tailscale/net/dns/direct.go

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// 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 dns
import (
"bufio"
"bytes"
"context"
"crypto/rand"
"fmt"
"io"
"io/ioutil"
"os"
"os/exec"
"path/filepath"
"runtime"
"strings"
"time"
"inet.af/netaddr"
"tailscale.com/types/logger"
"tailscale.com/util/dnsname"
)
const (
backupConf = "/etc/resolv.pre-tailscale-backup.conf"
resolvConf = "/etc/resolv.conf"
)
// writeResolvConf writes DNS configuration in resolv.conf format to the given writer.
func writeResolvConf(w io.Writer, servers []netaddr.IP, domains []dnsname.FQDN) {
io.WriteString(w, "# resolv.conf(5) file generated by tailscale\n")
io.WriteString(w, "# DO NOT EDIT THIS FILE BY HAND -- CHANGES WILL BE OVERWRITTEN\n\n")
for _, ns := range servers {
io.WriteString(w, "nameserver ")
io.WriteString(w, ns.String())
io.WriteString(w, "\n")
}
if len(domains) > 0 {
io.WriteString(w, "search")
for _, domain := range domains {
io.WriteString(w, " ")
io.WriteString(w, domain.WithoutTrailingDot())
}
io.WriteString(w, "\n")
}
}
func readResolv(r io.Reader) (config OSConfig, err error) {
scanner := bufio.NewScanner(r)
for scanner.Scan() {
line := strings.TrimSpace(scanner.Text())
i := strings.IndexByte(line, '#')
if i >= 0 {
line = line[:i]
}
if strings.HasPrefix(line, "nameserver") {
s := strings.TrimPrefix(line, "nameserver")
nameserver := strings.TrimSpace(s)
if len(nameserver) == len(s) {
return OSConfig{}, fmt.Errorf("missing space after \"nameserver\" in %q", line)
}
ip, err := netaddr.ParseIP(nameserver)
if err != nil {
return OSConfig{}, err
}
config.Nameservers = append(config.Nameservers, ip)
continue
}
if strings.HasPrefix(line, "search") {
s := strings.TrimPrefix(line, "search")
domain := strings.TrimSpace(s)
if len(domain) == len(s) {
// No leading space?!
return OSConfig{}, fmt.Errorf("missing space after \"domain\" in %q", line)
}
fqdn, err := dnsname.ToFQDN(domain)
if err != nil {
return OSConfig{}, fmt.Errorf("parsing search domains %q: %w", line, err)
}
config.SearchDomains = append(config.SearchDomains, fqdn)
continue
}
}
return config, nil
}
// resolvOwner returns the apparent owner of the resolv.conf
// configuration in bs - one of "resolvconf", "systemd-resolved" or
// "NetworkManager", or "" if no known owner was found.
func resolvOwner(bs []byte) string {
likely := ""
b := bytes.NewBuffer(bs)
for {
line, err := b.ReadString('\n')
if err != nil {
return likely
}
line = strings.TrimSpace(line)
if line == "" {
continue
}
if line[0] != '#' {
// First non-empty, non-comment line. Assume the owner
// isn't hiding further down.
return likely
}
if strings.Contains(line, "systemd-resolved") {
likely = "systemd-resolved"
} else if strings.Contains(line, "NetworkManager") {
likely = "NetworkManager"
} else if strings.Contains(line, "resolvconf") {
likely = "resolvconf"
}
}
}
// isResolvedRunning reports whether systemd-resolved is running on the system,
// even if it is not managing the system DNS settings.
func isResolvedRunning() bool {
if runtime.GOOS != "linux" {
return false
}
// systemd-resolved is never installed without systemd.
_, err := exec.LookPath("systemctl")
if err != nil {
return false
}
ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
defer cancel()
err = exec.CommandContext(ctx, "systemctl", "is-active", "systemd-resolved.service").Run()
// is-active exits with code 3 if the service is not active.
return err == nil
}
func restartResolved() error {
ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
defer cancel()
return exec.CommandContext(ctx, "systemctl", "restart", "systemd-resolved.service").Run()
}
// directManager is an OSConfigurator which replaces /etc/resolv.conf with a file
// generated from the given configuration, creating a backup of its old state.
//
// This way of configuring DNS is precarious, since it does not react
// to the disappearance of the Tailscale interface.
// The caller must call Down before program shutdown
// or as cleanup if the program terminates unexpectedly.
type directManager struct {
logf logger.Logf
fs wholeFileFS
// renameBroken is set if fs.Rename to or from /etc/resolv.conf
// fails. This can happen in some container runtimes, where
// /etc/resolv.conf is bind-mounted from outside the container,
// and therefore /etc and /etc/resolv.conf are different
// filesystems as far as rename(2) is concerned.
//
// In those situations, we fall back to emulating rename with file
// copies and truncations, which is not as good (opens up a race
// where a reader can see an empty or partial /etc/resolv.conf),
// but is better than having non-functioning DNS.
renameBroken bool
}
func newDirectManager(logf logger.Logf) *directManager {
return &directManager{
logf: logf,
fs: directFS{},
}
}
func newDirectManagerOnFS(logf logger.Logf, fs wholeFileFS) *directManager {
return &directManager{
logf: logf,
fs: fs,
}
}
func (m *directManager) readResolvFile(path string) (OSConfig, error) {
b, err := m.fs.ReadFile(path)
if err != nil {
return OSConfig{}, err
}
return readResolv(bytes.NewReader(b))
}
// ownedByTailscale reports whether /etc/resolv.conf seems to be a
// tailscale-managed file.
func (m *directManager) ownedByTailscale() (bool, error) {
isRegular, err := m.fs.Stat(resolvConf)
if err != nil {
if os.IsNotExist(err) {
return false, nil
}
return false, err
}
if !isRegular {
return false, nil
}
bs, err := m.fs.ReadFile(resolvConf)
if err != nil {
return false, err
}
if bytes.Contains(bs, []byte("generated by tailscale")) {
return true, nil
}
return false, nil
}
// backupConfig creates or updates a backup of /etc/resolv.conf, if
// resolv.conf does not currently contain a Tailscale-managed config.
func (m *directManager) backupConfig() error {
if _, err := m.fs.Stat(resolvConf); err != nil {
if os.IsNotExist(err) {
// No resolv.conf, nothing to back up. Also get rid of any
// existing backup file, to avoid restoring something old.
m.fs.Remove(backupConf)
return nil
}
return err
}
owned, err := m.ownedByTailscale()
if err != nil {
return err
}
if owned {
return nil
}
return m.rename(resolvConf, backupConf)
}
func (m *directManager) restoreBackup() (restored bool, err error) {
if _, err := m.fs.Stat(backupConf); err != nil {
if os.IsNotExist(err) {
// No backup, nothing we can do.
return false, nil
}
return false, err
}
owned, err := m.ownedByTailscale()
if err != nil {
return false, err
}
_, err = m.fs.Stat(resolvConf)
if err != nil && !os.IsNotExist(err) {
return false, err
}
resolvConfExists := !os.IsNotExist(err)
if resolvConfExists && !owned {
// There's already a non-tailscale config in place, get rid of
// our backup.
m.fs.Remove(backupConf)
return false, nil
}
// We own resolv.conf, and a backup exists.
if err := m.rename(backupConf, resolvConf); err != nil {
return false, err
}
return true, nil
}
// rename tries to rename old to new using m.fs.Rename, and falls back
// to hand-copying bytes and truncating old if that fails.
//
// This is a workaround to /etc/resolv.conf being a bind-mounted file
// some container environments, which cannot be moved elsewhere in
// /etc (because that would be a cross-filesystem move) or deleted
// (because that would break the bind in surprising ways).
func (m *directManager) rename(old, new string) error {
if !m.renameBroken {
err := m.fs.Rename(old, new)
if err == nil {
return nil
}
m.logf("rename of %q to %q failed (%v), falling back to copy+delete", old, new, err)
m.renameBroken = true
}
bs, err := m.fs.ReadFile(old)
if err != nil {
return fmt.Errorf("reading %q to rename: %v", old, err)
}
if err := m.fs.WriteFile(new, bs, 0644); err != nil {
return fmt.Errorf("writing to %q in rename of %q: %v", new, old, err)
}
if err := m.fs.Remove(old); err != nil {
err2 := m.fs.Truncate(old)
if err2 != nil {
return fmt.Errorf("remove of %q failed (%v) and so did truncate: %v", old, err, err2)
}
}
return nil
}
func (m *directManager) SetDNS(config OSConfig) (err error) {
var changed bool
if config.IsZero() {
changed, err = m.restoreBackup()
if err != nil {
return err
}
} else {
changed = true
if err := m.backupConfig(); err != nil {
return err
}
buf := new(bytes.Buffer)
writeResolvConf(buf, config.Nameservers, config.SearchDomains)
if err := m.atomicWriteFile(m.fs, resolvConf, buf.Bytes(), 0644); err != nil {
return err
}
}
// We might have taken over a configuration managed by resolved,
// in which case it will notice this on restart and gracefully
// start using our configuration. This shouldn't happen because we
// try to manage DNS through resolved when it's around, but as a
// best-effort fallback if we messed up the detection, try to
// restart resolved to make the system configuration consistent.
//
// We take care to only kick systemd-resolved if we've made some
// change to the system's DNS configuration, because this codepath
// can end up running in cases where the user has manually
// configured /etc/resolv.conf to point to systemd-resolved (but
// it's not managed explicitly by systemd-resolved), *and* has
// --accept-dns=false, meaning we pass an empty configuration to
// the running DNS manager. In that very edge-case scenario, we
// cause a disruptive DNS outage each time we reset an empty
// OS configuration.
if changed && isResolvedRunning() && !runningAsGUIDesktopUser() {
exec.Command("systemctl", "restart", "systemd-resolved.service").Run()
}
return nil
}
func (m *directManager) SupportsSplitDNS() bool {
return false
}
func (m *directManager) GetBaseConfig() (OSConfig, error) {
owned, err := m.ownedByTailscale()
if err != nil {
return OSConfig{}, err
}
fileToRead := resolvConf
if owned {
fileToRead = backupConf
}
return m.readResolvFile(fileToRead)
}
func (m *directManager) Close() error {
// We used to keep a file for the tailscale config and symlinked
// to it, but then we stopped because /etc/resolv.conf being a
// symlink to surprising places breaks snaps and other sandboxing
// things. Clean it up if it's still there.
m.fs.Remove("/etc/resolv.tailscale.conf")
if _, err := m.fs.Stat(backupConf); err != nil {
if os.IsNotExist(err) {
// No backup, nothing we can do.
return nil
}
return err
}
owned, err := m.ownedByTailscale()
if err != nil {
return err
}
_, err = m.fs.Stat(resolvConf)
if err != nil && !os.IsNotExist(err) {
return err
}
resolvConfExists := !os.IsNotExist(err)
if resolvConfExists && !owned {
// There's already a non-tailscale config in place, get rid of
// our backup.
m.fs.Remove(backupConf)
return nil
}
// We own resolv.conf, and a backup exists.
if err := m.rename(backupConf, resolvConf); err != nil {
return err
}
if isResolvedRunning() && !runningAsGUIDesktopUser() {
m.logf("restarting systemd-resolved...")
if err := restartResolved(); err != nil {
m.logf("restart of systemd-resolved failed: %v", err)
} else {
m.logf("restarted systemd-resolved")
}
}
return nil
}
func (m *directManager) atomicWriteFile(fs wholeFileFS, filename string, data []byte, perm os.FileMode) error {
var randBytes [12]byte
if _, err := rand.Read(randBytes[:]); err != nil {
return fmt.Errorf("atomicWriteFile: %w", err)
}
tmpName := fmt.Sprintf("%s.%x.tmp", filename, randBytes[:])
defer fs.Remove(tmpName)
if err := fs.WriteFile(tmpName, data, perm); err != nil {
return fmt.Errorf("atomicWriteFile: %w", err)
}
return m.rename(tmpName, filename)
}
// wholeFileFS is a high-level file system abstraction designed just for use
// by directManager, with the goal that it is easy to implement over wsl.exe.
//
// All name parameters are absolute paths.
type wholeFileFS interface {
Stat(name string) (isRegular bool, err error)
Rename(oldName, newName string) error
Remove(name string) error
ReadFile(name string) ([]byte, error)
Truncate(name string) error
WriteFile(name string, contents []byte, perm os.FileMode) error
}
// directFS is a wholeFileFS implemented directly on the OS.
type directFS struct {
// prefix is file path prefix.
//
// All name parameters are absolute paths so this is typically a
// testing temporary directory like "/tmp".
prefix string
}
func (fs directFS) path(name string) string { return filepath.Join(fs.prefix, name) }
func (fs directFS) Stat(name string) (isRegular bool, err error) {
fi, err := os.Stat(fs.path(name))
if err != nil {
return false, err
}
return fi.Mode().IsRegular(), nil
}
func (fs directFS) Rename(oldName, newName string) error {
return os.Rename(fs.path(oldName), fs.path(newName))
}
func (fs directFS) Remove(name string) error { return os.Remove(fs.path(name)) }
func (fs directFS) ReadFile(name string) ([]byte, error) {
return ioutil.ReadFile(fs.path(name))
}
func (fs directFS) Truncate(name string) error {
return os.Truncate(fs.path(name), 0)
}
func (fs directFS) WriteFile(name string, contents []byte, perm os.FileMode) error {
return ioutil.WriteFile(fs.path(name), contents, perm)
}
// runningAsGUIDesktopUser reports whether it seems that this code is
// being run as a regular user on a Linux desktop. This is a quick
// hack to fix Issue 2672 where PolicyKit pops up a GUI dialog asking
// to proceed we do a best effort attempt to restart
// systemd-resolved.service. There's surely a better way.
func runningAsGUIDesktopUser() bool {
return os.Getuid() != 0 && os.Getenv("DISPLAY") != ""
}