Xray-core/proxy/tun/tun_windows_wfp.go

//go:build windows

package tun

import (
	"net/netip"
	"os"
	"runtime"
	"slices"
	"strings"
	"unsafe"

	"github.com/xtls/xray-core/common/errors"
	"golang.org/x/sys/windows"
	"golang.zx2c4.com/wireguard/windows/tunnel/winipcfg"
)

var (
	modfwpuclnt = windows.NewLazySystemDLL("fwpuclnt.dll")
	moddnsapi   = windows.NewLazySystemDLL("dnsapi.dll")

	procFwpmEngineOpen0           = modfwpuclnt.NewProc("FwpmEngineOpen0")
	procFwpmEngineClose0          = modfwpuclnt.NewProc("FwpmEngineClose0")
	procFwpmTransactionBegin0     = modfwpuclnt.NewProc("FwpmTransactionBegin0")
	procFwpmTransactionCommit0    = modfwpuclnt.NewProc("FwpmTransactionCommit0")
	procFwpmTransactionAbort0     = modfwpuclnt.NewProc("FwpmTransactionAbort0")
	procFwpmSubLayerAdd0          = modfwpuclnt.NewProc("FwpmSubLayerAdd0")
	procFwpmFilterAdd0            = modfwpuclnt.NewProc("FwpmFilterAdd0")
	procFwpmGetAppIdFromFileName0 = modfwpuclnt.NewProc("FwpmGetAppIdFromFileName0")
	procFwpmFreeMemory0           = modfwpuclnt.NewProc("FwpmFreeMemory0")
	procDnsFlushResolverCache     = moddnsapi.NewProc("DnsFlushResolverCache")
)

// fwptypes.h and fwpmtypes.h
const (
	rpcCAuthnWinNT                 = 10 // RPC_C_AUTHN_WINNT
	fwpmSessionFlagDynamic         = 1  // FWPM_SESSION_FLAG_DYNAMIC
	fwpmFilterFlagClearActionRight = 8  // FWPM_FILTER_FLAG_CLEAR_ACTION_RIGHT

	fwpUint8                  = 1     // FWP_UINT8
	fwpUint16                 = 2     // FWP_UINT16
	fwpUint32                 = 3     // FWP_UINT32
	fwpUint64                 = 4     // FWP_UINT64
	fwpByteArray16Type        = 11    // FWP_BYTE_ARRAY16_TYPE
	fwpByteBlobType           = 12    // FWP_BYTE_BLOB_TYPE
	fwpSecurityDescriptorType = 14    // FWP_SECURITY_DESCRIPTOR_TYPE
	fwpRangeType              = 0x102 // FWP_RANGE_TYPE

	fwpMatchEqual       = 0 // FWP_MATCH_EQUAL
	fwpMatchRange       = 5 // FWP_MATCH_RANGE
	fwpMatchFlagsAllSet = 6 // FWP_MATCH_FLAGS_ALL_SET

	fwpConditionFlagIsLoopback = 1 // FWP_CONDITION_FLAG_IS_LOOPBACK

	fwpActionBlock  = 0x1001 // FWP_ACTION_BLOCK
	fwpActionPermit = 0x1002 // FWP_ACTION_PERMIT
)

// fwpmu.h
var (
	fwpmLayerALEAuthConnectV4    = windows.GUID{Data1: 0xc38d57d1, Data2: 0x05a7, Data3: 0x4c33, Data4: [8]byte{0x90, 0x4f, 0x7f, 0xbc, 0xee, 0xe6, 0x0e, 0x82}}
	fwpmLayerALEAuthConnectV6    = windows.GUID{Data1: 0x4a72393b, Data2: 0x319f, Data3: 0x44bc, Data4: [8]byte{0x84, 0xc3, 0xba, 0x54, 0xdc, 0xb3, 0xb6, 0xb4}}
	fwpmLayerALEAuthRecvAcceptV4 = windows.GUID{Data1: 0xe1cd9fe7, Data2: 0xf4b5, Data3: 0x4273, Data4: [8]byte{0x96, 0xc0, 0x59, 0x2e, 0x48, 0x7b, 0x86, 0x50}}
	fwpmLayerALEAuthRecvAcceptV6 = windows.GUID{Data1: 0xa3b42c97, Data2: 0x9f04, Data3: 0x4672, Data4: [8]byte{0xb8, 0x7e, 0xce, 0xe9, 0xc4, 0x83, 0x25, 0x7f}}

	fwpmConditionFlags              = windows.GUID{Data1: 0x632ce23b, Data2: 0x5167, Data3: 0x435c, Data4: [8]byte{0x86, 0xd7, 0xe9, 0x03, 0x68, 0x4a, 0xa8, 0x0c}}
	fwpmConditionIPArrivalInterface = windows.GUID{Data1: 0x618a9b6d, Data2: 0x386b, Data3: 0x4136, Data4: [8]byte{0xad, 0x6e, 0xb5, 0x15, 0x87, 0xcf, 0xb1, 0xcd}}
	fwpmConditionIPLocalInterface   = windows.GUID{Data1: 0x4cd62a49, Data2: 0x59c3, Data3: 0x4969, Data4: [8]byte{0xb7, 0xf3, 0xbd, 0xa5, 0xd3, 0x28, 0x90, 0xa4}}
	fwpmConditionIPLocalPort        = windows.GUID{Data1: 0x0c1ba1af, Data2: 0x5765, Data3: 0x453f, Data4: [8]byte{0xaf, 0x22, 0xa8, 0xf7, 0x91, 0xac, 0x77, 0x5b}} // also FWPM_CONDITION_ICMP_TYPE
	fwpmConditionIPNexthopInterface = windows.GUID{Data1: 0x93ae8f5b, Data2: 0x7f6f, Data3: 0x4719, Data4: [8]byte{0x98, 0xc8, 0x14, 0xe9, 0x74, 0x29, 0xef, 0x04}}
	fwpmConditionIPProtocol         = windows.GUID{Data1: 0x3971ef2b, Data2: 0x623e, Data3: 0x4f9a, Data4: [8]byte{0x8c, 0xb1, 0x6e, 0x79, 0xb8, 0x06, 0xb9, 0xa7}}
	fwpmConditionIPRemoteAddress    = windows.GUID{Data1: 0xb235ae9a, Data2: 0x1d64, Data3: 0x49b8, Data4: [8]byte{0xa4, 0x4c, 0x5f, 0xf3, 0xd9, 0x09, 0x50, 0x45}}
	fwpmConditionIPRemotePort       = windows.GUID{Data1: 0xc35a604d, Data2: 0xd22b, Data3: 0x4e1a, Data4: [8]byte{0x91, 0xb4, 0x68, 0xf6, 0x74, 0xee, 0x67, 0x4b}} // also FWPM_CONDITION_ICMP_CODE
	fwpmConditionALEAppID           = windows.GUID{Data1: 0xd78e1e87, Data2: 0x8644, Data3: 0x4ea5, Data4: [8]byte{0x94, 0x37, 0xd8, 0x09, 0xec, 0xef, 0xc9, 0x71}}
	fwpmConditionALEUserID          = windows.GUID{Data1: 0xaf043a0a, Data2: 0xb34d, Data3: 0x4f86, Data4: [8]byte{0x97, 0x9c, 0xc9, 0x03, 0x71, 0xaf, 0x6e, 0x66}}
)

// dnsClientSID is the SID of Windows' DNS Client service, NT SERVICE\Dnscache.
// Service SIDs derive from the service name, so it is the same everywhere (sc
// showsid dnscache).
const dnsClientSID = "S-1-5-80-859482183-879914841-863379149-1145462774-2388618682"

// ff02::1:2, where DHCPv6 clients send to. A package-level variable never
// moves, so conditions may refer to it through uintptr.
var ipv6AllDHCPv6Servers = [16]byte{0xff, 0x02, 13: 0x01, 15: 0x02}

type fwpByteBlob struct {
	size uint32
	data *byte
}

// fwpValue0 is FWP_VALUE0 as well as FWP_CONDITION_VALUE0. Their union holds
// a scalar of at most 32 bits, or a pointer for the larger types.
type fwpValue0 struct {
	typ   uint32
	value uintptr
}

// fwpRange0 is FWP_RANGE0, which an FWP_RANGE_TYPE condition points to.
type fwpRange0 struct {
	low, high fwpValue0
}

type fwpmDisplayData0 struct {
	name        *uint16
	description *uint16
}

type fwpmSession0 struct {
	sessionKey           windows.GUID
	displayData          fwpmDisplayData0
	flags                uint32
	txnWaitTimeoutInMSec uint32
	processID            uint32
	sid                  *windows.SID
	username             *uint16
	kernelMode           int32
}

type fwpmSublayer0 struct {
	subLayerKey  windows.GUID
	displayData  fwpmDisplayData0
	flags        uint32
	providerKey  *windows.GUID
	providerData fwpByteBlob
	weight       uint16
}

type fwpmFilterCondition0 struct {
	fieldKey       windows.GUID
	matchType      uint32
	conditionValue fwpValue0
}

type fwpmAction0 struct {
	typ        uint32
	filterType windows.GUID
}

type fwpmFilter0 struct {
	filterKey           windows.GUID
	displayData         fwpmDisplayData0
	flags               uint32
	providerKey         *windows.GUID
	providerData        fwpByteBlob
	layerKey            windows.GUID
	subLayerKey         windows.GUID
	weight              fwpValue0
	numFilterConditions uint32
	filterCondition     *fwpmFilterCondition0
	action              fwpmAction0
	_                   uint32 // C aligns the following union to 8 bytes, as it holds a UINT64
	providerContextKey  windows.GUID
	reserved            *windows.GUID
	_                   [8 - unsafe.Sizeof(uintptr(0))]byte // and filterId as well, also on 32-bit
	filterID            uint64
	effectiveWeight     fwpValue0
}

// fwpmResult converts the DWORD status the Fwpm functions return.
func fwpmResult(r1, _ uintptr, _ error) error {
	if r1 != 0 {
		return windows.Errno(r1)
	}
	return nil
}

func utf16Ptr(s string) *uint16 {
	p, _ := windows.UTF16PtrFromString(s)
	return p
}

func condition(field *windows.GUID, typ uint32, value uintptr) fwpmFilterCondition0 {
	return fwpmFilterCondition0{
		fieldKey:       *field,
		matchType:      fwpMatchEqual,
		conditionValue: fwpValue0{typ: typ, value: value},
	}
}

// leakFilters is what blockLeaks keeps from leaving outside the TUN.
type leakFilters struct {
	traffic    []leakTraffic // connections of these protocols and ports
	dnsClient  bool          // Windows' DNS Client service
	ipv4, ipv6 bool          // an IP version altogether
}

// newLeakFilters turns expanded autoSystemWfpBlockLeak values into the filters
// for a TUN that the routes of route4 (IPv4) and route6 (IPv6) lead to.
func newLeakFilters(leaks []string, route4, route6 bool) leakFilters {
	var block leakFilters
	for _, leak := range leaks {
		switch leak {
		case "windnsclient":
			block.dnsClient = dnsClientUsesAnyPort()
		case "misconfigtun":
			block.ipv4, block.ipv6 = block.ipv4 || !route4, block.ipv6 || !route6
		case "all":
			block.ipv4, block.ipv6 = true, true
		default:
			if traffic, ok := parseLeakTraffic(leak); ok {
				block.traffic = append(block.traffic, traffic)
			}
		}
	}
	return block
}

// String lists what block keeps inside the TUN, or is empty for nothing.
func (block leakFilters) String() string {
	var blocked []string
	for _, traffic := range block.traffic {
		blocked = append(blocked, traffic.String())
	}
	for _, b := range []struct {
		on   bool
		what string
	}{{block.dnsClient, "windnsclient"}, {block.ipv4, "IPv4"}, {block.ipv6, "IPv6"}} {
		if b.on {
			blocked = append(blocked, b.what)
		}
	}
	return strings.Join(blocked, ", ")
}

// systemDNS tells whether block keeps the system's DNS inside the TUN: port
// 53, the DNS Client service, or both IP versions altogether.
func (block leakFilters) systemDNS() bool {
	return block.dnsClient || block.ipv4 && block.ipv6 || slices.ContainsFunc(block.traffic, func(traffic leakTraffic) bool {
		return traffic.covers("udp", 53) || traffic.covers("tcp", 53)
	})
}

// dnsClientUsesAnyPort tells whether Windows' DNS Client service may send its
// queries over HTTPS or TLS, and so to any port, which it does since Windows 11
// and Server 2022 (build 20348). Earlier versions only query port 53, and may
// run the service in one process with others, which its filters would catch
// as well.
func dnsClientUsesAnyPort() bool {
	_, _, build := windows.RtlGetNtVersionNumbers()
	return build >= 20348
}

// blockLeaks keeps traffic from leaving through interfaces other than tun,
// for every program but Xray itself, whose outbounds (DNS included) use the
// other interfaces on purpose:
//
//   - traffic: connections of these protocols, TCP and UDP to some or all
//     ports, or ICMP, may only go through the TUN, those of DNS (53), DNS
//     over TLS (TCP 853) and DNS over QUIC (UDP 853) with "dns". Windows sends
//     a name query to the DNS servers of all interfaces, not only to those of
//     the TUN: to the first server of each interface, then to all of them when
//     no answer arrives within a second or two. It sends the queries for the
//     servers of an interface out through that interface, whatever the routes
//     say, and other programs reach an on-link resolver, like 192.168.1.1 from
//     DHCP, through its LAN route, which is more specific than the TUN's
//     default route.
//   - dnsClient: since Windows 11 and Server 2022, Windows may also send its
//     queries over HTTPS or TLS, to any port, so its DNS Client service may not
//     connect outside the TUN at all, except for name resolution on the local
//     link (mDNS, LLMNR).
//   - ipv4, ipv6: no IPv4, or no IPv6, at all outside the TUN, in either
//     direction: for a TUN that no route of that version leads to, or for
//     everything with "all". The TUN carries what is routed to it even without
//     an address of that IP version in gateway: Windows gives it link-local
//     ones itself, an IPv6 one at once, an IPv4 one from 169.254.0.0/16 after
//     some seconds (until then, IPv4 routed to the TUN is unreachable).
//
// Whatever is blocked, loopback, which cannot leave the system, and what
// Windows itself needs on the local link stay allowed: DHCP, and for IPv6
// neighbor and multicast listener discovery.
//
// The filters live in a dynamic WFP session: closing the returned engine handle
// with closeWFPEngine deletes them, and so does Windows when the process dies.
func blockLeaks(tun winipcfg.LUID, block leakFilters) (windows.Handle, error) {
	engine, err := openWFPEngine()
	if err != nil {
		return 0, err
	}
	if err := fwpmResult(procFwpmTransactionBegin0.Call(uintptr(engine), 0)); err != nil {
		closeWFPEngine(engine)
		return 0, errors.New("FwpmTransactionBegin0 failed").Base(err)
	}
	err = addLeakFilters(engine, tun, block)
	if err == nil {
		if err = fwpmResult(procFwpmTransactionCommit0.Call(uintptr(engine))); err != nil {
			err = errors.New("FwpmTransactionCommit0 failed").Base(err)
		}
	}
	if err != nil {
		procFwpmTransactionAbort0.Call(uintptr(engine))
		closeWFPEngine(engine)
		return 0, err
	}
	return engine, nil
}

func openWFPEngine() (windows.Handle, error) {
	if err := modfwpuclnt.Load(); err != nil {
		return 0, err
	}
	// txnWaitTimeoutInMSec stays 0 for BFE's default, so that a transaction
	// held by another program cannot hang the start forever.
	session := fwpmSession0{
		displayData: fwpmDisplayData0{name: utf16Ptr("Xray TUN")},
		flags:       fwpmSessionFlagDynamic,
	}
	var engine windows.Handle
	if err := fwpmResult(procFwpmEngineOpen0.Call(0, rpcCAuthnWinNT, 0, uintptr(unsafe.Pointer(&session)), uintptr(unsafe.Pointer(&engine)))); err != nil {
		return 0, errors.New("FwpmEngineOpen0 failed").Base(err)
	}
	return engine, nil
}

func closeWFPEngine(engine windows.Handle) {
	procFwpmEngineClose0.Call(uintptr(engine))
}

// addLeakFilters adds the filters of blockLeaks in a sublayer of their own.
// blockLeaks runs it in a transaction, so that they take effect all at once.
func addLeakFilters(engine windows.Handle, tun winipcfg.LUID, block leakFilters) error {
	exe, err := os.Executable()
	if err != nil {
		return err
	}
	exePath, err := windows.UTF16PtrFromString(exe)
	if err != nil {
		return err
	}
	var appID *fwpByteBlob
	if err := fwpmResult(procFwpmGetAppIdFromFileName0.Call(uintptr(unsafe.Pointer(exePath)), uintptr(unsafe.Pointer(&appID)))); err != nil {
		return errors.New("FwpmGetAppIdFromFileName0 failed for ", exe).Base(err)
	}
	defer func() { procFwpmFreeMemory0.Call(uintptr(unsafe.Pointer(&appID))) }()

	sublayer := fwpmSublayer0{
		displayData: fwpmDisplayData0{name: utf16Ptr("Xray TUN")},
		weight:      0xffff,
	}
	if sublayer.subLayerKey, err = windows.GenerateGUID(); err != nil {
		return err
	}
	if err := fwpmResult(procFwpmSubLayerAdd0.Call(uintptr(engine), uintptr(unsafe.Pointer(&sublayer)), 0)); err != nil {
		return errors.New("FwpmSubLayerAdd0 failed").Base(err)
	}
	add := func(layer *windows.GUID, name string, flags, action uint32, weight uint8, conditions ...fwpmFilterCondition0) error {
		return addFilter(engine, &sublayer.subLayerKey, layer, "Xray TUN: "+name, flags, action, weight, conditions...)
	}

	var pinner runtime.Pinner
	defer pinner.Unpin()
	tunLUID := new(uint64)
	*tunLUID = uint64(tun)
	pinner.Pin(tunLUID) // the condition only holds it as uintptr

	// The heaviest matching filter of a sublayer decides. All sublayers have
	// their say, though, and a block in any of them beats a permit, unless
	// the permit is hard: it clears the action right, and then the blocks of
	// lower sublayers, Windows Firewall rules among them, no longer override
	// it, only a callout's veto does. Xray's own connections out get such a
	// hard permit. Connections from outside to Xray get an ordinary one, so
	// that firewalls keep guarding its inbounds.
	self := condition(&fwpmConditionALEAppID, fwpByteBlobType, uintptr(unsafe.Pointer(appID)))
	// A connection goes through the TUN when its local address is the TUN's,
	// and it also leaves, or arrives, through the TUN. The local address alone
	// decides by default, but with weak host sending or receiving enabled,
	// packets of the TUN's address can use other interfaces. (The next hop,
	// the interface replies would leave by, is not known for arriving ones.)
	onTUN := func(field *windows.GUID) fwpmFilterCondition0 {
		return condition(field, fwpUint64, uintptr(unsafe.Pointer(tunLUID)))
	}
	// toPorts matches the ports of traffic, if not all of them.
	toPorts := func(traffic leakTraffic) []fwpmFilterCondition0 {
		switch {
		case traffic.low == 0:
			return nil
		case traffic.low == traffic.high:
			return []fwpmFilterCondition0{condition(&fwpmConditionIPRemotePort, fwpUint16, uintptr(traffic.low))}
		}
		ports := &fwpRange0{
			low:  fwpValue0{typ: fwpUint16, value: uintptr(traffic.low)},
			high: fwpValue0{typ: fwpUint16, value: uintptr(traffic.high)},
		}
		pinner.Pin(ports) // the condition only holds it as uintptr
		return []fwpmFilterCondition0{{
			fieldKey:       fwpmConditionIPRemotePort,
			matchType:      fwpMatchRange,
			conditionValue: fwpValue0{typ: fwpRangeType, value: uintptr(unsafe.Pointer(ports))},
		}}
	}

	// What stays allowed whatever is blocked, in both directions: replies to
	// a connection accepted from outside would leave through the physical
	// link as well.
	type permit struct {
		name       string
		conditions []fwpmFilterCondition0
	}
	loopback := permit{"loopback", []fwpmFilterCondition0{{
		fieldKey:       fwpmConditionFlags,
		matchType:      fwpMatchFlagsAllSet,
		conditionValue: fwpValue0{typ: fwpUint32, value: fwpConditionFlagIsLoopback},
	}}}
	// DHCP keeps the addresses of the other interfaces, which Xray's own
	// connections use.
	dhcp := permit{"DHCP", []fwpmFilterCondition0{
		condition(&fwpmConditionIPProtocol, fwpUint8, windows.IPPROTO_UDP),
		condition(&fwpmConditionIPLocalPort, fwpUint16, 68),
		condition(&fwpmConditionIPRemotePort, fwpUint16, 67),
	}}
	// Neighbor and multicast listener discovery, ICMPv6 130-137 and 143,
	// whose type and code sit where the local and remote port are.
	discovery := permit{"IPv6 neighbor and multicast listener discovery", []fwpmFilterCondition0{condition(&fwpmConditionIPProtocol, fwpUint8, windows.IPPROTO_ICMPV6)}}
	for _, typ := range []uintptr{130, 131, 132, 133, 134, 135, 136, 137, 143} {
		discovery.conditions = append(discovery.conditions, condition(&fwpmConditionIPLocalPort, fwpUint16, typ))
	}
	discovery.conditions = append(discovery.conditions, condition(&fwpmConditionIPRemotePort, fwpUint16, 0))
	dhcpv6 := []fwpmFilterCondition0{
		condition(&fwpmConditionIPProtocol, fwpUint8, windows.IPPROTO_UDP),
		condition(&fwpmConditionIPLocalPort, fwpUint16, 546),
		condition(&fwpmConditionIPRemotePort, fwpUint16, 547),
	}
	// The DHCPv6 client sends to the servers' multicast address, and they
	// answer from their own.
	dhcpv6Out := permit{"DHCPv6", slices.Concat(dhcpv6, []fwpmFilterCondition0{condition(&fwpmConditionIPRemoteAddress, fwpByteArray16Type, uintptr(unsafe.Pointer(&ipv6AllDHCPv6Servers)))})}
	dhcpv6In := permit{"DHCPv6", dhcpv6}

	for _, layer := range []struct {
		key       *windows.GUID
		selfFlags uint32
		via       *windows.GUID // the interface that has to be the TUN as well
		ipv6      bool
		permits   []permit
	}{
		{&fwpmLayerALEAuthConnectV4, fwpmFilterFlagClearActionRight, &fwpmConditionIPNexthopInterface, false, []permit{loopback, dhcp}},
		{&fwpmLayerALEAuthRecvAcceptV4, 0, &fwpmConditionIPArrivalInterface, false, []permit{loopback, dhcp}},
		{&fwpmLayerALEAuthConnectV6, fwpmFilterFlagClearActionRight, &fwpmConditionIPNexthopInterface, true, []permit{loopback, discovery, dhcpv6Out}},
		{&fwpmLayerALEAuthRecvAcceptV6, 0, &fwpmConditionIPArrivalInterface, true, []permit{loopback, discovery, dhcpv6In}},
	} {
		throughTUN := []fwpmFilterCondition0{onTUN(&fwpmConditionIPLocalInterface), onTUN(layer.via)}
		if err := add(layer.key, "permit Xray", layer.selfFlags, fwpActionPermit, 4, self); err != nil {
			return err
		}
		for _, p := range layer.permits {
			if err := add(layer.key, "permit "+p.name, 0, fwpActionPermit, 3, p.conditions...); err != nil {
				return err
			}
		}
		for _, traffic := range block.traffic {
			protocol := uintptr(windows.IPPROTO_UDP)
			switch {
			case traffic.protocol == "tcp":
				protocol = windows.IPPROTO_TCP
			case traffic.protocol == "icmp" && layer.ipv6:
				protocol = windows.IPPROTO_ICMPV6
			case traffic.protocol == "icmp":
				protocol = windows.IPPROTO_ICMP
			}
			matches := slices.Concat([]fwpmFilterCondition0{condition(&fwpmConditionIPProtocol, fwpUint8, protocol)}, toPorts(traffic))
			if err := add(layer.key, "permit "+traffic.String()+" through the TUN", 0, fwpActionPermit, 3, slices.Concat(matches, throughTUN)...); err != nil {
				return err
			}
			if err := add(layer.key, "block "+traffic.String(), 0, fwpActionBlock, 2, matches...); err != nil {
				return err
			}
		}
		if family := "IPv4"; layer.ipv6 && block.ipv6 || !layer.ipv6 && block.ipv4 {
			if layer.ipv6 {
				family = "IPv6"
			}
			if err := add(layer.key, "permit "+family+" through the TUN", 0, fwpActionPermit, 1, throughTUN...); err != nil {
				return err
			}
			if err := add(layer.key, "block "+family, 0, fwpActionBlock, 0); err != nil {
				return err
			}
		}
	}

	// Since Windows 11 and Server 2022, the DNS Client service may also send
	// the queries for an interface's servers over HTTPS or TLS, out through
	// that interface and to any port. So there it may only connect through
	// the TUN, except for mDNS and LLMNR, which stay on the local link (over
	// an IP version only while it is not blocked altogether). Like Windows
	// Firewall's rules for it, the filters recognize the service by its SID,
	// which Windows puts in the token of its process: the security descriptor
	// grants that SID the right to match (FWP_ACTRL_MATCH_FILTER, CC in SDDL).
	if block.dnsClient {
		sd, err := windows.SecurityDescriptorFromString("O:SYG:SYD:(A;;CCRC;;;" + dnsClientSID + ")")
		if err != nil {
			return err
		}
		sdBlob := &fwpByteBlob{size: sd.Length(), data: (*byte)(unsafe.Pointer(sd))}
		pinner.Pin(sdBlob) // the condition only holds it as uintptr
		dnsClient := condition(&fwpmConditionALEUserID, fwpSecurityDescriptorType, uintptr(unsafe.Pointer(sdBlob)))
		// Conditions on the same field match when any of them does.
		mdnsLLMNR := []fwpmFilterCondition0{dnsClient, condition(&fwpmConditionIPRemotePort, fwpUint16, 5353), condition(&fwpmConditionIPRemotePort, fwpUint16, 5355)}
		for _, layer := range []struct {
			key       *windows.GUID
			localLink bool
		}{
			{&fwpmLayerALEAuthConnectV4, !block.ipv4},
			{&fwpmLayerALEAuthConnectV6, !block.ipv6},
		} {
			if err := add(layer.key, "permit the DNS Client service through the TUN", 0, fwpActionPermit, 3, dnsClient, onTUN(&fwpmConditionIPLocalInterface), onTUN(&fwpmConditionIPNexthopInterface)); err != nil {
				return err
			}
			if layer.localLink {
				if err := add(layer.key, "permit the DNS Client service's mDNS and LLMNR", 0, fwpActionPermit, 3, mdnsLLMNR...); err != nil {
					return err
				}
			}
			if err := add(layer.key, "block the DNS Client service", 0, fwpActionBlock, 2, dnsClient); err != nil {
				return err
			}
		}
	}
	return nil
}

func addFilter(engine windows.Handle, sublayer, layer *windows.GUID, name string, flags, action uint32, weight uint8, conditions ...fwpmFilterCondition0) error {
	filter := fwpmFilter0{
		displayData:         fwpmDisplayData0{name: utf16Ptr(name)},
		flags:               flags,
		layerKey:            *layer,
		subLayerKey:         *sublayer,
		weight:              fwpValue0{typ: fwpUint8, value: uintptr(weight)},
		numFilterConditions: uint32(len(conditions)),
		action:              fwpmAction0{typ: action},
	}
	if len(conditions) > 0 {
		filter.filterCondition = &conditions[0]
	}
	if err := fwpmResult(procFwpmFilterAdd0.Call(uintptr(engine), uintptr(unsafe.Pointer(&filter)), 0, 0)); err != nil {
		return errors.New("FwpmFilterAdd0 failed for ", name).Base(err)
	}
	return nil
}

// dnsOutsideTUN returns the servers outside all of prefixes, the TUN's own
// subnets and routes: queries to them cannot go through the TUN.
func dnsOutsideTUN(servers []netip.Addr, prefixes []netip.Prefix) []netip.Addr {
	var outside []netip.Addr
	for _, server := range servers {
		server = server.Unmap()
		if !slices.ContainsFunc(prefixes, func(p netip.Prefix) bool { return p.Contains(server) }) {
			outside = append(outside, server)
		}
	}
	return outside
}

// flushDNSCache drops the answers Windows cached so far, like ipconfig
// /flushdns, so that names get resolved again with the current DNS setup.
func flushDNSCache() error {
	if err := procDnsFlushResolverCache.Find(); err != nil {
		return err
	}
	if r, _, err := procDnsFlushResolverCache.Call(); r == 0 {
		return err
	}
	return nil
}