esp8266/libraries/DNSServer/src/DNSServer.cpp

#include "DNSServer.h"
#include <lwip/def.h>
#include <Arduino.h>
#include <memory>

#ifdef DEBUG_ESP_PORT
#define DEBUG_OUTPUT DEBUG_ESP_PORT
#else
#define DEBUG_OUTPUT Serial
#endif

#define DNS_HEADER_SIZE sizeof(DNSHeader)

DNSServer::DNSServer()
{
  _ttl = lwip_htonl(60);
  _errorReplyCode = DNSReplyCode::NonExistentDomain;
}

bool DNSServer::start(const uint16_t &port, const String &domainName,
                     const IPAddress &resolvedIP)
{
  _port = port;
  
  _domainName = domainName;
  _resolvedIP[0] = resolvedIP[0];
  _resolvedIP[1] = resolvedIP[1];
  _resolvedIP[2] = resolvedIP[2];
  _resolvedIP[3] = resolvedIP[3];
  downcaseAndRemoveWwwPrefix(_domainName);
  return _udp.begin(_port) == 1;
}

void DNSServer::setErrorReplyCode(const DNSReplyCode &replyCode)
{
  _errorReplyCode = replyCode;
}

void DNSServer::setTTL(const uint32_t &ttl)
{
  _ttl = lwip_htonl(ttl);
}

void DNSServer::stop()
{
  _udp.stop();
}

void DNSServer::downcaseAndRemoveWwwPrefix(String &domainName)
{
  domainName.toLowerCase();
  if (domainName.startsWith("www."))
      domainName.remove(0, 4);
}

void DNSServer::respondToRequest(uint8_t *buffer, size_t length)
{
  DNSHeader *dnsHeader;
  uint8_t *query, *start;
  const char *matchString;
  size_t remaining, labelLength, queryLength;
  uint16_t qtype, qclass;

  dnsHeader = (DNSHeader *)buffer;

  // Must be a query for us to do anything with it
  if (dnsHeader->QR != DNS_QR_QUERY)
    return;

  // If operation is anything other than query, we don't do it
  if (dnsHeader->OPCode != DNS_OPCODE_QUERY)
    return replyWithError(dnsHeader, DNSReplyCode::NotImplemented);

  // Only support requests containing single queries - everything else
  // is badly defined
  if (dnsHeader->QDCount != lwip_htons(1))
    return replyWithError(dnsHeader, DNSReplyCode::FormError);

  // We must return a FormError in the case of a non-zero ARCount to
  // be minimally compatible with EDNS resolvers
  if (dnsHeader->ANCount != 0 || dnsHeader->NSCount != 0
      || dnsHeader->ARCount != 0)
    return replyWithError(dnsHeader, DNSReplyCode::FormError);

  // Even if we're not going to use the query, we need to parse it
  // so we can check the address type that's being queried

  query = start = buffer + DNS_HEADER_SIZE;
  remaining = length - DNS_HEADER_SIZE;
  while (remaining != 0 && *start != 0) {
    labelLength = *start;
    if (labelLength + 1 > remaining)
	return replyWithError(dnsHeader, DNSReplyCode::FormError);
    remaining -= (labelLength + 1);
    start += (labelLength + 1);
  }

  // 1 octet labelLength, 2 octet qtype, 2 octet qclass
  if (remaining < 5)
    return replyWithError(dnsHeader, DNSReplyCode::FormError);

  start += 1; // Skip the 0 length label that we found above

  memcpy(&qtype, start, sizeof(qtype));
  start += 2;
  memcpy(&qclass, start, sizeof(qclass));
  start += 2;

  queryLength = start - query;

  if (qclass != lwip_htons(DNS_QCLASS_ANY)
      && qclass != lwip_htons(DNS_QCLASS_IN))
    return replyWithError(dnsHeader, DNSReplyCode::NonExistentDomain,
			  query, queryLength);

  if (qtype != lwip_htons(DNS_QTYPE_A)
      && qtype != lwip_htons(DNS_QTYPE_ANY))
    return replyWithError(dnsHeader, DNSReplyCode::NonExistentDomain,
			  query, queryLength);

  // If we have no domain name configured, just return an error
  if (_domainName.isEmpty())
    return replyWithError(dnsHeader, _errorReplyCode,
			  query, queryLength);

  // If we're running with a wildcard we can just return a result now
  if (_domainName == "*")
    return replyWithIP(dnsHeader, query, queryLength);

  matchString = _domainName.c_str();

  start = query;

  // If there's a leading 'www', skip it
  if (*start == 3 && strncasecmp("www", (char *) start + 1, 3) == 0)
      start += 4;

  while (*start != 0) {
    labelLength = *start;
    start += 1;
    while (labelLength > 0) {
      if (tolower(*start) != *matchString)
	return replyWithError(dnsHeader, _errorReplyCode,
			      query, queryLength);
      ++start;
      ++matchString;
      --labelLength;
    }
    if (*start == 0 && *matchString == '\0')
      return replyWithIP(dnsHeader, query, queryLength);

    if (*matchString != '.')
      return replyWithError(dnsHeader, _errorReplyCode,
			    query, queryLength);
    ++matchString;
  }

  return replyWithError(dnsHeader, _errorReplyCode,
			query, queryLength);
}

void DNSServer::processNextRequest()
{
  size_t currentPacketSize;

  currentPacketSize = _udp.parsePacket();
  if (currentPacketSize == 0)
    return;

  // The DNS RFC requires that DNS packets be less than 512 bytes in size,
  // so just discard them if they are larger
  if (currentPacketSize > MAX_DNS_PACKETSIZE)
    return;

  // If the packet size is smaller than the DNS header, then someone is
  // messing with us
  if (currentPacketSize < DNS_HEADER_SIZE)
    return;

  std::unique_ptr<uint8_t[]> buffer(new (std::nothrow) uint8_t[currentPacketSize]);
  if (buffer == nullptr)
    return;

  _udp.read(buffer.get(), currentPacketSize);
  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;

  _udp.beginPacket(_udp.remoteIP(), _udp.remotePort());
  _udp.write((unsigned char *) dnsHeader, sizeof(DNSHeader));
  _udp.write(query, queryLength);

  // 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));

  // Answer is in the Internet Class
  writeNBOShort(lwip_htons(DNS_QCLASS_IN));

  // Output TTL (already NBO)
  _udp.write((unsigned char*)&_ttl, 4);

  // Length of RData is 4 bytes (because, in this case, RData is IPv4)
  writeNBOShort(lwip_htons(sizeof(_resolvedIP)));
  _udp.write(_resolvedIP, sizeof(_resolvedIP));
  _udp.endPacket();
}

void DNSServer::replyWithError(DNSHeader *dnsHeader,
			       DNSReplyCode rcode,
			       unsigned char *query,
			       size_t queryLength)
{
  dnsHeader->QR = DNS_QR_RESPONSE;
  dnsHeader->RCode = (unsigned char) rcode;
  if (query)
     dnsHeader->QDCount = lwip_htons(1);
  else
     dnsHeader->QDCount = 0;
  dnsHeader->ANCount = 0;
  dnsHeader->NSCount = 0;
  dnsHeader->ARCount = 0;

  _udp.beginPacket(_udp.remoteIP(), _udp.remotePort());
  _udp.write((unsigned char *)dnsHeader, sizeof(DNSHeader));
  if (query != NULL)
     _udp.write(query, queryLength);
  _udp.endPacket();
}

void DNSServer::replyWithError(DNSHeader *dnsHeader,
			       DNSReplyCode rcode)
{
  replyWithError(dnsHeader, rcode, NULL, 0);
}