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tailscale/net/dns/nrpt_windows.go

619 lines
17 KiB
Go

// Copyright (c) 2022 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"
"strings"
"sync"
"sync/atomic"
"golang.org/x/sys/windows"
"golang.org/x/sys/windows/registry"
"tailscale.com/types/logger"
"tailscale.com/util/dnsname"
"tailscale.com/util/winutil"
)
const (
dnsBaseGP = `SOFTWARE\Policies\Microsoft\Windows NT\DNSClient`
nrptBaseLocal = `SYSTEM\CurrentControlSet\Services\Dnscache\Parameters\DnsPolicyConfig`
nrptBaseGP = `SOFTWARE\Policies\Microsoft\Windows NT\DNSClient\DnsPolicyConfig`
nrptOverrideDNS = 0x8 // bitmask value for "use the provided override DNS resolvers"
// Apparently NRPT rules cannot handle > 50 domains.
nrptMaxDomainsPerRule = 50
// This is the legacy rule ID that previous versions used when we supported
// only a single rule. Now that we support multiple rules are required, we
// generate their GUIDs and store them under the Tailscale registry key.
nrptSingleRuleID = `{5abe529b-675b-4486-8459-25a634dacc23}`
// This is the name of the registry value we use to save Rule IDs under
// the Tailscale registry key.
nrptRuleIDValueName = `NRPTRuleIDs`
// This is the name of the registry value the NRPT uses for storing a rule's version number.
nrptRuleVersionName = `Version`
// This is the name of the registry value the NRPT uses for storing a rule's list of domains.
nrptRuleDomsName = `Name`
// This is the name of the registry value the NRPT uses for storing a rule's list of DNS servers.
nrptRuleServersName = `GenericDNSServers`
// This is the name of the registry value the NRPT uses for storing a rule's flags.
nrptRuleFlagsName = `ConfigOptions`
)
var (
libUserenv = windows.NewLazySystemDLL("userenv.dll")
procRefreshPolicyEx = libUserenv.NewProc("RefreshPolicyEx")
procRegisterGPNotification = libUserenv.NewProc("RegisterGPNotification")
procUnregisterGPNotification = libUserenv.NewProc("UnregisterGPNotification")
)
const _RP_FORCE = 1 // Flag for RefreshPolicyEx
// nrptRuleDatabase ensapsulates access to the Windows Name Resolution Policy
// Table (NRPT).
type nrptRuleDatabase struct {
logf logger.Logf
watcher *gpNotificationWatcher
isGPRefreshPending atomic.Bool
mu sync.Mutex // protects the fields below
ruleIDs []string
isGPDirty bool
writeAsGP bool
}
func newNRPTRuleDatabase(logf logger.Logf) *nrptRuleDatabase {
ret := &nrptRuleDatabase{logf: logf}
ret.loadRuleSubkeyNames()
ret.detectWriteAsGP()
ret.watchForGPChanges()
// 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.
ret.DelAllRuleKeys()
return ret
}
func (db *nrptRuleDatabase) loadRuleSubkeyNames() {
result := winutil.GetRegStrings(nrptRuleIDValueName, nil)
if result == nil {
// Use the legacy rule ID if none are specified in our registry key
result = []string{nrptSingleRuleID}
}
db.ruleIDs = result
}
// detectWriteAsGP determines which registry path should be used for writing
// NRPT rules. If there are rules in the GP path that don't belong to us, then
// we should use the GP path. When detectWriteAsGP determines that the desired
// path has changed, it moves the NRPT policies as appropriate.
func (db *nrptRuleDatabase) detectWriteAsGP() {
db.mu.Lock()
defer db.mu.Unlock()
writeAsGP := false
var err error
defer func() {
if err != nil {
return
}
prev := db.writeAsGP
db.writeAsGP = writeAsGP
db.logf("nrptRuleDatabase using group policy: %v, was %v\n", writeAsGP, prev)
// When db.watcher == nil, prev != writeAsGP because we're initializing, not
// because anything has changed. We do not invoke db.movePolicies in that case.
if db.watcher != nil && prev != writeAsGP {
db.movePolicies(writeAsGP)
}
}()
dnsKey, err := registry.OpenKey(registry.LOCAL_MACHINE, dnsBaseGP, registry.READ)
if err != nil {
db.logf("Failed to open key %q with error: %v\n", dnsBaseGP, err)
return
}
defer dnsKey.Close()
ki, err := dnsKey.Stat()
if err != nil {
db.logf("Failed to stat key %q with error: %v\n", dnsBaseGP, err)
return
}
// If the dnsKey contains any values, then we need to use the GP key.
if ki.ValueCount > 0 {
writeAsGP = true
return
}
if ki.SubKeyCount == 0 {
// If dnsKey contains no values and no subkeys, then we definitely don't
// need to use the GP key.
return
}
// Get a list of all the NRPT rules under the GP subkey.
nrptKey, err := registry.OpenKey(registry.LOCAL_MACHINE, nrptBaseGP, registry.READ)
if err != nil {
db.logf("Failed to open key %q with error: %v\n", nrptBaseGP, err)
return
}
defer nrptKey.Close()
gpSubkeyNames, err := nrptKey.ReadSubKeyNames(0)
if err != nil {
db.logf("Failed to list subkeys under %q with error: %v\n", nrptBaseGP, err)
return
}
// Add *all* rules from the GP subkey into a set.
gpSubkeyMap := make(map[string]struct{}, len(gpSubkeyNames))
for _, gpSubkey := range gpSubkeyNames {
gpSubkeyMap[strings.ToUpper(gpSubkey)] = struct{}{}
}
// Remove *our* rules from the set.
for _, ourRuleID := range db.ruleIDs {
delete(gpSubkeyMap, strings.ToUpper(ourRuleID))
}
// Any leftover rules do not belong to us. When group policy is being used
// by something else, we must also use the GP path.
writeAsGP = len(gpSubkeyMap) > 0
}
// DelAllRuleKeys removes any and all NRPT rules that are owned by Tailscale.
func (db *nrptRuleDatabase) DelAllRuleKeys() error {
db.mu.Lock()
defer db.mu.Unlock()
if err := db.delRuleKeys(db.ruleIDs); err != nil {
return err
}
if err := winutil.DeleteRegValue(nrptRuleIDValueName); err != nil {
db.logf("Error deleting registry value %q: %v", nrptRuleIDValueName, err)
return err
}
db.ruleIDs = nil
return nil
}
// delRuleKeys removes the NRPT rules specified by nrptRuleIDs from the
// Windows registry. It attempts to remove the rules from both possible registry
// keys: the local key and the group policy key.
func (db *nrptRuleDatabase) delRuleKeys(nrptRuleIDs []string) error {
for _, rid := range nrptRuleIDs {
keyNameLocal := nrptBaseLocal + `\` + rid
if err := registry.DeleteKey(registry.LOCAL_MACHINE, keyNameLocal); err != nil && err != registry.ErrNotExist {
db.logf("Error deleting NRPT rule key %q: %v", keyNameLocal, err)
return err
}
keyNameGP := nrptBaseGP + `\` + rid
err := registry.DeleteKey(registry.LOCAL_MACHINE, keyNameGP)
if err == nil {
// If this deleted subkey existed under the GP key, we will need to refresh.
db.isGPDirty = true
} else if err != registry.ErrNotExist {
db.logf("Error deleting NRPT rule key %q: %v", keyNameGP, err)
return err
}
}
if !db.isGPDirty {
return nil
}
// If we've removed keys from the Group Policy subkey, and the DNSPolicyConfig
// subkey is now empty, we need to remove that subkey.
isEmpty, err := isPolicyConfigSubkeyEmpty()
if err != nil || !isEmpty {
return err
}
return registry.DeleteKey(registry.LOCAL_MACHINE, nrptBaseGP)
}
// isPolicyConfigSubkeyEmpty returns true if and only if the nrptBaseGP exists
// and does not contain any values or subkeys.
func isPolicyConfigSubkeyEmpty() (bool, error) {
subKey, err := registry.OpenKey(registry.LOCAL_MACHINE, nrptBaseGP, registry.READ)
if err != nil {
if err == registry.ErrNotExist {
return false, nil
}
return false, err
}
defer subKey.Close()
ki, err := subKey.Stat()
if err != nil {
return false, err
}
return (ki.ValueCount == 0 && ki.SubKeyCount == 0), nil
}
func (db *nrptRuleDatabase) WriteSplitDNSConfig(servers []string, domains []dnsname.FQDN) error {
db.mu.Lock()
defer db.mu.Unlock()
// NRPT has an undocumented restriction that each rule may only be associated
// with a maximum of 50 domains. If we are setting rules for more domains
// than that, we need to split domains into chunks and write out a rule per chunk.
dq := len(domains) / nrptMaxDomainsPerRule
dr := len(domains) % nrptMaxDomainsPerRule
domainRulesLen := dq
if dr > 0 {
domainRulesLen++
}
db.loadRuleSubkeyNames()
for len(db.ruleIDs) < domainRulesLen {
guid, err := windows.GenerateGUID()
if err != nil {
return err
}
db.ruleIDs = append(db.ruleIDs, guid.String())
}
// Remove any surplus rules that are no longer needed.
ruleIDsToRemove := db.ruleIDs[domainRulesLen:]
db.delRuleKeys(ruleIDsToRemove)
// We need to save the list of rule IDs to our Tailscale registry key so that
// we know which rules are ours during subsequent modifications to NRPT rules.
ruleIDsToWrite := db.ruleIDs[:domainRulesLen]
if len(ruleIDsToWrite) == 0 {
if err := winutil.DeleteRegValue(nrptRuleIDValueName); err != nil {
return err
}
db.ruleIDs = nil
return nil
}
if err := winutil.SetRegStrings(nrptRuleIDValueName, ruleIDsToWrite); err != nil {
return err
}
db.ruleIDs = ruleIDsToWrite
curRuleID := 0
doms := make([]string, 0, nrptMaxDomainsPerRule)
for _, domain := range domains {
if len(doms) == nrptMaxDomainsPerRule {
if err := db.writeNRPTRule(db.ruleIDs[curRuleID], servers, doms); err != nil {
return err
}
curRuleID++
doms = doms[:0]
}
// NRPT rules must have a leading dot, which is not usual for
// DNS search paths.
doms = append(doms, "."+domain.WithoutTrailingDot())
}
if len(doms) > 0 {
if err := db.writeNRPTRule(db.ruleIDs[curRuleID], servers, doms); err != nil {
return err
}
}
return nil
}
// Refresh notifies the Windows group policy engine when policies have changed.
func (db *nrptRuleDatabase) Refresh() {
db.mu.Lock()
defer db.mu.Unlock()
db.refreshLocked()
}
func (db *nrptRuleDatabase) refreshLocked() {
if !db.isGPDirty {
return
}
// Record that we are about to initiate a refresh.
// (*nrptRuleDatabase).watchForGPChanges() checks this value to avoid false
// positives.
db.isGPRefreshPending.Store(true)
ok, _, err := procRefreshPolicyEx.Call(
uintptr(1), // Win32 TRUE: Refresh computer policy, not user policy.
uintptr(_RP_FORCE),
)
if ok == 0 {
db.logf("RefreshPolicyEx failed: %v", err)
return
}
db.isGPDirty = false
}
func (db *nrptRuleDatabase) writeNRPTRule(ruleID string, servers, doms []string) error {
var nrptBase string
if db.writeAsGP {
nrptBase = nrptBaseGP
} else {
nrptBase = nrptBaseLocal
}
keyStr := nrptBase + `\` + ruleID
// CreateKey is actually open-or-create, which suits us fine.
key, _, err := registry.CreateKey(registry.LOCAL_MACHINE, keyStr, registry.SET_VALUE)
if err != nil {
return fmt.Errorf("opening %s: %w", keyStr, err)
}
defer key.Close()
if err := writeNRPTValues(key, strings.Join(servers, "; "), doms); err != nil {
return err
}
db.isGPDirty = db.writeAsGP
return nil
}
func readNRPTValues(key registry.Key) (servers string, doms []string, err error) {
doms, _, err = key.GetStringsValue(nrptRuleDomsName)
if err != nil {
return servers, doms, err
}
servers, _, err = key.GetStringValue(nrptRuleServersName)
return servers, doms, err
}
func writeNRPTValues(key registry.Key, servers string, doms []string) error {
if err := key.SetDWordValue(nrptRuleVersionName, 1); err != nil {
return err
}
if err := key.SetStringsValue(nrptRuleDomsName, doms); err != nil {
return err
}
if err := key.SetStringValue(nrptRuleServersName, servers); err != nil {
return err
}
return key.SetDWordValue(nrptRuleFlagsName, nrptOverrideDNS)
}
func (db *nrptRuleDatabase) watchForGPChanges() {
db.isGPRefreshPending.Store(false)
watchHandler := func() {
// Do not invoke detectWriteAsGP when we ourselves were responsible for
// initiating the group policy refresh.
if db.isGPRefreshPending.CompareAndSwap(true, false) {
return
}
db.logf("Computer group policies refreshed, reconfiguring NRPT rule database.")
db.detectWriteAsGP()
}
watcher, err := newGPNotificationWatcher(watchHandler)
if err != nil {
return
}
db.watcher = watcher
}
// movePolicies moves each NRPT rule depending on the value of writeAsGP.
// When writeAsGP is true, each NRPT rule is moved from the local NRPT table
// to the group policy NRPT table. When writeAsGP is false, the move is
// executed in the opposite direction. db.mu should already be locked.
func (db *nrptRuleDatabase) movePolicies(writeAsGP bool) {
// Since we're moving either in or out of the group policy NRPT table, we need
// to refresh once this movePolicies is done.
defer db.refreshLocked()
var fromBase string
var toBase string
if writeAsGP {
fromBase = nrptBaseLocal
toBase = nrptBaseGP
} else {
fromBase = nrptBaseGP
toBase = nrptBaseLocal
}
fromBase += `\`
toBase += `\`
for _, id := range db.ruleIDs {
fromStr := fromBase + id
toStr := toBase + id
if err := executeMove(fromStr, toStr); err != nil {
db.logf("movePolicies: executeMove(\"%s\", \"%s\") failed with error %v", fromStr, toStr, err)
return
}
db.isGPDirty = true
}
if writeAsGP {
return
}
// Now that we have moved our rules out of the group policy subkey, it should
// now be empty. Let's verify that.
isEmpty, err := isPolicyConfigSubkeyEmpty()
if err != nil {
db.logf("movePolicies: isPolicyConfigSubkeyEmpty error %v", err)
return
}
if !isEmpty {
db.logf("movePolicies: policy config subkey should be empty, but isn't!")
return
}
// Delete the subkey itself. Group policy will continue to override local
// settings unless we do so.
if err := registry.DeleteKey(registry.LOCAL_MACHINE, nrptBaseGP); err != nil {
db.logf("movePolicies DeleteKey error %v", err)
}
db.isGPDirty = true
}
func executeMove(subKeyFrom, subKeyTo string) error {
err := func() error {
// Move the NRPT registry values from subKeyFrom to subKeyTo.
fromKey, err := registry.OpenKey(registry.LOCAL_MACHINE, subKeyFrom, registry.QUERY_VALUE)
if err != nil {
return err
}
defer fromKey.Close()
toKey, _, err := registry.CreateKey(registry.LOCAL_MACHINE, subKeyTo, registry.WRITE)
if err != nil {
return err
}
defer toKey.Close()
servers, doms, err := readNRPTValues(fromKey)
if err != nil {
return err
}
return writeNRPTValues(toKey, servers, doms)
}()
if err != nil {
return err
}
// This is a move operation, so we must delete subKeyFrom.
return registry.DeleteKey(registry.LOCAL_MACHINE, subKeyFrom)
}
func (db *nrptRuleDatabase) Close() error {
if db.watcher == nil {
return nil
}
err := db.watcher.Close()
db.watcher = nil
return err
}
type gpNotificationWatcher struct {
gpWaitEvents [2]windows.Handle
handler func()
done chan struct{}
}
// newGPNotificationWatcher creates an instance of gpNotificationWatcher that
// invokes handler every time Windows notifies it of a group policy change.
func newGPNotificationWatcher(handler func()) (*gpNotificationWatcher, error) {
var err error
// evtDone is signaled by (*gpNotificationWatcher).Close() to indicate that
// the doWatch goroutine should exit.
evtDone, err := windows.CreateEvent(nil, 0, 0, nil)
if err != nil {
return nil, err
}
defer func() {
if err != nil {
windows.CloseHandle(evtDone)
}
}()
// evtChanged is registered with the Windows policy engine to become
// signalled any time group policy has been refreshed.
evtChanged, err := windows.CreateEvent(nil, 0, 0, nil)
if err != nil {
return nil, err
}
defer func() {
if err != nil {
windows.CloseHandle(evtChanged)
}
}()
// Tell Windows to signal evtChanged whenever group policies are refreshed.
ok, _, e := procRegisterGPNotification.Call(
uintptr(evtChanged),
uintptr(1), // Win32 TRUE: We want to monitor computer policy changes, not user policy changes.
)
if ok == 0 {
err = e
return nil, err
}
result := &gpNotificationWatcher{
// Ordering of the event handles in gpWaitEvents is important:
// When calling windows.WaitForMultipleObjects and multiple objects are
// signalled simultaneously, it always returns the wait code for the
// lowest-indexed handle in its input array. evtDone is higher priority for
// us than evtChanged, so the former must be placed into the array ahead of
// the latter.
gpWaitEvents: [2]windows.Handle{
evtDone,
evtChanged,
},
handler: handler,
done: make(chan struct{}),
}
go result.doWatch()
return result, nil
}
func (w *gpNotificationWatcher) doWatch() {
// The wait code corresponding to the event that is signalled when a group
// policy change occurs.
const expectedWaitCode = windows.WAIT_OBJECT_0 + 1
for {
if waitCode, _ := windows.WaitForMultipleObjects(w.gpWaitEvents[:], false, windows.INFINITE); waitCode != expectedWaitCode {
break
}
w.handler()
}
close(w.done)
}
func (w *gpNotificationWatcher) Close() error {
// Notify doWatch that we're done and it should exit.
if err := windows.SetEvent(w.gpWaitEvents[0]); err != nil {
return err
}
procUnregisterGPNotification.Call(uintptr(w.gpWaitEvents[1]))
// Wait for doWatch to complete.
<-w.done
// Now we may safely clean up all the things.
for i, evt := range w.gpWaitEvents {
windows.CloseHandle(evt)
w.gpWaitEvents[i] = 0
}
w.handler = nil
return nil
}