net/interfaces: add func LikelyHomeRouterIP

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

net/interfaces/interfaces_darwin.go

@@ -0,0 +1,66 @@
+// 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 = likelyHomeRouterIPDarwin
+}
+
+/*
+Parse out 10.0.0.1 from:
+
+$ netstat -r -n -f inet
+Routing tables
+
+Internet:
+Destination        Gateway            Flags        Netif Expire
+default            10.0.0.1           UGSc           en0
+default            link#14            UCSI         utun2
+10/16              link#4             UCS            en0      !
+10.0.0.1/32        link#4             UCS            en0      !
+...
+
+*/
+func likelyHomeRouterIPDarwin() (ret netaddr.IP, ok bool) {
+	cmd := exec.Command("/usr/sbin/netstat", "-r", "-n", "-f", "inet")
+	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("default")) {
+			return nil
+		}
+		f = mem.AppendFields(f[:0], line)
+		if len(f) < 3 || !f[0].EqualString("default") {
+			return nil
+		}
+		ipm, flagsm := f[1], f[2]
+		if !mem.Contains(flagsm, mem.S("G")) {
+			return nil
+		}
+		ip, err := netaddr.ParseIP(string(mem.Append(nil, ipm)))
+		if err == nil && isPrivateIP(ip) {
+			ret = ip
+		}
+		return nil
+	})
+	return ret, !ret.IsZero()
+}