wgengine/tsdns: replace connections when net link changes (macOS)

When the network link changes, existing UDP sockets fail immediately
and permanently on macOS.

The forwarder set up a single UDP conn and never changed it.
As a result, any time there was a network link change,
all forwarded DNS queries failed.

To fix this, create a new connection when send requests
fail because of network unreachability.

This change is darwin-only, although extended it to other platforms
should be straightforward.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

wgengine/tsdns/neterr_darwin.go

@@ -0,0 +1,25 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package tsdns
+
+import (
+	"errors"
+	"syscall"
+)
+
+// Avoid allocation when calling errors.Is below
+// by converting syscall.Errno to error here.
+var (
+	networkDown        error = syscall.ENETDOWN
+	networkUnreachable error = syscall.ENETUNREACH
+)
+
+func networkIsDown(err error) bool {
+	return errors.Is(err, networkDown)
+}
+
+func networkIsUnreachable(err error) bool {
+	return errors.Is(err, networkUnreachable)
+}