control/controlclient: flip IPv6 to be on by default

The DEBUG_INCLUDE_IPV6 environment variable is now an opt-out.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/direct.go

@@ -447,13 +447,9 @@ func (c *Direct) PollNetMap(ctx context.Context, maxPolls int, cb func(*NetworkM
 	allowStream := maxPolls != 1
 	c.logf("PollNetMap: stream=%v :%v %v\n", maxPolls, localPort, ep)
 
-	// TODO(bradfitz): once this is verified to not be problematic, remove this
-	// knob and hard-code true below.
-	includeIPv6, _ := strconv.ParseBool(os.Getenv("DEBUG_INCLUDE_IPV6"))
-
 	request := tailcfg.MapRequest{
 		Version:     4,
-		IncludeIPv6: includeIPv6,
+		IncludeIPv6: includeIPv6(),
 		KeepAlive:   c.keepAlive,
 		NodeKey:     tailcfg.NodeKey(persist.PrivateNodeKey.Public()),
 		Endpoints:   ep,
@@ -704,3 +700,14 @@ func loadServerKey(ctx context.Context, httpc *http.Client, serverURL string) (w
 	}
 	return key, nil
 }
+
+// includeIPv6 reports whether we should enable IPv6 for magicsock
+// connections. This is only here temporarily (2020-03-26) as a
+// opt-out in case there are problems.
+func includeIPv6() bool {
+	if e := os.Getenv("DEBUG_INCLUDE_IPV6"); e != "" {
+		v, _ := strconv.ParseBool(e)
+		return v
+	}
+	return true
+}