/proc/sys/net/ipv6/conf/all/disable_ipv6")
+		if err == nil && strings.TrimSpace(string(content)) == "1" {
+			logger.Info("Kernel support disabled for family", "ipFamily", v1.IPv6Protocol)
+			ipv6Supported = false
+		}
+	} else {
 		logger.Info("No kernel support for family", "ipFamily", v1.IPv6Protocol)
 		ipv6Supported = false
 	}