DNSServer: Clarify and tidy writing the answer record

Modify the code used to write the answer record back to the server
so that it is clearer that we are writing network byte order
16-bit quantities, and to clarify what's happening with the pointer
used at the start of the answer.

Author: SimonWilkinson

Diff for: libraries/DNSServer/src/DNSServer.cpp

@@ -186,36 +186,43 @@ void DNSServer::processNextRequest()
   respondToRequest(buffer.get(), currentPacketSize);
 }
 
+void DNSServer::writeNBOShort(uint16_t value)
+{
+   _udp.write((unsigned char *)&value, 2);
+}
+
 void DNSServer::replyWithIP(DNSHeader *dnsHeader,
 			    unsigned char * query,
 			    size_t queryLength)
 {
+  uint16_t value;
+
   dnsHeader->QR = DNS_QR_RESPONSE;
   dnsHeader->QDCount = lwip_htons(1);
   dnsHeader->ANCount = lwip_htons(1);
   dnsHeader->NSCount = 0;
   dnsHeader->ARCount = 0;
 
-  //_dnsHeader->RA = 1;
-
   _udp.beginPacket(_udp.remoteIP(), _udp.remotePort());
   _udp.write((unsigned char *) dnsHeader, sizeof(DNSHeader));
   _udp.write(query, queryLength);
 
-  _udp.write((uint8_t)192); //  answer name is a pointer
-  _udp.write((uint8_t)12);  // pointer to offset at 0x00c
+  // Rather than restate the name here, we use a pointer to the name contained
+  // in the query section. Pointers have the top two bits set.
+  value = 0xC000 | DNS_HEADER_SIZE;
+  writeNBOShort(lwip_htons(value));
+
+  // Answer is type A (an IPv4 address)
+  writeNBOShort(lwip_htons(DNS_QTYPE_A));
 
-  _udp.write((uint8_t)0);   // 0x0001  answer is type A query (host address)
-  _udp.write((uint8_t)1);
+  // Answer is in the Internet Class
+  writeNBOShort(lwip_htons(DNS_QCLASS_IN));
 
-  _udp.write((uint8_t)0);   //0x0001 answer is class IN (internet address)
-  _udp.write((uint8_t)1);
- 
+  // Output TTL (already NBO)
   _udp.write((unsigned char*)&_ttl, 4);
 
   // Length of RData is 4 bytes (because, in this case, RData is IPv4)
-  _udp.write((uint8_t)0);
-  _udp.write((uint8_t)4);
+  writeNBOShort(lwip_htons(sizeof(_resolvedIP)));
   _udp.write(_resolvedIP, sizeof(_resolvedIP));
   _udp.endPacket();
 }

Diff for: libraries/DNSServer/src/DNSServer.h

@@ -82,5 +82,6 @@ class DNSServer
     void replyWithError(DNSHeader *dnsHeader,
 			DNSReplyCode rcode);
     void respondToRequest(uint8_t *buffer, size_t length);
+    void writeNBOShort(uint16_t value);
 };
 #endif