tailscale/net/dns/manager_windows.go

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// 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 dns
import (
"errors"
"fmt"
"os/exec"
"sort"
"strings"
"syscall"
"time"
"golang.org/x/sys/windows"
"golang.org/x/sys/windows/registry"
"golang.zx2c4.com/wireguard/windows/tunnel/winipcfg"
"inet.af/netaddr"
"tailscale.com/envknob"
"tailscale.com/types/logger"
"tailscale.com/util/dnsname"
)
const (
ipv4RegBase = `SYSTEM\CurrentControlSet\Services\Tcpip\Parameters`
ipv6RegBase = `SYSTEM\CurrentControlSet\Services\Tcpip6\Parameters`
// the GUID is randomly generated. At present, Tailscale installs
// zero or one NRPT rules, so hardcoding a single GUID everywhere
// is fine.
nrptBase = `SYSTEM\CurrentControlSet\services\Dnscache\Parameters\DnsPolicyConfig\{5abe529b-675b-4486-8459-25a634dacc23}`
nrptOverrideDNS = 0x8 // bitmask value for "use the provided override DNS resolvers"
versionKey = `SOFTWARE\Microsoft\Windows NT\CurrentVersion`
)
var configureWSL = envknob.Bool("TS_DEBUG_CONFIGURE_WSL")
type windowsManager struct {
logf logger.Logf
guid string
nrptWorks bool
wslManager *wslManager
}
func NewOSConfigurator(logf logger.Logf, interfaceName string) (OSConfigurator, error) {
ret := windowsManager{
logf: logf,
guid: interfaceName,
nrptWorks: isWindows10OrBetter(),
wslManager: newWSLManager(logf),
}
// Best-effort: if our NRPT rule exists, try to delete it. Unlike
// per-interface configuration, NRPT rules survive the unclean
// termination of the Tailscale process, and depending on the
// rule, it may prevent us from reaching login.tailscale.com to
// boot up. The bootstrap resolver logic will save us, but it
// slows down start-up a bunch.
if ret.nrptWorks {
ret.delKey(nrptBase)
}
// Log WSL status once at startup.
if distros, err := wslDistros(); err != nil {
logf("WSL: could not list distributions: %v", err)
} else {
logf("WSL: found %d distributions", len(distros))
}
return ret, nil
}
// keyOpenTimeout is how long we wait for a registry key to
// appear. For some reason, registry keys tied to ephemeral interfaces
// can take a long while to appear after interface creation, and we
// can end up racing with that.
const keyOpenTimeout = 20 * time.Second
func (m windowsManager) openKey(path string) (registry.Key, error) {
key, err := openKeyWait(registry.LOCAL_MACHINE, path, registry.SET_VALUE, keyOpenTimeout)
if err != nil {
return 0, fmt.Errorf("opening %s: %w", path, err)
}
return key, nil
}
func (m windowsManager) ifPath(basePath string) string {
return fmt.Sprintf(`%s\Interfaces\%s`, basePath, m.guid)
}
func (m windowsManager) delKey(path string) error {
if err := registry.DeleteKey(registry.LOCAL_MACHINE, path); err != nil && err != registry.ErrNotExist {
return err
}
return nil
}
func delValue(key registry.Key, name string) error {
if err := key.DeleteValue(name); err != nil && err != registry.ErrNotExist {
return err
}
return nil
}
// setSplitDNS configures an NRPT (Name Resolution Policy Table) rule
// to resolve queries for domains using resolvers, rather than the
// system's "primary" resolver.
//
// If no resolvers are provided, the Tailscale NRPT rule is deleted.
func (m windowsManager) setSplitDNS(resolvers []netaddr.IP, domains []dnsname.FQDN) error {
if len(resolvers) == 0 {
return m.delKey(nrptBase)
}
servers := make([]string, 0, len(resolvers))
for _, resolver := range resolvers {
servers = append(servers, resolver.String())
}
doms := make([]string, 0, len(domains))
for _, domain := range domains {
// NRPT rules must have a leading dot, which is not usual for
// DNS search paths.
doms = append(doms, "."+domain.WithoutTrailingDot())
}
// CreateKey is actually open-or-create, which suits us fine.
key, _, err := registry.CreateKey(registry.LOCAL_MACHINE, nrptBase, registry.SET_VALUE)
if err != nil {
return fmt.Errorf("opening %s: %w", nrptBase, err)
}
defer key.Close()
if err := key.SetDWordValue("Version", 1); err != nil {
return err
}
if err := key.SetStringsValue("Name", doms); err != nil {
return err
}
if err := key.SetStringValue("GenericDNSServers", strings.Join(servers, "; ")); err != nil {
return err
}
if err := key.SetDWordValue("ConfigOptions", nrptOverrideDNS); err != nil {
return err
}
return nil
}
// setPrimaryDNS sets the given resolvers and domains as the Tailscale
// interface's DNS configuration.
// If resolvers is non-empty, those resolvers become the system's
// "primary" resolvers.
// domains can be set without resolvers, which just contributes new
// paths to the global DNS search list.
func (m windowsManager) setPrimaryDNS(resolvers []netaddr.IP, domains []dnsname.FQDN) error {
var ipsv4 []string
var ipsv6 []string
for _, ip := range resolvers {
if ip.Is4() {
ipsv4 = append(ipsv4, ip.String())
} else {
ipsv6 = append(ipsv6, ip.String())
}
}
domStrs := make([]string, 0, len(domains))
for _, dom := range domains {
domStrs = append(domStrs, dom.WithoutTrailingDot())
}
key4, err := m.openKey(m.ifPath(ipv4RegBase))
if err != nil {
return err
}
defer key4.Close()
if len(ipsv4) == 0 {
if err := delValue(key4, "NameServer"); err != nil {
return err
}
} else if err := key4.SetStringValue("NameServer", strings.Join(ipsv4, ",")); err != nil {
return err
}
if len(domains) == 0 {
if err := delValue(key4, "SearchList"); err != nil {
return err
}
} else if err := key4.SetStringValue("SearchList", strings.Join(domStrs, ",")); err != nil {
return err
}
key6, err := m.openKey(m.ifPath(ipv6RegBase))
if err != nil {
return err
}
defer key6.Close()
if len(ipsv6) == 0 {
if err := delValue(key6, "NameServer"); err != nil {
return err
}
} else if err := key6.SetStringValue("NameServer", strings.Join(ipsv6, ",")); err != nil {
return err
}
if len(domains) == 0 {
if err := delValue(key6, "SearchList"); err != nil {
return err
}
} else if err := key6.SetStringValue("SearchList", strings.Join(domStrs, ",")); err != nil {
return err
}
// Disable LLMNR on the Tailscale interface. We don't do
// multicast, and we certainly don't do LLMNR, so it's pointless
// to make Windows try it.
if err := key4.SetDWordValue("EnableMulticast", 0); err != nil {
return err
}
if err := key6.SetDWordValue("EnableMulticast", 0); err != nil {
return err
}
return nil
}
func (m windowsManager) SetDNS(cfg OSConfig) error {
// We can configure Windows DNS in one of two ways:
//
// - In primary DNS mode, we set the NameServer and SearchList
// registry keys on our interface. Because our interface metric
// is very low, this turns us into the one and only "primary"
// resolver for the OS, i.e. all queries flow to the
// resolver(s) we specify.
// - In split DNS mode, we set the Domain registry key on our
// interface (which adds that domain to the global search list,
// but does not contribute other DNS configuration from the
// interface), and configure an NRPT (Name Resolution Policy
// Table) rule to route queries for our suffixes to the
// provided resolver.
//
// When switching modes, we delete all the configuration related
// to the other mode, so these two are an XOR.
//
// Windows actually supports much more advanced configurations as
// well, with arbitrary routing of hosts and suffixes to arbitrary
// resolvers. However, we use it in a "simple" split domain
// configuration only, routing one set of things to the "split"
// resolver and the rest to the primary.
if len(cfg.MatchDomains) == 0 {
if err := m.setSplitDNS(nil, nil); err != nil {
return err
}
if err := m.setPrimaryDNS(cfg.Nameservers, cfg.SearchDomains); err != nil {
return err
}
} else if !m.nrptWorks {
return errors.New("cannot set per-domain resolvers on Windows 7")
} else {
if err := m.setSplitDNS(cfg.Nameservers, cfg.MatchDomains); err != nil {
return err
}
// Still set search domains on the interface, since NRPT only
// handles query routing and not search domain expansion.
if err := m.setPrimaryDNS(nil, cfg.SearchDomains); err != nil {
return err
}
}
// 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
// have changed, which makes the DNS settings actually take
// effect.
//
// This command can take a few seconds to run, so run it async, best effort.
//
// After re-registering DNS, also flush the DNS cache to clear out
// any cached split-horizon queries that are no longer the correct
// answer.
go func() {
t0 := time.Now()
m.logf("running ipconfig /registerdns ...")
cmd := exec.Command("ipconfig", "/registerdns")
cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true}
err := cmd.Run()
d := time.Since(t0).Round(time.Millisecond)
if err != nil {
m.logf("error running ipconfig /registerdns after %v: %v", d, err)
} else {
m.logf("ran ipconfig /registerdns in %v", d)
}
t0 = time.Now()
m.logf("running ipconfig /flushdns ...")
cmd = exec.Command("ipconfig", "/flushdns")
cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true}
err = cmd.Run()
d = time.Since(t0).Round(time.Millisecond)
if err != nil {
m.logf("error running ipconfig /flushdns after %v: %v", d, err)
} else {
m.logf("ran ipconfig /flushdns in %v", d)
}
}()
// On initial setup of WSL, the restart caused by --shutdown is slow,
// so we do it out-of-line.
if configureWSL {
go func() {
if err := m.wslManager.SetDNS(cfg); err != nil {
m.logf("WSL SetDNS: %v", err) // continue
} else {
m.logf("WSL SetDNS: success")
}
}()
}
return nil
}
func (m windowsManager) SupportsSplitDNS() bool {
return m.nrptWorks
}
func (m windowsManager) Close() error {
return m.SetDNS(OSConfig{})
}
func (m windowsManager) GetBaseConfig() (OSConfig, error) {
resolvers, err := m.getBasePrimaryResolver()
if err != nil {
return OSConfig{}, err
}
return OSConfig{
Nameservers: resolvers,
// Don't return any search domains here, because even Windows
// 7 correctly handles blending search domains from multiple
// sources, and any search domains we add here will get tacked
// onto the Tailscale config unnecessarily.
}, nil
}
// getBasePrimaryResolver returns a guess of the non-Tailscale primary
// resolver on the system.
// It's used on Windows 7 to emulate split DNS by trying to figure out
// what the "previous" primary resolver was. It might be wrong, or
// incomplete.
func (m windowsManager) getBasePrimaryResolver() (resolvers []netaddr.IP, err error) {
tsGUID, err := windows.GUIDFromString(m.guid)
if err != nil {
return nil, err
}
tsLUID, err := winipcfg.LUIDFromGUID(&tsGUID)
if err != nil {
return nil, err
}
ifrows, err := winipcfg.GetIPInterfaceTable(windows.AF_INET)
if err == windows.ERROR_NOT_FOUND {
// IPv4 seems disabled, try to get interface metrics from IPv6 instead.
ifrows, err = winipcfg.GetIPInterfaceTable(windows.AF_INET6)
}
if err != nil {
return nil, err
}
type candidate struct {
id winipcfg.LUID
metric uint32
}
var candidates []candidate
for _, row := range ifrows {
if !row.Connected {
continue
}
if row.InterfaceLUID == tsLUID {
continue
}
candidates = append(candidates, candidate{row.InterfaceLUID, row.Metric})
}
if len(candidates) == 0 {
// No resolvers set outside of Tailscale.
return nil, nil
}
sort.Slice(candidates, func(i, j int) bool { return candidates[i].metric < candidates[j].metric })
for _, candidate := range candidates {
ips, err := candidate.id.DNS()
if err != nil {
return nil, err
}
ipLoop:
for _, stdip := range ips {
ip, ok := netaddr.FromStdIP(stdip)
if !ok {
continue
}
// Skip IPv6 site-local resolvers. These are an ancient
// and obsolete IPv6 RFC, which Windows still faithfully
// implements. The net result is that some low-metric
// interfaces can "have" DNS resolvers, but they're just
// site-local resolver IPs that don't go anywhere. So, we
// skip the site-local resolvers in order to find the
// first interface that has real DNS servers configured.
for _, sl := range siteLocalResolvers {
if ip.WithZone("") == sl {
continue ipLoop
}
}
resolvers = append(resolvers, ip)
}
if len(resolvers) > 0 {
// Found some resolvers, we're done.
break
}
}
return resolvers, nil
}
var siteLocalResolvers = []netaddr.IP{
netaddr.MustParseIP("fec0:0:0:ffff::1"),
netaddr.MustParseIP("fec0:0:0:ffff::2"),
netaddr.MustParseIP("fec0:0:0:ffff::3"),
}
func isWindows10OrBetter() bool {
key, err := registry.OpenKey(registry.LOCAL_MACHINE, versionKey, registry.READ)
if err != nil {
// Fail safe, assume old Windows.
return false
}
// This key above only exists in Windows 10 and above. Its mere
// presence is good enough.
if _, _, err := key.GetIntegerValue("CurrentMajorVersionNumber"); err != nil {
return false
}
return true
}