XDNS finalmask: Support AAAA & A (#6123)

https://github.com/XTLS/Xray-core/pull/6123#issuecomment-4439994266 https://github.com/XTLS/Xray-core/pull/6123#issuecomment-4441819696 https://github.com/XTLS/Xray-core/pull/6123#issuecomment-4522352460 Example: https://github.com/XTLS/Xray-core/pull/6123#issue-4436283689 Document: https://xtls.github.io/config/transports/finalmask.html#xdns
2026-06-08 14:11:54 +00:00 · 2026-05-23 17:24:37 +08:00
parent da9ba693cb
commit 359a28f876
6 changed files with 493 additions and 137 deletions
@@ -10,7 +10,6 @@ import (
 	"io"
 	"net"
 	"strconv"
-	"strings"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -40,6 +39,7 @@ type xdnsConnClient struct {
 	net.PacketConn

 	resolverAddrs []*net.UDPAddr
+	resolverTypes []uint16
 	resolverIdx   uint32
 	resolverSend  map[string]*atomic.Uint32

@@ -61,17 +61,15 @@ func NewConnClient(c *Config, raw net.PacketConn) (net.PacketConn, error) {

 	var domains []Name
 	var servers []string
+	var resolverTypes []uint16
 	for _, rs := range c.Resolvers {
-		parts := strings.Split(rs, "+udp://")
-		if len(parts) != 2 {
-			return nil, errors.New("invalid resolvers")
-		}
-		domain, err := ParseName(parts[0])
+		domain, server, resolverType, err := parseResolver(rs)
 		if err != nil {
-			return nil, err
+			return nil, errors.New("invalid resolvers").Base(err)
 		}
 		domains = append(domains, domain)
-		servers = append(servers, parts[1])
+		servers = append(servers, server)
+		resolverTypes = append(resolverTypes, resolverType)
 	}

 	var resolverAddrs []*net.UDPAddr
@@ -98,6 +96,7 @@ func NewConnClient(c *Config, raw net.PacketConn) (net.PacketConn, error) {
 		PacketConn: raw,

 		resolverAddrs: resolverAddrs,
+		resolverTypes: resolverTypes,
 		resolverIdx:   0,
 		resolverSend:  resolverSend,

@@ -216,7 +215,7 @@ func (c *xdnsConnClient) sendLoop() {
 			default:
 			}
 		} else {
-			encoded, _ := encode(nil, c.clientID, c.domains[c.resolverIdx])
+			encoded, _ := encode(nil, c.clientID, c.domains[c.resolverIdx], c.resolverTypes[c.resolverIdx])
 			p = &packet{
 				p: encoded,
 			}
@@ -276,7 +275,8 @@ func (c *xdnsConnClient) WriteTo(p []byte, addr net.Addr) (n int, err error) {
 		return 0, io.ErrClosedPipe
 	}

-	encoded, err := encode(p, c.clientID, c.domains[c.resolverIdx%uint32(len(c.resolverAddrs))])
+	idx := c.resolverIdx % uint32(len(c.resolverAddrs))
+	encoded, err := encode(p, c.clientID, c.domains[idx], c.resolverTypes[idx])
 	if err != nil {
 		errors.LogDebug(context.Background(), addr, " xdns wireformat err ", err, " ", len(p))
 		return 0, nil
@@ -299,7 +299,7 @@ func (c *xdnsConnClient) Close() error {
 	return c.PacketConn.Close()
 }

-func encode(p []byte, clientID []byte, domain Name) ([]byte, error) {
+func encode(p []byte, clientID []byte, domain Name, qtype uint16) ([]byte, error) {
 	var decoded []byte
 	{
 		if len(p) >= 224 {
@@ -338,7 +338,7 @@ func encode(p []byte, clientID []byte, domain Name) ([]byte, error) {
 		Question: []Question{
 			{
 				Name:  name,
-				Type:  RRTypeTXT,
+				Type:  qtype,
 				Class: ClassIN,
 			},
 		},
@@ -396,29 +396,22 @@ func dnsResponsePayload(resp *Message, domains []Name) []byte {
 		return nil
 	}

-	if len(resp.Answer) != 1 {
+	if len(resp.Answer) == 0 {
 		return nil
 	}
-	answer := resp.Answer[0]

-	var ok bool
-	for _, domain := range domains {
-		_, ok = answer.Name.TrimSuffix(domain)
-		if ok {
-			break
+	for _, answer := range resp.Answer {
+		var ok bool
+		for _, domain := range domains {
+			_, ok = answer.Name.TrimSuffix(domain)
+			if ok {
+				break
+			}
+		}
+		if !ok {
+			return nil
 		}
 	}
-	if !ok {
-		return nil
-	}

-	if answer.Type != RRTypeTXT {
-		return nil
-	}
-	payload, err := DecodeRDataTXT(answer.Data)
-	if err != nil {
-		return nil
-	}
-
-	return payload
+	return decodeResponsePayload(resp.Answer)
 }
@@ -45,8 +45,14 @@ var (
 )

 const (
+	// https://tools.ietf.org/html/rfc1035#section-3.2.2
+	RRTypeA = 1
+	// https://tools.ietf.org/html/rfc1035#section-3.2.2
+	RRTypeCNAME = 5
 	// https://tools.ietf.org/html/rfc1035#section-3.2.2
 	RRTypeTXT = 16
+	// https://tools.ietf.org/html/rfc3596#section-2.1
+	RRTypeAAAA = 28
 	// https://tools.ietf.org/html/rfc6891#section-6.1.1
 	RRTypeOPT = 41

@@ -593,3 +593,113 @@ func TestRDataTXTRoundTrip(t *testing.T) {
 		}
 	}
 }
+
+func TestIPAnswerPayloadRoundTrip(t *testing.T) {
+	for _, rrType := range []uint16{RRTypeA, RRTypeAAAA} {
+		for _, payload := range [][]byte{
+			{},
+			{0x01},
+			[]byte("hello world"),
+			bytes.Repeat([]byte{0xab}, payloadChunkSizeForType(rrType)*3+1),
+		} {
+			question := Question{
+				Name:  mustParseName("example.com"),
+				Type:  rrType,
+				Class: ClassIN,
+			}
+			answers, err := answersForPayload(question, responseTTL, payload)
+			if err != nil {
+				t.Fatalf("answersForPayload(%d) err = %v", rrType, err)
+			}
+
+			if len(answers) > 1 {
+				answers[0], answers[len(answers)-1] = answers[len(answers)-1], answers[0]
+			}
+
+			decoded := decodeResponsePayload(answers)
+			if !bytes.Equal(decoded, payload) {
+				t.Fatalf("rrType=%d decoded %x want %x", rrType, decoded, payload)
+			}
+		}
+	}
+}
+
+func TestParseResolver(t *testing.T) {
+	tests := []struct {
+		resolver string
+		rrType   uint16
+	}{
+		{"example.com+udp://1.1.1.1:53", RRTypeTXT},
+		{"example.com:txt+udp://1.1.1.1:53", RRTypeTXT},
+		{"example.com:a+udp://1.1.1.1:53", RRTypeA},
+		{"example.com:aaaa+udp://1.1.1.1:53", RRTypeAAAA},
+	}
+
+	for _, test := range tests {
+		domain, server, rrType, err := parseResolver(test.resolver)
+		if err != nil {
+			t.Fatalf("parseResolver(%q) err = %v", test.resolver, err)
+		}
+		if domain.String() != "example.com" || server != "1.1.1.1:53" || rrType != test.rrType {
+			t.Fatalf("parseResolver(%q) = (%q, %q, %d)", test.resolver, domain.String(), server, rrType)
+		}
+	}
+}
+
+func TestParseDomainSpec(t *testing.T) {
+	tests := []struct {
+		spec    string
+		def     string
+		rrType  uint16
+		wantErr bool
+	}{
+		{"example.com", "", 0, false},
+		{"example.com", "txt", RRTypeTXT, false},
+		{"example.com:a", "", RRTypeA, false},
+		{"example.com:aaaa", "", RRTypeAAAA, false},
+		{"example.com:doh", "", 0, true},
+	}
+
+	for _, test := range tests {
+		got, err := parseDomainSpec(test.spec, test.def)
+		if test.wantErr {
+			if err == nil {
+				t.Fatalf("parseDomainSpec(%q, %q) err = nil", test.spec, test.def)
+			}
+			continue
+		}
+		if err != nil {
+			t.Fatalf("parseDomainSpec(%q, %q) err = %v", test.spec, test.def, err)
+		}
+		if got.name.String() != "example.com" || got.rrType != test.rrType {
+			t.Fatalf("parseDomainSpec(%q, %q) = (%q, %d)", test.spec, test.def, got.name.String(), got.rrType)
+		}
+	}
+}
+
+func TestResponseForMethodRestriction(t *testing.T) {
+	query := &Message{
+		ID:    1,
+		Flags: 0x0100,
+		Question: []Question{{
+			Name:  mustParseName("abc.example.com"),
+			Type:  RRTypeTXT,
+			Class: ClassIN,
+		}},
+		Additional: []RR{{
+			Name:  Name{},
+			Type:  RRTypeOPT,
+			Class: 4096,
+		}},
+	}
+
+	resp, _ := responseFor(query, []domainSpec{{name: mustParseName("example.com"), rrType: RRTypeA}})
+	if resp == nil || resp.Rcode() != RcodeNameError {
+		t.Fatalf("responseFor method restriction rcode = %v", resp)
+	}
+
+	resp, _ = responseFor(query, []domainSpec{{name: mustParseName("example.com")}})
+	if resp == nil || resp.Rcode() != RcodeNoError {
+		t.Fatalf("responseFor unrestricted rcode = %v", resp)
+	}
+}
@@ -0,0 +1,226 @@
+package xdns
+
+import "bytes"
+
+const ipRecordHeaderSize = 2
+
+func maxEncodedPayloadForType(rrType uint16) int {
+	switch rrType {
+	case RRTypeA:
+		return maxEncodedPayloadA
+	case RRTypeAAAA:
+		return maxEncodedPayloadAAAA
+	default:
+		return maxEncodedPayloadTXT
+	}
+}
+
+func rrDataSizeForType(rrType uint16) int {
+	switch rrType {
+	case RRTypeA:
+		return 4
+	case RRTypeAAAA:
+		return 16
+	default:
+		return 0
+	}
+}
+
+func payloadChunkSizeForType(rrType uint16) int {
+	size := rrDataSizeForType(rrType)
+	if size <= ipRecordHeaderSize {
+		return 0
+	}
+	return size - ipRecordHeaderSize
+}
+
+func answersForPayload(question Question, ttl uint32, payload []byte) ([]RR, error) {
+	switch question.Type {
+	case RRTypeTXT:
+		return []RR{
+			{
+				Name:  question.Name,
+				Type:  question.Type,
+				Class: question.Class,
+				TTL:   ttl,
+				Data:  EncodeRDataTXT(payload),
+			},
+		}, nil
+	case RRTypeA, RRTypeAAAA:
+		return ipAnswersForPayload(question, ttl, payload)
+	default:
+		return nil, ErrIntegerOverflow
+	}
+}
+
+func ipAnswersForPayload(question Question, ttl uint32, payload []byte) ([]RR, error) {
+	chunkSize := payloadChunkSizeForType(question.Type)
+	rrDataSize := rrDataSizeForType(question.Type)
+	if chunkSize == 0 || rrDataSize == 0 {
+		return nil, ErrIntegerOverflow
+	}
+
+	numRecords := 1
+	if len(payload) > 0 {
+		numRecords = (len(payload) + chunkSize - 1) / chunkSize
+	}
+	if numRecords > 256 {
+		return nil, ErrIntegerOverflow
+	}
+
+	answers := make([]RR, 0, numRecords)
+	for i := 0; i < numRecords; i++ {
+		offset := i * chunkSize
+		n := len(payload) - offset
+		if n < 0 {
+			n = 0
+		}
+		if n > chunkSize {
+			n = chunkSize
+		}
+
+		data := make([]byte, rrDataSize)
+		data[0] = byte(i)
+		data[1] = byte(n)
+		copy(data[ipRecordHeaderSize:], payload[offset:offset+n])
+
+		answers = append(answers, RR{
+			Name:  question.Name,
+			Type:  question.Type,
+			Class: question.Class,
+			TTL:   ttl,
+			Data:  data,
+		})
+	}
+
+	return answers, nil
+}
+
+func decodeResponsePayload(answers []RR) []byte {
+	if len(answers) == 0 {
+		return nil
+	}
+
+	switch answers[0].Type {
+	case RRTypeTXT:
+		if len(answers) != 1 {
+			return nil
+		}
+		payload, err := DecodeRDataTXT(answers[0].Data)
+		if err != nil {
+			return nil
+		}
+		return payload
+	case RRTypeA, RRTypeAAAA:
+		return decodeIPAnswerPayload(answers, answers[0].Type)
+	default:
+		return nil
+	}
+}
+
+func decodeIPAnswerPayload(answers []RR, rrType uint16) []byte {
+	chunkSize := payloadChunkSizeForType(rrType)
+	rrDataSize := rrDataSizeForType(rrType)
+	if chunkSize == 0 || rrDataSize == 0 || len(answers) > 256 {
+		return nil
+	}
+
+	parts := make([][]byte, len(answers))
+	for _, answer := range answers {
+		if answer.Type != rrType || len(answer.Data) != rrDataSize {
+			return nil
+		}
+		idx := int(answer.Data[0])
+		n := int(answer.Data[1])
+		if idx >= len(answers) || n > chunkSize || parts[idx] != nil {
+			return nil
+		}
+
+		part := make([]byte, n)
+		copy(part, answer.Data[ipRecordHeaderSize:ipRecordHeaderSize+n])
+		parts[idx] = part
+	}
+
+	var payload bytes.Buffer
+	for _, part := range parts {
+		if part == nil {
+			return nil
+		}
+		payload.Write(part)
+	}
+	return payload.Bytes()
+}
+
+func computeMaxEncodedPayload(limit int) int {
+	return computeMaxEncodedPayloadForType(limit, RRTypeTXT)
+}
+
+func computeMaxEncodedPayloadForType(limit int, rrType uint16) int {
+	maxLengthName, err := NewName([][]byte{
+		[]byte("AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"),
+		[]byte("AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"),
+		[]byte("AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"),
+		[]byte("AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"),
+	})
+	if err != nil {
+		panic(err)
+	}
+	{
+		n := 0
+		for _, label := range maxLengthName {
+			n += len(label) + 1
+		}
+		n += 1
+		if n != 255 {
+			panic("computeMaxEncodedPayload n != 255")
+		}
+	}
+
+	queryLimit := uint16(limit)
+	if int(queryLimit) != limit {
+		queryLimit = 0xffff
+	}
+	query := &Message{
+		Question: []Question{
+			{
+				Name:  maxLengthName,
+				Type:  rrType,
+				Class: ClassIN,
+			},
+		},
+		Additional: []RR{
+			{
+				Name:  Name{},
+				Type:  RRTypeOPT,
+				Class: queryLimit,
+				TTL:   0,
+				Data:  []byte{},
+			},
+		},
+	}
+	resp, _ := responseFor(query, []domainSpec{{name: Name{[]byte{}}}})
+
+	low := 0
+	high := 32768
+	if chunkSize := payloadChunkSizeForType(rrType); chunkSize > 0 {
+		high = 256*chunkSize + 1
+	}
+	for low+1 < high {
+		mid := (low + high) / 2
+		resp.Answer, err = answersForPayload(query.Question[0], responseTTL, make([]byte, mid))
+		if err != nil {
+			panic(err)
+		}
+		buf, err := resp.WireFormat()
+		if err != nil {
+			panic(err)
+		}
+		if len(buf) <= limit {
+			low = mid
+		} else {
+			high = mid
+		}
+	}
+
+	return low
+}
@@ -21,8 +21,10 @@ const (
 )

 var (
-	maxUDPPayload     = 1280 - 40 - 8
-	maxEncodedPayload = computeMaxEncodedPayload(maxUDPPayload)
+	maxUDPPayload         = 1280 - 40 - 8
+	maxEncodedPayloadTXT  = computeMaxEncodedPayloadForType(maxUDPPayload, RRTypeTXT)
+	maxEncodedPayloadA    = computeMaxEncodedPayloadForType(maxUDPPayload, RRTypeA)
+	maxEncodedPayloadAAAA = computeMaxEncodedPayloadForType(maxUDPPayload, RRTypeAAAA)
 )

 func clientIDToAddr(clientID [8]byte) *net.UDPAddr {
@@ -44,15 +46,16 @@ type record struct {
 }

 type queue struct {
-	last  time.Time
-	queue chan []byte
-	stash chan []byte
+	last   time.Time
+	rrType uint16
+	queue  chan []byte
+	stash  chan []byte
 }

 type xdnsConnServer struct {
 	net.PacketConn

-	domains []Name
+	domains []domainSpec

 	ch            chan *record
 	readQueue     chan *packet
@@ -66,9 +69,9 @@ func NewConnServer(c *Config, raw net.PacketConn) (net.PacketConn, error) {
 	if len(c.Domains) == 0 {
 		return nil, errors.New("empty domains")
 	}
-	domains := make([]Name, 0, len(c.Domains))
+	domains := make([]domainSpec, 0, len(c.Domains))
 	for _, domain := range c.Domains {
-		domain, err := ParseName(domain)
+		domain, err := parseDomainSpec(domain, "")
 		if err != nil {
 			return nil, err
 		}
@@ -234,6 +237,7 @@ func (c *xdnsConnServer) recvLoop() {
 func (c *xdnsConnServer) sendLoop() {
 	var nextRec *record
 	for {
+		var err error
 		rec := nextRec
 		nextRec = nil

@@ -246,18 +250,8 @@ func (c *xdnsConnServer) sendLoop() {
 		}

 		if rec.Resp.Rcode() == RcodeNoError && len(rec.Resp.Question) == 1 {
-			rec.Resp.Answer = []RR{
-				{
-					Name:  rec.Resp.Question[0].Name,
-					Type:  rec.Resp.Question[0].Type,
-					Class: rec.Resp.Question[0].Class,
-					TTL:   responseTTL,
-					Data:  nil,
-				},
-			}
-
 			var payload bytes.Buffer
-			limit := maxEncodedPayload
+			limit := maxEncodedPayloadForType(rec.Resp.Question[0].Type)
 			timer := time.NewTimer(maxResponseDelay)

 			for {
@@ -267,6 +261,7 @@ func (c *xdnsConnServer) sendLoop() {
 					c.mutex.Unlock()
 					return
 				}
+				q.rrType = rec.Resp.Question[0].Type
 				c.mutex.Unlock()

 				var p []byte
@@ -294,7 +289,11 @@ func (c *xdnsConnServer) sendLoop() {
 				}

 				limit -= 2 + len(p)
-				if payload.Len() > 0 && limit < 0 {
+				if limit < 0 {
+					if payload.Len() == 0 {
+						errors.LogDebug(context.Background(), rec.Addr, " ", rec.ClientAddr, " xdns payload too large for rrtype ", rec.Resp.Question[0].Type, " ", len(p))
+						continue
+					}
 					c.stash(q, p)
 					break
 				}
@@ -308,7 +307,11 @@ func (c *xdnsConnServer) sendLoop() {
 			}

 			timer.Stop()
-			rec.Resp.Answer[0].Data = EncodeRDataTXT(payload.Bytes())
+			rec.Resp.Answer, err = answersForPayload(rec.Resp.Question[0], responseTTL, payload.Bytes())
+			if err != nil {
+				errors.LogDebug(context.Background(), rec.Addr, " ", rec.ClientAddr, " xdns encode err ", err)
+				continue
+			}
 		}

 		buf, err := rec.Resp.WireFormat()
@@ -349,11 +352,6 @@ func (c *xdnsConnServer) ReadFrom(p []byte) (n int, addr net.Addr, err error) {
 }

 func (c *xdnsConnServer) WriteTo(p []byte, addr net.Addr) (n int, err error) {
-	if len(p)+2 > maxEncodedPayload {
-		errors.LogDebug(context.Background(), addr, " mask write err short write ", len(p), "+2 > ", maxEncodedPayload)
-		return 0, nil
-	}
-
 	c.mutex.Lock()
 	defer c.mutex.Unlock()

@@ -361,6 +359,14 @@ func (c *xdnsConnServer) WriteTo(p []byte, addr net.Addr) (n int, err error) {
 	if q == nil {
 		return 0, io.ErrClosedPipe
 	}
+	limit := maxEncodedPayloadForType(q.rrType)
+	if q.rrType == 0 {
+		limit = maxEncodedPayloadTXT
+	}
+	if len(p)+2 > limit {
+		errors.LogDebug(context.Background(), addr, " mask write err short write ", len(p), "+2 > ", limit)
+		return 0, nil
+	}

 	buf := make([]byte, len(p))
 	copy(buf, p)
@@ -406,7 +412,7 @@ func nextPacketServer(r *bytes.Reader) ([]byte, error) {
 	}
 }

-func responseFor(query *Message, domains []Name) (*Message, []byte) {
+func responseFor(query *Message, domains []domainSpec) (*Message, []byte) {
 	resp := &Message{
 		ID:       query.ID,
 		Flags:    0x8000,
@@ -454,11 +460,15 @@ func responseFor(query *Message, domains []Name) (*Message, []byte) {
 	}
 	question := query.Question[0]

-	var prefix Name
-	var ok bool
+	var (
+		prefix Name
+		ok     bool
+		match  domainSpec
+	)
 	for _, domain := range domains {
-		prefix, ok = question.Name.TrimSuffix(domain)
+		prefix, ok = question.Name.TrimSuffix(domain.name)
 		if ok {
+			match = domain
 			break
 		}
 	}
@@ -473,7 +483,13 @@ func responseFor(query *Message, domains []Name) (*Message, []byte) {
 		return resp, nil
 	}

-	if question.Type != RRTypeTXT {
+	switch question.Type {
+	case RRTypeTXT, RRTypeA, RRTypeAAAA:
+	default:
+		resp.Flags |= RcodeNameError
+		return resp, nil
+	}
+	if match.rrType != 0 && question.Type != match.rrType {
 		resp.Flags |= RcodeNameError
 		return resp, nil
 	}
@@ -494,78 +510,3 @@ func responseFor(query *Message, domains []Name) (*Message, []byte) {

 	return resp, payload
 }
-
-func computeMaxEncodedPayload(limit int) int {
-	maxLengthName, err := NewName([][]byte{
-		[]byte("AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"),
-		[]byte("AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"),
-		[]byte("AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"),
-		[]byte("AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"),
-	})
-	if err != nil {
-		panic(err)
-	}
-	{
-		n := 0
-		for _, label := range maxLengthName {
-			n += len(label) + 1
-		}
-		n += 1
-		if n != 255 {
-			panic("computeMaxEncodedPayload n != 255")
-		}
-	}
-
-	queryLimit := uint16(limit)
-	if int(queryLimit) != limit {
-		queryLimit = 0xffff
-	}
-	query := &Message{
-		Question: []Question{
-			{
-				Name:  maxLengthName,
-				Type:  RRTypeTXT,
-				Class: RRTypeTXT,
-			},
-		},
-
-		Additional: []RR{
-			{
-				Name:  Name{},
-				Type:  RRTypeOPT,
-				Class: queryLimit,
-				TTL:   0,
-				Data:  []byte{},
-			},
-		},
-	}
-	resp, _ := responseFor(query, []Name{[][]byte{}})
-
-	resp.Answer = []RR{
-		{
-			Name:  query.Question[0].Name,
-			Type:  query.Question[0].Type,
-			Class: query.Question[0].Class,
-			TTL:   responseTTL,
-			Data:  nil,
-		},
-	}
-
-	low := 0
-	high := 32768
-	for low+1 < high {
-		mid := (low + high) / 2
-		resp.Answer[0].Data = EncodeRDataTXT(make([]byte, mid))
-		buf, err := resp.WireFormat()
-		if err != nil {
-			panic(err)
-		}
-		if len(buf) <= limit {
-			low = mid
-		} else {
-			high = mid
-		}
-	}
-
-	return low
-}
@@ -0,0 +1,80 @@
+package xdns
+
+import (
+	"strings"
+
+	"github.com/xtls/xray-core/common/errors"
+)
+
+type domainSpec struct {
+	name   Name
+	rrType uint16
+}
+
+func rrTypeFromMethod(method string) (uint16, error) {
+	switch strings.ToLower(method) {
+	case "", "txt":
+		return RRTypeTXT, nil
+	case "a":
+		return RRTypeA, nil
+	case "aaaa":
+		return RRTypeAAAA, nil
+	default:
+		return 0, errors.New("unsupported method")
+	}
+}
+
+func parseDomainSpec(s string, defaultMethod string) (domainSpec, error) {
+	domainPart := s
+	method := ""
+	hasMethod := false
+
+	if i := strings.LastIndex(s, ":"); i >= 0 {
+		domainPart = s[:i]
+		method = s[i+1:]
+		hasMethod = true
+	} else if defaultMethod != "" {
+		method = defaultMethod
+		hasMethod = true
+	}
+
+	if domainPart == "" {
+		return domainSpec{}, errors.New("empty domain")
+	}
+
+	name, err := ParseName(domainPart)
+	if err != nil {
+		return domainSpec{}, err
+	}
+
+	rrType := uint16(0)
+	if hasMethod {
+		var err error
+		rrType, err = rrTypeFromMethod(method)
+		if err != nil {
+			return domainSpec{}, err
+		}
+	}
+
+	return domainSpec{
+		name:   name,
+		rrType: rrType,
+	}, nil
+}
+
+func parseResolver(s string) (Name, string, uint16, error) {
+	head, server, ok := strings.Cut(s, "+udp://")
+	if !ok {
+		return nil, "", 0, errors.New("invalid resolver scheme")
+	}
+	if server == "" {
+		return nil, "", 0, errors.New("empty resolver server")
+	}
+
+	spec, err := parseDomainSpec(head, "txt")
+	if err != nil {
+		return nil, "", 0, err
+	}
+
+	return spec.name, server, spec.rrType, nil
+}