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// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
package netmon
import (
"bytes"
"log"
"net/netip"
"os/exec"
"sync/atomic"
"go4.org/mem"
"golang.org/x/sys/unix"
"tailscale.com/net/netaddr"
"tailscale.com/syncs"
"tailscale.com/util/lineiter"
)
var (
lastKnownDefaultRouteIfName syncs.AtomicValue[string]
)
var procNetRoutePath = "/proc/net/route"
// maxProcNetRouteRead is the max number of lines to read from
// /proc/net/route looking for a default route.
const maxProcNetRouteRead = 1000
func init() {
likelyHomeRouterIP = likelyHomeRouterIPAndroid
}
var procNetRouteErr atomic.Bool
/*
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 likelyHomeRouterIPAndroid() (ret netip.Addr, myIP netip.Addr, ok bool) {
if procNetRouteErr.Load() {
// If we failed to read /proc/net/route previously, don't keep trying.
return likelyHomeRouterIPHelper()
}
lineNum := 0
var f []mem.RO
for lr := range lineiter.File(procNetRoutePath) {
line, err := lr.Value()
if err != nil {
procNetRouteErr.Store(true)
return likelyHomeRouterIP()
}
lineNum++
if lineNum == 1 {
// Skip header line.
continue
}
if lineNum > maxProcNetRouteRead {
break
}
f = mem.AppendFields(f[:0], mem.B(line))
if len(f) < 4 {
continue
}
gwHex, flagsHex := f[2], f[3]
flags, err := mem.ParseUint(flagsHex, 16, 16)
if err != nil {
continue // ignore error, skip line and keep going
}
if flags&(unix.RTF_UP|unix.RTF_GATEWAY) != unix.RTF_UP|unix.RTF_GATEWAY {
continue
}
ipu32, err := mem.ParseUint(gwHex, 16, 32)
if err != nil {
continue // 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
break
}
}
if ret.IsValid() {
// Try to get the local IP of the interface associated with
// this route to short-circuit finding the IP associated with
// this gateway. This isn't fatal if it fails.
if len(f) > 0 && !disableLikelyHomeRouterIPSelf() {
ForeachInterface(func(ni Interface, pfxs []netip.Prefix) {
// Ensure this is the same interface
if !f[0].EqualString(ni.Name) {
return
}
// Find the first IPv4 address and use it.
for _, pfx := range pfxs {
if addr := pfx.Addr(); addr.Is4() {
myIP = addr
break
}
}
})
}
return ret, myIP, true
}
if lineNum >= maxProcNetRouteRead {
// If we went over our line limit without finding an answer, assume
// we're a big fancy Linux router (or at least not a home system)
// and set the error bit so we stop trying this in the future (and wasting CPU).
// See https://github.com/tailscale/tailscale/issues/7621.
//
// Remember that "likelyHomeRouterIP" exists purely to find the port
// mapping service (UPnP, PMP, PCP) often present on a home router. If we hit
// the route (line) limit without finding an answer, we're unlikely to ever
// find one in the future.
procNetRouteErr.Store(true)
}
return netip.Addr{}, netip.Addr{}, false
}
// Android apps don't have permission to read /proc/net/route, at
// least on Google devices and the Android emulator.
func likelyHomeRouterIPHelper() (ret netip.Addr, _ netip.Addr, 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 "
for lr := range lineiter.Reader(out) {
line, err := lr.Value()
if err != nil {
break
}
const pfx = "default via "
if !mem.HasPrefix(mem.B(line), mem.S(pfx)) {
continue
}
line = line[len(pfx):]
sp := bytes.IndexByte(line, ' ')
if sp == -1 {
continue
}
ipb := line[:sp]
if ip, err := netip.ParseAddr(string(ipb)); err == nil && ip.Is4() {
ret = ip
log.Printf("interfaces: found Android default route %v", ip)
}
}
cmd.Process.Kill()
cmd.Wait()
return ret, netip.Addr{}, ret.IsValid()
}
// UpdateLastKnownDefaultRouteInterface is called by libtailscale in the Android app when
// the connectivity manager detects a network path transition. If ifName is "", network has been lost.
// After updating the interface, Android calls Monitor.InjectEvent(), triggering a link change.
func UpdateLastKnownDefaultRouteInterface(ifName string) {
if old := lastKnownDefaultRouteIfName.Swap(ifName); old != ifName {
log.Printf("defaultroute: update from Android, ifName = %s (was %s)", ifName, old)
}
}
func defaultRoute() (d DefaultRouteDetails, err error) {
if ifName := lastKnownDefaultRouteIfName.Load(); ifName != "" {
d.InterfaceName = ifName
}
return d, nil
}
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