cmd/ctrld: use better approach for checking IPv6 available

Some operating systems may throw a confirmation dialog when attempting
to listen on any interface other than loopback. A better approach is
checking for any interface which is up and can be routed IP traffic.

cmd/ctrld/net.go

@@ -3,6 +3,8 @@ package main
 import (
 	"net"
 	"sync"
+
+	"golang.org/x/net/nettest"
 )
 
 var (
@@ -11,8 +13,8 @@ var (
 )
 
 func probeStack() {
-	if ln, err := net.Listen("tcp6", "[::]:0"); err == nil {
-		ln.Close()
+	// TODO(cuonglm): use nettest.SupportsIPv6 once https://github.com/golang/go/issues/57386 fixed.
+	if _, err := nettest.RoutedInterface("ip6", net.FlagUp); err == nil {
 		ipv6Enabled = true
 	}
 }

go.mod

@@ -13,6 +13,7 @@ require (
 	github.com/spf13/cobra v1.4.0
 	github.com/spf13/viper v1.9.0
 	github.com/stretchr/testify v1.8.0
+	golang.org/x/net v0.1.0
 	golang.org/x/sys v0.1.0
 	tailscale.com v1.34.1
 )
@@ -45,7 +46,6 @@ require (
 	golang.org/x/crypto v0.0.0-20220829220503-c86fa9a7ed90 // indirect
 	golang.org/x/exp v0.0.0-20220827204233-334a2380cb91 // indirect
 	golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4 // indirect
-	golang.org/x/net v0.1.0 // indirect
 	golang.org/x/text v0.4.0 // indirect
 	golang.org/x/tools v0.1.12 // indirect
 	gopkg.in/ini.v1 v1.66.2 // indirect