net/dns: set WSL /etc/resolv.conf

We also have to make a one-off change to /etc/wsl.conf to stop every
invocation of wsl.exe clobbering the /etc/resolv.conf. This appears to
be a safe change to make permanently, as even though the resolv.conf is
constantly clobbered, it is always the same stable internal IP that is
set as a nameserver. (I believe the resolv.conf clobbering predates the
MS stub resolver.)

Tested on WSL2, should work for WSL1 too.

Fixes #775

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

net/dns/direct.go

@@ -13,6 +13,7 @@ import (
 	"io/ioutil"
 	"os"
 	"os/exec"
+	"path/filepath"
 	"runtime"
 	"strings"
 
@@ -145,14 +146,14 @@ func isResolvedRunning() bool {
 // The caller must call Down before program shutdown
 // or as cleanup if the program terminates unexpectedly.
 type directManager struct {
-	fs pinholeFS
+	fs wholeFileFS
 }
 
 func newDirectManager() directManager {
 	return directManager{fs: directFS{}}
 }
 
-func newDirectManagerOnFS(fs pinholeFS) directManager {
+func newDirectManagerOnFS(fs wholeFileFS) directManager {
 	return directManager{fs: fs}
 }
 
@@ -325,7 +326,7 @@ func (m directManager) Close() error {
 	return nil
 }
 
-func atomicWriteFile(fs pinholeFS, filename string, data []byte, perm os.FileMode) error {
+func 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)
@@ -340,9 +341,11 @@ func atomicWriteFile(fs pinholeFS, filename string, data []byte, perm os.FileMod
 	return fs.Rename(tmpName, filename)
 }
 
-// pinholeFS is a high-level file system abstraction designed just for use
+// 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.
-type pinholeFS interface {
+//
+// 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
@@ -350,13 +353,19 @@ type pinholeFS interface {
 	WriteFile(name string, contents []byte, perm os.FileMode) error
 }
 
-// directFS is a pinholeFS implemented directly on the OS.
+// directFS is a wholeFileFS implemented directly on the OS.
 type directFS struct {
-	prefix string // file path prefix; used for testing
+	// 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.prefix + name)
+	fi, err := os.Stat(fs.path(name))
 	if err != nil {
 		return false, err
 	}
@@ -364,15 +373,15 @@ func (fs directFS) Stat(name string) (isRegular bool, err error) {
 }
 
 func (fs directFS) Rename(oldName, newName string) error {
-	return os.Rename(fs.prefix+oldName, fs.prefix+newName)
+	return os.Rename(fs.path(oldName), fs.path(newName))
 }
 
-func (fs directFS) Remove(name string) error { return os.Remove(fs.prefix + name) }
+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.prefix + name)
+	return ioutil.ReadFile(fs.path(name))
 }
 
 func (fs directFS) WriteFile(name string, contents []byte, perm os.FileMode) error {
-	return ioutil.WriteFile(fs.prefix+name, contents, perm)
+	return ioutil.WriteFile(fs.path(name), contents, perm)
 }

net/dns/ini.go

@@ -0,0 +1,29 @@
+// Copyright (c) 2021 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 (
+	"regexp"
+	"strings"
+)
+
+// parseIni parses a basic .ini file, used for wsl.conf.
+func parseIni(data string) map[string]map[string]string {
+	sectionRE := regexp.MustCompile(`^\[([^]]+)\]`)
+	kvRE := regexp.MustCompile(`^\s*(\w+)\s*=\s*([^#]*)`)
+
+	ini := map[string]map[string]string{}
+	var section string
+	for _, line := range strings.Split(data, "\n") {
+		if res := sectionRE.FindStringSubmatch(line); len(res) > 1 {
+			section = res[1]
+			ini[section] = map[string]string{}
+		} else if res := kvRE.FindStringSubmatch(line); len(res) > 2 {
+			k, v := strings.TrimSpace(res[1]), strings.TrimSpace(res[2])
+			ini[section][k] = v
+		}
+	}
+	return ini
+}

net/dns/ini_test.go

@@ -0,0 +1,37 @@
+// Copyright (c) 2021 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 (
+	"reflect"
+	"testing"
+)
+
+func TestParseIni(t *testing.T) {
+	var tests = []struct {
+		src  string
+		want map[string]map[string]string
+	}{
+		{
+			src: `# appended wsl.conf file
+[automount]
+	enabled = true
+	root=/mnt/
+# added by tailscale
+[network] # trailing comment
+generateResolvConf = false  # trailing comment`,
+			want: map[string]map[string]string{
+				"automount": map[string]string{"enabled": "true", "root": "/mnt/"},
+				"network":   map[string]string{"generateResolvConf": "false"},
+			},
+		},
+	}
+	for _, test := range tests {
+		got := parseIni(test.src)
+		if !reflect.DeepEqual(got, test.want) {
+			t.Errorf("for:\n%s\ngot:  %v\nwant: %v", test.src, got, test.want)
+		}
+	}
+}

net/dns/manager_windows.go

@@ -38,6 +38,7 @@ type windowsManager struct {
 	logf       logger.Logf
 	guid       string
 	nrptWorks  bool
+	wslManager *wslManager
 }
 
 func NewOSConfigurator(logf logger.Logf, interfaceName string) (OSConfigurator, error) {
@@ -57,6 +58,11 @@ func NewOSConfigurator(logf logger.Logf, interfaceName string) (OSConfigurator,
 		ret.delKey(nrptBase)
 	}
 
+	if distros := wslDistros(logf); len(distros) > 0 {
+		logf("WSL distributions: %v", distros)
+		ret.wslManager = newWSLManager(logf, distros)
+	}
+
 	return ret, nil
 }
 
@@ -296,6 +302,18 @@ func (m windowsManager) SetDNS(cfg OSConfig) error {
 		}
 	}()
 
+	// On initial setup of WSL, the restart caused by --shutdown is slow,
+	// so we do it out-of-line.
+	go func() {
+		if m.wslManager != nil {
+			if err := m.wslManager.SetDNS(cfg); err != nil {
+				m.logf("WSL SetDNS: %v", err) // continue
+			} else {
+				m.logf("WSL SetDNS: success")
+			}
+		}
+	}()
+
 	return nil
 }
 

net/dns/wsl_windows.go

@@ -0,0 +1,217 @@
+// Copyright (c) 2021 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 (
+	"bytes"
+	"fmt"
+	"os"
+	"os/exec"
+	"strings"
+	"syscall"
+	"unicode/utf16"
+
+	"tailscale.com/types/logger"
+)
+
+// wslDistros reports the names of the installed WSL2 linux distributions.
+func wslDistros(logf logger.Logf) []string {
+	cmd := exec.Command("wsl.exe", "-l")
+	cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true}
+	b, err := cmd.CombinedOutput()
+	if err != nil {
+		return nil
+	}
+
+	// The first line of output is a WSL header. E.g.
+	//
+	//	C:\tsdev>wsl.exe -l
+	//	Windows Subsystem for Linux Distributions:
+	//	Ubuntu-20.04 (Default)
+	//
+	// We can skip it by passing '-q', but here we put it to work.
+	// It turns out wsl.exe -l is broken, and outputs UTF-16 names
+	// that nothing can read. (Try `wsl.exe -l | more`.)
+	// So we look at the header to see if it's UTF-16.
+	// If so, we run the rest through a UTF-16 parser.
+	//
+	// https://github.com/microsoft/WSL/issues/4607
+	var output string
+	if bytes.HasPrefix(b, []byte("W\x00i\x00n\x00d\x00o\x00w\x00s\x00")) {
+		output, err = decodeUTF16(b)
+		if err != nil {
+			logf("failed to decode wsl.exe -l output %q: %v", b, err)
+			return nil
+		}
+	} else {
+		output = string(b)
+	}
+	fmt.Printf("wslDistros: %q\n", output)
+	lines := strings.Split(output, "\n")
+	if len(lines) < 1 {
+		return nil
+	}
+	lines = lines[1:] // drop "Windows Subsystem For Linux" header
+
+	var distros []string
+	for _, name := range lines {
+		name = strings.TrimSpace(name)
+		name = strings.TrimSuffix(name, " (Default)")
+		if name == "" {
+			continue
+		}
+		fmt.Printf("wslDistros: name=%q\n", name)
+		distros = append(distros, name)
+	}
+	return distros
+}
+
+func decodeUTF16(b []byte) (string, error) {
+	if len(b) == 0 {
+		return "", nil
+	} else if len(b)%2 != 0 {
+		return "", fmt.Errorf("decodeUTF16: invalid length %d", len(b))
+	}
+	var u16 []uint16
+	for i := 0; i < len(b); i += 2 {
+		u16 = append(u16, uint16(b[i])+(uint16(b[i+1])<<8))
+	}
+	return string(utf16.Decode(u16)), nil
+}
+
+// wslManager is a DNS manager for WSL2 linux distributions.
+// It configures /etc/wsl.conf and /etc/resolv.conf.
+type wslManager struct {
+	logf     logger.Logf
+	managers map[string]directManager // distro name -> manager
+}
+
+func newWSLManager(logf logger.Logf, distros []string) *wslManager {
+	m := &wslManager{
+		logf:     logf,
+		managers: make(map[string]directManager),
+	}
+	for _, distro := range distros {
+		m.managers[distro] = newDirectManagerOnFS(wslFS{
+			user:   "root",
+			distro: distro,
+		})
+	}
+	return m
+}
+
+func (wm *wslManager) SetDNS(cfg OSConfig) error {
+	if !cfg.IsZero() {
+		if wm.setWSLConf() {
+			// What's this? So glad you asked.
+			//
+			// WSL2 writes the /etc/resolv.conf.
+			// It is aggressive about it. Every time you execute wsl.exe,
+			// it writes it. (Opening a terminal is done by running wsl.exe.)
+			// You can turn this off using /etc/wsl.conf! But: this wsl.conf
+			// file is only parsed when the VM boots up. To do that, we
+			// have to shut down WSL2.
+			//
+			// So we do it here, before we call wsl.exe to write resolv.conf.
+			if b, err := wslCommand("--shutdown").CombinedOutput(); err != nil {
+				wm.logf("WSL SetDNS shutdown: %v: %s", err, b)
+			}
+		}
+	}
+
+	for distro, m := range wm.managers {
+		if err := m.SetDNS(cfg); err != nil {
+			wm.logf("WSL(%q) SetDNS: %v", distro, err)
+		}
+	}
+	return nil
+}
+
+const wslConf = "/etc/wsl.conf"
+const wslConfSection = `# added by tailscale
+[network]
+generateResolvConf = false
+`
+
+// setWSLConf attempts to disable generateResolvConf in each WSL2 linux.
+// If any are changed, it reports true.
+func (wm *wslManager) setWSLConf() (changed bool) {
+	for distro, m := range wm.managers {
+		b, err := m.fs.ReadFile(wslConf)
+		if err != nil && !os.IsNotExist(err) {
+			wm.logf("WSL(%q) wsl.conf: read: %v", distro, err)
+			continue
+		}
+		ini := parseIni(string(b))
+		if v := ini["network"]["generateResolvConf"]; v == "" {
+			b = append(b, wslConfSection...)
+			if err := m.fs.WriteFile(wslConf, b, 0644); err != nil {
+				wm.logf("WSL(%q) wsl.conf: write: %v", distro, err)
+				continue
+			}
+			changed = true
+		}
+	}
+	return changed
+}
+
+func (m *wslManager) SupportsSplitDNS() bool { return false }
+func (m *wslManager) Close() error           { return m.SetDNS(OSConfig{}) }
+
+// wslFS is a pinholeFS implemented on top of wsl.exe.
+//
+// We access WSL2 file systems via wsl.exe instead of \\wsl$\ because
+// the netpath appears to operate as the standard user, not root.
+type wslFS struct {
+	user   string
+	distro string
+}
+
+func (fs wslFS) Stat(name string) (isRegular bool, err error) {
+	err = fs.cmd("test", "-f", name).Run()
+	if ee, _ := err.(*exec.ExitError); ee != nil {
+		if ee.ExitCode() == 1 {
+			return false, os.ErrNotExist
+		}
+		return false, err
+	}
+	return true, nil
+}
+
+func (fs wslFS) Rename(oldName, newName string) error {
+	return fs.cmd("mv", "--", oldName, newName).Run()
+}
+func (fs wslFS) Remove(name string) error { return fs.cmd("rm", "--", name).Run() }
+
+func (fs wslFS) ReadFile(name string) ([]byte, error) {
+	b, err := fs.cmd("cat", "--", name).CombinedOutput()
+	if ee, _ := err.(*exec.ExitError); ee != nil && ee.ExitCode() == 1 {
+		return nil, os.ErrNotExist
+	}
+	return b, err
+}
+
+func (fs wslFS) WriteFile(name string, contents []byte, perm os.FileMode) error {
+	cmd := fs.cmd("tee", "--", name)
+	cmd.Stdin = bytes.NewReader(contents)
+	cmd.Stdout = nil
+	if err := cmd.Run(); err != nil {
+		return err
+	}
+	return fs.cmd("chmod", "--", fmt.Sprintf("%04o", perm), name).Run()
+}
+
+func (fs wslFS) cmd(args ...string) *exec.Cmd {
+	cmd := wslCommand("-u", fs.user, "-d", fs.distro, "-e")
+	cmd.Args = append(cmd.Args, args...)
+	fmt.Printf("wslFS.cmd: %v\n", cmd.Args)
+	return cmd
+}
+
+func wslCommand(args ...string) *exec.Cmd {
+	cmd := exec.Command("wsl.exe", args...)
+	cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true}
+	return cmd
+}