net/interfaces: add func LikelyHomeRouterIP

For discovering where we might direct NAT-PMP/PCP/UPnP queries at in
the future.

net/interfaces/interfaces_windows.go

@@ -0,0 +1,71 @@
+// 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 interfaces
+
+import (
+	"os/exec"
+
+	"go4.org/mem"
+	"inet.af/netaddr"
+	"tailscale.com/util/lineread"
+)
+
+func init() {
+	likelyHomeRouterIP = likelyHomeRouterIPWindows
+}
+
+/*
+Parse out 10.0.0.1 from:
+
+Z:\>route print -4
+===========================================================================
+Interface List
+ 15...aa 15 48 ff 1c 72 ......Red Hat VirtIO Ethernet Adapter
+  5...........................Tailscale Tunnel
+  1...........................Software Loopback Interface 1
+===========================================================================
+
+IPv4 Route Table
+===========================================================================
+Active Routes:
+Network Destination        Netmask          Gateway       Interface  Metric
+          0.0.0.0          0.0.0.0         10.0.0.1       10.0.28.63      5
+         10.0.0.0      255.255.0.0         On-link        10.0.28.63    261
+       10.0.28.63  255.255.255.255         On-link        10.0.28.63    261
+        10.0.42.0    255.255.255.0   100.103.42.106   100.103.42.106      5
+     10.0.255.255  255.255.255.255         On-link        10.0.28.63    261
+   34.193.248.174  255.255.255.255   100.103.42.106   100.103.42.106      5
+
+*/
+func likelyHomeRouterIPWindows() (ret netaddr.IP, ok bool) {
+	cmd := exec.Command("route", "print", "-4")
+	stdout, err := cmd.StdoutPipe()
+	if err != nil {
+		return
+	}
+	if err := cmd.Start(); err != nil {
+		return
+	}
+	defer cmd.Wait()
+
+	var f []mem.RO
+	lineread.Reader(stdout, func(lineb []byte) error {
+		line := mem.B(lineb)
+		if !mem.Contains(line, mem.S("0.0.0.0")) {
+			return nil
+		}
+		f = mem.AppendFields(f[:0], line)
+		if len(f) < 3 || !f[0].EqualString("0.0.0.0") || !f[1].EqualString("0.0.0.0") {
+			return nil
+		}
+		ipm := f[2]
+		ip, err := netaddr.ParseIP(string(mem.Append(nil, ipm)))
+		if err == nil && isPrivateIP(ip) {
+			ret = ip
+		}
+		return nil
+	})
+	return ret, !ret.IsZero()
+}