mirror of
https://github.com/tailscale/tailscale.git
synced 2024-11-29 21:15:39 +00:00
327 lines
8.0 KiB
Go
327 lines
8.0 KiB
Go
// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// +build linux freebsd openbsd
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package dns
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import (
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"bufio"
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"bytes"
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"fmt"
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"io"
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"io/ioutil"
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"os"
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"os/exec"
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"runtime"
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"strings"
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"inet.af/netaddr"
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"tailscale.com/atomicfile"
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"tailscale.com/util/dnsname"
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)
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const (
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backupConf = "/etc/resolv.pre-tailscale-backup.conf"
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resolvConf = "/etc/resolv.conf"
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)
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// writeResolvConf writes DNS configuration in resolv.conf format to the given writer.
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func writeResolvConf(w io.Writer, servers []netaddr.IP, domains []dnsname.FQDN) {
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io.WriteString(w, "# resolv.conf(5) file generated by tailscale\n")
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io.WriteString(w, "# DO NOT EDIT THIS FILE BY HAND -- CHANGES WILL BE OVERWRITTEN\n\n")
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for _, ns := range servers {
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io.WriteString(w, "nameserver ")
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io.WriteString(w, ns.String())
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io.WriteString(w, "\n")
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}
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if len(domains) > 0 {
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io.WriteString(w, "search")
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for _, domain := range domains {
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io.WriteString(w, " ")
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io.WriteString(w, domain.WithoutTrailingDot())
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}
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io.WriteString(w, "\n")
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}
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}
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func readResolv(r io.Reader) (config OSConfig, err error) {
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scanner := bufio.NewScanner(r)
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for scanner.Scan() {
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line := strings.TrimSpace(scanner.Text())
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if strings.HasPrefix(line, "nameserver") {
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nameserver := strings.TrimPrefix(line, "nameserver")
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nameserver = strings.TrimSpace(nameserver)
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ip, err := netaddr.ParseIP(nameserver)
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if err != nil {
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return OSConfig{}, err
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}
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config.Nameservers = append(config.Nameservers, ip)
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continue
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}
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if strings.HasPrefix(line, "search") {
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domain := strings.TrimPrefix(line, "search")
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domain = strings.TrimSpace(domain)
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fqdn, err := dnsname.ToFQDN(domain)
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if err != nil {
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return OSConfig{}, fmt.Errorf("parsing search domains %q: %w", line, err)
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}
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config.SearchDomains = append(config.SearchDomains, fqdn)
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continue
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}
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}
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return config, nil
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}
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func readResolvFile(path string) (OSConfig, error) {
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var config OSConfig
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f, err := os.Open(path)
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if err != nil {
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return config, err
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}
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defer f.Close()
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return readResolv(f)
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}
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// readResolvConf reads DNS configuration from /etc/resolv.conf.
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func readResolvConf() (OSConfig, error) {
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return readResolvFile(resolvConf)
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}
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// resolvOwner returns the apparent owner of the resolv.conf
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// configuration in bs - one of "resolvconf", "systemd-resolved" or
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// "NetworkManager", or "" if no known owner was found.
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func resolvOwner(bs []byte) string {
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b := bytes.NewBuffer(bs)
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for {
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line, err := b.ReadString('\n')
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if err != nil {
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return ""
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}
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line = strings.TrimSpace(line)
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if line == "" {
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continue
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}
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if line[0] != '#' {
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// First non-empty, non-comment line. Assume the owner
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// isn't hiding further down.
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return ""
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}
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if strings.Contains(line, "systemd-resolved") {
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return "systemd-resolved"
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} else if strings.Contains(line, "NetworkManager") {
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return "NetworkManager"
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} else if strings.Contains(line, "resolvconf") {
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return "resolvconf"
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}
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}
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}
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// isResolvedRunning reports whether systemd-resolved is running on the system,
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// even if it is not managing the system DNS settings.
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func isResolvedRunning() bool {
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if runtime.GOOS != "linux" {
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return false
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}
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// systemd-resolved is never installed without systemd.
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_, err := exec.LookPath("systemctl")
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if err != nil {
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return false
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}
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// is-active exits with code 3 if the service is not active.
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err = exec.Command("systemctl", "is-active", "systemd-resolved.service").Run()
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return err == nil
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}
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// directManager is a managerImpl which replaces /etc/resolv.conf with a file
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// generated from the given configuration, creating a backup of its old state.
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//
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// This way of configuring DNS is precarious, since it does not react
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// to the disappearance of the Tailscale interface.
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// The caller must call Down before program shutdown
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// or as cleanup if the program terminates unexpectedly.
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type directManager struct{}
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func newDirectManager() (directManager, error) {
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return directManager{}, nil
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}
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// ownedByTailscale reports whether /etc/resolv.conf seems to be a
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// tailscale-managed file.
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func (m directManager) ownedByTailscale() (bool, error) {
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st, err := os.Stat(resolvConf)
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if err != nil {
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if os.IsNotExist(err) {
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return false, nil
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}
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return false, err
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}
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if !st.Mode().IsRegular() {
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return false, nil
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}
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bs, err := ioutil.ReadFile(resolvConf)
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if err != nil {
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return false, err
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}
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if bytes.Contains(bs, []byte("generated by tailscale")) {
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return true, nil
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}
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return false, nil
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}
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// backupConfig creates or updates a backup of /etc/resolv.conf, if
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// resolv.conf does not currently contain a Tailscale-managed config.
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func (m directManager) backupConfig() error {
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if _, err := os.Stat(resolvConf); err != nil {
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if os.IsNotExist(err) {
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// No resolv.conf, nothing to back up. Also get rid of any
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// existing backup file, to avoid restoring something old.
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os.Remove(backupConf)
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return nil
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}
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return err
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}
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owned, err := m.ownedByTailscale()
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if err != nil {
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return err
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}
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if owned {
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return nil
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}
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return os.Rename(resolvConf, backupConf)
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}
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func (m directManager) restoreBackup() error {
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if _, err := os.Stat(backupConf); err != nil {
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if os.IsNotExist(err) {
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// No backup, nothing we can do.
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return nil
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}
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return err
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}
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owned, err := m.ownedByTailscale()
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if err != nil {
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return err
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}
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if _, err := os.Stat(resolvConf); err != nil && !os.IsNotExist(err) {
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return err
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}
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resolvConfExists := !os.IsNotExist(err)
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if resolvConfExists && !owned {
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// There's already a non-tailscale config in place, get rid of
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// our backup.
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os.Remove(backupConf)
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return nil
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}
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// We own resolv.conf, and a backup exists.
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if err := os.Rename(backupConf, resolvConf); err != nil {
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return err
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}
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return nil
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}
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func (m directManager) SetDNS(config OSConfig) error {
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if config.IsZero() {
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if err := m.restoreBackup(); err != nil {
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return err
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}
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} else {
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if err := m.backupConfig(); err != nil {
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return err
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}
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buf := new(bytes.Buffer)
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writeResolvConf(buf, config.Nameservers, config.SearchDomains)
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if err := atomicfile.WriteFile(resolvConf, buf.Bytes(), 0644); err != nil {
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return err
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}
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}
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// We might have taken over a configuration managed by resolved,
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// in which case it will notice this on restart and gracefully
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// start using our configuration. This shouldn't happen because we
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// try to manage DNS through resolved when it's around, but as a
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// best-effort fallback if we messed up the detection, try to
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// restart resolved to make the system configuration consistent.
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if isResolvedRunning() {
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exec.Command("systemctl", "restart", "systemd-resolved.service").Run()
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}
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return nil
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}
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func (m directManager) SupportsSplitDNS() bool {
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return false
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}
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func (m directManager) GetBaseConfig() (OSConfig, error) {
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owned, err := m.ownedByTailscale()
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if err != nil {
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return OSConfig{}, err
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}
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fileToRead := resolvConf
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if owned {
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fileToRead = backupConf
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}
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return readResolvFile(fileToRead)
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}
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func (m directManager) Close() error {
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// We used to keep a file for the tailscale config and symlinked
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// to it, but then we stopped because /etc/resolv.conf being a
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// symlink to surprising places breaks snaps and other sandboxing
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// things. Clean it up if it's still there.
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os.Remove("/etc/resolv.tailscale.conf")
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if _, err := os.Stat(backupConf); err != nil {
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if os.IsNotExist(err) {
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// No backup, nothing we can do.
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return nil
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}
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return err
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}
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owned, err := m.ownedByTailscale()
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if err != nil {
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return err
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}
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_, err = os.Stat(resolvConf)
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if err != nil && !os.IsNotExist(err) {
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return err
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}
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resolvConfExists := !os.IsNotExist(err)
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if resolvConfExists && !owned {
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// There's already a non-tailscale config in place, get rid of
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// our backup.
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os.Remove(backupConf)
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return nil
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}
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// We own resolv.conf, and a backup exists.
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if err := os.Rename(backupConf, resolvConf); err != nil {
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return err
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}
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if isResolvedRunning() {
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exec.Command("systemctl", "restart", "systemd-resolved.service").Run() // Best-effort.
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}
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return nil
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}
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