net/netns, net/interfaces: explicitly bind sockets to the default interface on all Darwin variants

We were previously only doing this for tailscaled-on-Darwin, but it also
appears to help on iOS. Otherwise, when we rebind magicsock UDP
connections after a cellular -> WiFi interface change they still keep
using cellular one.

To do this correctly when using exit nodes, we need to exclude the
Tailscale interface when getting the default route, otherwise packets
cannot leave the tunnel. There are native macOS/iOS APIs that we can
use to do this, so we allow those clients to override the implementation
of DefaultRouteInterfaceIndex.

Updates #6565, may also help with #5156

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

net/netns/netns_darwin.go

@@ -0,0 +1,81 @@
+// 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.
+
+//go:build darwin
+
+package netns
+
+import (
+	"errors"
+	"fmt"
+	"log"
+	"net"
+	"strings"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+	"tailscale.com/net/interfaces"
+	"tailscale.com/types/logger"
+)
+
+func control(logf logger.Logf) func(network, address string, c syscall.RawConn) error {
+	return func(network, address string, c syscall.RawConn) error {
+		return controlLogf(logf, network, address, c)
+	}
+}
+
+// controlLogf 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 controlLogf(logf logger.Logf, network, address string, c syscall.RawConn) error {
+	if isLocalhost(address) {
+		// Don't bind to an interface for localhost connections.
+		return nil
+	}
+	idx, err := interfaces.DefaultRouteInterfaceIndex()
+	if err != nil {
+		logf("[unexpected] netns: DefaultRouteInterfaceIndex: %v", err)
+		return nil
+	}
+
+	return bindConnToInterface(c, network, address, idx, logf)
+}
+
+// SetListenConfigInterfaceIndex sets lc.Control such that sockets are bound
+// to the provided interface index.
+func SetListenConfigInterfaceIndex(lc *net.ListenConfig, ifIndex int) error {
+	if lc == nil {
+		return errors.New("nil ListenConfig")
+	}
+	if lc.Control != nil {
+		return errors.New("ListenConfig.Control already set")
+	}
+	lc.Control = func(network, address string, c syscall.RawConn) error {
+		return bindConnToInterface(c, network, address, ifIndex, log.Printf)
+	}
+	return nil
+}
+
+func bindConnToInterface(c syscall.RawConn, network, address string, ifIndex int, logf logger.Logf) error {
+	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, ifIndex)
+	})
+	if sockErr != nil {
+		logf("[unexpected] netns: bindConnToInterface(%q, %q), v6=%v, index=%v: %v", network, address, v6, ifIndex, sockErr)
+	}
+	if err != nil {
+		return fmt.Errorf("RawConn.Control on %T: %w", c, err)
+	}
+	return sockErr
+}