wgengine/router/dns: move to net/dns.

Preparation for merging the APIs and whatnot.

Signed-off-by: David Anderson <danderson@tailscale.com>

net/dns/direct.go

@@ -0,0 +1,188 @@
+// 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
+}