net/interfaces: rewrite the darwin likelyHomeRouterIP from C to Go

We basically already had the RIB-parsing Go code for this in both
net/interfaces and wgengine/monitor, for other reasons.

Fixes #1426
Fixes #1471

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

net/interfaces/interfaces_darwin.go

@@ -7,76 +7,15 @@ package interfaces
 import (
 	"errors"
 	"fmt"
+	"log"
 	"net"
-	"os/exec"
 	"syscall"
 
-	"go4.org/mem"
 	"golang.org/x/net/route"
+	"golang.org/x/sys/unix"
 	"inet.af/netaddr"
-	"tailscale.com/util/lineread"
-	"tailscale.com/version"
 )
 
-/*
-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 likelyHomeRouterIPDarwinExec() (ret netaddr.IP, ok bool) {
-	if version.IsMobile() {
-		// Don't try to do subprocesses on iOS. Ends up with log spam like:
-		// kernel: "Sandbox: IPNExtension(86580) deny(1) process-fork"
-		// This is why we have likelyHomeRouterIPDarwinSyscall.
-		return ret, false
-	}
-	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
-			// We've found what we're looking for.
-			return errStopReadingNetstatTable
-		}
-		return nil
-	})
-	return ret, !ret.IsZero()
-}
-
-var errStopReadingNetstatTable = errors.New("found private gateway")
-
 func DefaultRouteInterface() (string, error) {
 	idx, err := DefaultRouteInterfaceIndex()
 	if err != nil {
@@ -138,3 +77,48 @@ func DefaultRouteInterfaceIndex() (int, error) {
 	}
 	return 0, fmt.Errorf("ambiguous gateway interfaces found: %v", indexSeen)
 }
+
+func init() {
+	likelyHomeRouterIP = likelyHomeRouterIPDarwinFetchRIB
+}
+
+func likelyHomeRouterIPDarwinFetchRIB() (ret netaddr.IP, ok bool) {
+	rib, err := route.FetchRIB(syscall.AF_UNSPEC, syscall.NET_RT_DUMP2, 0)
+	if err != nil {
+		log.Printf("routerIP/FetchRIB: %v", err)
+		return ret, false
+	}
+	msgs, err := route.ParseRIB(syscall.NET_RT_IFLIST2, rib)
+	if err != nil {
+		log.Printf("routerIP/ParseRIB: %v", err)
+		return ret, false
+	}
+	for _, m := range msgs {
+		rm, ok := m.(*route.RouteMessage)
+		if !ok {
+			continue
+		}
+		const RTF_GATEWAY = 0x2
+		const RTF_IFSCOPE = 0x1000000
+		if rm.Flags&RTF_GATEWAY == 0 {
+			continue
+		}
+		if rm.Flags&RTF_IFSCOPE != 0 {
+			continue
+		}
+		if len(rm.Addrs) > unix.RTAX_GATEWAY {
+			dst4, ok := rm.Addrs[unix.RTAX_DST].(*route.Inet4Addr)
+			if !ok || dst4.IP != ([4]byte{0, 0, 0, 0}) {
+				// Expect 0.0.0.0 as DST field.
+				continue
+			}
+			gw, ok := rm.Addrs[unix.RTAX_GATEWAY].(*route.Inet4Addr)
+			if !ok {
+				continue
+			}
+			return netaddr.IPv4(gw.IP[0], gw.IP[1], gw.IP[2], gw.IP[3]), true
+		}
+	}
+
+	return ret, false
+}