Synthesize a truncated response if the response wouldn't fit the local MSS

dnscrypt-proxy/common.go

@@ -27,10 +27,6 @@ var (
 	CertRefreshDelay       = 30 * time.Minute
 )
 
-func HasTCFlag(packet []byte) bool {
-	return packet[2]&2 == 2
-}
-
 func PrefixWithSize(packet []byte) ([]byte, error) {
 	packet_len := len(packet)
 	if packet_len > 0xffff {

dnscrypt-proxy/crypto.go

@@ -12,6 +12,7 @@ const (
 	NonceSize        = xsecretbox.NonceSize
 	HalfNonceSize    = xsecretbox.NonceSize / 2
 	TagSize          = xsecretbox.TagSize
+	ResponseOverhead = len(ServerMagic) + NonceSize + TagSize
 )
 
 func pad(packet []byte, minSize int) []byte {
@@ -40,22 +41,22 @@ func (proxy *Proxy) Encrypt(serverInfo *ServerInfo, packet []byte, proto string)
 	nonce, clientNonce := make([]byte, NonceSize), make([]byte, HalfNonceSize)
 	rand.Read(clientNonce)
 	copy(nonce, clientNonce)
-	minQuestionSize := len(packet)
+	minQuestionSize := ResponseOverhead + len(packet)
 	if proto == "udp" {
-		minQuestionSize = proxy.questionSizeEstimator.MinQuestionSize()
+		minQuestionSize = Max(proxy.questionSizeEstimator.MinQuestionSize(), minQuestionSize)
 	} else {
 		var xpad [1]byte
 		rand.Read(xpad[:])
 		minQuestionSize += int(xpad[0])
 	}
-	paddedLength := Min((minQuestionSize+63)&^63, MaxDNSUDPPacketSize-1)
-	if paddedLength <= 0 || len(packet) >= paddedLength {
+	paddedLength := Min(MaxDNSUDPPacketSize, (Max(minQuestionSize, ResponseOverhead)+63) & ^63)
+	if ResponseOverhead+len(packet)+1 > paddedLength {
 		err = errors.New("Question too large; cannot be padded")
 		return
 	}
 	encrypted = append(serverInfo.MagicQuery[:], proxy.proxyPublicKey[:]...)
 	encrypted = append(encrypted, nonce[:HalfNonceSize]...)
-	encrypted = xsecretbox.Seal(encrypted, nonce, pad(packet, paddedLength), serverInfo.SharedKey[:])
+	encrypted = xsecretbox.Seal(encrypted, nonce, pad(packet, paddedLength-ResponseOverhead), serverInfo.SharedKey[:])
 	return
 }
 
@@ -63,8 +64,9 @@ func (proxy *Proxy) Decrypt(serverInfo *ServerInfo, encrypted []byte, nonce []by
 	serverMagicLen := len(ServerMagic)
 	responseHeaderLen := serverMagicLen + NonceSize
 	if len(encrypted) < responseHeaderLen+TagSize+int(MinDNSPacketSize) ||
+		len(encrypted) > responseHeaderLen+TagSize+int(MaxDNSPacketSize) ||
 		!bytes.Equal(encrypted[:serverMagicLen], ServerMagic[:]) {
-		return encrypted, errors.New("Short message")
+		return encrypted, errors.New("Invalid message size or prefix")
 	}
 	serverNonce := encrypted[serverMagicLen:responseHeaderLen]
 	if !bytes.Equal(nonce[:HalfNonceSize], serverNonce[:HalfNonceSize]) {

dnscrypt-proxy/dnsutils.go

@@ -0,0 +1,23 @@
+package main
+
+import (
+	"github.com/miekg/dns"
+)
+
+func TruncatedResponse(packet []byte) ([]byte, error) {
+	srcMsg := new(dns.Msg)
+	if err := srcMsg.Unpack(packet); err != nil {
+		return nil, err
+	}
+	dstMsg := srcMsg
+	dstMsg.Response = true
+	dstMsg.Answer = make([]dns.RR, 0)
+	dstMsg.Ns = make([]dns.RR, 0)
+	dstMsg.Extra = make([]dns.RR, 0)
+	dstMsg.Truncated = true
+	return dstMsg.Pack()
+}
+
+func HasTCFlag(packet []byte) bool {
+	return packet[2]&2 == 2
+}

dnscrypt-proxy/estimators.go

@@ -20,7 +20,7 @@ func (questionSizeEstimator *QuestionSizeEstimator) MinQuestionSize() int {
 
 func (questionSizeEstimator *QuestionSizeEstimator) blindAdjust() {
 	questionSizeEstimator.Lock()
-	if MaxDNSPacketSize-questionSizeEstimator.minQuestionSize < questionSizeEstimator.minQuestionSize {
+	if MaxDNSUDPPacketSize-questionSizeEstimator.minQuestionSize < questionSizeEstimator.minQuestionSize {
 		questionSizeEstimator.minQuestionSize = MaxDNSUDPPacketSize
 	} else {
 		questionSizeEstimator.minQuestionSize *= 2

dnscrypt-proxy/main.go

@@ -142,22 +142,28 @@ func (proxy *Proxy) processIncomingQuery(serverInfo *ServerInfo, serverProto str
 	if err != nil {
 		return
 	}
-	var encryptedResponse []byte
+	var response []byte
 	if serverProto == "udp" {
-		encryptedResponse, err = proxy.exchangeWithUDPServer(serverInfo, encryptedQuery, clientNonce)
+		response, err = proxy.exchangeWithUDPServer(serverInfo, encryptedQuery, clientNonce)
 	} else {
-		encryptedResponse, err = proxy.exchangeWithTCPServer(serverInfo, encryptedQuery, clientNonce)
+		response, err = proxy.exchangeWithTCPServer(serverInfo, encryptedQuery, clientNonce)
 	}
 	if err != nil {
 		return
 	}
 	if clientAddr != nil {
-		clientPc.(net.PacketConn).WriteTo(encryptedResponse, *clientAddr)
-		if HasTCFlag(encryptedResponse) {
+		if len(response) > MaxDNSUDPPacketSize {
+			response, err = TruncatedResponse(response)
+			if err != nil {
+				return
+			}
+		}
+		clientPc.(net.PacketConn).WriteTo(response, *clientAddr)
+		if HasTCFlag(response) {
 			proxy.questionSizeEstimator.blindAdjust()
 		}
 	} else {
-		response, err := PrefixWithSize(encryptedResponse)
+		response, err = PrefixWithSize(response)
 		if err != nil {
 			return
 		}