DNS Server : bug fix and prettifying (#1011)

* Retrieve some code from what has been done on the ESP8266. Clarify a bit the signification of several bytes in the response.

* Add the type and class as members of the DNS class for an eventual future use.

* Clarify the sense of a magic number present in DNS server.

* A bit of aesthetics for the DNS server.

* 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: LaurentLouf

libraries/DNSServer/src/DNSServer.cpp

@@ -5,10 +5,11 @@
 
 DNSServer::DNSServer()
 {
-  _ttl = htonl(60);
+  _ttl = htonl(DNS_DEFAULT_TTL);
   _errorReplyCode = DNSReplyCode::NonExistentDomain;
-  _dnsHeader = NULL;
-  _buffer = NULL;
+  _dnsHeader    = (DNSHeader*) malloc( sizeof(DNSHeader) ) ;
+  _dnsQuestion  = (DNSQuestion*) malloc( sizeof(DNSQuestion) ) ;     
+  _buffer     = NULL;
   _currentPacketSize = 0;
   _port = 0;
 }
@@ -55,11 +56,37 @@ void DNSServer::processNextRequest()
   _currentPacketSize = _udp.parsePacket();
   if (_currentPacketSize)
   {
-    if (_buffer != NULL) free(_buffer);
+    // Allocate buffer for the DNS query
+    if (_buffer != NULL) 
+      free(_buffer);
     _buffer = (unsigned char*)malloc(_currentPacketSize * sizeof(char));
-    if (_buffer == NULL) return;
+    if (_buffer == NULL) 
+      return;
+
+    // Put the packet received in the buffer and get DNS header (beginning of message)
+    // and the question
     _udp.read(_buffer, _currentPacketSize);
-    _dnsHeader = (DNSHeader*) _buffer;
+    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 &&
@@ -87,15 +114,21 @@ bool DNSServer::requestIncludesOnlyOneQuestion()
          _dnsHeader->ARCount == 0;
 }
 
+
 String DNSServer::getDomainNameWithoutWwwPrefix()
 {
+  // Error checking : if the buffer containing the DNS request is a null pointer, return an empty domain
   String parsedDomainName = "";
-  if (_buffer == NULL) return parsedDomainName;
-  unsigned char *start = _buffer + 12;
+  if (_buffer == NULL) 
+    return parsedDomainName;
+  
+  // Set the start of the domain just after the header (12 bytes). If equal to null character, return an empty domain
+  unsigned char *start = _buffer + DNS_OFFSET_DOMAIN_NAME;
   if (*start == 0)
   {
     return parsedDomainName;
   }
+
   int pos = 0;
   while(true)
   {
@@ -121,16 +154,35 @@ String DNSServer::getDomainNameWithoutWwwPrefix()
 void DNSServer::replyWithIP()
 {
   if (_buffer == NULL) return;
-  _dnsHeader->QR = DNS_QR_RESPONSE;
-  _dnsHeader->ANCount = _dnsHeader->QDCount;
-  _dnsHeader->QDCount = 0;
-
+  
   _udp.beginPacket(_udp.remoteIP(), _udp.remotePort());
-  _udp.write(_buffer, _currentPacketSize);
-  _udp.write((unsigned char*)&_ttl, 4);
-  _udp.write((uint8_t)0);
-  _udp.write((uint8_t)4);
-  _udp.write(_resolvedIP, 4);
+  
+  // 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;
+  _udp.write( (unsigned char*) _dnsHeader, DNS_HEADER_SIZE ) ;
+
+  // 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, 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

@@ -5,20 +5,43 @@
 #define DNS_QR_QUERY 0
 #define DNS_QR_RESPONSE 1
 #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
 {
-  NoError = 0,
+  NoError   = 0,
   FormError = 1,
-  ServerFailure = 2,
+  ServerFailure     = 2,
   NonExistentDomain = 3,
-  NotImplemented = 4,
-  Refused = 5,
-  YXDomain = 6,
-  YXRRSet = 7,
-  NXRRSet = 8
+  NotImplemented    = 4,
+  Refused   = 5,
+  YXDomain  = 6,
+  YXRRSet   = 7,
+  NXRRSet   = 8
 };
 
+enum DNSType
+{
+  DNS_TYPE_A      = 1,  // Host Address
+  DNS_TYPE_AAAA   = 28, // IPv6 Address
+  DNS_TYPE_SOA    = 6,  // Start Of a zone of Authority
+  DNS_TYPE_PTR    = 12, // Domain name PoinTeR
+  DNS_TYPE_DNAME  = 39  // Delegation Name
+} ; 
+
+enum DNSClass
+{
+  DNS_CLASS_IN = 1, // INternet
+  DNS_CLASS_CH = 3  // CHaos
+} ; 
+
+enum DNSRDLength
+{
+  DNS_RDLENGTH_IPV4 = 4 // 4 bytes for an IPv4 address 
+} ; 
+
 struct DNSHeader
 {
   uint16_t ID;               // identification number
@@ -41,6 +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:
@@ -66,6 +97,8 @@ class DNSServer
     DNSHeader* _dnsHeader;
     uint32_t _ttl;
     DNSReplyCode _errorReplyCode;
+    DNSQuestion*    _dnsQuestion ; 
+
 
     void downcaseAndRemoveWwwPrefix(String &domainName);
     String getDomainNameWithoutWwwPrefix();