// Copyright (c) Tailscale Inc & contributors
// SPDX-License-Identifier: BSD-3-Clause

package dns

import (
	"bufio"
	"bytes"
	"errors"
	"fmt"
	"io/fs"
	"maps"
	"net/netip"
	"os"
	"os/exec"
	"path/filepath"
	"slices"
	"sort"
	"strings"
	"syscall"
	"time"

	"golang.org/x/sys/windows"
	"golang.org/x/sys/windows/registry"
	"golang.zx2c4.com/wireguard/windows/tunnel/winipcfg"
	"tailscale.com/atomicfile"
	"tailscale.com/control/controlknobs"
	"tailscale.com/envknob"
	"tailscale.com/health"
	"tailscale.com/syncs"
	"tailscale.com/types/logger"
	"tailscale.com/util/dnsname"
	"tailscale.com/util/eventbus"
	"tailscale.com/util/syspolicy/pkey"
	"tailscale.com/util/syspolicy/policyclient"
	"tailscale.com/util/syspolicy/ptype"
	"tailscale.com/util/winutil"
	"tailscale.com/util/winutil/winenv"
)

const (
	versionKey = `SOFTWARE\Microsoft\Windows NT\CurrentVersion`
)

var configureWSL = envknob.RegisterBool("TS_DEBUG_CONFIGURE_WSL")

type windowsManager struct {
	logf       logger.Logf
	guid       string
	knobs      *controlknobs.Knobs // or nil
	nrptDB     *nrptRuleDatabase
	wslManager *wslManager
	polc       policyclient.Client

	unregisterPolicyChangeCb func() // called when the manager is closing

	mu      syncs.Mutex
	closing bool
}

// NewOSConfigurator created a new OS configurator.
//
// The health tracker, eventbus and the knobs may be nil.
func NewOSConfigurator(logf logger.Logf, health *health.Tracker, bus *eventbus.Bus, polc policyclient.Client, knobs *controlknobs.Knobs, interfaceName string) (OSConfigurator, error) {
	if polc == nil {
		panic("nil policyclient.Client")
	}
	ret := &windowsManager{
		logf:       logf,
		guid:       interfaceName,
		knobs:      knobs,
		polc:       polc,
		wslManager: newWSLManager(logf, health),
	}

	if isWindows10OrBetter() {
		ret.nrptDB = newNRPTRuleDatabase(logf)
	}

	var err error
	if ret.unregisterPolicyChangeCb, err = polc.RegisterChangeCallback("", ret.sysPolicyChanged); err != nil {
		logf("error registering policy change callback: %v", err) // non-fatal
	}

	go func() {
		// 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
}

func (m *windowsManager) openInterfaceKey(pfx winutil.RegistryPathPrefix) (registry.Key, error) {
	var key registry.Key
	var err error
	path := pfx.WithSuffix(m.guid)

	m.mu.Lock()
	closing := m.closing
	m.mu.Unlock()
	if closing {
		// Do not wait for the interface key to appear if the manager is being closed.
		// If it's being closed due to the removal of the wintun adapter,
		// the key would already be gone by now and will not reappear until tailscaled is restarted.
		key, err = registry.OpenKey(registry.LOCAL_MACHINE, string(path), registry.SET_VALUE)
	} else {
		key, err = winutil.OpenKeyWait(registry.LOCAL_MACHINE, path, registry.SET_VALUE)
	}
	if err != nil {
		return 0, fmt.Errorf("opening %s: %w", path, err)
	}
	return key, nil
}

func (m *windowsManager) muteKeyNotFoundIfClosing(err error) error {
	m.mu.Lock()
	defer m.mu.Unlock()
	if !m.closing || (!errors.Is(err, windows.ERROR_FILE_NOT_FOUND) && !errors.Is(err, windows.ERROR_PATH_NOT_FOUND)) {
		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 one or more NRPT (Name Resolution Policy Table) rules
// to resolve queries for domains using resolvers, rather than the
// system's "primary" resolver.
//
// If no resolvers are provided, the Tailscale NRPT rules are deleted.
func (m *windowsManager) setSplitDNS(resolvers []netip.Addr, domains []dnsname.FQDN) error {
	if m.nrptDB == nil {
		if resolvers == nil {
			// Just a no-op in this case.
			return nil
		}
		return fmt.Errorf("Split DNS unsupported on this Windows version")
	}

	defer m.nrptDB.NotifyPolicyChanged()
	if len(resolvers) == 0 {
		return m.nrptDB.DelAllRuleKeys()
	}

	servers := make([]string, 0, len(resolvers))
	for _, resolver := range resolvers {
		servers = append(servers, resolver.String())
	}

	return m.nrptDB.WriteSplitDNSConfig(servers, domains)
}

func setTailscaleHosts(logf logger.Logf, prevHostsFile []byte, hosts []*HostEntry) ([]byte, error) {
	sc := bufio.NewScanner(bytes.NewReader(prevHostsFile))
	const (
		header = "# TailscaleHostsSectionStart"
		footer = "# TailscaleHostsSectionEnd"
	)
	comments := []string{
		"# This section contains MagicDNS entries for Tailscale.",
		"# Do not edit this section manually.",
	}

	prevEntries := make(map[netip.Addr][]string)
	addPrevEntry := func(line string) {
		if line == "" || line[0] == '#' {
			return
		}

		parts := strings.Split(line, " ")
		if len(parts) < 1 {
			return
		}

		addr, err := netip.ParseAddr(parts[0])
		if err != nil {
			logf("Parsing address from hosts: %v", err)
			return
		}

		prevEntries[addr] = parts[1:]
	}

	nextEntries := make(map[netip.Addr][]string, len(hosts))
	for _, he := range hosts {
		nextEntries[he.Addr] = he.Hosts
	}

	var out bytes.Buffer
	var inSection bool
	for sc.Scan() {
		line := strings.TrimSpace(sc.Text())
		if line == header {
			inSection = true
			continue
		}
		if line == footer {
			inSection = false
			continue
		}
		if inSection {
			addPrevEntry(line)
			continue
		}
		fmt.Fprintf(&out, "%s\r\n", line)
	}
	if err := sc.Err(); err != nil {
		return nil, err
	}

	unchanged := maps.EqualFunc(prevEntries, nextEntries, func(a, b []string) bool {
		return slices.Equal(a, b)
	})
	if unchanged {
		return nil, nil
	}

	if len(hosts) > 0 {
		fmt.Fprintf(&out, "%s\r\n", header)
		for _, c := range comments {
			fmt.Fprintf(&out, "%s\r\n", c)
		}
		fmt.Fprintf(&out, "\r\n")
		for _, he := range hosts {
			fmt.Fprintf(&out, "%s %s\r\n", he.Addr, strings.Join(he.Hosts, " "))
		}
		fmt.Fprintf(&out, "\r\n%s\r\n", footer)
	}
	return out.Bytes(), nil
}

// setHosts sets the hosts file to contain the given host entries.
func (m *windowsManager) setHosts(hosts []*HostEntry) error {
	systemDir, err := windows.GetSystemDirectory()
	if err != nil {
		return err
	}
	hostsFile := filepath.Join(systemDir, "drivers", "etc", "hosts")
	b, err := os.ReadFile(hostsFile)
	switch {
	case err == nil:
		// Continue.
	case errors.Is(err, fs.ErrNotExist):
		// Non-fatal, we'll just create a new hosts file.
		m.logf("failed to read the hosts file: %v", err)
	default:
		return err
	}
	outB, err := setTailscaleHosts(m.logf, b, hosts)
	if err != nil {
		return err
	}
	if outB == nil {
		// No change to hosts file, therefore no write necessary.
		return nil
	}

	const fileMode = 0 // ignored on windows.

	// This can fail spuriously with an access denied error, so retry it a
	// few times.
	for range 5 {
		if err = atomicfile.WriteFile(hostsFile, outB, fileMode); err == nil {
			return nil
		}
		time.Sleep(10 * time.Millisecond)
	}
	return err
}

// 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 []netip.Addr, 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.openInterfaceKey(winutil.IPv4TCPIPInterfacePrefix)
	if err != nil {
		return m.muteKeyNotFoundIfClosing(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.openInterfaceKey(winutil.IPv6TCPIPInterfacePrefix)
	if err != nil {
		return m.muteKeyNotFoundIfClosing(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. It is
	// being deprecated.
	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) disableLocalDNSOverrideViaNRPT() bool {
	return m.knobs != nil && m.knobs.DisableLocalDNSOverrideViaNRPT.Load()
}

func (m *windowsManager) disableHostsFileUpdates() bool {
	return m.knobs != nil && m.knobs.DisableHostsFileUpdates.Load()
}

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.

	// Reconfigure DNS registration according to the [syspolicy.DNSRegistration]
	// policy setting, and unconditionally disable NetBIOS on our interfaces.
	m.reconfigureDNSRegistration()
	if err := m.disableNetBIOS(); err != nil {
		m.logf("disableNetBIOS error: %v\n", err)
	}

	if len(cfg.MatchDomains) == 0 {
		var resolvers []netip.Addr
		var domains []dnsname.FQDN
		if !m.disableLocalDNSOverrideViaNRPT() {
			// Create a default catch-all rule to make ourselves the actual primary resolver.
			// Without this rule, Windows 8.1 and newer devices issue parallel DNS requests to DNS servers
			// associated with all network adapters, even when "Override local DNS" is enabled and/or
			// a Mullvad exit node is being used, resulting in DNS leaks.
			resolvers = cfg.Nameservers
			domains = []dnsname.FQDN{"."}
		}
		if err := m.setSplitDNS(resolvers, domains); err != nil {
			return err
		}
		var hosts []*HostEntry
		if !m.disableHostsFileUpdates() && winenv.IsDomainJoined() {
			// On domain-joined Windows devices the primary search domain (the one the device is joined to)
			// always takes precedence over other search domains. This breaks MagicDNS when we are the primary
			// resolver on the device (see #18712). To work around this Windows behavior, we should write MagicDNS
			// host names the hosts file just as we do when we're not the primary resolver.
			hosts = cfg.Hosts
		}
		if err := m.setHosts(hosts); err != nil {
			return err
		}
		if err := m.setPrimaryDNS(cfg.Nameservers, cfg.SearchDomains); err != nil {
			return err
		}
	} else {
		if err := m.setSplitDNS(cfg.Nameservers, cfg.MatchDomains); err != nil {
			return err
		}
		// Unset the resolver on the interface to ensure that we do not become
		// the primary resolver. Although this is what we want, at the moment
		// (2022-08-13) it causes single label resolutions from the OS resolver
		// to wait for a MDNS response from the Tailscale interface.
		// See #1659 and #5366 for more details.
		//
		// 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
		}

		if !m.disableHostsFileUpdates() {
			// As we are not the primary resolver in this setup, we need to
			// explicitly set some single name hosts to ensure that we can resolve
			// them quickly and get around the 2.3s delay that otherwise occurs due
			// to multicast timeouts.
			if err := m.setHosts(cfg.Hosts); 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{
			CreationFlags: windows.DETACHED_PROCESS,
		}
		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{
			CreationFlags: windows.DETACHED_PROCESS,
		}
		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.nrptDB != nil
}

func (m *windowsManager) Close() error {
	m.mu.Lock()
	if m.closing {
		m.mu.Unlock()
		return nil
	}
	m.closing = true
	m.mu.Unlock()

	if m.unregisterPolicyChangeCb != nil {
		m.unregisterPolicyChangeCb()
	}

	err := m.SetDNS(OSConfig{})
	if m.nrptDB != nil {
		m.nrptDB.Close()
		m.nrptDB = nil
	}
	return err
}

// sysPolicyChanged is a callback triggered by [syspolicy] when it detects
// a change in one or more syspolicy settings.
func (m *windowsManager) sysPolicyChanged(policy policyclient.PolicyChange) {
	if policy.HasChanged(pkey.EnableDNSRegistration) {
		m.reconfigureDNSRegistration()
	}
}

// reconfigureDNSRegistration configures the DNS registration settings
// using the [syspolicy.DNSRegistration] policy setting, if it is set.
// If the policy is not configured, it disables DNS registration.
func (m *windowsManager) reconfigureDNSRegistration() {
	// Disable DNS registration by default (if the policy setting is not configured).
	// This is primarily for historical reasons and to avoid breaking existing
	// setups that rely on this behavior.
	enableDNSRegistration, err := m.polc.GetPreferenceOption(pkey.EnableDNSRegistration, ptype.NeverByPolicy)
	if err != nil {
		m.logf("error getting DNSRegistration policy setting: %v", err) // non-fatal; we'll use the default
	}

	if enableDNSRegistration.Show() {
		// "Show" reports whether the policy setting is configured as "user-decides".
		// The name is a bit unfortunate in this context, as we don't actually "show" anything.
		// Still, if the admin configured the policy as "user-decides", we shouldn't modify
		// the adapter's settings and should leave them up to the user (admin rights required)
		// or the system defaults.
		return
	}

	// Otherwise, if the policy setting is configured as "always" or "never",
	// we should configure the adapter accordingly.
	if err := m.configureDNSRegistration(enableDNSRegistration.IsAlways()); err != nil {
		m.logf("error configuring DNS registration: %v", err)
	}
}

// configureDNSRegistration sets the appropriate registry values to allow or prevent
// the Windows DHCP client from registering Tailscale IP addresses with DNS
// and sending dynamic updates for our interface to AD domain controllers.
func (m *windowsManager) configureDNSRegistration(enabled bool) error {
	prefixen := []winutil.RegistryPathPrefix{
		winutil.IPv4TCPIPInterfacePrefix,
		winutil.IPv6TCPIPInterfacePrefix,
	}

	var (
		registrationEnabled            = uint32(0)
		disableDynamicUpdate           = uint32(1)
		maxNumberOfAddressesToRegister = uint32(0)
	)
	if enabled {
		registrationEnabled = 1
		disableDynamicUpdate = 0
		maxNumberOfAddressesToRegister = 1
	}

	for _, prefix := range prefixen {
		k, err := m.openInterfaceKey(prefix)
		if err != nil {
			return m.muteKeyNotFoundIfClosing(err)
		}
		defer k.Close()

		if err := k.SetDWordValue("RegistrationEnabled", registrationEnabled); err != nil {
			return err
		}
		if err := k.SetDWordValue("DisableDynamicUpdate", disableDynamicUpdate); err != nil {
			return err
		}
		if err := k.SetDWordValue("MaxNumberOfAddressesToRegister", maxNumberOfAddressesToRegister); err != nil {
			return err
		}
	}
	return nil
}

// setSingleDWORD opens the Registry Key in HKLM for the interface associated
// with the windowsManager and sets the "keyPrefix\value" to data.
func (m *windowsManager) setSingleDWORD(prefix winutil.RegistryPathPrefix, value string, data uint32) error {
	k, err := m.openInterfaceKey(prefix)
	if err != nil {
		return m.muteKeyNotFoundIfClosing(err)
	}
	defer k.Close()
	return k.SetDWordValue(value, data)
}

// disableNetBIOS sets the appropriate registry values to prevent Windows from
// sending NetBIOS name resolution requests for our interface which we do not
// handle nor want to. By leaving it enabled and not handling it we introduce
// short-name resolution delays in certain conditions as Windows waits for
// NetBIOS responses from our interface (#1659).
//
// Further, LLMNR and NetBIOS are being deprecated anyway in favor of MDNS.
// https://techcommunity.microsoft.com/t5/networking-blog/aligning-on-mdns-ramping-down-netbios-name-resolution-and-llmnr/ba-p/3290816
func (m *windowsManager) disableNetBIOS() error {
	return m.setSingleDWORD(winutil.NetBTInterfacePrefix, "NetbiosOptions", 2)
}

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 []netip.Addr, 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 _, ip := range ips {
			ip = ip.Unmap()
			// 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 = []netip.Addr{
	netip.MustParseAddr("fec0:0:0:ffff::1"),
	netip.MustParseAddr("fec0:0:0:ffff::2"),
	netip.MustParseAddr("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
}
