Improved detect in ipv6IsEnabled() (#555)

* Improved detect in ipv6IsEnabled()

* Added comments in ipv6IsEnabled.

Problem described in #155

pkg/controllers/routing/utils.go

@@ -83,7 +83,14 @@ func ipv4IsEnabled() bool {
 }
 
 func ipv6IsEnabled() bool {
-	l, err := net.Listen("tcp6", "")
+	// If ipv6 is disabled with;
+	//
+	//  sysctl -w net.ipv6.conf.all.disable_ipv6=1
+	//
+	// It is still possible to listen on the any-address "::". So this
+	// function tries the loopback address "::1" which must be present
+	// if ipv6 is enabled.
+	l, err := net.Listen("tcp6", "[::1]:0")
 	if err != nil {
 		return false
 	}