net/{interfaces,ns}: add tailscaled-mode darwin routing looping prevention

Fixes #1331

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

cmd/tailscale/depaware.txt

@@ -72,7 +72,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         golang.org/x/net/http2/hpack                                 from net/http
         golang.org/x/net/idna                                        from golang.org/x/net/http/httpguts+
         golang.org/x/net/proxy                                       from tailscale.com/net/netns
-   D    golang.org/x/net/route                                       from net
+   D    golang.org/x/net/route                                       from net+
         golang.org/x/oauth2                                          from tailscale.com/ipn+
         golang.org/x/oauth2/internal                                 from golang.org/x/oauth2
         golang.org/x/sync/errgroup                                   from tailscale.com/derp

cmd/tailscaled/depaware.txt

@@ -157,7 +157,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         golang.org/x/net/ipv4                                        from github.com/tailscale/wireguard-go/device
         golang.org/x/net/ipv6                                        from github.com/tailscale/wireguard-go/device+
         golang.org/x/net/proxy                                       from tailscale.com/net/netns
-   D    golang.org/x/net/route                                       from net
+   D    golang.org/x/net/route                                       from net+
         golang.org/x/oauth2                                          from tailscale.com/control/controlclient+
         golang.org/x/oauth2/internal                                 from golang.org/x/oauth2
         golang.org/x/sync/errgroup                                   from tailscale.com/derp

net/interfaces/interfaces_darwin_tailscaled.go

@@ -0,0 +1,81 @@
+// 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.
+
+// +build darwin,!redo,!ios
+// (Exclude redo, because we don't want this code in the App Store
+// version's sandbox, where it won't work, and also don't want it on
+// iOS. This is just for utun-using non-sandboxed cmd/tailscaled on macOS.
+
+package interfaces
+
+import (
+	"errors"
+	"fmt"
+	"net"
+	"syscall"
+
+	"golang.org/x/net/route"
+)
+
+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("FetchRIB: %w", err)
+	}
+	msgs, err := route.ParseRIB(syscall.NET_RT_IFLIST2, rib)
+	if err != nil {
+		return 0, fmt.Errorf("Parse: %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)
+}

net/interfaces/interfaces_defaultrouteif_todo.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !linux,!windows
+// +build !linux,!windows,!darwin darwin,redo
 
 package interfaces
 

net/netns/netns_darwin_tailscaled.go

@@ -0,0 +1,52 @@
+// Copyright (c) 2021 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.
+
+// +build darwin,!redo
+
+package netns
+
+import (
+	"fmt"
+	"log"
+	"strings"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+	"tailscale.com/net/interfaces"
+)
+
+// control marks c as necessary to dial in a separate network namespace.
+//
+// It's intentionally the same signature as net.Dialer.Control
+// and net.ListenConfig.Control.
+func control(network, address string, c syscall.RawConn) error {
+	if strings.HasPrefix(address, "127.") || address == "::1" {
+		// Don't bind to an interface for localhost connections.
+		return nil
+	}
+	idx, err := interfaces.DefaultRouteInterfaceIndex()
+	if err != nil {
+		log.Printf("netns: DefaultRouteInterfaceIndex: %v", err)
+		return nil
+	}
+	v6 := strings.Contains(address, "]:") || strings.HasSuffix(network, "6") // hacky test for v6
+	proto := unix.IPPROTO_IP
+	opt := unix.IP_BOUND_IF
+	if v6 {
+		proto = unix.IPPROTO_IPV6
+		opt = unix.IPV6_BOUND_IF
+	}
+
+	var sockErr error
+	err = c.Control(func(fd uintptr) {
+		sockErr = unix.SetsockoptInt(int(fd), proto, opt, idx)
+	})
+	if err != nil {
+		return fmt.Errorf("RawConn.Control on %T: %w", c, err)
+	}
+	if sockErr != nil {
+		log.Printf("netns: control(%q, %q), v6=%v, index=%v: %v", network, address, v6, idx, sockErr)
+	}
+	return sockErr
+}

net/netns/netns_default.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !linux,!windows
+// +build !linux,!windows,!darwin darwin,redo
 
 package netns