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tun_linux.go
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tun_linux.go
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/*
* Copyright (c) 2017, Psiphon Inc.
* All rights reserved.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
*/
package tun
import (
"fmt"
"net"
"os"
"strconv"
"strings"
"syscall"
"unsafe"
"github.com/Psiphon-Labs/psiphon-tunnel-core/psiphon/common"
"github.com/Psiphon-Labs/psiphon-tunnel-core/psiphon/common/errors"
"golang.org/x/sys/unix"
)
const (
DEFAULT_PUBLIC_INTERFACE_NAME = "eth0"
)
func IsSupported() bool {
return true
}
func makeDeviceInboundBuffer(MTU int) []byte {
return make([]byte, MTU)
}
func makeDeviceOutboundBuffer(MTU int) []byte {
// On Linux, no outbound buffer is used
return nil
}
// OpenTunDevice opens a file for performing device I/O with
// either a specified tun device, or a new tun device (when
// name is "").
func OpenTunDevice(name string) (*os.File, string, error) {
// Prevent fork between creating fd and setting CLOEXEC
// TODO: is this still necessary with unix.Open?
syscall.ForkLock.RLock()
defer syscall.ForkLock.RUnlock()
// Requires process to run as root or have CAP_NET_ADMIN
// As explained in https://github.com/golang/go/issues/30426, the fd must
// not be added to the Go poller before the following TUNSETIFF ioctl
// call. This is achieved by using unix.Open -- which opens a raw fd --
// instead of os.FileOpen, followed by the ioctl and finally os.NewFile
// to add the fd to the Go poller.
//
// Set CLOEXEC so file descriptor not leaked to network config command
// subprocesses.
fileName := "/dev/net/tun"
fd, err := unix.Open(fileName, os.O_RDWR|syscall.O_CLOEXEC, 0)
if err != nil {
return nil, "", errors.Trace(err)
}
// This code follows snippets in this thread:
// https://groups.google.com/forum/#!msg/golang-nuts/x_c_pZ6p95c/8T0JBZLpTwAJ;
// Definitions from <linux/if.h>, <linux/if_tun.h>
// Note: using IFF_NO_PI, so packets have no size/flags header. This does mean
// that if the MTU is changed after the tun device is initialized, packets could
// be truncated when read.
const (
IFNAMSIZ = 16
IF_REQ_PAD_SIZE = 40 - 18
IFF_TUN = 0x0001
IFF_NO_PI = 0x1000
)
var ifName [IFNAMSIZ]byte
if name == "" {
copy(ifName[:], []byte("tun%d"))
} else {
copy(ifName[:], []byte(name))
}
ifReq := struct {
name [IFNAMSIZ]byte
flags uint16
pad [IF_REQ_PAD_SIZE]byte
}{
ifName,
uint16(IFF_TUN | IFF_NO_PI),
[IF_REQ_PAD_SIZE]byte{},
}
_, _, errno := syscall.Syscall(
syscall.SYS_IOCTL,
uintptr(fd),
uintptr(syscall.TUNSETIFF),
uintptr(unsafe.Pointer(&ifReq)))
if errno != 0 {
unix.Close(fd)
return nil, "", errors.Trace(errno)
}
err = unix.SetNonblock(fd, true)
if err != nil {
unix.Close(fd)
return nil, "", errors.Trace(err)
}
file := os.NewFile(uintptr(fd), fileName)
deviceName := strings.Trim(string(ifReq.name[:]), "\x00")
return file, deviceName, nil
}
func (device *Device) readTunPacket() (int, int, error) {
// Assumes MTU passed to makeDeviceInboundBuffer is actual MTU and
// so buffer is sufficiently large to always read a complete packet.
n, err := device.deviceIO.Read(device.inboundBuffer)
if err != nil {
return 0, 0, errors.Trace(err)
}
return 0, n, nil
}
func (device *Device) writeTunPacket(packet []byte) error {
// Doesn't need outboundBuffer since there's no header; write directly to device.
_, err := device.deviceIO.Write(packet)
if err != nil {
return errors.Trace(err)
}
return nil
}
func resetNATTables(
config *ServerConfig,
IPAddress net.IP) error {
// Uses the "conntrack" command, which is often not installed by default.
// conntrack --delete -src-nat --orig-src <address> will clear NAT tables of existing
// connections, making it less likely that traffic for a previous client using the
// specified address will be forwarded to a new client using this address. This is in
// the already unlikely event that there's still in-flight traffic when the address is
// recycled.
err := common.RunNetworkConfigCommand(
config.Logger,
config.SudoNetworkConfigCommands,
"conntrack",
"--delete",
"--src-nat",
"--orig-src",
IPAddress.String())
if err != nil {
// conntrack exits with this error message when there are no flows
// to delete, which is not a failure condition.
if strings.Contains(err.Error(), "0 flow entries have been deleted") {
return nil
}
return errors.Trace(err)
}
return nil
}
func configureServerInterface(
config *ServerConfig,
tunDeviceName string) error {
// Set tun device network addresses and MTU
IPv4Address, IPv4Netmask, err := splitIPMask(serverIPv4AddressCIDR)
if err != nil {
return errors.Trace(err)
}
err = common.RunNetworkConfigCommand(
config.Logger,
config.SudoNetworkConfigCommands,
"ifconfig",
tunDeviceName,
IPv4Address, "netmask", IPv4Netmask,
"mtu", strconv.Itoa(getMTU(config.MTU)),
"up")
if err != nil {
return errors.Trace(err)
}
err = common.RunNetworkConfigCommand(
config.Logger,
config.SudoNetworkConfigCommands,
"ifconfig",
tunDeviceName,
"add", serverIPv6AddressCIDR)
if err != nil {
if config.AllowNoIPv6NetworkConfiguration {
config.Logger.WithTraceFields(
common.LogFields{
"error": err}).Warning(
"assign IPv6 address failed")
} else {
return errors.Trace(err)
}
}
egressInterface := config.EgressInterface
if egressInterface == "" {
egressInterface = DEFAULT_PUBLIC_INTERFACE_NAME
}
// NAT tun device to external interface
// TODO: need only set forwarding for specific interfaces?
err = common.RunNetworkConfigCommand(
config.Logger,
config.SudoNetworkConfigCommands,
"sysctl",
"net.ipv4.conf.all.forwarding=1")
if err != nil {
return errors.Trace(err)
}
err = common.RunNetworkConfigCommand(
config.Logger,
config.SudoNetworkConfigCommands,
"sysctl",
"net.ipv6.conf.all.forwarding=1")
if err != nil {
if config.AllowNoIPv6NetworkConfiguration {
config.Logger.WithTraceFields(
common.LogFields{
"error": err}).Warning(
"allow IPv6 forwarding failed")
} else {
return errors.Trace(err)
}
}
// To avoid duplicates, first try to drop existing rule, then add
for _, mode := range []string{"-D", "-A"} {
err = common.RunNetworkConfigCommand(
config.Logger,
config.SudoNetworkConfigCommands,
"iptables",
"-t", "nat",
mode, "POSTROUTING",
"-s", privateSubnetIPv4.String(),
"-o", egressInterface,
"-j", "MASQUERADE")
if mode != "-D" && err != nil {
return errors.Trace(err)
}
err = common.RunNetworkConfigCommand(
config.Logger,
config.SudoNetworkConfigCommands,
"ip6tables",
"-t", "nat",
mode, "POSTROUTING",
"-s", privateSubnetIPv6.String(),
"-o", egressInterface,
"-j", "MASQUERADE")
if mode != "-D" && err != nil {
if config.AllowNoIPv6NetworkConfiguration {
config.Logger.WithTraceFields(
common.LogFields{
"error": err}).Warning(
"configure IPv6 masquerading failed")
} else {
return errors.Trace(err)
}
}
}
return nil
}
func configureClientInterface(
config *ClientConfig,
tunDeviceName string) error {
// Set tun device network addresses and MTU
IPv4Address, IPv4Netmask, err := splitIPMask(config.IPv4AddressCIDR)
if err != nil {
return errors.Trace(err)
}
err = common.RunNetworkConfigCommand(
config.Logger,
config.SudoNetworkConfigCommands,
"ifconfig",
tunDeviceName,
IPv4Address,
"netmask", IPv4Netmask,
"mtu", strconv.Itoa(getMTU(config.MTU)),
"up")
if err != nil {
return errors.Trace(err)
}
err = common.RunNetworkConfigCommand(
config.Logger,
config.SudoNetworkConfigCommands,
"ifconfig",
tunDeviceName,
"add", config.IPv6AddressCIDR)
if err != nil {
if config.AllowNoIPv6NetworkConfiguration {
config.Logger.WithTraceFields(
common.LogFields{
"error": err}).Warning(
"assign IPv6 address failed")
} else {
return errors.Trace(err)
}
}
// Set routing. Routes set here should automatically
// drop when the tun device is removed.
// TODO: appear to need explicit routing only for IPv6?
for _, destination := range config.RouteDestinations {
// Destination may be host (IP) or network (CIDR)
IP := net.ParseIP(destination)
if IP == nil {
var err error
IP, _, err = net.ParseCIDR(destination)
if err != nil {
return errors.Trace(err)
}
}
if IP.To4() != nil {
continue
}
// Note: use "replace" instead of "add" as route from
// previous run (e.g., tun_test case) may not yet be cleared.
err = common.RunNetworkConfigCommand(
config.Logger,
config.SudoNetworkConfigCommands,
"ip",
"-6",
"route", "replace",
destination,
"dev", tunDeviceName)
if err != nil {
if config.AllowNoIPv6NetworkConfiguration {
config.Logger.WithTraceFields(
common.LogFields{
"error": err}).Warning("add IPv6 route failed")
} else {
return errors.Trace(err)
}
}
}
return nil
}
// BindToDevice binds a socket to the specified interface.
func BindToDevice(fd int, deviceName string) error {
err := syscall.BindToDevice(fd, deviceName)
if err != nil {
return errors.Trace(err)
}
return nil
}
func fixBindToDevice(logger common.Logger, useSudo bool, tunDeviceName string) error {
// Fix the problem described here:
// https://stackoverflow.com/questions/24011205/cant-perform-tcp-handshake-through-a-nat-between-two-nics-with-so-bindtodevice/
//
// > the linux kernel is configured on certain mainstream distributions
// > (Ubuntu...) to act as a router and drop packets where the source
// > address is suspect in order to prevent spoofing (search "rp_filter" on
// > https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt and
// > RFC3704)
err := common.RunNetworkConfigCommand(
logger,
useSudo,
"sysctl",
"net.ipv4.conf.all.accept_local=1")
if err != nil {
return errors.Trace(err)
}
err = common.RunNetworkConfigCommand(
logger,
useSudo,
"sysctl",
"net.ipv4.conf.all.rp_filter=0")
if err != nil {
return errors.Trace(err)
}
err = common.RunNetworkConfigCommand(
logger,
useSudo,
"sysctl",
fmt.Sprintf("net.ipv4.conf.%s.rp_filter=0", tunDeviceName))
if err != nil {
return errors.Trace(err)
}
return nil
}