// 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.
package interfaces
import (
"bufio"
"bytes"
"errors"
"fmt"
"io"
"log"
"os"
"os/exec"
"runtime"
"strings"
"go4.org/mem"
"inet.af/netaddr"
"tailscale.com/syncs"
"tailscale.com/util/lineread"
)
func init() {
likelyHomeRouterIP = likelyHomeRouterIPLinux
}
var procNetRouteErr syncs.AtomicBool
/*
Parse 10.0.0.1 out of:
$ cat /proc/net/route
Iface Destination Gateway Flags RefCnt Use Metric Mask MTU Window IRTT
ens18 00000000 0100000A 0003 0 0 0 00000000 0 0 0
ens18 0000000A 00000000 0001 0 0 0 0000FFFF 0 0 0
*/
func likelyHomeRouterIPLinux() (ret netaddr.IP, ok bool) {
if procNetRouteErr.Get() {
// If we failed to read /proc/net/route previously, don't keep trying.
// But if we're on Android, go into the Android path.
if runtime.GOOS == "android" {
return likelyHomeRouterIPAndroid()
}
return ret, false
}
lineNum := 0
var f []mem.RO
err := lineread.File("/proc/net/route", func(line []byte) error {
lineNum++
if lineNum == 1 {
// Skip header line.
return nil
}
f = mem.AppendFields(f[:0], mem.B(line))
if len(f) < 4 {
return nil
}
gwHex, flagsHex := f[2], f[3]
flags, err := mem.ParseUint(flagsHex, 16, 16)
if err != nil {
return nil // ignore error, skip line and keep going
}
const RTF_UP = 0x0001
const RTF_GATEWAY = 0x0002
if flags&(RTF_UP|RTF_GATEWAY) != RTF_UP|RTF_GATEWAY {
return nil
}
ipu32, err := mem.ParseUint(gwHex, 16, 32)
if err != nil {
return nil // ignore error, skip line and keep going
}
ip := netaddr.IPv4(byte(ipu32), byte(ipu32>>8), byte(ipu32>>16), byte(ipu32>>24))
if ip.IsPrivate() {
ret = ip
}
return nil
})
if err != nil {
procNetRouteErr.Set(true)
if runtime.GOOS == "android" {
return likelyHomeRouterIPAndroid()
}
log.Printf("interfaces: failed to read /proc/net/route: %v", err)
}
return ret, !ret.IsZero()
}
// Android apps don't have permission to read /proc/net/route, at
// least on Google devices and the Android emulator.
func likelyHomeRouterIPAndroid() (ret netaddr.IP, ok bool) {
cmd := exec.Command("/system/bin/ip", "route", "show", "table", "0")
out, err := cmd.StdoutPipe()
if err != nil {
return
}
if err := cmd.Start(); err != nil {
log.Printf("interfaces: running /system/bin/ip: %v", err)
return
}
// Search for line like "default via 10.0.2.2 dev radio0 table 1016 proto static mtu 1500 "
lineread.Reader(out, func(line []byte) error {
const pfx = "default via "
if !mem.HasPrefix(mem.B(line), mem.S(pfx)) {
return nil
}
line = line[len(pfx):]
sp := bytes.IndexByte(line, ' ')
if sp == -1 {
return nil
}
ipb := line[:sp]
if ip, err := netaddr.ParseIP(string(ipb)); err == nil && ip.Is4() {
ret = ip
log.Printf("interfaces: found Android default route %v", ip)
}
return nil
})
cmd.Process.Kill()
cmd.Wait()
return ret, !ret.IsZero()
}
// DefaultRouteInterface returns the name of the network interface that owns
// the default route, not including any tailscale interfaces.
func DefaultRouteInterface() (string, error) {
v, err := defaultRouteInterfaceProcNet()
if err == nil {
return v, nil
}
if runtime.GOOS == "android" {
return defaultRouteInterfaceAndroidIPRoute()
}
return v, err
}
var zeroRouteBytes = []byte("00000000")
var procNetRoutePath = "/proc/net/route"
func defaultRouteInterfaceProcNetInternal(bufsize int) (string, error) {
f, err := os.Open(procNetRoutePath)
if err != nil {
return "", err
}
defer f.Close()
br := bufio.NewReaderSize(f, bufsize)
for {
line, err := br.ReadSlice('\n')
if err == io.EOF {
return "", fmt.Errorf("no default routes found: %w", err)
}
if err != nil {
return "", err
}
if !bytes.Contains(line, zeroRouteBytes) {
continue
}
fields := strings.Fields(string(line))
ifc := fields[0]
ip := fields[1]
netmask := fields[7]
if strings.HasPrefix(ifc, "tailscale") ||
strings.HasPrefix(ifc, "wg") {
continue
}
if ip == "00000000" && netmask == "00000000" {
// default route
return ifc, nil // interface name
}
}
}
// returns string interface name and an error.
// io.EOF: full route table processed, no default route found.
// other io error: something went wrong reading the route file.
func defaultRouteInterfaceProcNet() (string, error) {
rc, err := defaultRouteInterfaceProcNetInternal(128)
if rc == "" && (errors.Is(err, io.EOF) || err == nil) {
// https://github.com/google/gvisor/issues/5732
// On a regular Linux kernel you can read the first 128 bytes of /proc/net/route,
// then come back later to read the next 128 bytes and so on.
//
// In Google Cloud Run, where /proc/net/route comes from gVisor, you have to
// read it all at once. If you read only the first few bytes then the second
// read returns 0 bytes no matter how much originally appeared to be in the file.
//
// At the time of this writing (Mar 2021) Google Cloud Run has eth0 and eth1
// with a 384 byte /proc/net/route. We allocate a large buffer to ensure we'll
// read it all in one call.
return defaultRouteInterfaceProcNetInternal(4096)
}
return rc, err
}
// defaultRouteInterfaceAndroidIPRoute tries to find the machine's default route interface name
// by parsing the "ip route" command output. We use this on Android where /proc/net/route
// can be missing entries or have locked-down permissions.
// See also comments in https://github.com/tailscale/tailscale/pull/666.
func defaultRouteInterfaceAndroidIPRoute() (ifname string, err error) {
cmd := exec.Command("/system/bin/ip", "route", "show", "table", "0")
out, err := cmd.StdoutPipe()
if err != nil {
return "", err
}
if err := cmd.Start(); err != nil {
log.Printf("interfaces: running /system/bin/ip: %v", err)
return "", err
}
// Search for line like "default via 10.0.2.2 dev radio0 table 1016 proto static mtu 1500 "
lineread.Reader(out, func(line []byte) error {
const pfx = "default via "
if !mem.HasPrefix(mem.B(line), mem.S(pfx)) {
return nil
}
ff := strings.Fields(string(line))
for i, v := range ff {
if i > 0 && ff[i-1] == "dev" && ifname == "" {
ifname = v
}
}
return nil
})
cmd.Process.Kill()
cmd.Wait()
if ifname == "" {
return "", errors.New("no default routes found")
}
return ifname, nil
}