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tailscale/net/netutil/ip_forward.go

340 lines
11 KiB
Go

// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
// Package netutil contains misc shared networking code & types.
package netutil
import (
"bytes"
"fmt"
"net/netip"
"os"
"os/exec"
"path/filepath"
"runtime"
"strconv"
"strings"
"tailscale.com/net/interfaces"
)
// protocolsRequiredForForwarding reports whether IPv4 and/or IPv6 protocols are
// required to forward the specified routes.
// The state param must be specified.
func protocolsRequiredForForwarding(routes []netip.Prefix, state *interfaces.State) (v4, v6 bool) {
if len(routes) == 0 {
// Nothing to route, so no need to warn.
return false, false
}
localIPs := make(map[netip.Addr]bool)
for _, addrs := range state.InterfaceIPs {
for _, pfx := range addrs {
localIPs[pfx.Addr()] = true
}
}
for _, r := range routes {
// It's possible to advertise a route to one of the local
// machine's local IPs. IP forwarding isn't required for this
// to work, so we shouldn't warn for such exports.
if r.IsSingleIP() && localIPs[r.Addr()] {
continue
}
if r.Addr().Is4() {
v4 = true
} else {
v6 = true
}
}
return v4, v6
}
// CheckIPForwarding reports whether IP forwarding is enabled correctly
// for subnet routing and exit node functionality on any interface.
// The state param must not be nil.
// The routes should only be advertised routes, and should not contain the
// nodes Tailscale IPs.
// It returns an error if it is unable to determine if IP forwarding is enabled.
// It returns a warning describing configuration issues if IP forwarding is
// non-functional or partly functional.
func CheckIPForwarding(routes []netip.Prefix, state *interfaces.State) (warn, err error) {
if runtime.GOOS != "linux" {
switch runtime.GOOS {
case "dragonfly", "freebsd", "netbsd", "openbsd":
return fmt.Errorf("Subnet routing and exit nodes only work with additional manual configuration on %v, and is not currently officially supported.", runtime.GOOS), nil
}
return nil, nil
}
if state == nil {
return nil, fmt.Errorf("Couldn't check system's IP forwarding configuration; no link state")
}
const kbLink = "\nSee https://tailscale.com/s/ip-forwarding"
wantV4, wantV6 := protocolsRequiredForForwarding(routes, state)
if !wantV4 && !wantV6 {
return nil, nil
}
v4e, err := ipForwardingEnabledLinux(ipv4, "")
if err != nil {
return nil, fmt.Errorf("Couldn't check system's IP forwarding configuration, subnet routing/exit nodes may not work: %w%s", err, kbLink)
}
v6e, err := ipForwardingEnabledLinux(ipv6, "")
if err != nil {
return nil, fmt.Errorf("Couldn't check system's IP forwarding configuration, subnet routing/exit nodes may not work: %w%s", err, kbLink)
}
if v4e && v6e {
// IP forwarding is enabled systemwide, all is well.
return nil, nil
}
if !wantV4 {
if !v6e {
return nil, fmt.Errorf("IPv6 forwarding is disabled, subnet routing/exit nodes may not work.%s", kbLink)
}
return nil, nil
}
// IP forwarding isn't enabled globally, but it might be enabled
// on a per-interface basis. Check if it's on for all interfaces,
// and warn appropriately if it's not.
// Note: you might be wondering why we check only the state of
// ipv6.conf.all.forwarding, rather than per-interface forwarding
// configuration. According to kernel documentation, it seems
// that to actually forward packets, you need to enable
// forwarding globally, and the per-interface forwarding
// setting only alters other things such as how router
// advertisements are handled. The kernel itself warns that
// enabling forwarding per-interface and not globally will
// probably not work, so I feel okay calling those configs
// broken until we have proof otherwise.
var (
anyEnabled bool
warnings []string
)
if wantV6 && !v6e {
warnings = append(warnings, "IPv6 forwarding is disabled.")
}
for _, iface := range state.Interface {
if iface.Name == "lo" {
continue
}
v4e, err := ipForwardingEnabledLinux(ipv4, iface.Name)
if err != nil {
return nil, fmt.Errorf("Couldn't check system's IP forwarding configuration, subnet routing/exit nodes may not work: %w%s", err, kbLink)
} else if !v4e {
warnings = append(warnings, fmt.Sprintf("Traffic received on %s won't be forwarded (%s disabled)", iface.Name, ipForwardSysctlKey(dotFormat, ipv4, iface.Name)))
} else {
anyEnabled = true
}
}
if !anyEnabled {
// IP forwarding is completely disabled, just say that rather
// than enumerate all the interfaces on the system.
return fmt.Errorf("IP forwarding is disabled, subnet routing/exit nodes will not work.%s", kbLink), nil
}
if len(warnings) > 0 {
// If partially enabled, enumerate the bits that won't work.
return fmt.Errorf("%s\nSubnet routes and exit nodes may not work correctly.%s", strings.Join(warnings, "\n"), kbLink), nil
}
return nil, nil
}
// CheckReversePathFiltering reports whether reverse path filtering is either
// disabled or set to 'loose' mode for exit node functionality on any
// interface.
//
// The state param can be nil, in which case interfaces.GetState is used.
//
// The routes should only be advertised routes, and should not contain the
// node's Tailscale IPs.
//
// This function returns an error if it is unable to determine whether reverse
// path filtering is enabled, or a warning describing configuration issues if
// reverse path fitering is non-functional or partly functional.
func CheckReversePathFiltering(routes []netip.Prefix, state *interfaces.State) (warn []string, err error) {
if runtime.GOOS != "linux" {
return nil, nil
}
if state == nil {
var err error
state, err = interfaces.GetState()
if err != nil {
return nil, err
}
}
// Reverse path filtering as a syscall is only implemented on Linux for IPv4.
wantV4, _ := protocolsRequiredForForwarding(routes, state)
if !wantV4 {
return nil, nil
}
// The kernel uses the maximum value for rp_filter between the 'all'
// setting and each per-interface config, so we need to fetch both.
allSetting, err := reversePathFilterValueLinux("all")
if err != nil {
return nil, fmt.Errorf("reading global rp_filter value: %w", err)
}
const (
filtOff = 0
filtStrict = 1
filtLoose = 2
)
// Because the kernel use the max rp_filter value, each interface will use 'loose', so we
// can abort early.
if allSetting == filtLoose {
return nil, nil
}
for _, iface := range state.Interface {
if iface.IsLoopback() {
continue
}
iSetting, err := reversePathFilterValueLinux(iface.Name)
if err != nil {
return nil, fmt.Errorf("reading interface rp_filter value for %q: %w", iface.Name, err)
}
// Perform the same max() that the kernel does
if allSetting > iSetting {
iSetting = allSetting
}
if iSetting == filtStrict {
warn = append(warn, fmt.Sprintf("Interface %q has strict reverse-path filtering enabled", iface.Name))
}
}
return warn, nil
}
// ipForwardSysctlKey returns the sysctl key for the given protocol and iface.
// When the dotFormat parameter is true the output is formatted as `net.ipv4.ip_forward`,
// else it is `net/ipv4/ip_forward`
func ipForwardSysctlKey(format sysctlFormat, p protocol, iface string) string {
if iface == "" {
if format == dotFormat {
if p == ipv4 {
return "net.ipv4.ip_forward"
}
return "net.ipv6.conf.all.forwarding"
}
if p == ipv4 {
return "net/ipv4/ip_forward"
}
return "net/ipv6/conf/all/forwarding"
}
var k string
if p == ipv4 {
k = "net/ipv4/conf/%s/forwarding"
} else {
k = "net/ipv6/conf/%s/forwarding"
}
if format == dotFormat {
// Swap the delimiters.
iface = strings.ReplaceAll(iface, ".", "/")
k = strings.ReplaceAll(k, "/", ".")
}
return fmt.Sprintf(k, iface)
}
// rpFilterSysctlKey returns the sysctl key for the given iface.
//
// Format controls whether the output is formatted as
// `net.ipv4.conf.iface.rp_filter` or `net/ipv4/conf/iface/rp_filter`.
func rpFilterSysctlKey(format sysctlFormat, iface string) string {
// No iface means all interfaces
if iface == "" {
iface = "all"
}
k := "net/ipv4/conf/%s/rp_filter"
if format == dotFormat {
// Swap the delimiters.
iface = strings.ReplaceAll(iface, ".", "/")
k = strings.ReplaceAll(k, "/", ".")
}
return fmt.Sprintf(k, iface)
}
type sysctlFormat int
const (
dotFormat sysctlFormat = iota
slashFormat
)
type protocol int
const (
ipv4 protocol = iota
ipv6
)
// ipForwardingEnabledLinux reports whether the IP Forwarding is enabled for the
// given interface.
// The iface param determines which interface to check against, "" means to check
// global config.
// This is Linux-specific: it only reads from /proc/sys and doesn't shell out to
// sysctl (which on Linux just reads from /proc/sys anyway).
func ipForwardingEnabledLinux(p protocol, iface string) (bool, error) {
k := ipForwardSysctlKey(slashFormat, p, iface)
bs, err := os.ReadFile(filepath.Join("/proc/sys", k))
if err != nil {
if os.IsNotExist(err) {
// If IPv6 is disabled, sysctl keys like "net.ipv6.conf.all.forwarding" just don't
// exist on disk. But first diagnose whether procfs is even mounted before assuming
// absence means false.
if fi, err := os.Stat("/proc/sys"); err != nil {
return false, fmt.Errorf("failed to check sysctl %v; no procfs? %w", k, err)
} else if !fi.IsDir() {
return false, fmt.Errorf("failed to check sysctl %v; /proc/sys isn't a directory, is %v", k, fi.Mode())
}
return false, nil
}
return false, err
}
val, err := strconv.ParseInt(string(bytes.TrimSpace(bs)), 10, 32)
if err != nil {
return false, fmt.Errorf("couldn't parse %s: %w", k, err)
}
// 0 = disabled, 1 = enabled, 2 = enabled (but uncommon)
// https://github.com/tailscale/tailscale/issues/8375
if val < 0 || val > 2 {
return false, fmt.Errorf("unexpected value %d for %s", val, k)
}
on := val == 1 || val == 2
return on, nil
}
// reversePathFilterValueLinux reports the reverse path filter setting on Linux
// for the given interface.
//
// The iface param determines which interface to check against; the empty
// string means to check the global config.
//
// This function tries to look up the value directly from `/proc/sys`, and
// falls back to using the `sysctl` command on failure.
func reversePathFilterValueLinux(iface string) (int, error) {
k := rpFilterSysctlKey(slashFormat, iface)
bs, err := os.ReadFile(filepath.Join("/proc/sys", k))
if err != nil {
// Fall back to the sysctl command
k := rpFilterSysctlKey(dotFormat, iface)
bs, err = exec.Command("sysctl", "-n", k).Output()
if err != nil {
return -1, fmt.Errorf("couldn't check %s (%v)", k, err)
}
}
v, err := strconv.Atoi(string(bytes.TrimSpace(bs)))
if err != nil {
return -1, fmt.Errorf("couldn't parse %s (%v)", k, err)
}
return v, nil
}