// 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 }