wgengine/router: dns: unify on *BSD, multimode on Linux, Magic DNS (#536)

Signed-off-by: Dmytro Shynkevych <dmytro@tailscale.com>

go.mod

@@ -9,6 +9,7 @@ require (
 	github.com/flynn/go-shlex v0.0.0-20150515145356-3f9db97f8568 // indirect
 	github.com/gliderlabs/ssh v0.2.2
 	github.com/go-ole/go-ole v1.2.4
+	github.com/godbus/dbus/v5 v5.0.3
 	github.com/golang/groupcache v0.0.0-20200121045136-8c9f03a8e57e
 	github.com/google/go-cmp v0.4.0
 	github.com/goreleaser/nfpm v1.1.10

go.sum

@@ -30,6 +30,9 @@ github.com/gliderlabs/ssh v0.2.2 h1:6zsha5zo/TWhRhwqCD3+EarCAgZ2yN28ipRnGPnwkI0=
 github.com/gliderlabs/ssh v0.2.2/go.mod h1:U7qILu1NlMHj9FlMhZLlkCdDnU1DBEAqr0aevW3Awn0=
 github.com/go-ole/go-ole v1.2.4 h1:nNBDSCOigTSiarFpYE9J/KtEA1IOW4CNeqT9TQDqCxI=
 github.com/go-ole/go-ole v1.2.4/go.mod h1:XCwSNxSkXRo4vlyPy93sltvi/qJq0jqQhjqQNIwKuxM=
+github.com/godbus/dbus v4.1.0+incompatible h1:WqqLRTsQic3apZUK9qC5sGNfXthmPXzUZ7nQPrNITa4=
+github.com/godbus/dbus/v5 v5.0.3 h1:ZqHaoEF7TBzh4jzPmqVhE/5A1z9of6orkAe5uHoAeME=
+github.com/godbus/dbus/v5 v5.0.3/go.mod h1:xhWf0FNVPg57R7Z0UbKHbJfkEywrmjJnf7w5xrFpKfA=
 github.com/golang/groupcache v0.0.0-20200121045136-8c9f03a8e57e h1:1r7pUrabqp18hOBcwBwiTsbnFeTZHV9eER/QT5JVZxY=
 github.com/golang/groupcache v0.0.0-20200121045136-8c9f03a8e57e/go.mod h1:cIg4eruTrX1D+g88fzRXU5OdNfaM+9IcxsU14FzY7Hc=
 github.com/golang/protobuf v1.2.0 h1:P3YflyNX/ehuJFLhxviNdFxQPkGK5cDcApsge1SqnvM=

internal/deepprint/deepprint_test.go

@@ -50,7 +50,10 @@ func getVal() []interface{} {
 			},
 		},
 		&router.Config{
-			DNS: []netaddr.IP{netaddr.IPv4(8, 8, 8, 8)},
+			DNSConfig: router.DNSConfig{
+				Nameservers: []netaddr.IP{netaddr.IPv4(8, 8, 8, 8)},
+				Domains:     []string{"tailscale.net"},
+			},
 		},
 		map[string]string{
 			"key1": "val1",

ipn/local.go

@@ -471,7 +471,7 @@ func (b *LocalBackend) updateDNSMap(netMap *controlclient.NetworkMap) {
 		// Like PeerStatus.SimpleHostName()
 		domain = strings.TrimSuffix(domain, ".local")
 		domain = strings.TrimSuffix(domain, ".localdomain")
-		domain = domain + ".tailscale.us"
+		domain = domain + ".b.tailscale.net"
 		domainToIP[domain] = netaddr.IPFrom16(peer.Addresses[0].IP.Addr)
 	}
 	b.e.SetDNSMap(tsdns.NewMap(domainToIP))
@@ -868,11 +868,13 @@ func routerConfig(cfg *wgcfg.Config, prefs *Prefs, dnsDomains []string) *router.
 
 	rs := &router.Config{
 		LocalAddrs:       wgCIDRToNetaddr(addrs),
-		DNS:              wgIPToNetaddr(cfg.DNS),
-		DNSDomains:       dnsDomains,
 		SubnetRoutes:     wgCIDRToNetaddr(prefs.AdvertiseRoutes),
 		SNATSubnetRoutes: !prefs.NoSNAT,
 		NetfilterMode:    prefs.NetfilterMode,
+		DNSConfig: router.DNSConfig{
+			Nameservers: wgIPToNetaddr(cfg.DNS),
+			Domains:     dnsDomains,
+		},
 	}
 
 	for _, peer := range cfg.Peers {

wgengine/router/dns.go

@@ -0,0 +1,83 @@
+// 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.
+
+package router
+
+import (
+	"time"
+
+	"inet.af/netaddr"
+)
+
+// DNSConfig is the subset of Config that contains DNS parameters.
+type DNSConfig struct {
+	// Nameservers are the IP addresses of the nameservers to use.
+	Nameservers []netaddr.IP
+	// Domains are the search domains to use.
+	Domains []string
+}
+
+// EquivalentTo determines whether its argument and receiver
+// represent equivalent DNS configurations (then DNS reconfig is a no-op).
+func (lhs DNSConfig) EquivalentTo(rhs DNSConfig) bool {
+	if len(lhs.Nameservers) != len(rhs.Nameservers) {
+		return false
+	}
+
+	if len(lhs.Domains) != len(rhs.Domains) {
+		return false
+	}
+
+	// With how we perform resolution order shouldn't matter,
+	// but it is unlikely that we will encounter different orders.
+	for i, server := range lhs.Nameservers {
+		if rhs.Nameservers[i] != server {
+			return false
+		}
+	}
+
+	for i, domain := range lhs.Domains {
+		if rhs.Domains[i] != domain {
+			return false
+		}
+	}
+
+	return true
+}
+
+// dnsReconfigTimeout is the timeout for DNS reconfiguration.
+//
+// This is useful because certain conditions can cause indefinite hangs
+// (such as improper dbus auth followed by contextless dbus.Object.Call).
+// Such operations should be wrapped in a timeout context.
+const dnsReconfigTimeout = time.Second
+
+// dnsMode determines how DNS settings are managed.
+type dnsMode uint8
+
+const (
+	// dnsDirect indicates that /etc/resolv.conf is edited directly.
+	dnsDirect dnsMode = iota
+	// dnsResolvconf indicates that a resolvconf binary is used.
+	dnsResolvconf
+	// dnsNetworkManager indicates that the NetworkManaer DBus API is used.
+	dnsNetworkManager
+	// dnsResolved indicates that the systemd-resolved DBus API is used.
+	dnsResolved
+)
+
+func (m dnsMode) String() string {
+	switch m {
+	case dnsDirect:
+		return "direct"
+	case dnsResolvconf:
+		return "resolvconf"
+	case dnsNetworkManager:
+		return "networkmanager"
+	case dnsResolved:
+		return "resolved"
+	default:
+		return "???"
+	}
+}

wgengine/router/dns_direct.go

@@ -0,0 +1,151 @@
+// 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 router
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"os"
+	"strings"
+
+	"inet.af/netaddr"
+	"tailscale.com/atomicfile"
+)
+
+const (
+	tsConf     = "/etc/resolv.tailscale.conf"
+	backupConf = "/etc/resolv.pre-tailscale-backup.conf"
+	resolvConf = "/etc/resolv.conf"
+)
+
+// dnsWriteConfig writes DNS configuration in resolv.conf format to the given writer.
+func dnsWriteConfig(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")
+	}
+}
+
+// dnsReadConfig reads DNS configuration from /etc/resolv.conf.
+func dnsReadConfig() (DNSConfig, error) {
+	var config DNSConfig
+
+	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
+}
+
+// dnsDirectUp 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 dnsDirectDown before program shutdown
+// and ensure that router.Cleanup is run if the program terminates unexpectedly.
+func dnsDirectUp(config DNSConfig) error {
+	// Write the tsConf file.
+	buf := new(bytes.Buffer)
+	dnsWriteConfig(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 && !os.IsNotExist(err) {
+			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
+	}
+
+	return nil
+}
+
+// dnsDirectDown restores /etc/resolv.conf to its state before dnsDirectUp.
+// It is idempotent and behaves correctly even if dnsDirectUp has never been run.
+func dnsDirectDown() 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)
+	return nil
+}

wgengine/router/dns_networkmanager.go

@@ -0,0 +1,209 @@
+// 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
+
+package router
+
+import (
+	"bufio"
+	"bytes"
+	"context"
+	"encoding/binary"
+	"fmt"
+	"os"
+	"os/exec"
+
+	"github.com/godbus/dbus/v5"
+)
+
+type nmSettings map[string]map[string]dbus.Variant
+
+// nmIsActive determines if NetworkManager is currently managing system DNS settings.
+func nmIsActive() bool {
+	// This is somewhat tricky because NetworkManager supports a number
+	// of DNS configuration modes. In all cases, we expect it to be installed
+	// and /etc/resolv.conf to contain a mention of NetworkManager in the comments.
+	_, err := exec.LookPath("NetworkManager")
+	if err != nil {
+		return false
+	}
+
+	f, err := os.Open("/etc/resolv.conf")
+	if err != nil {
+		return false
+	}
+	defer f.Close()
+
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		line := scanner.Bytes()
+		// Look for the word "NetworkManager" until comments end.
+		if len(line) > 0 && line[0] != '#' {
+			return false
+		}
+		if bytes.Contains(line, []byte("NetworkManager")) {
+			return true
+		}
+	}
+	return false
+}
+
+// dnsNetworkManagerUp updates the DNS config for the Tailscale interface
+// through the NetworkManager DBus API.
+func dnsNetworkManagerUp(config DNSConfig, interfaceName string) error {
+	ctx, cancel := context.WithTimeout(context.Background(), dnsReconfigTimeout)
+	defer cancel()
+
+	conn, err := dbus.SystemBus()
+	if err != nil {
+		return fmt.Errorf("connecting to system bus: %w", err)
+	}
+	defer conn.Close()
+
+	// This is how we get at the DNS settings:
+	//               org.freedesktop.NetworkManager
+	//                              ⇩
+	//           org.freedesktop.NetworkManager.Device
+	//              (describes a network interface)
+	//                              ⇩
+	//       org.freedesktop.NetworkManager.Connection.Active
+	//  (active instance of a connection initialized from settings)
+	//                              ⇩
+	//         org.freedesktop.NetworkManager.Connection
+	//                   (connection settings)
+	//          contains {dns, dns-priority, dns-search}
+	//
+	// Ref: https://developer.gnome.org/NetworkManager/stable/settings-ipv4.html.
+
+	nm := conn.Object(
+		"org.freedesktop.NetworkManager",
+		dbus.ObjectPath("/org/freedesktop/NetworkManager"),
+	)
+
+	var devicePath dbus.ObjectPath
+	err = nm.CallWithContext(
+		ctx, "org.freedesktop.NetworkManager.GetDeviceByIpIface", 0,
+		interfaceName,
+	).Store(&devicePath)
+	if err != nil {
+		return fmt.Errorf("GetDeviceByIpIface: %w", err)
+	}
+	device := conn.Object("org.freedesktop.NetworkManager", devicePath)
+
+	var activeConnPath dbus.ObjectPath
+	err = device.CallWithContext(
+		ctx, "org.freedesktop.DBus.Properties.Get", 0,
+		"org.freedesktop.NetworkManager.Device", "ActiveConnection",
+	).Store(&activeConnPath)
+	if err != nil {
+		return fmt.Errorf("getting ActiveConnection: %w", err)
+	}
+	activeConn := conn.Object("org.freedesktop.NetworkManager", activeConnPath)
+
+	var connPath dbus.ObjectPath
+	err = activeConn.CallWithContext(
+		ctx, "org.freedesktop.DBus.Properties.Get", 0,
+		"org.freedesktop.NetworkManager.Connection.Active", "Connection",
+	).Store(&connPath)
+	if err != nil {
+		return fmt.Errorf("getting Connection: %w", err)
+	}
+	connection := conn.Object("org.freedesktop.NetworkManager", connPath)
+
+	// Note: strictly speaking, the following is not safe.
+	//
+	// It appears that the way to update connection settings
+	// in NetworkManager is to get an entire connection settings object,
+	// modify the fields we are interested in, then submit the modified object.
+	//
+	// This is unfortunate: if the network state changes in the meantime
+	// (most relevantly to us, if routes change), we will overwrite those changes.
+	//
+	// That said, fortunately, this should have no real effect, as Tailscale routes
+	// do not seem to show up in NetworkManager at all,
+	// so they are presumably immune from being tampered with.
+
+	var settings nmSettings
+	err = connection.CallWithContext(
+		ctx, "org.freedesktop.NetworkManager.Settings.Connection.GetSettings", 0,
+	).Store(&settings)
+	if err != nil {
+		return fmt.Errorf("getting Settings: %w", err)
+	}
+
+	// Frustratingly, NetworkManager represents IPv4 addresses as uint32s,
+	// although IPv6 addresses are represented as byte arrays.
+	// Perform the conversion here.
+	var (
+		dnsv4 []uint32
+		dnsv6 [][]byte
+	)
+	for _, ip := range config.Nameservers {
+		b := ip.As16()
+		if ip.Is4() {
+			dnsv4 = append(dnsv4, binary.BigEndian.Uint32(b[12:]))
+		} else {
+			dnsv6 = append(dnsv6, b[:])
+		}
+	}
+
+	ipv4Map := settings["ipv4"]
+	ipv4Map["dns"] = dbus.MakeVariant(dnsv4)
+	ipv4Map["dns-search"] = dbus.MakeVariant(config.Domains)
+	// dns-priority = -1 ensures that we have priority
+	// over other interfaces, except those exploiting this same trick.
+	// Ref: https://bugs.launchpad.net/ubuntu/+source/network-manager/+bug/1211110/comments/92.
+	ipv4Map["dns-priority"] = dbus.MakeVariant(-1)
+	// In principle, we should not need set this to true,
+	// as our interface does not configure any automatic DNS settings (presumably via DHCP).
+	// All the same, better to be safe.
+	ipv4Map["ignore-auto-dns"] = dbus.MakeVariant(true)
+
+	ipv6Map := settings["ipv6"]
+	// This is a hack.
+	// Methods "disabled", "ignore", "link-local" (IPv6 default) prevent us from setting DNS.
+	// It seems that our only recourse is "manual" or "auto".
+	// "manual" requires addresses, so we use "auto", which will assign us a random IPv6 /64.
+	ipv6Map["method"] = dbus.MakeVariant("auto")
+	// Our IPv6 config is a fake, so it should never become the default route.
+	ipv6Map["never-default"] = dbus.MakeVariant(true)
+	// Moreover, we should ignore all autoconfigured routes (hopefully none), as they are bogus.
+	ipv6Map["ignore-auto-routes"] = dbus.MakeVariant(true)
+
+	// Finally, set the actual DNS config.
+	ipv6Map["dns"] = dbus.MakeVariant(dnsv6)
+	ipv6Map["dns-search"] = dbus.MakeVariant(config.Domains)
+	ipv6Map["dns-priority"] = dbus.MakeVariant(-1)
+	ipv6Map["ignore-auto-dns"] = dbus.MakeVariant(true)
+
+	// deprecatedProperties are the properties in interface settings
+	// that are deprecated by NetworkManager.
+	//
+	// In practice, this means that they are returned for reading,
+	// but submitting a settings object with them present fails
+	// with hard-to-diagnose errors. They must be removed.
+	deprecatedProperties := []string{
+		"addresses", "routes",
+	}
+
+	for _, property := range deprecatedProperties {
+		delete(ipv4Map, property)
+		delete(ipv6Map, property)
+	}
+
+	err = connection.CallWithContext(
+		ctx, "org.freedesktop.NetworkManager.Settings.Connection.UpdateUnsaved", 0, settings,
+	).Store()
+	if err != nil {
+		return fmt.Errorf("setting Settings: %w", err)
+	}
+
+	return nil
+}
+
+// dnsNetworkManagerDown undoes the changes made by dnsNetworkManagerUp.
+func dnsNetworkManagerDown(interfaceName string) error {
+	return dnsNetworkManagerUp(DNSConfig{Nameservers: nil, Domains: nil}, interfaceName)
+}

wgengine/router/dns_resolvconf.go

@@ -0,0 +1,84 @@
+// 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
+
+package router
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"os"
+	"os/exec"
+)
+
+// resolvconfIsActive indicates whether the system appears to be using resolvconf.
+// If this is true, then dnsManualUp should be avoided:
+// resolvconf has exclusive ownership of /etc/resolv.conf.
+func resolvconfIsActive() bool {
+	// Sanity-check first: if there is no resolvconf binary, then this is fruitless.
+	//
+	// However, this binary may be a shim like the one systemd-resolved provides.
+	// Such a shim may not behave as expected: in particular, systemd-resolved
+	// does not seem to respect the exclusive mode -x, saying:
+	//   -x            Send DNS traffic preferably over this interface
+	// whereas e.g. openresolv sends DNS traffix _exclusively_ over that interface,
+	// or not at all (in case of another exclusive-mode request later in time).
+	//
+	// Moreover, resolvconf may be installed but unused, in which case we should
+	// not use it either, lest we clobber existing configuration.
+	//
+	// To handle all the above correctly, we scan the comments in /etc/resolv.conf
+	// to ensure that it was generated by a resolvconf implementation.
+	_, err := exec.LookPath("resolvconf")
+	if err != nil {
+		return false
+	}
+
+	f, err := os.Open("/etc/resolv.conf")
+	if err != nil {
+		return false
+	}
+	defer f.Close()
+
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		line := scanner.Bytes()
+		// Look for the word "resolvconf" until comments end.
+		if len(line) > 0 && line[0] != '#' {
+			return false
+		}
+		if bytes.Contains(line, []byte("resolvconf")) {
+			return true
+		}
+	}
+	return false
+}
+
+// dnsResolvconfUp invokes the resolvconf binary to associate
+// the given DNS configuration the Tailscale interface.
+func dnsResolvconfUp(config DNSConfig, interfaceName string) error {
+	stdin := new(bytes.Buffer)
+	dnsWriteConfig(stdin, config.Nameservers, config.Domains) // dns_direct.go
+
+	cmd := exec.Command("resolvconf", "-m", "0", "-x", "-a", interfaceName+".inet")
+	cmd.Stdin = stdin
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return fmt.Errorf("running %s: %s", cmd, out)
+	}
+
+	return nil
+}
+
+// dnsResolvconfDown undoes the action of dnsResolvconfUp.
+func dnsResolvconfDown(interfaceName string) error {
+	cmd := exec.Command("resolvconf", "-f", "-d", interfaceName+".inet")
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return fmt.Errorf("running %s: %s", cmd, out)
+	}
+	return nil
+}

wgengine/router/dns_resolved.go

@@ -0,0 +1,177 @@
+// 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
+
+package router
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os/exec"
+
+	"github.com/godbus/dbus/v5"
+	"golang.org/x/sys/unix"
+	"inet.af/netaddr"
+	"tailscale.com/net/interfaces"
+)
+
+// resolvedListenAddr is the listen address of the resolved stub resolver.
+//
+// We only consider resolved to be the system resolver if the stub resolver is;
+// that is, if this address is the sole nameserver in /etc/resolved.conf.
+// In other cases, resolved may still be managing the system DNS configuration directly.
+// Then the nameserver list will be a concatenation of those for all
+// the interfaces that register their interest in being a default resolver with
+//   SetLinkDomains([]{{"~.", true}, ...})
+// which includes at least the interface with the default route, i.e. not us.
+// This does not work for us: there is a possibility of getting NXDOMAIN
+// from the other nameservers before we are asked or get a chance to respond.
+// We consider this case as lacking resolved support and fall through to dnsDirect.
+//
+// While it may seem that we need to read a config option to get at this,
+// this address is, in fact, hard-coded into resolved.
+var resolvedListenAddr = netaddr.IPv4(127, 0, 0, 53)
+
+var errNotReady = errors.New("interface not ready")
+
+type resolvedLinkNameserver struct {
+	Family  int32
+	Address []byte
+}
+
+type resolvedLinkDomain struct {
+	Domain      string
+	RoutingOnly bool
+}
+
+// resolvedIsActive determines if resolved is currently managing system DNS settings.
+func resolvedIsActive() bool {
+	// 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").Run()
+	if err != nil {
+		return false
+	}
+
+	config, err := dnsReadConfig()
+	if err != nil {
+		return false
+	}
+
+	// The sole nameserver must be the systemd-resolved stub.
+	if len(config.Nameservers) == 1 && config.Nameservers[0] == resolvedListenAddr {
+		return true
+	}
+
+	return false
+}
+
+// dnsResolvedUp sets the DNS parameters for the Tailscale interface
+// to given nameservers and search domains using the resolved DBus API.
+func dnsResolvedUp(config DNSConfig) error {
+	ctx, cancel := context.WithTimeout(context.Background(), dnsReconfigTimeout)
+	defer cancel()
+
+	conn, err := dbus.SystemBus()
+	if err != nil {
+		return fmt.Errorf("connecting to system bus: %w", err)
+	}
+	defer conn.Close()
+
+	resolved := conn.Object(
+		"org.freedesktop.resolve1",
+		dbus.ObjectPath("/org/freedesktop/resolve1"),
+	)
+
+	_, iface, err := interfaces.Tailscale()
+	if err != nil {
+		return fmt.Errorf("getting interface index: %w", err)
+	}
+	if iface == nil {
+		return errNotReady
+	}
+
+	var linkNameservers = make([]resolvedLinkNameserver, len(config.Nameservers))
+	for i, server := range config.Nameservers {
+		ip := server.As16()
+		if server.Is4() {
+			linkNameservers[i] = resolvedLinkNameserver{
+				Family:  unix.AF_INET,
+				Address: ip[12:],
+			}
+		} else {
+			linkNameservers[i] = resolvedLinkNameserver{
+				Family:  unix.AF_INET6,
+				Address: ip[:],
+			}
+		}
+	}
+
+	err = resolved.CallWithContext(
+		ctx, "org.freedesktop.resolve1.Manager.SetLinkDNS", 0,
+		iface.Index, linkNameservers,
+	).Store()
+	if err != nil {
+		return fmt.Errorf("SetLinkDNS: %w", err)
+	}
+
+	var linkDomains = make([]resolvedLinkDomain, len(config.Domains))
+	for i, domain := range config.Domains {
+		linkDomains[i] = resolvedLinkDomain{
+			Domain:      domain,
+			RoutingOnly: false,
+		}
+	}
+
+	err = resolved.CallWithContext(
+		ctx, "org.freedesktop.resolve1.Manager.SetLinkDomains", 0,
+		iface.Index, linkDomains,
+	).Store()
+	if err != nil {
+		return fmt.Errorf("SetLinkDomains: %w", err)
+	}
+
+	return nil
+}
+
+// dnsResolvedDown undoes the changes made by dnsResolvedUp.
+func dnsResolvedDown() error {
+	ctx, cancel := context.WithTimeout(context.Background(), dnsReconfigTimeout)
+	defer cancel()
+
+	conn, err := dbus.SystemBus()
+	if err != nil {
+		return fmt.Errorf("connecting to system bus: %w", err)
+	}
+
+	resolved := conn.Object(
+		"org.freedesktop.resolve1",
+		dbus.ObjectPath("/org/freedesktop/resolve1"),
+	)
+
+	_, iface, err := interfaces.Tailscale()
+	if err != nil {
+		return fmt.Errorf("getting interface index: %w", err)
+	}
+	if iface == nil {
+		return errNotReady
+	}
+
+	err = resolved.CallWithContext(
+		ctx, "org.freedesktop.resolve1.Manager.RevertLink", 0,
+		iface.Index,
+	).Store()
+	if err != nil {
+		return fmt.Errorf("RevertLink: %w", err)
+	}
+
+	return nil
+}

wgengine/router/ifconfig_windows.go

@@ -262,7 +262,7 @@ func configureInterface(cfg *Config, tun *tun.NativeTun) error {
 		}
 	}()
 
-	setDNSDomains(guid, cfg.DNSDomains)
+	setDNSDomains(guid, cfg.Domains)
 
 	routes := []winipcfg.RouteData{}
 	var firstGateway4 *net.IP
@@ -359,7 +359,7 @@ func configureInterface(cfg *Config, tun *tun.NativeTun) error {
 	}
 
 	var dnsIPs []net.IP
-	for _, ip := range cfg.DNS {
+	for _, ip := range cfg.Nameservers {
 		dnsIPs = append(dnsIPs, ip.IPAddr().IP)
 	}
 	err = iface.SetDNS(dnsIPs)

wgengine/router/router.go

@@ -32,6 +32,7 @@ type Router interface {
 // New returns a new Router for the current platform, using the
 // provided tun device.
 func New(logf logger.Logf, wgdev *device.Device, tundev tun.Device) (Router, error) {
+	logf = logger.WithPrefix(logf, "router: ")
 	return newUserspaceRouter(logf, wgdev, tundev)
 }
 
@@ -39,7 +40,7 @@ func New(logf logger.Logf, wgdev *device.Device, tundev tun.Device) (Router, err
 // in case the Tailscale daemon terminated without closing the router.
 // No other state needs to be instantiated before this runs.
 func Cleanup(logf logger.Logf, interfaceName string) {
-	// TODO(dmytro): implement this.
+	cleanup(logf, interfaceName)
 }
 
 // NetfilterMode is the firewall management mode to use when
@@ -69,10 +70,10 @@ func (m NetfilterMode) String() string {
 // the OS's network stack.
 type Config struct {
 	LocalAddrs []netaddr.IPPrefix
-	DNS        []netaddr.IP
-	DNSDomains []string
 	Routes     []netaddr.IPPrefix // routes to point into the Tailscale interface
 
+	DNSConfig
+
 	// Linux-only things below, ignored on other platforms.
 
 	SubnetRoutes     []netaddr.IPPrefix // subnets being advertised to other Tailscale nodes

wgengine/router/router_darwin.go

@@ -52,3 +52,13 @@ func (r *darwinRouter) Up() error {
 	}
 	return r.Router.Up()
 }
+
+func upDNS(config DNSConfig, interfaceName string) error {
+	// Handled by IPNExtension
+	return nil
+}
+
+func downDNS(interfaceName string) error {
+	// Handled by IPNExtension
+	return nil
+}

wgengine/router/router_default.go

@@ -15,3 +15,7 @@ import (
 func newUserspaceRouter(logf logger.Logf, tunname string, dev *device.Device, tuntap tun.Device, netChanged func()) Router {
 	return NewFakeRouter(logf, tunname, dev, tuntap, netChanged)
 }
+
+func cleanup() error {
+	return nil
+}

wgengine/router/router_freebsd.go

@@ -5,6 +5,8 @@
 package router
 
 import (
+	"fmt"
+
 	"github.com/tailscale/wireguard-go/device"
 	"github.com/tailscale/wireguard-go/tun"
 	"tailscale.com/types/logger"
@@ -18,3 +20,35 @@ import (
 func newUserspaceRouter(logf logger.Logf, _ *device.Device, tundev tun.Device) (Router, error) {
 	return newUserspaceBSDRouter(logf, nil, tundev)
 }
+
+func upDNS(config DNSConfig, interfaceName string) error {
+	if len(config.Nameservers) == 0 {
+		return downDNS(interfaceName)
+	}
+
+	if resolvconfIsActive() {
+		if err := dnsResolvconfUp(config, interfaceName); err != nil {
+			return fmt.Errorf("resolvconf: %w")
+		}
+		return nil
+	}
+
+	if err := dnsDirectUp(config); err != nil {
+		return fmt.Errorf("direct: %w")
+	}
+	return nil
+}
+
+func downDNS(interfaceName string) error {
+	if resolvconfIsActive() {
+		if err := dnsResolvconfDown(interfaceName); err != nil {
+			return fmt.Errorf("resolvconf: %w")
+		}
+		return nil
+	}
+
+	if err := dnsDirectDown(); err != nil {
+		return fmt.Errorf("direct: %w")
+	}
+	return nil
+}

wgengine/router/router_linux.go

@@ -5,19 +5,14 @@
 package router
 
 import (
-	"bytes"
 	"fmt"
-	"io/ioutil"
-	"os"
 	"os/exec"
-	"path/filepath"
 	"strings"
 
 	"github.com/coreos/go-iptables/iptables"
 	"github.com/tailscale/wireguard-go/device"
 	"github.com/tailscale/wireguard-go/tun"
 	"inet.af/netaddr"
-	"tailscale.com/atomicfile"
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/types/logger"
 )
@@ -73,6 +68,9 @@ type linuxRouter struct {
 	snatSubnetRoutes bool
 	netfilterMode    NetfilterMode
 
+	dnsMode   dnsMode
+	dnsConfig DNSConfig
+
 	ipt4 netfilterRunner
 	cmd  commandRunner
 }
@@ -119,10 +117,27 @@ func (r *linuxRouter) Up() error {
 		return err
 	}
 
+	switch {
+	// TODO(dmytro): enable resolved when per-domain resolvers are desired.
+	case resolvedIsActive():
+		r.dnsMode = dnsDirect
+	//	r.dnsMode = dnsResolved
+	case nmIsActive():
+		r.dnsMode = dnsNetworkManager
+	case resolvconfIsActive():
+		r.dnsMode = dnsResolvconf
+	default:
+		r.dnsMode = dnsDirect
+	}
+	r.logf("dns mode: %v", r.dnsMode)
+
 	return nil
 }
 
-func (r *linuxRouter) down() error {
+func (r *linuxRouter) Close() error {
+	if err := r.downDNS(); err != nil {
+		return err
+	}
 	if err := r.downInterface(); err != nil {
 		return err
 	}
@@ -139,20 +154,6 @@ func (r *linuxRouter) down() error {
 	return nil
 }
 
-func (r *linuxRouter) Close() error {
-	var ret error
-	if ret = r.restoreResolvConf(); ret != nil {
-		r.logf("failed to restore system resolv.conf: %v", ret)
-	}
-	if err := r.down(); err != nil {
-		if ret == nil {
-			ret = err
-		}
-	}
-
-	return ret
-}
-
 // Set implements the Router interface.
 func (r *linuxRouter) Set(cfg *Config) error {
 	if cfg == nil {
@@ -189,12 +190,14 @@ func (r *linuxRouter) Set(cfg *Config) error {
 	}
 	r.snatSubnetRoutes = cfg.SNATSubnetRoutes
 
-	// TODO: this:
-	if false {
-		if err := r.replaceResolvConf(cfg.DNS, cfg.DNSDomains); err != nil {
-			return fmt.Errorf("replacing resolv.conf failed: %w", err)
+	if !r.dnsConfig.EquivalentTo(cfg.DNSConfig) {
+		if err := r.upDNS(cfg.DNSConfig); err != nil {
+			r.logf("dns up: %v", err)
+		} else {
+			r.dnsConfig = cfg.DNSConfig
 		}
 	}
+
 	return nil
 }
 
@@ -315,102 +318,6 @@ func (r *linuxRouter) setNetfilterMode(mode NetfilterMode) error {
 	return nil
 }
 
-const (
-	tsConf     = "/etc/resolv.tailscale.conf"
-	backupConf = "/etc/resolv.pre-tailscale-backup.conf"
-	resolvConf = "/etc/resolv.conf"
-)
-
-func (r *linuxRouter) replaceResolvConf(servers []netaddr.IP, domains []string) error {
-	if len(servers) == 0 {
-		return r.restoreResolvConf()
-	}
-
-	// First 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")
-	}
-	f, err := ioutil.TempFile(filepath.Dir(tsConf), filepath.Base(tsConf)+".*")
-	if err != nil {
-		return err
-	}
-	f.Close()
-	if err := atomicfile.WriteFile(f.Name(), buf.Bytes(), 0644); err != nil {
-		return err
-	}
-	os.Chmod(f.Name(), 0644) // ioutil.TempFile creates the file with 0600
-	if err := os.Rename(f.Name(), 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
-	}
-
-	out, _ := exec.Command("service", "systemd-resolved", "restart").CombinedOutput()
-	if len(out) > 0 {
-		r.logf("service systemd-resolved restart: %s", out)
-	}
-	return nil
-}
-
-func (r *linuxRouter) 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 of tsConf file
-	out, _ := exec.Command("service", "systemd-resolved", "restart").CombinedOutput()
-	if len(out) > 0 {
-		r.logf("service systemd-resolved restart: %s", out)
-	}
-	return nil
-}
-
 // addAddress adds an IP/mask to the tunnel interface. Fails if the
 // address is already assigned to the interface, or if the addition
 // fails.
@@ -932,3 +839,69 @@ func normalizeCIDR(cidr netaddr.IPPrefix) string {
 	nip := ncidr.IP.Mask(ncidr.Mask)
 	return fmt.Sprintf("%s/%d", nip, cidr.Bits)
 }
+
+// upDNS updates the system DNS configuration to the given one.
+func (r *linuxRouter) upDNS(config DNSConfig) error {
+	if len(config.Nameservers) == 0 {
+		return r.downDNS()
+	}
+
+	switch r.dnsMode {
+	case dnsResolved:
+		if err := dnsResolvedUp(config); err != nil {
+			return fmt.Errorf("resolved: %w", err)
+		}
+	case dnsResolvconf:
+		if err := dnsResolvconfUp(config, r.tunname); err != nil {
+			return fmt.Errorf("resolvconf: %w", err)
+		}
+	case dnsNetworkManager:
+		if err := dnsNetworkManagerUp(config, r.tunname); err != nil {
+			return fmt.Errorf("network manager: %w", err)
+		}
+	case dnsDirect:
+		if err := dnsDirectUp(config); err != nil {
+			return fmt.Errorf("direct: %w", err)
+		}
+	}
+	return nil
+}
+
+// downDNS restores system DNS configuration to its state before upDNS.
+// It is idempotent (in particular, it does nothing if upDNS was never run).
+func (r *linuxRouter) downDNS() error {
+	switch r.dnsMode {
+	case dnsResolved:
+		if err := dnsResolvedDown(); err != nil {
+			return fmt.Errorf("resolved: %w", err)
+		}
+	case dnsResolvconf:
+		if err := dnsResolvconfDown(r.tunname); err != nil {
+			return fmt.Errorf("resolvconf: %w", err)
+		}
+	case dnsNetworkManager:
+		if err := dnsNetworkManagerDown(r.tunname); err != nil {
+			return fmt.Errorf("network manager: %w", err)
+		}
+	case dnsDirect:
+		if err := dnsDirectDown(); err != nil {
+			return fmt.Errorf("direct: %w", err)
+		}
+	}
+	return nil
+}
+
+func cleanup(logf logger.Logf, interfaceName string) {
+	// Note: we need not do anything for dnsResolved,
+	// as its settings are interface-bound and get cleaned up for us.
+	switch {
+	case resolvconfIsActive():
+		if err := dnsResolvconfDown(interfaceName); err != nil {
+			logf("down down: resolvconf: %v", err)
+		}
+	default:
+		if err := dnsDirectDown(); err != nil {
+			logf("dns down: direct: %v", err)
+		}
+	}
+}

wgengine/router/router_openbsd.go

@@ -5,20 +5,14 @@
 package router
 
 import (
-	"bytes"
 	"errors"
 	"fmt"
-	"io/ioutil"
 	"log"
-	"os"
 	"os/exec"
-	"path/filepath"
-	"strings"
 
 	"github.com/tailscale/wireguard-go/device"
 	"github.com/tailscale/wireguard-go/tun"
 	"inet.af/netaddr"
-	"tailscale.com/atomicfile"
 	"tailscale.com/types/logger"
 )
 
@@ -31,6 +25,8 @@ type openbsdRouter struct {
 	tunname string
 	local   netaddr.IPPrefix
 	routes  map[netaddr.IPPrefix]struct{}
+
+	dnsConfig DNSConfig
 }
 
 func newUserspaceRouter(logf logger.Logf, _ *device.Device, tundev tun.Device) (Router, error) {
@@ -159,112 +155,28 @@ func (r *openbsdRouter) Set(cfg *Config) error {
 	r.local = localAddr
 	r.routes = newRoutes
 
-	if err := r.replaceResolvConf(cfg.DNS, cfg.DNSDomains); err != nil {
-		errq = fmt.Errorf("replacing resolv.conf failed: %v", err)
+	if !r.dnsConfig.EquivalentTo(cfg.DNSConfig) {
+		if err := dnsDirectUp(cfg.DNSConfig); err != nil {
+			errq = fmt.Errorf("dns up: direct: %v", err)
+		} else {
+			r.dnsConfig = cfg.DNSConfig
+		}
 	}
 
 	return errq
 }
 
 func (r *openbsdRouter) Close() error {
-	out, err := cmd("ifconfig", r.tunname, "down").CombinedOutput()
+	cleanup(r.logf, r.tunname)
+	return nil
+}
+
+func cleanup(logf logger.Logf, interfaceName string) {
+	if err := dnsDirectDown(); err != nil {
+		logf("dns down: direct: %v", err)
+	}
+	out, err := cmd("ifconfig", interfaceName, "down").CombinedOutput()
 	if err != nil {
-		r.logf("running ifconfig failed: %v\n%s", err, out)
+		logf("ifconfig down: %v\n%s", err, out)
 	}
-
-	if err := r.restoreResolvConf(); err != nil {
-		r.logf("failed to restore system resolv.conf: %v", err)
-	}
-
-	return nil
-}
-
-const (
-	tsConf     = "/etc/resolv.tailscale.conf"
-	backupConf = "/etc/resolv.pre-tailscale-backup.conf"
-	resolvConf = "/etc/resolv.conf"
-)
-
-func (r *openbsdRouter) replaceResolvConf(servers []netaddr.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
-}
-
-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
 }

wgengine/router/router_userspace_bsd.go

@@ -23,6 +23,8 @@ type userspaceBSDRouter struct {
 	tunname string
 	local   netaddr.IPPrefix
 	routes  map[netaddr.IPPrefix]struct{}
+
+	dnsConfig DNSConfig
 }
 
 func newUserspaceBSDRouter(logf logger.Logf, _ *device.Device, tundev tun.Device) (Router, error) {
@@ -36,7 +38,7 @@ func newUserspaceBSDRouter(logf logger.Logf, _ *device.Device, tundev tun.Device
 	}, nil
 }
 
-func (r *userspaceBSDRouter) cmd(args ...string) *exec.Cmd {
+func cmd(args ...string) *exec.Cmd {
 	if len(args) == 0 {
 		log.Fatalf("exec.Cmd(%#v) invalid; need argv[0]\n", args)
 	}
@@ -45,7 +47,7 @@ func (r *userspaceBSDRouter) cmd(args ...string) *exec.Cmd {
 
 func (r *userspaceBSDRouter) Up() error {
 	ifup := []string{"ifconfig", r.tunname, "up"}
-	if out, err := r.cmd(ifup...).CombinedOutput(); err != nil {
+	if out, err := cmd(ifup...).CombinedOutput(); err != nil {
 		r.logf("running ifconfig failed: %v\n%s", err, out)
 		return err
 	}
@@ -73,7 +75,7 @@ func (r *userspaceBSDRouter) Set(cfg *Config) error {
 		if r.local != (netaddr.IPPrefix{}) {
 			addrdel := []string{"ifconfig", r.tunname,
 				"inet", r.local.String(), "-alias"}
-			out, err := r.cmd(addrdel...).CombinedOutput()
+			out, err := cmd(addrdel...).CombinedOutput()
 			if err != nil {
 				r.logf("addr del failed: %v: %v\n%s", addrdel, err, out)
 				if errq == nil {
@@ -85,7 +87,7 @@ func (r *userspaceBSDRouter) Set(cfg *Config) error {
 		// Add the interface.
 		addradd := []string{"ifconfig", r.tunname,
 			"inet", localAddr.String(), localAddr.IP.String()}
-		out, err := r.cmd(addradd...).CombinedOutput()
+		out, err := cmd(addradd...).CombinedOutput()
 		if err != nil {
 			r.logf("addr add failed: %v: %v\n%s", addradd, err, out)
 			if errq == nil {
@@ -107,7 +109,7 @@ func (r *userspaceBSDRouter) Set(cfg *Config) error {
 			routedel := []string{"route", "-q", "-n",
 				"del", "-inet", nstr,
 				"-iface", r.tunname}
-			out, err := r.cmd(routedel...).CombinedOutput()
+			out, err := cmd(routedel...).CombinedOutput()
 			if err != nil {
 				r.logf("route del failed: %v: %v\n%s", routedel, err, out)
 				if errq == nil {
@@ -125,7 +127,7 @@ func (r *userspaceBSDRouter) Set(cfg *Config) error {
 			routeadd := []string{"route", "-q", "-n",
 				"add", "-inet", nstr,
 				"-iface", r.tunname}
-			out, err := r.cmd(routeadd...).CombinedOutput()
+			out, err := cmd(routeadd...).CombinedOutput()
 			if err != nil {
 				r.logf("addr add failed: %v: %v\n%s", routeadd, err, out)
 				if errq == nil {
@@ -139,18 +141,29 @@ func (r *userspaceBSDRouter) Set(cfg *Config) error {
 	r.local = localAddr
 	r.routes = newRoutes
 
-	if err := r.replaceResolvConf(cfg.DNS, cfg.DNSDomains); err != nil {
-		errq = fmt.Errorf("replacing resolv.conf failed: %v", err)
+	if !r.dnsConfig.EquivalentTo(cfg.DNSConfig) {
+		if err := upDNS(cfg.DNSConfig, r.tunname); err != nil {
+			errq = fmt.Errorf("dns up: %v", err)
+		} else {
+			r.dnsConfig = cfg.DNSConfig
+		}
 	}
 
 	return errq
 }
 
 func (r *userspaceBSDRouter) Close() error {
+	cleanup(r.logf, r.tunname)
 	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 *userspaceBSDRouter) replaceResolvConf(_ []netaddr.IP, _ []string) error { return nil }
-func (r *userspaceBSDRouter) restoreResolvConf() error                           { return nil }
+func cleanup(logf logger.Logf, interfaceName string) {
+	if err := downDNS(interfaceName); err != nil {
+		logf("dns down: %v", err)
+	}
+
+	ifup := []string{"ifconfig", interfaceName, "down"}
+	if out, err := cmd(ifup...).CombinedOutput(); err != nil {
+		logf("ifconfig down: %v\n%s", err, out)
+	}
+}

wgengine/router/router_windows.go

@@ -64,3 +64,7 @@ func (r *winRouter) Close() error {
 	}
 	return nil
 }
+
+func cleanup(logf logger.Logf, interfaceName string) {
+	// DNS is interface-bound, so nothing to do here.
+}

wgengine/userspace.go

@@ -57,6 +57,9 @@ const (
 	magicDNSPort = 53
 )
 
+// magicDNSDomain is the parent domain for Tailscale nodes.
+const magicDNSDomain = "b.tailscale.net"
+
 type userspaceEngine struct {
 	logf            logger.Logf
 	reqCh           chan struct{}
@@ -180,7 +183,7 @@ func newUserspaceEngineAdvanced(conf EngineConfig) (_ Engine, reterr error) {
 		reqCh:           make(chan struct{}, 1),
 		waitCh:          make(chan struct{}),
 		tundev:          tstun.WrapTUN(logf, conf.TUN),
-		resolver:        tsdns.NewResolver(logf, "tailscale.us"),
+		resolver:        tsdns.NewResolver(logf, magicDNSDomain),
 		useTailscaleDNS: conf.UseTailscaleDNS,
 		pingers:         make(map[wgcfg.Key]*pinger),
 	}
@@ -548,8 +551,7 @@ func (e *userspaceEngine) Reconfig(cfg *wgcfg.Config, routerCfg *router.Config)
 		if !addr.IP.Is4() {
 			continue
 		}
-		bs := addr.IP.As16()
-		localAddrs[packet.NewIP(net.IP(bs[12:16]))] = true
+		localAddrs[packet.IPFromNetaddr(addr.IP)] = true
 	}
 	e.localAddrs.Store(localAddrs)
 
@@ -565,6 +567,13 @@ func (e *userspaceEngine) Reconfig(cfg *wgcfg.Config, routerCfg *router.Config)
 	}
 	e.mu.Unlock()
 
+	// If the only nameserver is quad 100 (Magic DNS), set up the resolver appropriately.
+	if len(routerCfg.Nameservers) == 1 && routerCfg.Nameservers[0] == packet.IP(magicDNSIP).Netaddr() {
+		// TODO(dmytro): plumb dnsReadConfig here instead of hardcoding this.
+		e.resolver.SetNameservers([]string{"8.8.8.8:53"})
+		routerCfg.Domains = append([]string{magicDNSDomain}, routerCfg.Domains...)
+	}
+
 	engineChanged := updateSig(&e.lastEngineSig, cfg)
 	routerChanged := updateSig(&e.lastRouterSig, routerCfg)
 	if !engineChanged && !routerChanged {