avoid send not tcp dns packet (#262)

Author: Leonardo Parente

src/handlers/dns/DnsStreamHandler.cpp

@@ -180,6 +180,10 @@ void DnsStreamHandler::process_udp_packet_cb(pcpp::Packet &payload, PacketDirect
 
 void TcpSessionData::receive_dns_wire_data(const uint8_t *data, size_t len)
 {
+    if (_invalid_data) {
+        return;
+    }
+
     _buffer.append(reinterpret_cast<const char *>(data), len);
 
     for (;;) {
@@ -193,6 +197,13 @@ void TcpSessionData::receive_dns_wire_data(const uint8_t *data, size_t len)
         // dns packet size is in network byte order.
         size = static_cast<unsigned char>(_buffer[1]) | static_cast<unsigned char>(_buffer[0]) << 8;
 
+        //if size is less than MIN_DNS_QUERY_SIZE, it is not a dns packet
+        if (size < MIN_DNS_QUERY_SIZE) {
+            _buffer.clear();
+            _invalid_data = true;
+            break;
+        }
+
         if (_buffer.size() >= sizeof(size) + size) {
             auto data = std::make_unique<uint8_t[]>(size);
             std::memcpy(data.get(), _buffer.data() + sizeof(size), size);

src/handlers/dns/DnsStreamHandler.h

@@ -213,11 +213,13 @@ class TcpSessionData final
 private:
     std::string _buffer;
     got_msg_cb _got_dns_msg;
+    bool _invalid_data;
 
 public:
     TcpSessionData(
         got_msg_cb got_data_handler)
         : _got_dns_msg{std::move(got_data_handler)}
+        , _invalid_data(false)
     {
     }