path: root/dnsdist-ecs.cc

/*
 * This file is part of PowerDNS or dnsdist.
 * Copyright -- PowerDNS.COM B.V. and its contributors
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of version 2 of the GNU General Public License as
 * published by the Free Software Foundation.
 *
 * In addition, for the avoidance of any doubt, permission is granted to
 * link this program with OpenSSL and to (re)distribute the binaries
 * produced as the result of such linking.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */
#include "dolog.hh"
#include "dnsdist.hh"
#include "dnsdist-ecs.hh"
#include "dnsparser.hh"
#include "dnswriter.hh"
#include "ednsoptions.hh"
#include "ednssubnet.hh"

/* when we add EDNS to a query, we don't want to advertise
   a large buffer size */
size_t g_EdnsUDPPayloadSize = 512;
static const uint16_t defaultPayloadSizeSelfGenAnswers = 1232;
static_assert(defaultPayloadSizeSelfGenAnswers < s_udpIncomingBufferSize, "The UDP responder's payload size should be smaller or equal to our incoming buffer size");
uint16_t g_PayloadSizeSelfGenAnswers{defaultPayloadSizeSelfGenAnswers};

/* draft-ietf-dnsop-edns-client-subnet-04 "11.1.  Privacy" */
uint16_t g_ECSSourcePrefixV4 = 24;
uint16_t g_ECSSourcePrefixV6 = 56;

bool g_ECSOverride{false};
bool g_addEDNSToSelfGeneratedResponses{true};

int rewriteResponseWithoutEDNS(const PacketBuffer& initialPacket, PacketBuffer& newContent)
{
  assert(initialPacket.size() >= sizeof(dnsheader));
  const struct dnsheader* dh = reinterpret_cast<const struct dnsheader*>(initialPacket.data());

  if (ntohs(dh->arcount) == 0)
    return ENOENT;

  if (ntohs(dh->qdcount) == 0)
    return ENOENT;

  PacketReader pr(pdns_string_view(reinterpret_cast<const char*>(initialPacket.data()), initialPacket.size()));

  size_t idx = 0;
  DNSName rrname;
  uint16_t qdcount = ntohs(dh->qdcount);
  uint16_t ancount = ntohs(dh->ancount);
  uint16_t nscount = ntohs(dh->nscount);
  uint16_t arcount = ntohs(dh->arcount);
  uint16_t rrtype;
  uint16_t rrclass;
  string blob;
  struct dnsrecordheader ah;

  rrname = pr.getName();
  rrtype = pr.get16BitInt();
  rrclass = pr.get16BitInt();

  GenericDNSPacketWriter<PacketBuffer> pw(newContent, rrname, rrtype, rrclass, dh->opcode);
  pw.getHeader()->id=dh->id;
  pw.getHeader()->qr=dh->qr;
  pw.getHeader()->aa=dh->aa;
  pw.getHeader()->tc=dh->tc;
  pw.getHeader()->rd=dh->rd;
  pw.getHeader()->ra=dh->ra;
  pw.getHeader()->ad=dh->ad;
  pw.getHeader()->cd=dh->cd;
  pw.getHeader()->rcode=dh->rcode;

  /* consume remaining qd if any */
  if (qdcount > 1) {
    for(idx = 1; idx < qdcount; idx++) {
      rrname = pr.getName();
      rrtype = pr.get16BitInt();
      rrclass = pr.get16BitInt();
      (void) rrtype;
      (void) rrclass;
    }
  }

  /* copy AN and NS */
  for (idx = 0; idx < ancount; idx++) {
    rrname = pr.getName();
    pr.getDnsrecordheader(ah);

    pw.startRecord(rrname, ah.d_type, ah.d_ttl, ah.d_class, DNSResourceRecord::ANSWER, true);
    pr.xfrBlob(blob);
    pw.xfrBlob(blob);
  }

  for (idx = 0; idx < nscount; idx++) {
    rrname = pr.getName();
    pr.getDnsrecordheader(ah);

    pw.startRecord(rrname, ah.d_type, ah.d_ttl, ah.d_class, DNSResourceRecord::AUTHORITY, true);
    pr.xfrBlob(blob);
    pw.xfrBlob(blob);
  }
  /* consume AR, looking for OPT */
  for (idx = 0; idx < arcount; idx++) {
    rrname = pr.getName();
    pr.getDnsrecordheader(ah);

    if (ah.d_type != QType::OPT) {
      pw.startRecord(rrname, ah.d_type, ah.d_ttl, ah.d_class, DNSResourceRecord::ADDITIONAL, true);
      pr.xfrBlob(blob);
      pw.xfrBlob(blob);
    } else {

      pr.skip(ah.d_clen);
    }
  }
  pw.commit();

  return 0;
}

static bool addOrReplaceEDNSOption(std::vector<std::pair<uint16_t, std::string>>& options, uint16_t optionCode, bool& optionAdded, bool overrideExisting, const string& newOptionContent)
{
  for (auto it = options.begin(); it != options.end(); ) {
    if (it->first == optionCode) {
      optionAdded = false;

      if (!overrideExisting) {
        return false;
      }

      it = options.erase(it);
    }
    else {
      ++it;
    }
  }

  options.emplace_back(optionCode, std::string(&newOptionContent.at(EDNS_OPTION_CODE_SIZE + EDNS_OPTION_LENGTH_SIZE), newOptionContent.size() - (EDNS_OPTION_CODE_SIZE + EDNS_OPTION_LENGTH_SIZE)));
  return true;
}

bool slowRewriteEDNSOptionInQueryWithRecords(const PacketBuffer& initialPacket, PacketBuffer& newContent, bool& ednsAdded, uint16_t optionToReplace, bool& optionAdded, bool overrideExisting, const string& newOptionContent)
{
  assert(initialPacket.size() >= sizeof(dnsheader));
  const struct dnsheader* dh = reinterpret_cast<const struct dnsheader*>(initialPacket.data());

  if (ntohs(dh->qdcount) == 0) {
    return false;
  }

  if (ntohs(dh->ancount) == 0 && ntohs(dh->nscount) == 0 && ntohs(dh->arcount) == 0) {
    throw std::runtime_error(std::string(__PRETTY_FUNCTION__) + " should not be called for queries that have no records");
  }

  optionAdded = false;
  ednsAdded = true;

  PacketReader pr(pdns_string_view(reinterpret_cast<const char*>(initialPacket.data()), initialPacket.size()));

  size_t idx = 0;
  DNSName rrname;
  uint16_t qdcount = ntohs(dh->qdcount);
  uint16_t ancount = ntohs(dh->ancount);
  uint16_t nscount = ntohs(dh->nscount);
  uint16_t arcount = ntohs(dh->arcount);
  uint16_t rrtype;
  uint16_t rrclass;
  string blob;
  struct dnsrecordheader ah;

  rrname = pr.getName();
  rrtype = pr.get16BitInt();
  rrclass = pr.get16BitInt();

  GenericDNSPacketWriter<PacketBuffer> pw(newContent, rrname, rrtype, rrclass, dh->opcode);
  pw.getHeader()->id=dh->id;
  pw.getHeader()->qr=dh->qr;
  pw.getHeader()->aa=dh->aa;
  pw.getHeader()->tc=dh->tc;
  pw.getHeader()->rd=dh->rd;
  pw.getHeader()->ra=dh->ra;
  pw.getHeader()->ad=dh->ad;
  pw.getHeader()->cd=dh->cd;
  pw.getHeader()->rcode=dh->rcode;

  /* consume remaining qd if any */
  if (qdcount > 1) {
    for(idx = 1; idx < qdcount; idx++) {
      rrname = pr.getName();
      rrtype = pr.get16BitInt();
      rrclass = pr.get16BitInt();
      (void) rrtype;
      (void) rrclass;
    }
  }

  /* copy AN and NS */
  for (idx = 0; idx < ancount; idx++) {
    rrname = pr.getName();
    pr.getDnsrecordheader(ah);

    pw.startRecord(rrname, ah.d_type, ah.d_ttl, ah.d_class, DNSResourceRecord::ANSWER, true);
    pr.xfrBlob(blob);
    pw.xfrBlob(blob);
  }

  for (idx = 0; idx < nscount; idx++) {
    rrname = pr.getName();
    pr.getDnsrecordheader(ah);

    pw.startRecord(rrname, ah.d_type, ah.d_ttl, ah.d_class, DNSResourceRecord::AUTHORITY, true);
    pr.xfrBlob(blob);
    pw.xfrBlob(blob);
  }

  /* consume AR, looking for OPT */
  for (idx = 0; idx < arcount; idx++) {
    rrname = pr.getName();
    pr.getDnsrecordheader(ah);

    if (ah.d_type != QType::OPT) {
      pw.startRecord(rrname, ah.d_type, ah.d_ttl, ah.d_class, DNSResourceRecord::ADDITIONAL, true);
      pr.xfrBlob(blob);
      pw.xfrBlob(blob);
    } else {

      ednsAdded = false;
      pr.xfrBlob(blob);

      std::vector<std::pair<uint16_t, std::string>> options;
      getEDNSOptionsFromContent(blob, options);

      EDNS0Record edns0;
      static_assert(sizeof(edns0) == sizeof(ah.d_ttl), "sizeof(EDNS0Record) must match sizeof(uint32_t) AKA RR TTL size");
      memcpy(&edns0, &ah.d_ttl, sizeof(edns0));

      /* addOrReplaceEDNSOption will set it to false if there is already an existing option */
      optionAdded = true;
      addOrReplaceEDNSOption(options, optionToReplace, optionAdded, overrideExisting, newOptionContent);
      pw.addOpt(ah.d_class, edns0.extRCode, edns0.extFlags, options, edns0.version);
    }
  }

  if (ednsAdded) {
    pw.addOpt(g_EdnsUDPPayloadSize, 0, 0, {{optionToReplace, std::string(&newOptionContent.at(EDNS_OPTION_CODE_SIZE + EDNS_OPTION_LENGTH_SIZE), newOptionContent.size() - (EDNS_OPTION_CODE_SIZE + EDNS_OPTION_LENGTH_SIZE))}}, 0);
    optionAdded = true;
  }

  pw.commit();

  return true;
}

static bool slowParseEDNSOptions(const PacketBuffer& packet, std::shared_ptr<std::map<uint16_t, EDNSOptionView> >& options)
{
  if (packet.size() < sizeof(dnsheader)) {
    return false;
  }

  const struct dnsheader* dh = reinterpret_cast<const struct dnsheader*>(packet.data());

  if (ntohs(dh->qdcount) == 0) {
    return false;
  }

  if (ntohs(dh->arcount) == 0) {
    throw std::runtime_error("slowParseEDNSOptions() should not be called for queries that have no EDNS");
  }

  try {
    uint64_t numrecords = ntohs(dh->ancount) + ntohs(dh->nscount) + ntohs(dh->arcount);
    DNSPacketMangler dpm(const_cast<char*>(reinterpret_cast<const char*>(&packet.at(0))), packet.size());
    uint64_t n;
    for(n=0; n < ntohs(dh->qdcount) ; ++n) {
      dpm.skipDomainName();
      /* type and class */
      dpm.skipBytes(4);
    }

    for(n=0; n < numrecords; ++n) {
      dpm.skipDomainName();

      uint8_t section = n < ntohs(dh->ancount) ? 1 : (n < (ntohs(dh->ancount) + ntohs(dh->nscount)) ? 2 : 3);
      uint16_t dnstype = dpm.get16BitInt();
      dpm.get16BitInt();
      dpm.skipBytes(4); /* TTL */

      if(section == 3 && dnstype == QType::OPT) {
        uint32_t offset = dpm.getOffset();
        if (offset >= packet.size()) {
          return false;
        }
        /* if we survive this call, we can parse it safely */
        dpm.skipRData();
        return getEDNSOptions(reinterpret_cast<const char*>(&packet.at(offset)), packet.size() - offset, *options) == 0;
      }
      else {
        dpm.skipRData();
      }
    }
  }
  catch(...)
  {
    return false;
  }

  return true;
}

int locateEDNSOptRR(const PacketBuffer& packet, uint16_t * optStart, size_t * optLen, bool * last)
{
  assert(optStart != NULL);
  assert(optLen != NULL);
  assert(last != NULL);
  const struct dnsheader* dh = reinterpret_cast<const struct dnsheader*>(packet.data());

  if (ntohs(dh->arcount) == 0)
    return ENOENT;

  PacketReader pr(pdns_string_view(reinterpret_cast<const char*>(packet.data()), packet.size()));

  size_t idx = 0;
  DNSName rrname;
  uint16_t qdcount = ntohs(dh->qdcount);
  uint16_t ancount = ntohs(dh->ancount);
  uint16_t nscount = ntohs(dh->nscount);
  uint16_t arcount = ntohs(dh->arcount);
  uint16_t rrtype;
  uint16_t rrclass;
  struct dnsrecordheader ah;

  /* consume qd */
  for(idx = 0; idx < qdcount; idx++) {
    rrname = pr.getName();
    rrtype = pr.get16BitInt();
    rrclass = pr.get16BitInt();
    (void) rrtype;
    (void) rrclass;
  }

  /* consume AN and NS */
  for (idx = 0; idx < ancount + nscount; idx++) {
    rrname = pr.getName();
    pr.getDnsrecordheader(ah);
    pr.skip(ah.d_clen);
  }

  /* consume AR, looking for OPT */
  for (idx = 0; idx < arcount; idx++) {
    uint16_t start = pr.getPosition();
    rrname = pr.getName();
    pr.getDnsrecordheader(ah);

    if (ah.d_type == QType::OPT) {
      *optStart = start;
      *optLen = (pr.getPosition() - start) + ah.d_clen;

      if (packet.size() < (*optStart + *optLen)) {
        throw std::range_error("Opt record overflow");
      }

      if (idx == ((size_t) arcount - 1)) {
        *last = true;
      }
      else {
        *last = false;
      }
      return 0;
    }
    pr.skip(ah.d_clen);
  }

  return ENOENT;
}

/* extract the start of the OPT RR in a QUERY packet if any */
int getEDNSOptionsStart(const PacketBuffer& packet, const size_t offset, uint16_t* optRDPosition, size_t* remaining)
{
  assert(optRDPosition != nullptr);
  assert(remaining != nullptr);
  const struct dnsheader* dh = reinterpret_cast<const struct dnsheader*>(packet.data());

  if (offset >= packet.size()) {
    return ENOENT;
  }

  if (ntohs(dh->qdcount) != 1 || ntohs(dh->ancount) != 0 || ntohs(dh->arcount) != 1 || ntohs(dh->nscount) != 0)
    return ENOENT;

  size_t pos = sizeof(dnsheader) + offset;
  pos += DNS_TYPE_SIZE + DNS_CLASS_SIZE;

  if (pos >= packet.size())
    return ENOENT;

  if ((pos + /* root */ 1 + DNS_TYPE_SIZE + DNS_CLASS_SIZE) >= packet.size()) {
    return ENOENT;
  }

  if (packet[pos] != 0) {
    /* not the root so not an OPT record */
    return ENOENT;
  }
  pos += 1;

  uint16_t qtype = packet.at(pos)*256 + packet.at(pos+1);
  pos += DNS_TYPE_SIZE;
  pos += DNS_CLASS_SIZE;

  if (qtype != QType::OPT || (packet.size() - pos) < (DNS_TTL_SIZE + DNS_RDLENGTH_SIZE)) {
    return ENOENT;
  }

  pos += DNS_TTL_SIZE;
  *optRDPosition = pos;
  *remaining = packet.size() - pos;

  return 0;
}

void generateECSOption(const ComboAddress& source, string& res, uint16_t ECSPrefixLength)
{
  Netmask sourceNetmask(source, ECSPrefixLength);
  EDNSSubnetOpts ecsOpts;
  ecsOpts.source = sourceNetmask;
  string payload = makeEDNSSubnetOptsString(ecsOpts);
  generateEDNSOption(EDNSOptionCode::ECS, payload, res);
}

bool generateOptRR(const std::string& optRData, PacketBuffer& res, size_t maximumSize, uint16_t udpPayloadSize, uint8_t ednsrcode, bool dnssecOK)
{
  const uint8_t name = 0;
  dnsrecordheader dh;
  EDNS0Record edns0;
  edns0.extRCode = ednsrcode;
  edns0.version = 0;
  edns0.extFlags = dnssecOK ? htons(EDNS_HEADER_FLAG_DO) : 0;

  if ((maximumSize - res.size()) < (sizeof(name) + sizeof(dh) + optRData.length())) {
    return false;
  }

  dh.d_type = htons(QType::OPT);
  dh.d_class = htons(udpPayloadSize);
  static_assert(sizeof(EDNS0Record) == sizeof(dh.d_ttl), "sizeof(EDNS0Record) must match sizeof(dnsrecordheader.d_ttl)");
  memcpy(&dh.d_ttl, &edns0, sizeof edns0);
  dh.d_clen = htons(static_cast<uint16_t>(optRData.length()));

  res.reserve(res.size() + sizeof(name) + sizeof(dh) + optRData.length());
  res.insert(res.end(), reinterpret_cast<const uint8_t*>(&name), reinterpret_cast<const uint8_t*>(&name) + sizeof(name));
  res.insert(res.end(), reinterpret_cast<const uint8_t*>(&dh), reinterpret_cast<const uint8_t*>(&dh) + sizeof(dh));
  res.insert(res.end(), reinterpret_cast<const uint8_t*>(optRData.data()), reinterpret_cast<const uint8_t*>(optRData.data()) + optRData.length());

  return true;
}

static bool replaceEDNSClientSubnetOption(PacketBuffer& packet, size_t maximumSize, size_t const oldEcsOptionStartPosition, size_t const oldEcsOptionSize, size_t const optRDLenPosition, const string& newECSOption)
{
  assert(oldEcsOptionStartPosition < packet.size());
  assert(optRDLenPosition < packet.size());

  if (newECSOption.size() == oldEcsOptionSize) {
    /* same size as the existing option */
    memcpy(&packet.at(oldEcsOptionStartPosition), newECSOption.c_str(), oldEcsOptionSize);
  }
  else {
    /* different size than the existing option */
    const unsigned int newPacketLen = packet.size() + (newECSOption.length() - oldEcsOptionSize);
    const size_t beforeOptionLen = oldEcsOptionStartPosition;
    const size_t dataBehindSize = packet.size() - beforeOptionLen - oldEcsOptionSize;

    /* check that it fits in the existing buffer */
    if (newPacketLen > packet.size()) {
      if (newPacketLen > maximumSize) {
        return false;
      }

      packet.resize(newPacketLen);
    }

    /* fix the size of ECS Option RDLen */
    uint16_t newRDLen = (packet.at(optRDLenPosition) * 256) + packet.at(optRDLenPosition + 1);
    newRDLen += (newECSOption.size() - oldEcsOptionSize);
    packet.at(optRDLenPosition) = newRDLen / 256;
    packet.at(optRDLenPosition + 1) = newRDLen % 256;

    if (dataBehindSize > 0) {
      memmove(&packet.at(oldEcsOptionStartPosition), &packet.at(oldEcsOptionStartPosition + oldEcsOptionSize), dataBehindSize);
    }
    memcpy(&packet.at(oldEcsOptionStartPosition + dataBehindSize), newECSOption.c_str(), newECSOption.size());
    packet.resize(newPacketLen);
  }

  return true;
}

/* This function looks for an OPT RR, return true if a valid one was found (even if there was no options)
   and false otherwise. */
bool parseEDNSOptions(const DNSQuestion& dq)
{
  const auto dh = dq.getHeader();
  if (dq.ednsOptions != nullptr) {
    return true;
  }

  dq.ednsOptions = std::make_shared<std::map<uint16_t, EDNSOptionView> >();

  if (ntohs(dh->arcount) == 0) {
    /* nothing in additional so no EDNS */
    return false;
  }

  if (ntohs(dh->ancount) != 0 || ntohs(dh->nscount) != 0 || ntohs(dh->arcount) > 1) {
    return slowParseEDNSOptions(dq.getData(), dq.ednsOptions);
  }

  size_t remaining = 0;
  uint16_t optRDPosition;
  int res = getEDNSOptionsStart(dq.getData(), dq.qname->wirelength(), &optRDPosition, &remaining);

  if (res == 0) {
    res = getEDNSOptions(reinterpret_cast<const char*>(&dq.getData().at(optRDPosition)), remaining, *dq.ednsOptions);
    return (res == 0);
  }

  return false;
}

static bool addECSToExistingOPT(PacketBuffer& packet, size_t maximumSize, const string& newECSOption, size_t optRDLenPosition, bool& ecsAdded)
{
  /* we need to add one EDNS0 ECS option, fixing the size of EDNS0 RDLENGTH */
  /* getEDNSOptionsStart has already checked that there is exactly one AR,
     no NS and no AN */
  uint16_t oldRDLen = (packet.at(optRDLenPosition) * 256) + packet.at(optRDLenPosition + 1);
  if (packet.size() != (optRDLenPosition + sizeof(uint16_t) + oldRDLen)) {
    /* we are supposed to be the last record, do we have some trailing data to remove? */
    uint32_t realPacketLen = getDNSPacketLength(reinterpret_cast<const char*>(packet.data()), packet.size());
    packet.resize(realPacketLen);
  }

  if ((maximumSize - packet.size()) < newECSOption.size()) {
    return false;
  }

  uint16_t newRDLen = oldRDLen + newECSOption.size();
  packet.at(optRDLenPosition) = newRDLen / 256;
  packet.at(optRDLenPosition + 1) = newRDLen % 256;

  packet.insert(packet.end(), newECSOption.begin(), newECSOption.end());
  ecsAdded = true;

  return true;
}

static bool addEDNSWithECS(PacketBuffer& packet, size_t maximumSize, const string& newECSOption, bool& ednsAdded, bool& ecsAdded)
{
  if (!generateOptRR(newECSOption, packet, maximumSize, g_EdnsUDPPayloadSize, 0, false)) {
    return false;
  }

  struct dnsheader* dh = reinterpret_cast<struct dnsheader*>(packet.data());
  uint16_t arcount = ntohs(dh->arcount);
  arcount++;
  dh->arcount = htons(arcount);
  ednsAdded = true;
  ecsAdded = true;

  return true;
}

bool handleEDNSClientSubnet(PacketBuffer& packet, const size_t maximumSize, const size_t qnameWireLength, bool& ednsAdded, bool& ecsAdded, bool overrideExisting, const string& newECSOption)
{
  assert(qnameWireLength <= packet.size());

  const struct dnsheader* dh = reinterpret_cast<const struct dnsheader*>(packet.data());

  if (ntohs(dh->ancount) != 0 || ntohs(dh->nscount) != 0 || (ntohs(dh->arcount) != 0 && ntohs(dh->arcount) != 1)) {
    PacketBuffer newContent;
    newContent.reserve(packet.size());

    if (!slowRewriteEDNSOptionInQueryWithRecords(packet, newContent, ednsAdded, EDNSOptionCode::ECS, ecsAdded, overrideExisting, newECSOption)) {
      return false;
    }

    if (newContent.size() > maximumSize) {
      ednsAdded = false;
      ecsAdded = false;
      return false;
    }

    packet = std::move(newContent);
    return true;
  }

  uint16_t optRDPosition = 0;
  size_t remaining = 0;

  int res = getEDNSOptionsStart(packet, qnameWireLength, &optRDPosition, &remaining);

  if (res != 0) {
    /* no EDNS but there might be another record in additional (TSIG?) */
    /* Careful, this code assumes that ANCOUNT == 0 && NSCOUNT == 0 */
    size_t minimumPacketSize = sizeof(dnsheader) + qnameWireLength + sizeof(uint16_t) + sizeof(uint16_t);
    if (packet.size() > minimumPacketSize) {
      if (ntohs(dh->arcount) == 0) {
        /* well now.. */
        packet.resize(minimumPacketSize);
      }
      else {
        uint32_t realPacketLen = getDNSPacketLength(reinterpret_cast<const char*>(packet.data()), packet.size());
        packet.resize(realPacketLen);
      }
    }

    return addEDNSWithECS(packet, maximumSize, newECSOption, ednsAdded, ecsAdded);
  }

  size_t ecsOptionStartPosition = 0;
  size_t ecsOptionSize = 0;

  res = getEDNSOption(reinterpret_cast<const char*>(&packet.at(optRDPosition)), remaining, EDNSOptionCode::ECS, &ecsOptionStartPosition, &ecsOptionSize);

  if (res == 0) {
    /* there is already an ECS value */
    if (!overrideExisting) {
      return true;
    }

    return replaceEDNSClientSubnetOption(packet, maximumSize, optRDPosition + ecsOptionStartPosition, ecsOptionSize, optRDPosition, newECSOption);
  } else {
    /* we have an EDNS OPT RR but no existing ECS option */
    return addECSToExistingOPT(packet, maximumSize, newECSOption, optRDPosition, ecsAdded);
  }

  return true;
}

bool handleEDNSClientSubnet(DNSQuestion& dq, bool& ednsAdded, bool& ecsAdded)
{
  assert(dq.remote != nullptr);
  string newECSOption;
  generateECSOption(dq.ecsSet ? dq.ecs.getNetwork() : *dq.remote, newECSOption, dq.ecsSet ? dq.ecs.getBits() : dq.ecsPrefixLength);

  return handleEDNSClientSubnet(dq.getMutableData(), dq.getMaximumSize(), dq.qname->wirelength(), ednsAdded, ecsAdded, dq.ecsOverride, newECSOption);
}

static int removeEDNSOptionFromOptions(unsigned char* optionsStart, const uint16_t optionsLen, const uint16_t optionCodeToRemove, uint16_t* newOptionsLen)
{
  unsigned char* p = optionsStart;
  size_t pos = 0;
  while ((pos + 4) <= optionsLen) {
    unsigned char* optionBegin = p;
    const uint16_t optionCode = 0x100*p[0] + p[1];
    p += sizeof(optionCode);
    pos += sizeof(optionCode);
    const uint16_t optionLen = 0x100*p[0] + p[1];
    p += sizeof(optionLen);
    pos += sizeof(optionLen);
    if ((pos + optionLen) > optionsLen) {
      return EINVAL;
    }
    if (optionCode == optionCodeToRemove) {
      if (pos + optionLen < optionsLen) {
        /* move remaining options over the removed one,
           if any */
        memmove(optionBegin, p + optionLen, optionsLen - (pos + optionLen));
      }
      *newOptionsLen = optionsLen - (sizeof(optionCode) + sizeof(optionLen) + optionLen);
      return 0;
    }
    p += optionLen;
    pos += optionLen;
  }
  return ENOENT;
}

int removeEDNSOptionFromOPT(char* optStart, size_t* optLen, const uint16_t optionCodeToRemove)
{
  if (*optLen < optRecordMinimumSize) {
    return EINVAL;
  }
  const unsigned char* end = (const unsigned char*) optStart + *optLen;
  unsigned char* p = (unsigned char*) optStart + 9;
  unsigned char* rdLenPtr = p;
  uint16_t rdLen = (0x100*p[0] + p[1]);
  p += sizeof(rdLen);
  if (p + rdLen != end) {
    return EINVAL;
  }
  uint16_t newRdLen = 0;
  int res = removeEDNSOptionFromOptions(p, rdLen, optionCodeToRemove, &newRdLen);
  if (res != 0) {
    return res;
  }
  *optLen -= (rdLen - newRdLen);
  rdLenPtr[0] = newRdLen / 0x100;
  rdLenPtr[1] = newRdLen % 0x100;
  return 0;
}

bool isEDNSOptionInOpt(const PacketBuffer& packet, const size_t optStart, const size_t optLen, const uint16_t optionCodeToFind, size_t* optContentStart, uint16_t* optContentLen)
{
  if (optLen < optRecordMinimumSize) {
    return false;
  }
  size_t p = optStart + 9;
  uint16_t rdLen = (0x100*static_cast<unsigned char>(packet.at(p)) + static_cast<unsigned char>(packet.at(p+1)));
  p += sizeof(rdLen);
  if (rdLen > (optLen - optRecordMinimumSize)) {
    return false;
  }

  size_t rdEnd = p + rdLen;
  while ((p + 4) <= rdEnd) {
    const uint16_t optionCode = 0x100*static_cast<unsigned char>(packet.at(p)) + static_cast<unsigned char>(packet.at(p+1));
    p += sizeof(optionCode);
    const uint16_t optionLen = 0x100*static_cast<unsigned char>(packet.at(p)) + static_cast<unsigned char>(packet.at(p+1));
    p += sizeof(optionLen);

    if ((p + optionLen) > rdEnd) {
      return false;
    }

    if (optionCode == optionCodeToFind) {
      if (optContentStart != nullptr) {
        *optContentStart = p;
      }

      if (optContentLen != nullptr) {
        *optContentLen = optionLen;
      }

      return true;
    }
    p += optionLen;
  }
  return false;
}

int rewriteResponseWithoutEDNSOption(const PacketBuffer& initialPacket, const uint16_t optionCodeToSkip, PacketBuffer& newContent)
{
  assert(initialPacket.size() >= sizeof(dnsheader));
  const struct dnsheader* dh = reinterpret_cast<const struct dnsheader*>(initialPacket.data());

  if (ntohs(dh->arcount) == 0)
    return ENOENT;

  if (ntohs(dh->qdcount) == 0)
    return ENOENT;

  PacketReader pr(pdns_string_view(reinterpret_cast<const char*>(initialPacket.data()), initialPacket.size()));

  size_t idx = 0;
  DNSName rrname;
  uint16_t qdcount = ntohs(dh->qdcount);
  uint16_t ancount = ntohs(dh->ancount);
  uint16_t nscount = ntohs(dh->nscount);
  uint16_t arcount = ntohs(dh->arcount);
  uint16_t rrtype;
  uint16_t rrclass;
  string blob;
  struct dnsrecordheader ah;

  rrname = pr.getName();
  rrtype = pr.get16BitInt();
  rrclass = pr.get16BitInt();

  GenericDNSPacketWriter<PacketBuffer> pw(newContent, rrname, rrtype, rrclass, dh->opcode);
  pw.getHeader()->id=dh->id;
  pw.getHeader()->qr=dh->qr;
  pw.getHeader()->aa=dh->aa;
  pw.getHeader()->tc=dh->tc;
  pw.getHeader()->rd=dh->rd;
  pw.getHeader()->ra=dh->ra;
  pw.getHeader()->ad=dh->ad;
  pw.getHeader()->cd=dh->cd;
  pw.getHeader()->rcode=dh->rcode;

  /* consume remaining qd if any */
  if (qdcount > 1) {
    for(idx = 1; idx < qdcount; idx++) {
      rrname = pr.getName();
      rrtype = pr.get16BitInt();
      rrclass = pr.get16BitInt();
      (void) rrtype;
      (void) rrclass;
    }
  }

  /* copy AN and NS */
  for (idx = 0; idx < ancount; idx++) {
    rrname = pr.getName();
    pr.getDnsrecordheader(ah);

    pw.startRecord(rrname, ah.d_type, ah.d_ttl, ah.d_class, DNSResourceRecord::ANSWER, true);
    pr.xfrBlob(blob);
    pw.xfrBlob(blob);
  }

  for (idx = 0; idx < nscount; idx++) {
    rrname = pr.getName();
    pr.getDnsrecordheader(ah);

    pw.startRecord(rrname, ah.d_type, ah.d_ttl, ah.d_class, DNSResourceRecord::AUTHORITY, true);
    pr.xfrBlob(blob);
    pw.xfrBlob(blob);
  }

  /* consume AR, looking for OPT */
  for (idx = 0; idx < arcount; idx++) {
    rrname = pr.getName();
    pr.getDnsrecordheader(ah);

    if (ah.d_type != QType::OPT) {
      pw.startRecord(rrname, ah.d_type, ah.d_ttl, ah.d_class, DNSResourceRecord::ADDITIONAL, true);
      pr.xfrBlob(blob);
      pw.xfrBlob(blob);
    } else {
      pw.startRecord(rrname, ah.d_type, ah.d_ttl, ah.d_class, DNSResourceRecord::ADDITIONAL, false);
      pr.xfrBlob(blob);
      uint16_t rdLen = blob.length();
      removeEDNSOptionFromOptions((unsigned char*)blob.c_str(), rdLen, optionCodeToSkip, &rdLen);
      /* xfrBlob(string, size) completely ignores size.. */
      if (rdLen > 0) {
        blob.resize((size_t)rdLen);
        pw.xfrBlob(blob);
      } else {
        pw.commit();
      }
    }
  }
  pw.commit();

  return 0;
}

bool addEDNS(PacketBuffer& packet, size_t maximumSize, bool dnssecOK, uint16_t payloadSize, uint8_t ednsrcode)
{
  if (!generateOptRR(std::string(), packet, maximumSize, payloadSize, ednsrcode, dnssecOK)) {
    return false;
  }

  auto dh = reinterpret_cast<dnsheader*>(packet.data());
  dh->arcount = htons(ntohs(dh->arcount) + 1);

  return true;
}

/*
  This function keeps the existing header and DNSSECOK bit (if any) but wipes anything else,
  generating a NXD or NODATA answer with a SOA record in the additional section.
*/
bool setNegativeAndAdditionalSOA(DNSQuestion& dq, bool nxd, const DNSName& zone, uint32_t ttl, const DNSName& mname, const DNSName& rname, uint32_t serial, uint32_t refresh, uint32_t retry, uint32_t expire, uint32_t minimum)
{
  auto& packet = dq.getMutableData();
  auto dh = dq.getHeader();
  if (ntohs(dh->qdcount) != 1) {
    return false;
  }

  size_t queryPartSize = sizeof(dnsheader) + dq.qname->wirelength() + DNS_TYPE_SIZE + DNS_CLASS_SIZE;
  if (packet.size() < queryPartSize) {
    /* something is already wrong, don't build on flawed foundations */
    return false;
  }

  uint16_t qtype = htons(QType::SOA);
  uint16_t qclass = htons(QClass::IN);
  uint16_t rdLength = mname.wirelength() + rname.wirelength() + sizeof(serial) + sizeof(refresh) + sizeof(retry) + sizeof(expire) + sizeof(minimum);
  size_t soaSize = zone.wirelength() + sizeof(qtype) + sizeof(qclass) + sizeof(ttl) + sizeof(rdLength) + rdLength;
  bool hadEDNS = false;
  bool dnssecOK = false;

  if (g_addEDNSToSelfGeneratedResponses) {
    uint16_t payloadSize = 0;
    uint16_t z = 0;
    hadEDNS = getEDNSUDPPayloadSizeAndZ(reinterpret_cast<const char*>(packet.data()), packet.size(), &payloadSize, &z);
    if (hadEDNS) {
      dnssecOK = z & EDNS_HEADER_FLAG_DO;
    }
  }

  /* chop off everything after the question */
  packet.resize(queryPartSize);
  dh = dq.getHeader();
  if (nxd) {
    dh->rcode = RCode::NXDomain;
  }
  else {
    dh->rcode = RCode::NoError;
  }
  dh->qr = true;
  dh->ancount = 0;
  dh->nscount = 0;
  dh->arcount = 0;

  rdLength = htons(rdLength);
  ttl = htonl(ttl);
  serial = htonl(serial);
  refresh = htonl(refresh);
  retry = htonl(retry);
  expire = htonl(expire);
  minimum = htonl(minimum);

  std::string soa;
  soa.reserve(soaSize);
  soa.append(zone.toDNSString());
  soa.append(reinterpret_cast<const char*>(&qtype), sizeof(qtype));
  soa.append(reinterpret_cast<const char*>(&qclass), sizeof(qclass));
  soa.append(reinterpret_cast<const char*>(&ttl), sizeof(ttl));
  soa.append(reinterpret_cast<const char*>(&rdLength), sizeof(rdLength));
  soa.append(mname.toDNSString());
  soa.append(rname.toDNSString());
  soa.append(reinterpret_cast<const char*>(&serial), sizeof(serial));
  soa.append(reinterpret_cast<const char*>(&refresh), sizeof(refresh));
  soa.append(reinterpret_cast<const char*>(&retry), sizeof(retry));
  soa.append(reinterpret_cast<const char*>(&expire), sizeof(expire));
  soa.append(reinterpret_cast<const char*>(&minimum), sizeof(minimum));

  if (soa.size() != soaSize) {
    throw std::runtime_error("Unexpected SOA response size: " + std::to_string(soa.size()) + " vs " + std::to_string(soaSize));
  }

  packet.insert(packet.end(), soa.begin(), soa.end());
  dh = dq.getHeader();
  dh->arcount = htons(1);

  if (hadEDNS) {
    /* now we need to add a new OPT record */
    return addEDNS(packet, dq.getMaximumSize(), dnssecOK, g_PayloadSizeSelfGenAnswers, dq.ednsRCode);
  }

  return true;
}

bool addEDNSToQueryTurnedResponse(DNSQuestion& dq)
{
  uint16_t optRDPosition;
  /* remaining is at least the size of the rdlen + the options if any + the following records if any */
  size_t remaining = 0;

  auto& packet = dq.getMutableData();
  int res = getEDNSOptionsStart(packet, dq.qname->wirelength(), &optRDPosition, &remaining);

  if (res != 0) {
    /* if the initial query did not have EDNS0, we are done */
    return true;
  }

  const size_t existingOptLen = /* root */ 1 + DNS_TYPE_SIZE + DNS_CLASS_SIZE + EDNS_EXTENDED_RCODE_SIZE + EDNS_VERSION_SIZE + /* Z */ 2 + remaining;
  if (existingOptLen >= packet.size()) {
    /* something is wrong, bail out */
    return false;
  }

  uint8_t* optRDLen = &packet.at(optRDPosition);
  uint8_t* optPtr = (optRDLen - (/* root */ 1 + DNS_TYPE_SIZE + DNS_CLASS_SIZE + EDNS_EXTENDED_RCODE_SIZE + EDNS_VERSION_SIZE + /* Z */ 2));

  const uint8_t* zPtr = optPtr + /* root */ 1 + DNS_TYPE_SIZE + DNS_CLASS_SIZE + EDNS_EXTENDED_RCODE_SIZE + EDNS_VERSION_SIZE;
  uint16_t z = 0x100 * (*zPtr) + *(zPtr + 1);
  bool dnssecOK = z & EDNS_HEADER_FLAG_DO;

  /* remove the existing OPT record, and everything else that follows (any SIG or TSIG would be useless anyway) */
  packet.resize(packet.size() - existingOptLen);
  dq.getHeader()->arcount = 0;

  if (g_addEDNSToSelfGeneratedResponses) {
    /* now we need to add a new OPT record */
    return addEDNS(packet, dq.getMaximumSize(), dnssecOK, g_PayloadSizeSelfGenAnswers, dq.ednsRCode);
  }

  /* otherwise we are just fine */
  return true;
}

// goal in life - if you send us a reasonably normal packet, we'll get Z for you, otherwise 0
int getEDNSZ(const DNSQuestion& dq)
{
  try
  {
    const auto& dh = dq.getHeader();
    if (ntohs(dh->qdcount) != 1 || dh->ancount != 0 || ntohs(dh->arcount) != 1 || dh->nscount != 0) {
      return 0;
    }

    if (dq.getData().size() <= sizeof(dnsheader)) {
      return 0;
    }

    size_t pos = sizeof(dnsheader) + dq.qname->wirelength() + DNS_TYPE_SIZE + DNS_CLASS_SIZE;

    if (dq.getData().size() <= (pos + /* root */ 1 + DNS_TYPE_SIZE + DNS_CLASS_SIZE)) {
      return 0;
    }

    auto& packet = dq.getData();

    if (packet.at(pos) != 0) {
      /* not root, so not a valid OPT record */
      return 0;
    }

    pos++;

    uint16_t qtype = packet.at(pos)*256 + packet.at(pos+1);
    pos += DNS_TYPE_SIZE;
    pos += DNS_CLASS_SIZE;

    if (qtype != QType::OPT || (pos + EDNS_EXTENDED_RCODE_SIZE + EDNS_VERSION_SIZE + 1) >= packet.size()) {
      return 0;
    }

    const uint8_t* z = &packet.at(pos + EDNS_EXTENDED_RCODE_SIZE + EDNS_VERSION_SIZE);
    return 0x100 * (*z) + *(z+1);
  }
  catch(...)
  {
    return 0;
  }
}

bool queryHasEDNS(const DNSQuestion& dq)
{
  uint16_t optRDPosition;
  size_t ecsRemaining = 0;

  int res = getEDNSOptionsStart(dq.getData(), dq.qname->wirelength(), &optRDPosition, &ecsRemaining);
  if (res == 0) {
    return true;
  }

  return false;
}

bool getEDNS0Record(const DNSQuestion& dq, EDNS0Record& edns0)
{
  uint16_t optStart;
  size_t optLen = 0;
  bool last = false;
  const auto& packet = dq.getData();
  int res = locateEDNSOptRR(packet, &optStart, &optLen, &last);
  if (res != 0) {
    // no EDNS OPT RR
    return false;
  }

  if (optLen < optRecordMinimumSize) {
    return false;
  }

  if (optStart < packet.size() && packet.at(optStart) != 0) {
    // OPT RR Name != '.'
    return false;
  }

  static_assert(sizeof(EDNS0Record) == sizeof(uint32_t), "sizeof(EDNS0Record) must match sizeof(uint32_t) AKA RR TTL size");
  // copy out 4-byte "ttl" (really the EDNS0 record), after root label (1) + type (2) + class (2).
  memcpy(&edns0, &packet.at(optStart + 5), sizeof edns0);
  return true;
}