wgengine: add OpenBSD router DNS update handling

Also removes validated TODO comments.

Signed-off-by: Martin Baillie <martin@baillie.email>

wgengine/router_openbsd.go

@@ -5,24 +5,25 @@
 package wgengine
 
 import (
+	"bytes"
 	"fmt"
+	"io/ioutil"
 	"log"
+	"os"
 	"os/exec"
+	"path/filepath"
+	"strings"
 
 	"github.com/tailscale/wireguard-go/device"
 	"github.com/tailscale/wireguard-go/tun"
 	"github.com/tailscale/wireguard-go/wgcfg"
+	"tailscale.com/atomicfile"
 	"tailscale.com/types/logger"
 )
 
-// For now this router only supports the userspace WireGuard implementations.
-//
-// There is an experimental kernel version in the works:
+// For now this router only supports the WireGuard userspace implementation.
+// There is an experimental kernel version in the works for OpenBSD:
 // https://git.zx2c4.com/wireguard-openbsd.
-//
-// TODO(mbaillie): netlink-style monitoring might be possible through
-// `ifstated(8)`/`devd(8)`, or become possible with the OpenBSD kernel
-// implementation. This merits further investigation.
 
 type openbsdRouter struct {
 	logf    logger.Logf
@@ -42,7 +43,6 @@ func newUserspaceRouter(logf logger.Logf, _ *device.Device, tundev tun.Device) (
 	}, nil
 }
 
-// TODO(mbaillie): extract as identical to linux version
 func cmd(args ...string) *exec.Cmd {
 	if len(args) == 0 {
 		log.Fatalf("exec.Cmd(%#v) invalid; need argv[0]\n", args)
@@ -167,12 +167,95 @@ func (r *openbsdRouter) Close() error {
 		r.logf("failed to restore system resolv.conf: %v", err)
 	}
 
-	// TODO(mbaillie): wipe routes
+	return nil
+}
+
+const (
+	tsConf     = "/etc/resolv.tailscale.conf"
+	backupConf = "/etc/resolv.pre-tailscale-backup.conf"
+	resolvConf = "/etc/resolv.conf"
+)
+
+func (r *openbsdRouter) replaceResolvConf(servers []wgcfg.IP, domains []string) error {
+	if len(servers) == 0 {
+		return r.restoreResolvConf()
+	}
+
+	// Write the tsConf file.
+	buf := new(bytes.Buffer)
+	fmt.Fprintf(buf, "# resolv.conf(5) file generated by tailscale\n")
+	fmt.Fprintf(buf, "# DO NOT EDIT THIS FILE BY HAND -- CHANGES WILL BE OVERWRITTEN\n\n")
+	for _, ns := range servers {
+		fmt.Fprintf(buf, "nameserver %s\n", ns)
+	}
+	if len(domains) > 0 {
+		fmt.Fprintf(buf, "search "+strings.Join(domains, " ")+"\n")
+	}
+	tf, err := ioutil.TempFile(filepath.Dir(tsConf), filepath.Base(tsConf)+".*")
+	if err != nil {
+		return err
+	}
+	tempName := tf.Name()
+	tf.Close()
+
+	if err := atomicfile.WriteFile(tempName, buf.Bytes(), 0644); err != nil {
+		return err
+	}
+	if err := os.Rename(tempName, tsConf); 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 os.IsNotExist(err) {
+			// No existing /etc/resolv.conf file to backup.
+			// Nothing to do.
+			return nil
+		} else if err != nil {
+			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 nil
+	}
 
 	return nil
 }
 
-// TODO(mbaillie): these are no-ops for now. They could re-use the Linux funcs
-// (sans systemd parts), but I note Linux DNS is disabled(?) so leaving for now.
-func (r *openbsdRouter) replaceResolvConf(_ []wgcfg.IP, _ []string) error { return nil }
-func (r *openbsdRouter) restoreResolvConf() error                         { return nil }
+func (r *openbsdRouter) restoreResolvConf() error {
+	if _, err := os.Stat(backupConf); err != nil {
+		if os.IsNotExist(err) {
+			return nil // No backup resolv.conf to restore.
+		}
+		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) // Best effort removal.
+
+	return nil
+}