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tailscale/net/dns/direct.go

248 lines
6.1 KiB
Go

// 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.
// +build linux freebsd openbsd
package dns
import (
"bufio"
"bytes"
"io"
"io/ioutil"
"os"
"os/exec"
"runtime"
"strings"
"inet.af/netaddr"
"tailscale.com/atomicfile"
)
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 []string) {
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)
}
io.WriteString(w, "\n")
}
}
func readResolvFile(path string) (OSConfig, error) {
var config OSConfig
f, err := os.Open(path)
if err != nil {
return config, err
}
scanner := bufio.NewScanner(f)
for scanner.Scan() {
line := strings.TrimSpace(scanner.Text())
if strings.HasPrefix(line, "nameserver") {
nameserver := strings.TrimPrefix(line, "nameserver")
nameserver = strings.TrimSpace(nameserver)
ip, err := netaddr.ParseIP(nameserver)
if err != nil {
return config, err
}
config.Nameservers = append(config.Nameservers, ip)
continue
}
if strings.HasPrefix(line, "search") {
domain := strings.TrimPrefix(line, "search")
domain = strings.TrimSpace(domain)
config.SearchDomains = append(config.SearchDomains, domain)
continue
}
}
return config, nil
}
// readResolvConf reads DNS configuration from /etc/resolv.conf.
func readResolvConf() (OSConfig, error) {
return readResolvFile(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
}
// is-active exits with code 3 if the service is not active.
err = exec.Command("systemctl", "is-active", "systemd-resolved.service").Run()
return err == nil
}
// directManager is a managerImpl 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{}
func newDirectManager() directManager {
return directManager{}
}
// ownedByTailscale reports whether /etc/resolv.conf seems to be a
// tailscale-managed file.
func (m directManager) ownedByTailscale() (bool, error) {
st, err := os.Stat(resolvConf)
if err != nil {
if os.IsNotExist(err) {
return false, nil
}
return false, err
}
if !st.Mode().IsRegular() {
return false, nil
}
bs, err := ioutil.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 := os.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.
os.Remove(backupConf)
return nil
}
return err
}
owned, err := m.ownedByTailscale()
if err != nil {
return err
}
if owned {
return nil
}
return os.Rename(resolvConf, backupConf)
}
func (m directManager) SetDNS(config OSConfig) error {
if err := m.backupConfig(); err != nil {
return err
}
buf := new(bytes.Buffer)
writeResolvConf(buf, config.Nameservers, config.SearchDomains)
if err := atomicfile.WriteFile(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.
if isResolvedRunning() {
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 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.
os.Remove("/etc/resolv.tailscale.conf")
if _, err := os.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 = os.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.
os.Remove(backupConf)
return nil
}
// We own resolv.conf, and a backup exists.
if err := os.Rename(backupConf, resolvConf); err != nil {
return err
}
if isResolvedRunning() {
exec.Command("systemctl", "restart", "systemd-resolved.service").Run() // Best-effort.
}
return nil
}