feat: implement IKE session relay and transport layer for ePDG communication

2026-08-20 14:53:42 +08:00 · 2026-08-20 13:14:45 +08:00
parent 497cd24c8d
commit b8df7f43f8
8 changed files with 580 additions and 91 deletions
@@ -280,6 +280,239 @@ func decryptPayloads(
 	return header, payloads, nil
 }

+const defaultIKEFragmentSize = 1100
+
+func encryptPayloadsFragmented(
+	header ikeHeader,
+	inner []payload,
+	suite negotiatedSuite,
+	encryptionKey []byte,
+	integrityKey []byte,
+	maxFragmentSize int,
+	random io.Reader,
+) ([][]byte, error) {
+	if random == nil {
+		random = rand.Reader
+	}
+	if maxFragmentSize <= 0 {
+		maxFragmentSize = defaultIKEFragmentSize
+	}
+	first, plaintext, err := marshalPayloadChain(inner)
+	if err != nil {
+		return nil, err
+	}
+	block, err := aes.NewCipher(encryptionKey)
+	if err != nil {
+		return nil, fmt.Errorf("ike: initialize AES: %w", err)
+	}
+	_, checksumLength, err := suite.integrityLengths()
+	if err != nil {
+		return nil, err
+	}
+
+	maxChunk := maxFragmentSize - ikeHeaderLength - 8 - block.BlockSize() - block.BlockSize() - checksumLength
+	if maxChunk < 64 {
+		maxChunk = 64
+	}
+
+	var chunks [][]byte
+	for len(plaintext) > 0 {
+		take := len(plaintext)
+		if take > maxChunk {
+			take = maxChunk
+		}
+		chunks = append(chunks, plaintext[:take])
+		plaintext = plaintext[take:]
+	}
+	totalFragments := uint16(len(chunks))
+	if totalFragments == 0 {
+		totalFragments = 1
+		chunks = [][]byte{nil}
+	}
+
+	var packets [][]byte
+	for index, chunk := range chunks {
+		fragNum := uint16(index + 1)
+		fragNext := uint8(payloadNone)
+		if fragNum == 1 {
+			fragNext = first
+		}
+
+		paddingLength := block.BlockSize() - (len(chunk)+1)%block.BlockSize()
+		if paddingLength == block.BlockSize() {
+			paddingLength = 0
+		}
+		padding := make([]byte, paddingLength)
+		if _, err := io.ReadFull(random, padding); err != nil {
+			return nil, fmt.Errorf("ike: generate encrypted payload padding: %w", err)
+		}
+		paddedChunk := append(append([]byte(nil), chunk...), padding...)
+		paddedChunk = append(paddedChunk, byte(paddingLength))
+
+		iv := make([]byte, block.BlockSize())
+		if _, err := io.ReadFull(random, iv); err != nil {
+			return nil, fmt.Errorf("ike: generate encrypted payload IV: %w", err)
+		}
+		ciphertext := make([]byte, len(paddedChunk))
+		cipher.NewCBCEncrypter(block, iv).CryptBlocks(ciphertext, paddedChunk)
+
+		skfLength := 4 + 4 + len(iv) + len(ciphertext) + checksumLength
+		if skfLength > 65535 {
+			return nil, errors.New("ike: encrypted fragment exceeds 65535 bytes")
+		}
+
+		body := make([]byte, skfLength)
+		body[0] = fragNext
+		body[1] = 0
+		binary.BigEndian.PutUint16(body[2:4], uint16(skfLength))
+		binary.BigEndian.PutUint16(body[4:6], fragNum)
+		binary.BigEndian.PutUint16(body[6:8], totalFragments)
+		copy(body[8:], iv)
+		copy(body[8+len(iv):], ciphertext)
+
+		fragHeader := header
+		fragHeader.NextPayload = payloadEncryptedFragment
+		packet := fragHeader.marshal(body)
+		checksum, err := integrityMAC(suite, integrityKey, packet[:len(packet)-checksumLength])
+		if err != nil {
+			return nil, err
+		}
+		copy(packet[len(packet)-checksumLength:], checksum)
+		packets = append(packets, packet)
+	}
+	return packets, nil
+}
+
+func decryptSingleFragment(
+	packet []byte,
+	suite negotiatedSuite,
+	encryptionKey []byte,
+	integrityKey []byte,
+) (ikeHeader, uint8, uint16, uint16, []byte, error) {
+	header, body, err := parseIKEPacket(packet)
+	if err != nil {
+		return ikeHeader{}, 0, 0, 0, nil, err
+	}
+	if header.NextPayload != payloadEncryptedFragment || len(body) < 8 {
+		return ikeHeader{}, 0, 0, 0, nil, fmt.Errorf("%w: message is not an encrypted IKE fragment", errUnexpectedPacket)
+	}
+	skfLength := int(binary.BigEndian.Uint16(body[2:4]))
+	if skfLength != len(body) {
+		return ikeHeader{}, 0, 0, 0, nil, fmt.Errorf("%w: encrypted fragment length mismatch", errMalformedPacket)
+	}
+	block, err := aes.NewCipher(encryptionKey)
+	if err != nil {
+		return ikeHeader{}, 0, 0, 0, nil, fmt.Errorf("ike: initialize AES: %w", err)
+	}
+	_, checksumLength, err := suite.integrityLengths()
+	if err != nil {
+		return ikeHeader{}, 0, 0, 0, nil, err
+	}
+	if len(body) < 8+block.BlockSize()+block.BlockSize()+checksumLength {
+		return ikeHeader{}, 0, 0, 0, nil, fmt.Errorf("%w: encrypted fragment is too short", errMalformedPacket)
+	}
+	expected, err := integrityMAC(suite, integrityKey, packet[:len(packet)-checksumLength])
+	if err != nil {
+		return ikeHeader{}, 0, 0, 0, nil, err
+	}
+	actual := packet[len(packet)-checksumLength:]
+	if subtle.ConstantTimeCompare(actual, expected) != 1 {
+		return ikeHeader{}, 0, 0, 0, nil, errIntegrityMismatch
+	}
+
+	fragNext := body[0]
+	fragNum := binary.BigEndian.Uint16(body[4:6])
+	totalFrags := binary.BigEndian.Uint16(body[6:8])
+	if fragNum == 0 || totalFrags == 0 || fragNum > totalFrags {
+		return ikeHeader{}, 0, 0, 0, nil, fmt.Errorf("%w: invalid fragment numbers %d/%d", errMalformedPacket, fragNum, totalFrags)
+	}
+
+	ivStart := 8
+	ciphertextStart := ivStart + block.BlockSize()
+	ciphertextEnd := len(body) - checksumLength
+	ciphertext := body[ciphertextStart:ciphertextEnd]
+	if len(ciphertext) == 0 || len(ciphertext)%block.BlockSize() != 0 {
+		return ikeHeader{}, 0, 0, 0, nil, fmt.Errorf("%w: fragment ciphertext is not block aligned", errMalformedPacket)
+	}
+	plaintext := make([]byte, len(ciphertext))
+	cipher.NewCBCDecrypter(block, body[ivStart:ciphertextStart]).CryptBlocks(plaintext, ciphertext)
+	paddingLength := int(plaintext[len(plaintext)-1])
+	if paddingLength+1 > len(plaintext) {
+		return ikeHeader{}, 0, 0, 0, nil, fmt.Errorf("%w: invalid encrypted fragment padding", errMalformedPacket)
+	}
+	plaintext = plaintext[:len(plaintext)-paddingLength-1]
+	return header, fragNext, fragNum, totalFrags, plaintext, nil
+}
+
+func decryptPayloadsAny(
+	packet []byte,
+	fragments [][]byte,
+	suite negotiatedSuite,
+	encryptionKey []byte,
+	integrityKey []byte,
+) (ikeHeader, []payload, error) {
+	if len(fragments) > 0 {
+		var (
+			firstHeader   ikeHeader
+			firstNext     uint8
+			totalExpected uint16
+			plaintexts    = make(map[uint16][]byte)
+		)
+		for _, fragPacket := range fragments {
+			hdr, next, num, total, plain, err := decryptSingleFragment(fragPacket, suite, encryptionKey, integrityKey)
+			if err != nil {
+				return ikeHeader{}, nil, err
+			}
+			if totalExpected == 0 {
+				firstHeader = hdr
+				totalExpected = total
+			} else if total != totalExpected || hdr.MessageID != firstHeader.MessageID || hdr.Exchange != firstHeader.Exchange {
+				return ikeHeader{}, nil, fmt.Errorf("%w: inconsistent fragment headers", errMalformedPacket)
+			}
+			if num == 1 {
+				firstNext = next
+			}
+			plaintexts[num] = plain
+		}
+		if uint16(len(plaintexts)) != totalExpected {
+			return ikeHeader{}, nil, fmt.Errorf("%w: missing fragments: received %d of %d", errMalformedPacket, len(plaintexts), totalExpected)
+		}
+		var fullPlaintext []byte
+		for i := uint16(1); i <= totalExpected; i++ {
+			chunk, ok := plaintexts[i]
+			if !ok {
+				return ikeHeader{}, nil, fmt.Errorf("%w: missing fragment %d", errMalformedPacket, i)
+			}
+			fullPlaintext = append(fullPlaintext, chunk...)
+		}
+		payloads, err := parsePayloadChain(firstNext, fullPlaintext)
+		if err != nil {
+			return ikeHeader{}, nil, err
+		}
+		return firstHeader, payloads, nil
+	}
+
+	header, _, err := parseIKEPacket(packet)
+	if err != nil {
+		return ikeHeader{}, nil, err
+	}
+	if header.NextPayload == payloadEncryptedFragment {
+		hdr, next, num, total, plain, err := decryptSingleFragment(packet, suite, encryptionKey, integrityKey)
+		if err != nil {
+			return ikeHeader{}, nil, err
+		}
+		if num != 1 || total != 1 {
+			return ikeHeader{}, nil, fmt.Errorf("%w: standalone fragment with total=%d", errMalformedPacket, total)
+		}
+		payloads, err := parsePayloadChain(next, plain)
+		if err != nil {
+			return ikeHeader{}, nil, err
+		}
+		return hdr, payloads, nil
+	}
+	return decryptPayloads(packet, suite, encryptionKey, integrityKey)
+}
+
 var modpPrimes = map[uint16]string{
 	dhMODP1024: "FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD1" +
 		"29024E088A67CC74020BBEA63B139B22514A08798E3404DD" +
@@ -113,3 +113,80 @@ func TestIKEKeyDerivationSeparatesDirections(t *testing.T) {
 		t.Fatal("initiator and responder keys were not separated")
 	}
 }
+
+func TestRFC7383FragmentationAndReassembly(t *testing.T) {
+	suite := legacyTestSuite()
+	encryptionKey := bytes.Repeat([]byte{0x11}, 16)
+	integrityKey := bytes.Repeat([]byte{0x22}, 20)
+	header := ikeHeader{
+		InitiatorSPI: [8]byte{1, 2, 3, 4, 5, 6, 7, 8},
+		ResponderSPI: [8]byte{8, 7, 6, 5, 4, 3, 2, 1},
+		Exchange:     exchangeIKEAuth,
+		Flags:        flagInitiator,
+		MessageID:    9,
+	}
+
+	largeCertData := bytes.Repeat([]byte{0xAB, 0xCD, 0xEF, 0x01}, 400) // 1600 bytes
+	inner := []payload{
+		{Type: payloadIDi, Body: []byte{3, 0, 0, 0, 'u', 's', 'e', 'r'}},
+		{Type: payloadCert, Body: largeCertData},
+		{Type: payloadAuth, Body: bytes.Repeat([]byte{0x55}, 64)},
+	}
+
+	// Fragment into chunks with max fragment size 600 bytes
+	packets, err := encryptPayloadsFragmented(
+		header,
+		inner,
+		suite,
+		encryptionKey,
+		integrityKey,
+		600,
+		bytes.NewReader(bytes.Repeat([]byte{0x77}, 1024)),
+	)
+	if err != nil {
+		t.Fatalf("encryptPayloadsFragmented() error = %v", err)
+	}
+
+	if len(packets) < 3 {
+		t.Fatalf("expected at least 3 fragments for large payload, got %d", len(packets))
+	}
+
+	for i, pkt := range packets {
+		hdr, body, parseErr := parseIKEPacket(pkt)
+		if parseErr != nil {
+			t.Fatalf("fragment %d parse error: %v", i+1, parseErr)
+		}
+		if hdr.NextPayload != payloadEncryptedFragment {
+			t.Fatalf("fragment %d NextPayload = %d, want %d (payloadEncryptedFragment)", i+1, hdr.NextPayload, payloadEncryptedFragment)
+		}
+		if len(body) < 8 {
+			t.Fatalf("fragment %d body too short", i+1)
+		}
+	}
+
+	// Decrypt and reassemble
+	decodedHeader, decoded, err := decryptPayloadsAny(nil, packets, suite, encryptionKey, integrityKey)
+	if err != nil {
+		t.Fatalf("decryptPayloadsAny() error = %v", err)
+	}
+
+	if decodedHeader.MessageID != header.MessageID || len(decoded) != len(inner) {
+		t.Fatalf("reassembled payload mismatch: header=%#v, count=%d, want=%d", decodedHeader, len(decoded), len(inner))
+	}
+
+	for index := range inner {
+		if decoded[index].Type != inner[index].Type || !bytes.Equal(decoded[index].Body, inner[index].Body) {
+			t.Fatalf("decoded payload %d = %#v, want %#v", index, decoded[index], inner[index])
+		}
+	}
+
+	// Test tamper detection on second fragment
+	tamperedPackets := make([][]byte, len(packets))
+	for i := range packets {
+		tamperedPackets[i] = append([]byte(nil), packets[i]...)
+	}
+	tamperedPackets[1][len(tamperedPackets[1])-1] ^= 0x55
+	if _, _, err := decryptPayloadsAny(nil, tamperedPackets, suite, encryptionKey, integrityKey); !errors.Is(err, errIntegrityMismatch) {
+		t.Fatalf("tampered fragment decrypt error = %v, want errIntegrityMismatch", err)
+	}
+}
@@ -187,6 +187,7 @@ func (provider *Provider) start(ctx context.Context, request vowifi.TunnelReques
 		{Type: payloadNonce, Body: initiatorNonce},
 		makeNotify(notifyNATSource, sourceHash),
 		makeNotify(notifyNATDestination, destinationHash),
+		makeNotify(notifyFragmentationSupported, nil),
 	}
 	var (
 		initRequest          []byte
@@ -243,6 +244,7 @@ func (provider *Provider) start(ctx context.Context, request vowifi.TunnelReques
 		}
 		break
 	}
+	peerSupportsFragmentation := hasNotifyType(initResponsePayloads, notifyFragmentationSupported)
 	saPayload, err := onePayload(initResponsePayloads, payloadSA)
 	if err != nil {
 		return nil, err
@@ -345,21 +347,19 @@ func (provider *Provider) start(ctx context.Context, request vowifi.TunnelReques
 		Flags:        flagInitiator,
 		MessageID:    1,
 	}
-	authRequest, err := encryptPayloads(authHeader, firstAuthPayloads, ikeSuite, keys.SKei, keys.SKai, provider.config.Random)
-	if err != nil {
-		return nil, err
-	}
-	authResponse, err := transport.RoundTrip(ctx, authRequest)
-	if err != nil {
-		return nil, err
-	}
-	authResponseHeader, authResponsePayloads, err := decryptAndValidate(
-		authResponse, initiatorSPI, responseHeader.ResponderSPI, exchangeIKEAuth, 1, ikeSuite, keys,
+	_, authResponsePayloads, err := sendAndReceiveIKEPayloads(
+		ctx,
+		transport,
+		authHeader,
+		firstAuthPayloads,
+		ikeSuite,
+		keys,
+		peerSupportsFragmentation,
+		provider.config.Random,
 	)
 	if err != nil {
 		return nil, err
 	}
-	_ = authResponseHeader
 	serverName := strings.TrimSpace(provider.config.ServerName)
 	if serverName == "" {
 		serverName = epdg
@@ -409,27 +409,27 @@ func (provider *Provider) start(ctx context.Context, request vowifi.TunnelReques
 				requestPayloads = append(requestPayloads, deviceIdentity)
 			}
 		}
-		eapRequest, err := encryptPayloads(ikeHeader{
+		eapHeader := ikeHeader{
 			InitiatorSPI: initiatorSPI,
 			ResponderSPI: responseHeader.ResponderSPI,
 			Exchange:     exchangeIKEAuth,
 			Flags:        flagInitiator,
 			MessageID:    messageID,
-		}, requestPayloads, ikeSuite, keys.SKei, keys.SKai, provider.config.Random)
-		if err != nil {
-			return nil, err
 		}
 		if requested, notifyErr := deviceIdentityRequested(currentPayloads); notifyErr != nil {
 			return nil, notifyErr
 		} else if requested {
 			deviceIdentityPending = true
 		}
-		eapResponse, err := transport.RoundTrip(ctx, eapRequest)
-		if err != nil {
-			return nil, err
-		}
-		_, currentPayloads, err = decryptAndValidate(
-			eapResponse, initiatorSPI, responseHeader.ResponderSPI, exchangeIKEAuth, messageID, ikeSuite, keys,
+		_, currentPayloads, err = sendAndReceiveIKEPayloads(
+			ctx,
+			transport,
+			eapHeader,
+			requestPayloads,
+			ikeSuite,
+			keys,
+			peerSupportsFragmentation,
+			provider.config.Random,
 		)
 		if err != nil {
 			return nil, err
@@ -454,22 +454,22 @@ func (provider *Provider) start(ctx context.Context, request vowifi.TunnelReques
 	}
 	messageID++
 	cleanupMessageID = messageID + 1
-	finalRequest, err := encryptPayloads(ikeHeader{
+	finalHeader := ikeHeader{
 		InitiatorSPI: initiatorSPI,
 		ResponderSPI: responseHeader.ResponderSPI,
 		Exchange:     exchangeIKEAuth,
 		Flags:        flagInitiator,
 		MessageID:    messageID,
-	}, []payload{initiatorAUTH}, ikeSuite, keys.SKei, keys.SKai, provider.config.Random)
-	if err != nil {
-		return nil, err
 	}
-	finalResponse, err := transport.RoundTrip(ctx, finalRequest)
-	if err != nil {
-		return nil, err
-	}
-	_, finalPayloads, err := decryptAndValidate(
-		finalResponse, initiatorSPI, responseHeader.ResponderSPI, exchangeIKEAuth, messageID, ikeSuite, keys,
+	_, finalPayloads, err := sendAndReceiveIKEPayloads(
+		ctx,
+		transport,
+		finalHeader,
+		[]payload{initiatorAUTH},
+		ikeSuite,
+		keys,
+		peerSupportsFragmentation,
+		provider.config.Random,
 	)
 	if err != nil {
 		return nil, err
@@ -763,7 +763,7 @@ func decryptAndValidate(
 	suite negotiatedSuite,
 	keys ikeKeys,
 ) (ikeHeader, []payload, error) {
-	header, payloads, err := decryptPayloads(packet, suite, keys.SKer, keys.SKar)
+	header, payloads, err := decryptPayloadsAny(packet, nil, suite, keys.SKer, keys.SKar)
 	if err != nil {
 		return ikeHeader{}, nil, err
 	}
@@ -778,6 +778,77 @@ func decryptAndValidate(
 	return header, payloads, nil
 }

+func decryptAndValidateFragments(
+	packets [][]byte,
+	initiatorSPI [8]byte,
+	responderSPI [8]byte,
+	exchange uint8,
+	messageID uint32,
+	suite negotiatedSuite,
+	keys ikeKeys,
+) (ikeHeader, []payload, error) {
+	if len(packets) == 0 {
+		return ikeHeader{}, nil, errors.New("ike: empty exchange response")
+	}
+	if len(packets) == 1 {
+		return decryptAndValidate(packets[0], initiatorSPI, responderSPI, exchange, messageID, suite, keys)
+	}
+	header, payloads, err := decryptPayloadsAny(nil, packets, suite, keys.SKer, keys.SKar)
+	if err != nil {
+		return ikeHeader{}, nil, err
+	}
+	if header.InitiatorSPI != initiatorSPI ||
+		header.ResponderSPI != responderSPI ||
+		header.Exchange != exchange ||
+		header.MessageID != messageID ||
+		header.Flags&flagResponse == 0 ||
+		header.Flags&flagInitiator != 0 {
+		return ikeHeader{}, nil, fmt.Errorf("%w: encrypted response header does not match the request", errUnexpectedPacket)
+	}
+	return header, payloads, nil
+}
+
+func sendAndReceiveIKEPayloads(
+	ctx context.Context,
+	transport datagramTransport,
+	header ikeHeader,
+	payloads []payload,
+	suite negotiatedSuite,
+	keys ikeKeys,
+	peerSupportsFragmentation bool,
+	random io.Reader,
+) (ikeHeader, []payload, error) {
+	var outboundPackets [][]byte
+	var err error
+	if peerSupportsFragmentation {
+		outboundPackets, err = encryptPayloadsFragmented(header, payloads, suite, keys.SKei, keys.SKai, defaultIKEFragmentSize, random)
+	} else {
+		pkt, encryptErr := encryptPayloads(header, payloads, suite, keys.SKei, keys.SKai, random)
+		if encryptErr != nil {
+			return ikeHeader{}, nil, encryptErr
+		}
+		outboundPackets = [][]byte{pkt}
+	}
+	if err != nil {
+		return ikeHeader{}, nil, err
+	}
+	inboundPackets, err := transport.RoundTripExchange(ctx, outboundPackets)
+	if err != nil {
+		return ikeHeader{}, nil, err
+	}
+	return decryptAndValidateFragments(inboundPackets, header.InitiatorSPI, header.ResponderSPI, header.Exchange, header.MessageID, suite, keys)
+}
+
+func hasNotifyType(payloads []payload, notifyType uint16) bool {
+	for _, item := range payloadsOfType(payloads, payloadNotify) {
+		kind, _, err := parseNotify(item)
+		if err == nil && kind == notifyType {
+			return true
+		}
+	}
+	return false
+}
+
 var errNoProposalChosen = errors.New("ike: responder reported NO_PROPOSAL_CHOSEN")

 type invalidKEPayloadError struct {
@@ -76,7 +76,7 @@ func (transport *firstAuthCaptureTransport) Float(context.Context) error {
 	return nil
 }

-func (transport *firstAuthCaptureTransport) RoundTrip(_ context.Context, packet []byte) ([]byte, error) {
+func (transport *firstAuthCaptureTransport) RoundTrip(ctx context.Context, packet []byte) ([]byte, error) {
 	transport.calls++
 	if len(transport.cookieChallenge) > 0 {
 		switch transport.calls {
@@ -106,6 +106,17 @@ func (transport *firstAuthCaptureTransport) RoundTrip(_ context.Context, packet
 	}
 }

+func (transport *firstAuthCaptureTransport) RoundTripExchange(ctx context.Context, packets [][]byte) ([][]byte, error) {
+	if len(packets) == 0 {
+		return nil, errors.New("test: empty outbound packets")
+	}
+	resp, err := transport.RoundTrip(ctx, packets[0])
+	if err != nil {
+		return nil, err
+	}
+	return [][]byte{resp}, nil
+}
+
 func (transport *firstAuthCaptureTransport) answerIKECookie(packet []byte) ([]byte, error) {
 	header, _, err := parseIKEPacket(packet)
 	if err != nil {
@@ -146,8 +157,8 @@ func (transport *firstAuthCaptureTransport) verifyIKECookie(packet []byte) error
 		return errors.New("test: first retried IKE_SA_INIT payload is not the expected COOKIE")
 	}
 	cookies := payloadsOfType(payloads, payloadNotify)
-	if len(cookies) != 3 {
-		return fmt.Errorf("test: retried IKE_SA_INIT has %d notify payloads, want 3", len(cookies))
+	if len(cookies) != 4 {
+		return fmt.Errorf("test: retried IKE_SA_INIT has %d notify payloads, want 4", len(cookies))
 	}
 	found := false
 	for _, item := range cookies {
@@ -39,7 +39,7 @@ func newSessionRelay(
 	keepalive time.Duration,
 ) *sessionRelay {
 	if keepalive <= 0 {
-		keepalive = 20 * time.Second
+		keepalive = 15 * time.Second
 	}
 	ctx, cancel := context.WithCancel(context.Background())
 	relay := &sessionRelay{
@@ -3,6 +3,7 @@ package ike
 import (
 	"bytes"
 	"context"
+	"errors"
 	"net"
 	"sync"
 	"sync/atomic"
@@ -49,6 +50,16 @@ func (*fakeSessionTransport) Float(context.Context) error { return nil }
 func (*fakeSessionTransport) RoundTrip(context.Context, []byte) ([]byte, error) {
 	return nil, context.DeadlineExceeded
 }
+func (t *fakeSessionTransport) RoundTripExchange(ctx context.Context, packets [][]byte) ([][]byte, error) {
+	if len(packets) == 0 {
+		return nil, errors.New("empty outbound packets")
+	}
+	resp, err := t.RoundTrip(ctx, packets[0])
+	if err != nil {
+		return nil, err
+	}
+	return [][]byte{resp}, nil
+}
 func (transport *fakeSessionTransport) SendESP(ctx context.Context, packet []byte) error {
 	return transport.SendSessionPacket(ctx, packet, false)
 }
@@ -20,6 +20,7 @@ type datagramTransport interface {
 	RemoteAddr() *net.UDPAddr
 	Float(context.Context) error
 	RoundTrip(context.Context, []byte) ([]byte, error)
+	RoundTripExchange(context.Context, [][]byte) ([][]byte, error)
 	SendESP(context.Context, []byte) error
 	ReceiveESP(context.Context, []byte) (int, error)
 	SendSessionPacket(context.Context, []byte, bool) error
@@ -96,6 +97,29 @@ func roundTripDatagram(
 	read func([]byte, time.Time) (int, error),
 	packet []byte,
 ) ([]byte, error) {
+	writeAll := func(values [][]byte) error {
+		if len(values) > 0 {
+			return write(values[0])
+		}
+		return nil
+	}
+	responses, err := roundTripFragments(ctx, timeout, writeAll, read, [][]byte{packet})
+	if err != nil {
+		return nil, err
+	}
+	if len(responses) == 0 {
+		return nil, errors.New("ike: empty datagram response")
+	}
+	return responses[0], nil
+}
+
+func roundTripFragments(
+	ctx context.Context,
+	timeout time.Duration,
+	writeAll func([][]byte) error,
+	read func([]byte, time.Time) (int, error),
+	packets [][]byte,
+) ([][]byte, error) {
 	if ctx == nil {
 		ctx = context.Background()
 	}
@@ -110,20 +134,44 @@ func roundTripDatagram(
 		if err := ctx.Err(); err != nil {
 			return nil, err
 		}
-		if err := write(packet); err != nil {
+		if err := writeAll(packets); err != nil {
 			return nil, err
 		}
 		attemptDeadline := time.Now().Add(interval)
 		if deadline.Before(attemptDeadline) {
 			attemptDeadline = deadline
 		}
+		var (
+			totalExpected uint16
+			fragments     = make(map[uint16][]byte)
+		)
 		for time.Now().Before(attemptDeadline) {
 			if err := ctx.Err(); err != nil {
 				return nil, err
 			}
 			n, err := read(buffer, attemptDeadline)
 			if err == nil {
-				return append([]byte(nil), buffer[:n]...), nil
+				pkt := append([]byte(nil), buffer[:n]...)
+				header, body, parseErr := parseIKEPacket(pkt)
+				if parseErr == nil && header.NextPayload == payloadEncryptedFragment && len(body) >= 8 {
+					fragNum := binary.BigEndian.Uint16(body[4:6])
+					total := binary.BigEndian.Uint16(body[6:8])
+					if total > 1 {
+						if totalExpected == 0 {
+							totalExpected = total
+						}
+						fragments[fragNum] = pkt
+						if uint16(len(fragments)) == totalExpected {
+							res := make([][]byte, 0, totalExpected)
+							for i := uint16(1); i <= totalExpected; i++ {
+								res = append(res, fragments[i])
+							}
+							return res, nil
+						}
+						continue
+					}
+				}
+				return [][]byte{pkt}, nil
 			}
 			if timeoutError, ok := err.(net.Error); ok && timeoutError.Timeout() {
 				lastErr = err
@@ -222,25 +270,47 @@ func (transport *directUDP) Float(ctx context.Context) error {
 }

 func (transport *directUDP) RoundTrip(ctx context.Context, packet []byte) ([]byte, error) {
+	responses, err := transport.RoundTripExchange(ctx, [][]byte{packet})
+	if err != nil {
+		return nil, err
+	}
+	if len(responses) == 0 {
+		return nil, errors.New("ike: empty exchange response")
+	}
+	return responses[0], nil
+}
+
+func (transport *directUDP) RoundTripExchange(ctx context.Context, packets [][]byte) ([][]byte, error) {
 	transport.mu.Lock()
 	defer transport.mu.Unlock()
 	if transport.conn == nil {
 		return nil, errors.New("ike: UDP transport is closed")
 	}
-	requestHeader, _, err := parseIKEPacket(packet)
+	if len(packets) == 0 {
+		return nil, errors.New("ike: outbound packet list is empty")
+	}
+	requestHeader, _, err := parseIKEPacket(packets[0])
 	if err != nil {
 		return nil, fmt.Errorf("ike: invalid outbound packet: %w", err)
 	}
-	wirePacket := packet
-	if transport.floated {
-		wirePacket = append([]byte{0, 0, 0, 0}, packet...)
-	}
-	write := func(value []byte) error {
-		if err := transport.conn.SetWriteDeadline(deadlineFor(ctx, transport.config.Timeout)); err != nil {
-			return err
+	var wirePackets [][]byte
+	for _, pkt := range packets {
+		wire := pkt
+		if transport.floated {
+			wire = append([]byte{0, 0, 0, 0}, pkt...)
 		}
-		_, err := transport.conn.Write(value)
-		return err
+		wirePackets = append(wirePackets, wire)
+	}
+	writeAll := func(values [][]byte) error {
+		for _, value := range values {
+			if err := transport.conn.SetWriteDeadline(deadlineFor(ctx, transport.config.Timeout)); err != nil {
+				return err
+			}
+			if _, err := transport.conn.Write(value); err != nil {
+				return err
+			}
+		}
+		return nil
 	}
 	read := func(buffer []byte, attemptDeadline time.Time) (int, error) {
 		for {
@@ -252,10 +322,6 @@ func (transport *directUDP) RoundTrip(ctx context.Context, packet []byte) ([]byt
 				return 0, err
 			}
 			if transport.floated {
-				// IKE and ESP legitimately share UDP/4500. An ESP packet can
-				// arrive immediately before the IKE response that completes
-				// CHILD_SA setup; discard it here and keep the same absolute
-				// attempt deadline while waiting for marked IKE.
 				if !hasNonESPMarker(buffer[:n]) {
 					continue
 				}
@@ -268,7 +334,7 @@ func (transport *directUDP) RoundTrip(ctx context.Context, packet []byte) ([]byt
 			return n, nil
 		}
 	}
-	return roundTripDatagram(ctx, transport.config.Timeout, write, read, wirePacket)
+	return roundTripFragments(ctx, transport.config.Timeout, writeAll, read, wirePackets)
 }

 func (transport *directUDP) SendESP(ctx context.Context, packet []byte) error {
@@ -561,12 +627,26 @@ func (transport *socks5UDP) Float(_ context.Context) error {
 }

 func (transport *socks5UDP) RoundTrip(ctx context.Context, packet []byte) ([]byte, error) {
+	responses, err := transport.RoundTripExchange(ctx, [][]byte{packet})
+	if err != nil {
+		return nil, err
+	}
+	if len(responses) == 0 {
+		return nil, errors.New("ike: empty exchange response")
+	}
+	return responses[0], nil
+}
+
+func (transport *socks5UDP) RoundTripExchange(ctx context.Context, packets [][]byte) ([][]byte, error) {
 	transport.mu.Lock()
 	defer transport.mu.Unlock()
 	if transport.udp == nil {
 		return nil, errors.New("ike: SOCKS5 UDP transport is closed")
 	}
-	requestHeader, _, err := parseIKEPacket(packet)
+	if len(packets) == 0 {
+		return nil, errors.New("ike: outbound packet list is empty")
+	}
+	requestHeader, _, err := parseIKEPacket(packets[0])
 	if err != nil {
 		return nil, fmt.Errorf("ike: invalid outbound packet: %w", err)
 	}
@@ -576,18 +656,18 @@ func (transport *socks5UDP) RoundTrip(ctx context.Context, packet []byte) ([]byt
 	// Once a gateway answers, keep it pinned for the lifetime of the IKE SA.
 	if !transport.floated && requestHeader.Exchange == exchangeIKEInit && requestHeader.MessageID == 0 && len(transport.remotes) > 1 {
 		var lastErr error
-		var cookieResponse []byte
+		var cookieResponse [][]byte
 		for _, candidate := range transport.remotes {
 			transport.remote = cloneUDPAddr(candidate)
-			response, attemptErr := transport.roundTripLocked(ctx, packet, requestHeader)
+			responses, attemptErr := transport.roundTripFragmentsLocked(ctx, packets, requestHeader)
 			if attemptErr == nil {
-				if ikeInitResponseHasCookie(response) {
+				if len(responses) > 0 && ikeInitResponseHasCookie(responses[0]) {
 					if cookieResponse == nil {
-						cookieResponse = append([]byte(nil), response...)
+						cookieResponse = responses
 					}
 					continue
 				}
-				return response, nil
+				return responses, nil
 			}
 			lastErr = attemptErr
 			if ctx.Err() != nil || !isNetworkTimeout(attemptErr) {
@@ -599,24 +679,32 @@ func (transport *socks5UDP) RoundTrip(ctx context.Context, packet []byte) ([]byt
 		}
 		return nil, fmt.Errorf("ike: all %d resolved ePDG addresses timed out: %w", len(transport.remotes), lastErr)
 	}
-	return transport.roundTripLocked(ctx, packet, requestHeader)
+	return transport.roundTripFragmentsLocked(ctx, packets, requestHeader)
 }

-func (transport *socks5UDP) roundTripLocked(ctx context.Context, packet []byte, requestHeader ikeHeader) ([]byte, error) {
-	wireIKE := packet
-	if transport.floated {
-		wireIKE = append([]byte{0, 0, 0, 0}, packet...)
-	}
-	datagram, err := marshalSOCKS5Datagram(transport.remote, wireIKE)
-	if err != nil {
-		return nil, err
-	}
-	write := func(value []byte) error {
-		if err := transport.udp.SetWriteDeadline(deadlineFor(ctx, transport.config.Timeout)); err != nil {
-			return err
+func (transport *socks5UDP) roundTripFragmentsLocked(ctx context.Context, packets [][]byte, requestHeader ikeHeader) ([][]byte, error) {
+	var datagrams [][]byte
+	for _, pkt := range packets {
+		wireIKE := pkt
+		if transport.floated {
+			wireIKE = append([]byte{0, 0, 0, 0}, pkt...)
 		}
-		_, err := transport.udp.Write(value)
-		return err
+		datagram, err := marshalSOCKS5Datagram(transport.remote, wireIKE)
+		if err != nil {
+			return nil, err
+		}
+		datagrams = append(datagrams, datagram)
+	}
+	writeAll := func(values [][]byte) error {
+		for _, value := range values {
+			if err := transport.udp.SetWriteDeadline(deadlineFor(ctx, transport.config.Timeout)); err != nil {
+				return err
+			}
+			if _, err := transport.udp.Write(value); err != nil {
+				return err
+			}
+		}
+		return nil
 	}
 	read := func(buffer []byte, attemptDeadline time.Time) (int, error) {
 		for {
@@ -630,10 +718,6 @@ func (transport *socks5UDP) roundTripLocked(ctx context.Context, packet []byte,
 				return 0, err
 			}
 			if transport.floated {
-				// The relay can deliver ESP before the marked IKE response on
-				// the same UDP/4500 association. Do not accept it as IKE, and
-				// do not abort the exchange; keep waiting within the original
-				// deadline.
 				if !hasNonESPMarker(payload) {
 					continue
 				}
@@ -646,7 +730,7 @@ func (transport *socks5UDP) roundTripLocked(ctx context.Context, packet []byte,
 			return len(payload), nil
 		}
 	}
-	return roundTripDatagram(ctx, transport.config.Timeout, write, read, datagram)
+	return roundTripFragments(ctx, transport.config.Timeout, writeAll, read, datagrams)
 }

 func isNetworkTimeout(err error) bool {
@@ -31,9 +31,10 @@ const (
 	payloadDelete    = 42
 	payloadTSi       = 44
 	payloadTSr       = 45
-	payloadEncrypted = 46
-	payloadCP        = 47
-	payloadEAP       = 48
+	payloadEncrypted         = 46
+	payloadCP                = 47
+	payloadEAP               = 48
+	payloadEncryptedFragment = 53

 	protocolIKE = 1
 	protocolESP = 3
@@ -55,15 +56,16 @@ const (
 	dhMODP2048               = 14
 	transformAttributeKeyLen = 14

-	notifyInitialContact  = 16384
-	notifyMOBIKESupported = 16396
-	notifyNATSource       = 16388
-	notifyNATDestination  = 16389
-	notifyCookie          = 16390
-	notifyEAPOnlyAuth     = 16417
-	notifyDeviceIdentity  = 41101
-	notifyInvalidKE       = 17
-	notifyNoProposal      = 14
+	notifyInitialContact         = 16384
+	notifyMOBIKESupported        = 16396
+	notifyNATSource              = 16388
+	notifyNATDestination         = 16389
+	notifyCookie                 = 16390
+	notifyEAPOnlyAuth            = 16417
+	notifyFragmentationSupported = 16430
+	notifyDeviceIdentity         = 41101
+	notifyInvalidKE              = 17
+	notifyNoProposal             = 14
 )

 var (