tailscale/net/netns/netns_darwin.go

// 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"
	"net/netip"
	"os"
	"strings"
	"syscall"

	"golang.org/x/net/route"
	"golang.org/x/sys/unix"
	"tailscale.com/envknob"
	"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)
	}
}

var bindToInterfaceByRouteEnv = envknob.RegisterBool("TS_BIND_TO_INTERFACE_BY_ROUTE")

var errInterfaceIndexInvalid = errors.New("interface index invalid")

// 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 := getInterfaceIndex(logf, address)
	if err != nil {
		// callee logged
		return nil
	}

	return bindConnToInterface(c, network, address, idx, logf)
}

func getInterfaceIndex(logf logger.Logf, address string) (int, error) {
	// Helper so we can log errors.
	defaultIdx := func() (int, error) {
		idx, err := interfaces.DefaultRouteInterfaceIndex()
		if err != nil {
			logf("[unexpected] netns: DefaultRouteInterfaceIndex: %v", err)
			return -1, err
		}
		return idx, nil
	}

	useRoute := bindToInterfaceByRoute.Load() || bindToInterfaceByRouteEnv()
	if !useRoute {
		return defaultIdx()
	}

	host, _, err := net.SplitHostPort(address)
	if err != nil {
		// No port number; use the string directly.
		host = address
	}

	// If the address doesn't parse, use the default index.
	addr, err := netip.ParseAddr(host)
	if err != nil {
		logf("[unexpected] netns: error parsing address %q: %v", host, err)
		return defaultIdx()
	}

	idx, err := interfaceIndexFor(addr, true /* canRecurse */)
	if err != nil {
		logf("netns: error in interfaceIndexFor: %v", err)
		return defaultIdx()
	}

	// Verify that we didn't just choose the Tailscale interface;
	// if so, we fall back to binding from the default.
	_, tsif, err2 := interfaces.Tailscale()
	if err2 == nil && tsif.Index == idx {
		logf("[unexpected] netns: interfaceIndexFor returned Tailscale interface")
		return defaultIdx()
	}

	return idx, err
}

// interfaceIndexFor returns the interface index that we should bind to in
// order to send traffic to the provided address.
func interfaceIndexFor(addr netip.Addr, canRecurse bool) (int, error) {
	fd, err := unix.Socket(unix.AF_ROUTE, unix.SOCK_RAW, unix.AF_UNSPEC)
	if err != nil {
		return 0, fmt.Errorf("creating AF_ROUTE socket: %w", err)
	}
	defer unix.Close(fd)

	var routeAddr route.Addr
	if addr.Is4() {
		routeAddr = &route.Inet4Addr{IP: addr.As4()}
	} else {
		routeAddr = &route.Inet6Addr{IP: addr.As16()}
	}

	rm := route.RouteMessage{
		// NOTE: This is unix.RTM_VERSION, but we want to pin this to a
		// particular constant so that it doesn't change under us if
		// the x/sys/unix package changes down the road. Currently this
		// is 0x5 on both Darwin x86 and ARM64.
		Version: 0x5,
		Type:    unix.RTM_GET,
		Flags:   unix.RTF_UP,
		ID:      uintptr(os.Getpid()),
		Seq:     1,
		Addrs: []route.Addr{
			unix.RTAX_DST: routeAddr,
		},
	}
	b, err := rm.Marshal()
	if err != nil {
		return 0, fmt.Errorf("marshaling RouteMessage: %w", err)
	}
	_, err = unix.Write(fd, b)
	if err != nil {
		return 0, fmt.Errorf("writing message: %w", err)
	}

	// On macOS, the RTM_GET call should return exactly one route message.
	// Given the following sizes and constants:
	//    - sizeof(struct rt_msghdr) = 92
	//    - RTAX_MAX = 8
	//    - sizeof(struct sockaddr_in6) = 28
	//    - sizeof(struct sockaddr_in) = 16
	//    - sizeof(struct sockaddr_dl) = 20
	//
	// The maximum buffer size should be:
	//    sizeof(struct rt_msghdr) + RTAX_MAX*sizeof(struct sockaddr_in6)
	//    = 92 + 8*28
	//    = 316
	//
	// During my testing, responses are typically ~120 bytes.
	//
	// We provide a much larger buffer just in case we're off by a bit, or
	// the kernel decides to return more than one message; 2048 bytes
	// should be plenty here. This also means we can do a single Read.
	var buf [2048]byte
	n, err := unix.Read(fd, buf[:])
	if err != nil {
		return 0, fmt.Errorf("reading message: %w", err)
	}
	msgs, err := route.ParseRIB(route.RIBTypeRoute, buf[:n])
	if err != nil {
		return 0, fmt.Errorf("route.ParseRIB: %w", err)
	}
	if len(msgs) == 0 {
		return 0, fmt.Errorf("no messages")
	}

	for _, msg := range msgs {
		rm, ok := msg.(*route.RouteMessage)
		if !ok {
			continue
		}
		if rm.Version < 3 || rm.Version > 5 || rm.Type != unix.RTM_GET {
			continue
		}
		if len(rm.Addrs) < unix.RTAX_GATEWAY {
			continue
		}

		switch addr := rm.Addrs[unix.RTAX_GATEWAY].(type) {
		case *route.LinkAddr:
			return addr.Index, nil
		case *route.Inet4Addr:
			// We can get a gateway IP; recursively call ourselves
			// (exactly once) to get the link (and thus index) for
			// the gateway IP.
			if canRecurse {
				return interfaceIndexFor(netip.AddrFrom4(addr.IP), false)
			}
		case *route.Inet6Addr:
			// As above.
			if canRecurse {
				return interfaceIndexFor(netip.AddrFrom16(addr.IP), false)
			}
		default:
			// Unknown type; skip it
			continue
		}
	}

	return 0, fmt.Errorf("no valid address found")
}

// 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
}