Proper EDNS handling and cleaner NOERROR response

Author: Kolkman

libraries/DNSServer/library.properties

@@ -1,5 +1,5 @@
 name=DNSServer
-version=3.2.0
+version=3.3.0
 author=Kristijan Novoselić
 maintainer=Kristijan Novoselić, <[email protected]>
 sentence=A simple DNS server for ESP32.

libraries/DNSServer/src/DNSServer.cpp

@@ -15,7 +15,7 @@
 DNSServer::DNSServer() : _port(DNS_DEFAULT_PORT), _ttl(htonl(DNS_DEFAULT_TTL)), _errorReplyCode(DNSReplyCode::NonExistentDomain) {}
 
 DNSServer::DNSServer(const String &domainName)
-  : _port(DNS_DEFAULT_PORT), _ttl(htonl(DNS_DEFAULT_TTL)), _errorReplyCode(DNSReplyCode::NonExistentDomain), _domainName(domainName){};
+  : _port(DNS_DEFAULT_PORT), _ttl(htonl(DNS_DEFAULT_TTL)), _errorReplyCode(DNSReplyCode::NonExistentDomain), _domainName(domainName) {};
 
 bool DNSServer::start() {
   if (_resolvedIP.operator uint32_t() == 0) {  // no address is set, try to obtain AP interface's IP
@@ -111,16 +111,22 @@ void DNSServer::_handleUDP(AsyncUDPPacket &pkt) {
   // will reply with IP only to "*" or if domain matches without www. subdomain
   if (dnsHeader.OPCode == DNS_OPCODE_QUERY && requestIncludesOnlyOneQuestion(dnsHeader)
       && (_domainName.isEmpty() || getDomainNameWithoutWwwPrefix(static_cast<const unsigned char *>(dnsQuestion.QName), dnsQuestion.QNameLength) == _domainName)) {
-    replyWithIP(pkt, dnsHeader, dnsQuestion);
+
+    // Qtype = A (1) or ANY (255): send an A record otherwise an empty response
+    if (ntohs(dnsQuestion.QType) == 1 || ntohs(dnsQuestion.QType) == 255) {
+      replyWithIP(pkt, dnsHeader, dnsQuestion);
+    } else {
+      replyWithNoAnsw(pkt, dnsHeader, dnsQuestion);
+    }
     return;
   }
-
   // otherwise reply with custom code
   replyWithCustomCode(pkt, dnsHeader);
 }
 
 bool DNSServer::requestIncludesOnlyOneQuestion(DNSHeader &dnsHeader) {
-  return ntohs(dnsHeader.QDCount) == 1 && dnsHeader.ANCount == 0 && dnsHeader.NSCount == 0 && dnsHeader.ARCount == 0;
+  dnsHeader.ARCount = 0;  // We assume that if ARCount !=0 there is a EDNS OPT packet, just ignore
+  return ntohs(dnsHeader.QDCount) == 1 && dnsHeader.ANCount == 0 && dnsHeader.NSCount == 0;  // && dnsHeader.ARCount == 0;
 }
 
 String DNSServer::getDomainNameWithoutWwwPrefix(const unsigned char *start, size_t len) {
@@ -139,7 +145,6 @@ String DNSServer::getDomainNameWithoutWwwPrefix(const unsigned char *start, size
 
 void DNSServer::replyWithIP(AsyncUDPPacket &req, DNSHeader &dnsHeader, DNSQuestion &dnsQuestion) {
   AsyncUDPMessage rpl;
-
   // 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;
@@ -187,3 +192,77 @@ void DNSServer::replyWithCustomCode(AsyncUDPPacket &req, DNSHeader &dnsHeader) {
   rpl.write(reinterpret_cast<const uint8_t *>(&dnsHeader), sizeof(DNSHeader));
   _udp.sendTo(rpl, req.remoteIP(), req.remotePort());
 }
+
+void DNSServer::replyWithNoAnsw(AsyncUDPPacket &req, DNSHeader &dnsHeader, DNSQuestion &dnsQuestion) {
+
+  dnsHeader.QR = DNS_QR_RESPONSE;
+  // dnsHeader.QDCount = 1;
+  dnsHeader.ANCount = 0;
+  dnsHeader.NSCount = htons(1);
+
+  AsyncUDPMessage rpl;
+  rpl.write(reinterpret_cast<const uint8_t *>(&dnsHeader), sizeof(DNSHeader));
+
+  // Write the question
+  rpl.write(dnsQuestion.QName, dnsQuestion.QNameLength);
+  rpl.write((uint8_t *)&dnsQuestion.QType, 2);
+  rpl.write((uint8_t *)&dnsQuestion.QClass, 2);
+
+  // An empty answer contains an authority section with a SOA,
+  // We take the name of the query as the root of the zone for which the SOA is generated
+  // and use a value of DNS_MINIMAL_TTL seconds  in order to minimize negative caching
+  // Write the authority section:
+  // The SOA RR's ownername is set equal to the query name, and we use made up names for
+  // the MNAME and  RNAME - it doesn't really matter from a protocol perspective - as for
+  // a no such QTYPE answer only the timing fields are used.
+  // a protocol perspective - it
+  // Use DNS name compression : instead of repeating the name in this RNAME occurrence,
+  // 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)
+  rpl.write((uint8_t)0xC0);
+  rpl.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_SOA), answerClass = htons(DNS_CLASS_IN);
+  uint32_t Serial = htonl(DNS_SOA_SERIAL);    // Date type serial based on the date this piece of code was written
+  uint32_t Refresh = htonl(DNS_SOA_REFRESH);  // These timers don't matter, we don't serve zone transfers
+  uint32_t Retry = htonl(DNS_SOA_RETRY);
+  uint32_t Expire = htonl(DNS_SOA_EXPIRE);
+  uint32_t MinTTL = htonl(DNS_MINIMAL_TTL);  // See RFC2308 section 5
+  char MLabel[] = DNS_SOA_MNAME_LABEL;
+  char RLabel[] = DNS_SOA_RNAME_LABEL;
+  char PostFixLabel[] = DNS_SOA_POSTFIX_LABEL;
+
+  // 4 accounts for len fields and  for both rname
+  // and lname and their postfix labels and there are 5 32 bit fields
+
+  uint16_t RdataLength = htons((uint16_t)(strlen(MLabel) + strlen(RLabel) + 2 * strlen(PostFixLabel) + 4 + 5 * sizeof(Serial)));
+
+  rpl.write((unsigned char *)&answerType, 2);
+  rpl.write((unsigned char *)&answerClass, 2);
+  rpl.write((unsigned char *)&MinTTL, 4);  // DNS Time To Live
+
+  rpl.write((unsigned char *)&RdataLength, 2);
+
+  rpl.write((uint8_t)strlen(MLabel));
+  rpl.write((unsigned char *)&MLabel, strlen(MLabel));
+
+  rpl.write((unsigned char *)&PostFixLabel, strlen(PostFixLabel));
+  rpl.write((uint8_t)0);
+  //  rpl.write((uint8_t)0xC0);
+  //  rpl.write((uint8_t)DNS_OFFSET_DOMAIN_NAME);
+
+  rpl.write((uint8_t)strlen(RLabel));
+  rpl.write((unsigned char *)&RLabel, strlen(RLabel));
+  rpl.write((unsigned char *)&PostFixLabel, strlen(PostFixLabel));
+  rpl.write((uint8_t)0);
+
+  rpl.write((unsigned char *)&Serial, 4);
+  rpl.write((unsigned char *)&Refresh, 4);
+  rpl.write((unsigned char *)&Retry, 4);
+  rpl.write((unsigned char *)&Expire, 4);
+  rpl.write((unsigned char *)&MinTTL, 4);
+
+  _udp.sendTo(rpl, req.remoteIP(), req.remotePort());
+}

libraries/DNSServer/src/DNSServer.h

@@ -9,6 +9,23 @@
 #define DNS_OFFSET_DOMAIN_NAME DNS_HEADER_SIZE  // Offset in bytes to reach the domain name labels in the DNS message
 #define DNS_DEFAULT_PORT       53
 
+#define DNS_SOA_MNAME_LABEL "ns"
+#define DNS_SOA_RNAME_LABEL "esp32"
+// The POSTFIX_LABEL will be concatinated to the RName and MName Label  label
+// do not use a multilabel name here. "local" is a good choice as it is reserved for
+// local use by IANA
+// The postfix label is defined as an array of characters that follows the
+// definition of RFC1035 3.1
+// for instance, a postfix of  example.com would be defined as:
+// #define DNS_SOA_POSTFIX_LABEL {'\7', 'e', 'x', 'a', 'm', 'p', 'l', 'e', '\3', 'c', 'o', 'm', '\0'}
+#define DNS_SOA_POSTFIX_LABEL {'\5', 'l', 'o', 'c', 'a', 'l', '\0'}
+// From the following values only the MINIMAL_TTL has relevance
+// in the context of client-server protocol interactions.
+#define DNS_SOA_SERIAL  2025052900  // Arbitrary
+#define DNS_SOA_REFRESH 100000      // Arbitrary
+#define DNS_SOA_RETRY   10000       // Arbitrary
+#define DNS_SOA_EXPIRE  1000000     // Arbitrary
+#define DNS_MINIMAL_TTL 5           // Time to live for negative answers RFC2308
 enum class DNSReplyCode : uint16_t {
   NoError = 0,
   FormError = 1,
@@ -82,7 +99,7 @@ class DNSServer {
      * @param domainName - domain name to serve
      */
   DNSServer(const String &domainName);
-  ~DNSServer(){};  // default d-tor
+  ~DNSServer() {};  // default d-tor
 
   // Copy semantics not implemented (won't run on same UDP port anyway)
   DNSServer(const DNSServer &) = delete;
@@ -179,5 +196,7 @@ class DNSServer {
   inline bool requestIncludesOnlyOneQuestion(DNSHeader &dnsHeader);
   void replyWithIP(AsyncUDPPacket &req, DNSHeader &dnsHeader, DNSQuestion &dnsQuestion);
   inline void replyWithCustomCode(AsyncUDPPacket &req, DNSHeader &dnsHeader);
+  inline void replyWithNoAnsw(AsyncUDPPacket &req, DNSHeader &dnsHeader, DNSQuestion &dnsQuestion);
+
   void _handleUDP(AsyncUDPPacket &pkt);
 };