net/interfaces: merge darwin files for DefaultRouteInterface in sandbox

DefaultRouteInterface was previously guarded by build tags such that
it was only accessible to tailscaled-on-macos, but there was no reason
for that. It runs fine in the sandbox and gives better default info,
so merge its file into interfaces_darwin.go.

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

net/interfaces/interfaces_darwin.go

@@ -6,9 +6,13 @@ package interfaces
 
 import (
 	"errors"
+	"fmt"
+	"net"
 	"os/exec"
+	"syscall"
 
 	"go4.org/mem"
+	"golang.org/x/net/route"
 	"inet.af/netaddr"
 	"tailscale.com/util/lineread"
 	"tailscale.com/version"
@@ -72,3 +76,65 @@ func likelyHomeRouterIPDarwinExec() (ret netaddr.IP, ok bool) {
 }
 
 var errStopReadingNetstatTable = errors.New("found private gateway")
+
+func DefaultRouteInterface() (string, error) {
+	idx, err := DefaultRouteInterfaceIndex()
+	if err != nil {
+		return "", err
+	}
+	iface, err := net.InterfaceByIndex(idx)
+	if err != nil {
+		return "", err
+	}
+	return iface.Name, nil
+}
+
+func DefaultRouteInterfaceIndex() (int, error) {
+	// $ netstat -nr
+	// Routing tables
+	// Internet:
+	// Destination        Gateway            Flags        Netif Expire
+	// default            10.0.0.1           UGSc           en0         <-- want this one
+	// default            10.0.0.1           UGScI          en1
+
+	// From man netstat:
+	// U       RTF_UP           Route usable
+	// G       RTF_GATEWAY      Destination requires forwarding by intermediary
+	// S       RTF_STATIC       Manually added
+	// c       RTF_PRCLONING    Protocol-specified generate new routes on use
+	// I       RTF_IFSCOPE      Route is associated with an interface scope
+
+	rib, err := route.FetchRIB(syscall.AF_UNSPEC, syscall.NET_RT_DUMP2, 0)
+	if err != nil {
+		return 0, fmt.Errorf("route.FetchRIB: %w", err)
+	}
+	msgs, err := route.ParseRIB(syscall.NET_RT_IFLIST2, rib)
+	if err != nil {
+		return 0, fmt.Errorf("route.ParseRIB: %w", err)
+	}
+	indexSeen := map[int]int{} // index => count
+	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
+		}
+		indexSeen[rm.Index]++
+	}
+	if len(indexSeen) == 0 {
+		return 0, errors.New("no gateway index found")
+	}
+	if len(indexSeen) == 1 {
+		for idx := range indexSeen {
+			return idx, nil
+		}
+	}
+	return 0, fmt.Errorf("ambiguous gateway interfaces found: %v", indexSeen)
+}