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tailscale/net/dns/direct.go

189 lines
4.8 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"
"errors"
"fmt"
"io"
"io/ioutil"
"os"
"os/exec"
"runtime"
"strings"
"inet.af/netaddr"
"tailscale.com/atomicfile"
)
const (
tsConf = "/etc/resolv.tailscale.conf"
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")
}
}
// readResolvConf reads DNS configuration from /etc/resolv.conf.
func readResolvConf() (Config, error) {
var config Config
f, err := os.Open("/etc/resolv.conf")
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.Domains = append(config.Domains, domain)
continue
}
}
return config, nil
}
// 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(mconfig ManagerConfig) managerImpl {
return directManager{}
}
// Up implements managerImpl.
func (m directManager) Up(config Config) error {
// Write the tsConf file.
buf := new(bytes.Buffer)
writeResolvConf(buf, config.Nameservers, config.Domains)
if err := atomicfile.WriteFile(tsConf, buf.Bytes(), 0644); err != nil {
return err
}
if linkPath, err := os.Readlink(resolvConf); err != nil {
// Remove any old backup that may exist.
os.Remove(backupConf)
// Backup the existing /etc/resolv.conf file.
contents, err := ioutil.ReadFile(resolvConf)
// If the original did not exist, still back up an empty file.
// The presence of a backup file is the way we know that Up ran.
if err != nil && !errors.Is(err, os.ErrNotExist) {
return err
}
if err := atomicfile.WriteFile(backupConf, contents, 0644); err != nil {
return err
}
} else if linkPath != tsConf {
// Backup the existing symlink.
os.Remove(backupConf)
if err := os.Symlink(linkPath, backupConf); err != nil {
return err
}
} else {
// Nothing to do, resolvConf already points to tsConf.
return nil
}
os.Remove(resolvConf)
if err := os.Symlink(tsConf, resolvConf); err != nil {
return err
}
if isResolvedRunning() {
exec.Command("systemctl", "restart", "systemd-resolved.service").Run() // Best-effort.
}
return nil
}
// Down implements managerImpl.
func (m directManager) Down() error {
if _, err := os.Stat(backupConf); err != nil {
// If the backup file does not exist, then Up never ran successfully.
if os.IsNotExist(err) {
return nil
}
return err
}
if ln, err := os.Readlink(resolvConf); err != nil {
return err
} else if ln != tsConf {
return fmt.Errorf("resolv.conf is not a symlink to %s", tsConf)
}
if err := os.Rename(backupConf, resolvConf); err != nil {
return err
}
os.Remove(tsConf)
if isResolvedRunning() {
exec.Command("systemctl", "restart", "systemd-resolved.service").Run() // Best-effort.
}
return nil
}