Add a structure for the DNS question, use it DNS server to store the question data and to create the DNS answer from scratch.

Author: Laurent Louf

libraries/DNSServer/src/DNSServer.cpp

@@ -7,7 +7,8 @@ DNSServer::DNSServer()
 {
   _ttl = htonl(DNS_DEFAULT_TTL);
   _errorReplyCode = DNSReplyCode::NonExistentDomain;
-  _dnsHeader  = NULL;
+  _dnsHeader    = (DNSHeader*) malloc( sizeof(DNSHeader) ) ;
+  _dnsQuestion  = (DNSQuestion*) malloc( sizeof(DNSQuestion) ) ;     
   _buffer     = NULL;
   _currentPacketSize = 0;
   _port = 0;
@@ -62,12 +63,30 @@ void DNSServer::processNextRequest()
     if (_buffer == NULL) 
       return;
 
-    // Put the packet received in the buffer and get DNS header (beginning of message), 
-    // type and class (last 4 bytes of the message)
+    // Put the packet received in the buffer and get DNS header (beginning of message)
+    // and the question
     _udp.read(_buffer, _currentPacketSize);
-    _dnsHeader  = (DNSHeader*) _buffer;
-    _type       = (uint16_t) ( (_buffer[_currentPacketSize - 4] << 8) + _buffer[_currentPacketSize - 3] ) ; 
-    _class      = (uint16_t) ( (_buffer[_currentPacketSize - 2] << 8) + _buffer[_currentPacketSize - 1] ) ; 
+    memcpy( _dnsHeader, _buffer, DNS_HEADER_SIZE ) ; 
+    if ( requestIncludesOnlyOneQuestion() )
+    {
+      // The QName has a variable length, maximum 255 bytes and is comprised of multiple labels.
+      // Each label contains a byte to describe its length and the label itself. The list of 
+      // labels terminates with a zero-valued byte. In "github.com", we have two labels "github" & "com"
+      // Iterate through the labels and copy them as they come into a single buffer (for simplicity's sake)
+      _dnsQuestion->QNameLength = 0 ;
+      while ( _buffer[ DNS_HEADER_SIZE + _dnsQuestion->QNameLength ] != 0 )
+      {
+        memcpy( (void*) &_dnsQuestion->QName[_dnsQuestion->QNameLength], (void*) &_buffer[DNS_HEADER_SIZE + _dnsQuestion->QNameLength], _buffer[DNS_HEADER_SIZE + _dnsQuestion->QNameLength] + 1 ) ;
+        _dnsQuestion->QNameLength += _buffer[DNS_HEADER_SIZE + _dnsQuestion->QNameLength] + 1 ; 
+      }
+      _dnsQuestion->QName[_dnsQuestion->QNameLength] = 0 ; 
+      _dnsQuestion->QNameLength++ ;   
+
+      // Copy the QType and QClass 
+      memcpy( &_dnsQuestion->QType, (void*) &_buffer[DNS_HEADER_SIZE + _dnsQuestion->QNameLength], sizeof(_dnsQuestion->QType) ) ;
+      memcpy( &_dnsQuestion->QClass, (void*) &_buffer[DNS_HEADER_SIZE + _dnsQuestion->QNameLength + sizeof(_dnsQuestion->QType)], sizeof(_dnsQuestion->QClass) ) ;
+    }
+    
 
     if (_dnsHeader->QR == DNS_QR_QUERY &&
         _dnsHeader->OPCode == DNS_OPCODE_QUERY &&
@@ -135,31 +154,35 @@ String DNSServer::getDomainNameWithoutWwwPrefix()
 void DNSServer::replyWithIP()
 {
   if (_buffer == NULL) return;
-  _dnsHeader->QR = DNS_QR_RESPONSE;
+  
+  _udp.beginPacket(_udp.remoteIP(), _udp.remotePort());
+  
+  // Change the type of message to a response and set the number of answers equal to 
+  // the number of questions in the header
+  _dnsHeader->QR      = DNS_QR_RESPONSE;
   _dnsHeader->ANCount = _dnsHeader->QDCount;
-  //_dnsHeader->QDCount = 0;
+  _udp.write( (unsigned char*) _dnsHeader, DNS_HEADER_SIZE ) ;
 
-  _udp.beginPacket(_udp.remoteIP(), _udp.remotePort());
-  _udp.write(_buffer, _currentPacketSize);
+  // Write the question
+  _udp.write(_dnsQuestion->QName, _dnsQuestion->QNameLength) ;
+  _udp.write( (unsigned char*) &_dnsQuestion->QType, 2 ) ;
+  _udp.write( (unsigned char*) &_dnsQuestion->QClass, 2 ) ;
 
+  // Write the answer 
   // Use DNS name compression : instead of repeating the name in this RNAME occurence,
   // set the two MSB of the byte corresponding normally to the length to 1. The following
   // 14 bits must be used to specify the offset of the domain name in the message 
   // (<255 here so the first byte has the 6 LSB at 0) 
   _udp.write((uint8_t) 0xC0); 
   _udp.write((uint8_t) DNS_OFFSET_DOMAIN_NAME);  
-  // DNS type A : host address
-  _udp.write((uint8_t) (DNS_TYPE_A >> 8) ); 
-  _udp.write((uint8_t) (DNS_TYPE_A & 0xFF) );
-  // DNS class IN for INternet
-  _udp.write((uint8_t) (DNS_CLASS_IN >> 8) );
-  _udp.write((uint8_t) (DNS_CLASS_IN & 0xFF) );
-  // DNS Time To Live
-  _udp.write((unsigned char*)&_ttl, 4);
-  // Returning an IPv4 address
-  _udp.write((uint8_t) (DNS_RDLENGTH_IPV4 >> 8) );
-  _udp.write((uint8_t) (DNS_RDLENGTH_IPV4 & 0xFF) );
-  _udp.write(_resolvedIP, sizeof(_resolvedIP));
+
+  // DNS type A : host address, DNS class IN for INternet, returning an IPv4 address 
+  uint16_t answerType = htons(DNS_TYPE_A), answerClass = htons(DNS_CLASS_IN), answerIPv4 = htons(DNS_RDLENGTH_IPV4)  ; 
+  _udp.write((unsigned char*) &answerType, 2 );
+  _udp.write((unsigned char*) &answerClass, 2 );
+  _udp.write((unsigned char*) &_ttl, 4);        // DNS Time To Live
+  _udp.write((unsigned char*) &answerIPv4, 2 );
+  _udp.write(_resolvedIP, sizeof(_resolvedIP)); // The IP address to return
   _udp.endPacket();
 }
 

libraries/DNSServer/src/DNSServer.h

@@ -7,6 +7,7 @@
 #define DNS_OPCODE_QUERY 0
 #define DNS_DEFAULT_TTL 60        // Default Time To Live : time interval in seconds that the resource record should be cached before being discarded
 #define DNS_OFFSET_DOMAIN_NAME 12 // Offset in bytes to reach the domain name in the DNS message 
+#define DNS_HEADER_SIZE 12 
 
 enum class DNSReplyCode
 {
@@ -63,6 +64,14 @@ struct DNSHeader
   uint16_t ARCount;          // number of resource entries
 };
 
+struct DNSQuestion
+{
+  uint8_t   QName[255] ;
+  int8_t    QNameLength ; 
+  uint16_t  QType ; 
+  uint16_t  QClass ; 
+} ; 
+
 class DNSServer
 {
   public:
@@ -88,8 +97,8 @@ class DNSServer
     DNSHeader* _dnsHeader;
     uint32_t _ttl;
     DNSReplyCode _errorReplyCode;
-    uint16_t _type ; 
-    uint16_t _class ; 
+    DNSQuestion*    _dnsQuestion ; 
+
 
     void downcaseAndRemoveWwwPrefix(String &domainName);
     String getDomainNameWithoutWwwPrefix();