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interfaces_linux.go
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interfaces_linux.go
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// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
//go:build !android
package netmon
import (
"bufio"
"bytes"
"errors"
"fmt"
"io"
"log"
"net"
"net/netip"
"os"
"os/exec"
"runtime"
"strings"
"sync/atomic"
"github.com/jsimonetti/rtnetlink"
"github.com/mdlayher/netlink"
"go4.org/mem"
"golang.org/x/sys/unix"
"tailscale.com/net/netaddr"
"tailscale.com/util/lineread"
)
func init() {
likelyHomeRouterIP = likelyHomeRouterIPLinux
}
var procNetRouteErr atomic.Bool
// errStopReading is a sentinel error value used internally by
// lineread.File callers to stop reading. It doesn't escape to
// callers/users.
var errStopReading = errors.New("stop reading")
/*
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 netip.Addr, myIP netip.Addr, ok bool) {
if procNetRouteErr.Load() {
// If we failed to read /proc/net/route previously, don't keep trying.
if runtime.GOOS == "android" {
return likelyHomeRouterIPAndroid()
}
return ret, myIP, false
}
lineNum := 0
var f []mem.RO
err := lineread.File(procNetRoutePath, func(line []byte) error {
lineNum++
if lineNum == 1 {
// Skip header line.
return nil
}
if lineNum > maxProcNetRouteRead {
return errStopReading
}
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
}
if flags&(unix.RTF_UP|unix.RTF_GATEWAY) != unix.RTF_UP|unix.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 errStopReading
}
return nil
})
if errors.Is(err, errStopReading) {
err = nil
}
if err != nil {
procNetRouteErr.Store(true)
if runtime.GOOS == "android" {
return likelyHomeRouterIPAndroid()
}
log.Printf("interfaces: failed to read /proc/net/route: %v", err)
}
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 likelyHomeRouterIPAndroid() (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 "
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 := netip.ParseAddr(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, netip.Addr{}, ret.IsValid()
}
func defaultRoute() (d DefaultRouteDetails, err error) {
v, err := defaultRouteInterfaceProcNet()
if err == nil {
d.InterfaceName = v
return d, nil
}
// Issue 4038: the default route (such as on Unifi UDM Pro)
// might be in a non-default table, so it won't show up in
// /proc/net/route. Use netlink to find the default route.
//
// TODO(bradfitz): this allocates a fair bit. We should track
// this in net/interfaces/monitor instead and have
// interfaces.GetState take a netmon.Monitor or similar so the
// routing table can be cached and the monitor's existing
// subscription to route changes can update the cached state,
// rather than querying the whole thing every time like
// defaultRouteFromNetlink does.
//
// Then we should just always try to use the cached route
// table from netlink every time, and only use /proc/net/route
// as a fallback for weird environments where netlink might be
// banned but /proc/net/route is emulated (e.g. stuff like
// Cloud Run?).
return defaultRouteFromNetlink()
}
func defaultRouteFromNetlink() (d DefaultRouteDetails, err error) {
c, err := rtnetlink.Dial(&netlink.Config{Strict: true})
if err != nil {
return d, fmt.Errorf("defaultRouteFromNetlink: Dial: %w", err)
}
defer c.Close()
rms, err := c.Route.List()
if err != nil {
return d, fmt.Errorf("defaultRouteFromNetlink: List: %w", err)
}
for _, rm := range rms {
if rm.Attributes.Gateway == nil {
// A default route has a gateway. If it doesn't, skip it.
continue
}
if rm.Attributes.Dst != nil {
// A default route has a nil destination to mean anything
// so ignore any route for a specific destination.
// TODO(bradfitz): better heuristic?
// empirically this seems like enough.
continue
}
// TODO(bradfitz): care about address family, if
// callers ever start caring about v4-vs-v6 default
// route differences.
idx := int(rm.Attributes.OutIface)
if idx == 0 {
continue
}
if iface, err := net.InterfaceByIndex(idx); err == nil {
d.InterfaceName = iface.Name
d.InterfaceIndex = idx
return d, nil
}
}
return d, errNoDefaultRoute
}
var zeroRouteBytes = []byte("00000000")
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
var errNoDefaultRoute = errors.New("no default route found")
func defaultRouteInterfaceProcNetInternal(bufsize int) (string, error) {
f, err := os.Open(procNetRoutePath)
if err != nil {
return "", err
}
defer f.Close()
br := bufio.NewReaderSize(f, bufsize)
lineNum := 0
for {
lineNum++
line, err := br.ReadSlice('\n')
if err == io.EOF || lineNum > maxProcNetRouteRead {
return "", errNoDefaultRoute
}
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
}