net/dns: some WSL wiring

net/dns/manager_windows.go

@@ -259,6 +259,10 @@ func (m windowsManager) SetDNS(cfg OSConfig) error {
 		}
 	}
 
+	if len(cfg.SearchDomains) != 0 {
+		go w.setWSL2DNS()
+	}
+
 	// Force DNS re-registration in Active Directory. What we actually
 	// care about is that this command invokes the undocumented hidden
 	// function that forces Windows to notice that adapter settings
@@ -299,6 +303,19 @@ func (m windowsManager) SetDNS(cfg OSConfig) error {
 	return nil
 }
 
+func (w windowsManager) setWSL2DNS() {
+	cmd := exec.Command("wsl.exe", "-l", "-q")
+	cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true}
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		m.logf("wsl.exe did not list distros, skipping WSL DNS config")
+		return
+	}
+	for _, distroName := range strings.Split(string(out), "\n") {
+		distroName = strings.TrimSpace(distroName)
+	}
+}
+
 func (m windowsManager) SupportsSplitDNS() bool {
 	return m.nrptWorks
 }

net/dns/wsl_windows.go

@@ -0,0 +1,52 @@
+// 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 (
+	"fmt"
+	"io"
+	"os/exec"
+	"strings"
+	"syscall"
+)
+
+// We access WSL2 file systems via wsl.exe instead of \\wsl$\ because
+// the netpath appears to operate as the standard user, not root.
+
+func wslFileExists(distro string, fileName string) (bool, error) {
+	cmd := exec.Command("wsl.exe", "-u", "root", "-d", distro, "/bin/test", fileName)
+	cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true}
+	err := cmd.Run()
+	if err == nil {
+		return true, nil
+	}
+	if ee, _ := err.(*exec.ExitError); ee != nil {
+		if ee.ExitCode() == 1 {
+			return false, nil
+		}
+	}
+	return false, fmt.Errorf("wslFileExists(%q): %w", fileName, err)
+}
+
+func wslReadFile(distro, fileName string) (string, error) {
+	cmd := exec.Command("wsl.exe", "-u", "root", "-d", distro, "/bin/cat", fileName)
+	cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true}
+	b, err := cmd.CombinedOutput()
+	if err != nil {
+		return "", fmt.Errorf("wslReadFile(%q): %w", fileName, err)
+	}
+	return strings.TrimSpace(string(b)), nil
+}
+
+func wslWriteFile(distro, fileName string, r io.Reader) error {
+	cmd := exec.Command("wsl.exe", "-u", "root", "-d", distro, "tee", fileName)
+	cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true}
+	cmd.Stdin = r
+	cmd.Stdout = null
+	if err := cmd.Run(); err != nil {
+		return "", fmt.Errorf("wslWriteFile(%q): %w", fileName, err)
+	}
+	return nil
+}