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      1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
      2 // Use of this source code is governed by a BSD-style license that can be
      3 // found in the LICENSE file.
      4 
      5 #include "net/dns/dns_response.h"
      6 
      7 #include "base/big_endian.h"
      8 #include "base/strings/string_util.h"
      9 #include "base/sys_byteorder.h"
     10 #include "net/base/address_list.h"
     11 #include "net/base/dns_util.h"
     12 #include "net/base/io_buffer.h"
     13 #include "net/base/net_errors.h"
     14 #include "net/dns/dns_protocol.h"
     15 #include "net/dns/dns_query.h"
     16 
     17 namespace net {
     18 
     19 DnsResourceRecord::DnsResourceRecord() {
     20 }
     21 
     22 DnsResourceRecord::~DnsResourceRecord() {
     23 }
     24 
     25 DnsRecordParser::DnsRecordParser() : packet_(NULL), length_(0), cur_(0) {
     26 }
     27 
     28 DnsRecordParser::DnsRecordParser(const void* packet,
     29                                  size_t length,
     30                                  size_t offset)
     31     : packet_(reinterpret_cast<const char*>(packet)),
     32       length_(length),
     33       cur_(packet_ + offset) {
     34   DCHECK_LE(offset, length);
     35 }
     36 
     37 unsigned DnsRecordParser::ReadName(const void* const vpos,
     38                                    std::string* out) const {
     39   const char* const pos = reinterpret_cast<const char*>(vpos);
     40   DCHECK(packet_);
     41   DCHECK_LE(packet_, pos);
     42   DCHECK_LE(pos, packet_ + length_);
     43 
     44   const char* p = pos;
     45   const char* end = packet_ + length_;
     46   // Count number of seen bytes to detect loops.
     47   unsigned seen = 0;
     48   // Remember how many bytes were consumed before first jump.
     49   unsigned consumed = 0;
     50 
     51   if (pos >= end)
     52     return 0;
     53 
     54   if (out) {
     55     out->clear();
     56     out->reserve(dns_protocol::kMaxNameLength);
     57   }
     58 
     59   for (;;) {
     60     // The first two bits of the length give the type of the length. It's
     61     // either a direct length or a pointer to the remainder of the name.
     62     switch (*p & dns_protocol::kLabelMask) {
     63       case dns_protocol::kLabelPointer: {
     64         if (p + sizeof(uint16) > end)
     65           return 0;
     66         if (consumed == 0) {
     67           consumed = p - pos + sizeof(uint16);
     68           if (!out)
     69             return consumed;  // If name is not stored, that's all we need.
     70         }
     71         seen += sizeof(uint16);
     72         // If seen the whole packet, then we must be in a loop.
     73         if (seen > length_)
     74           return 0;
     75         uint16 offset;
     76         base::ReadBigEndian<uint16>(p, &offset);
     77         offset &= dns_protocol::kOffsetMask;
     78         p = packet_ + offset;
     79         if (p >= end)
     80           return 0;
     81         break;
     82       }
     83       case dns_protocol::kLabelDirect: {
     84         uint8 label_len = *p;
     85         ++p;
     86         // Note: root domain (".") is NOT included.
     87         if (label_len == 0) {
     88           if (consumed == 0) {
     89             consumed = p - pos;
     90           }  // else we set |consumed| before first jump
     91           return consumed;
     92         }
     93         if (p + label_len >= end)
     94           return 0;  // Truncated or missing label.
     95         if (out) {
     96           if (!out->empty())
     97             out->append(".");
     98           out->append(p, label_len);
     99         }
    100         p += label_len;
    101         seen += 1 + label_len;
    102         break;
    103       }
    104       default:
    105         // unhandled label type
    106         return 0;
    107     }
    108   }
    109 }
    110 
    111 bool DnsRecordParser::ReadRecord(DnsResourceRecord* out) {
    112   DCHECK(packet_);
    113   size_t consumed = ReadName(cur_, &out->name);
    114   if (!consumed)
    115     return false;
    116   base::BigEndianReader reader(cur_ + consumed,
    117                                packet_ + length_ - (cur_ + consumed));
    118   uint16 rdlen;
    119   if (reader.ReadU16(&out->type) &&
    120       reader.ReadU16(&out->klass) &&
    121       reader.ReadU32(&out->ttl) &&
    122       reader.ReadU16(&rdlen) &&
    123       reader.ReadPiece(&out->rdata, rdlen)) {
    124     cur_ = reader.ptr();
    125     return true;
    126   }
    127   return false;
    128 }
    129 
    130 bool DnsRecordParser::SkipQuestion() {
    131   size_t consumed = ReadName(cur_, NULL);
    132   if (!consumed)
    133     return false;
    134 
    135   const char* next = cur_ + consumed + 2 * sizeof(uint16);  // QTYPE + QCLASS
    136   if (next > packet_ + length_)
    137     return false;
    138 
    139   cur_ = next;
    140 
    141   return true;
    142 }
    143 
    144 DnsResponse::DnsResponse()
    145     : io_buffer_(new IOBufferWithSize(dns_protocol::kMaxUDPSize + 1)) {
    146 }
    147 
    148 DnsResponse::DnsResponse(size_t length)
    149     : io_buffer_(new IOBufferWithSize(length)) {
    150 }
    151 
    152 DnsResponse::DnsResponse(const void* data,
    153                          size_t length,
    154                          size_t answer_offset)
    155     : io_buffer_(new IOBufferWithSize(length)),
    156       parser_(io_buffer_->data(), length, answer_offset) {
    157   DCHECK(data);
    158   memcpy(io_buffer_->data(), data, length);
    159 }
    160 
    161 DnsResponse::~DnsResponse() {
    162 }
    163 
    164 bool DnsResponse::InitParse(int nbytes, const DnsQuery& query) {
    165   DCHECK_GE(nbytes, 0);
    166   // Response includes query, it should be at least that size.
    167   if (nbytes < query.io_buffer()->size() || nbytes >= io_buffer_->size())
    168     return false;
    169 
    170   // Match the query id.
    171   if (base::NetToHost16(header()->id) != query.id())
    172     return false;
    173 
    174   // Match question count.
    175   if (base::NetToHost16(header()->qdcount) != 1)
    176     return false;
    177 
    178   // Match the question section.
    179   const size_t hdr_size = sizeof(dns_protocol::Header);
    180   const base::StringPiece question = query.question();
    181   if (question != base::StringPiece(io_buffer_->data() + hdr_size,
    182                                     question.size())) {
    183     return false;
    184   }
    185 
    186   // Construct the parser.
    187   parser_ = DnsRecordParser(io_buffer_->data(),
    188                             nbytes,
    189                             hdr_size + question.size());
    190   return true;
    191 }
    192 
    193 bool DnsResponse::InitParseWithoutQuery(int nbytes) {
    194   DCHECK_GE(nbytes, 0);
    195 
    196   size_t hdr_size = sizeof(dns_protocol::Header);
    197 
    198   if (nbytes < static_cast<int>(hdr_size) || nbytes >= io_buffer_->size())
    199     return false;
    200 
    201   parser_ = DnsRecordParser(
    202       io_buffer_->data(), nbytes, hdr_size);
    203 
    204   unsigned qdcount = base::NetToHost16(header()->qdcount);
    205   for (unsigned i = 0; i < qdcount; ++i) {
    206     if (!parser_.SkipQuestion()) {
    207       parser_ = DnsRecordParser();  // Make parser invalid again.
    208       return false;
    209     }
    210   }
    211 
    212   return true;
    213 }
    214 
    215 bool DnsResponse::IsValid() const {
    216   return parser_.IsValid();
    217 }
    218 
    219 uint16 DnsResponse::flags() const {
    220   DCHECK(parser_.IsValid());
    221   return base::NetToHost16(header()->flags) & ~(dns_protocol::kRcodeMask);
    222 }
    223 
    224 uint8 DnsResponse::rcode() const {
    225   DCHECK(parser_.IsValid());
    226   return base::NetToHost16(header()->flags) & dns_protocol::kRcodeMask;
    227 }
    228 
    229 unsigned DnsResponse::answer_count() const {
    230   DCHECK(parser_.IsValid());
    231   return base::NetToHost16(header()->ancount);
    232 }
    233 
    234 unsigned DnsResponse::additional_answer_count() const {
    235   DCHECK(parser_.IsValid());
    236   return base::NetToHost16(header()->arcount);
    237 }
    238 
    239 base::StringPiece DnsResponse::qname() const {
    240   DCHECK(parser_.IsValid());
    241   // The response is HEADER QNAME QTYPE QCLASS ANSWER.
    242   // |parser_| is positioned at the beginning of ANSWER, so the end of QNAME is
    243   // two uint16s before it.
    244   const size_t hdr_size = sizeof(dns_protocol::Header);
    245   const size_t qname_size = parser_.GetOffset() - 2 * sizeof(uint16) - hdr_size;
    246   return base::StringPiece(io_buffer_->data() + hdr_size, qname_size);
    247 }
    248 
    249 uint16 DnsResponse::qtype() const {
    250   DCHECK(parser_.IsValid());
    251   // QTYPE starts where QNAME ends.
    252   const size_t type_offset = parser_.GetOffset() - 2 * sizeof(uint16);
    253   uint16 type;
    254   base::ReadBigEndian<uint16>(io_buffer_->data() + type_offset, &type);
    255   return type;
    256 }
    257 
    258 std::string DnsResponse::GetDottedName() const {
    259   return DNSDomainToString(qname());
    260 }
    261 
    262 DnsRecordParser DnsResponse::Parser() const {
    263   DCHECK(parser_.IsValid());
    264   // Return a copy of the parser.
    265   return parser_;
    266 }
    267 
    268 const dns_protocol::Header* DnsResponse::header() const {
    269   return reinterpret_cast<const dns_protocol::Header*>(io_buffer_->data());
    270 }
    271 
    272 DnsResponse::Result DnsResponse::ParseToAddressList(
    273     AddressList* addr_list,
    274     base::TimeDelta* ttl) const {
    275   DCHECK(IsValid());
    276   // DnsTransaction already verified that |response| matches the issued query.
    277   // We still need to determine if there is a valid chain of CNAMEs from the
    278   // query name to the RR owner name.
    279   // We err on the side of caution with the assumption that if we are too picky,
    280   // we can always fall back to the system getaddrinfo.
    281 
    282   // Expected owner of record. No trailing dot.
    283   std::string expected_name = GetDottedName();
    284 
    285   uint16 expected_type = qtype();
    286   DCHECK(expected_type == dns_protocol::kTypeA ||
    287          expected_type == dns_protocol::kTypeAAAA);
    288 
    289   size_t expected_size = (expected_type == dns_protocol::kTypeAAAA)
    290       ? kIPv6AddressSize : kIPv4AddressSize;
    291 
    292   uint32 ttl_sec = kuint32max;
    293   IPAddressList ip_addresses;
    294   DnsRecordParser parser = Parser();
    295   DnsResourceRecord record;
    296   unsigned ancount = answer_count();
    297   for (unsigned i = 0; i < ancount; ++i) {
    298     if (!parser.ReadRecord(&record))
    299       return DNS_MALFORMED_RESPONSE;
    300 
    301     if (record.type == dns_protocol::kTypeCNAME) {
    302       // Following the CNAME chain, only if no addresses seen.
    303       if (!ip_addresses.empty())
    304         return DNS_CNAME_AFTER_ADDRESS;
    305 
    306       if (base::strcasecmp(record.name.c_str(), expected_name.c_str()) != 0)
    307         return DNS_NAME_MISMATCH;
    308 
    309       if (record.rdata.size() !=
    310           parser.ReadName(record.rdata.begin(), &expected_name))
    311         return DNS_MALFORMED_CNAME;
    312 
    313       ttl_sec = std::min(ttl_sec, record.ttl);
    314     } else if (record.type == expected_type) {
    315       if (record.rdata.size() != expected_size)
    316         return DNS_SIZE_MISMATCH;
    317 
    318       if (base::strcasecmp(record.name.c_str(), expected_name.c_str()) != 0)
    319         return DNS_NAME_MISMATCH;
    320 
    321       ttl_sec = std::min(ttl_sec, record.ttl);
    322       ip_addresses.push_back(IPAddressNumber(record.rdata.begin(),
    323                                              record.rdata.end()));
    324     }
    325   }
    326 
    327   // TODO(szym): Extract TTL for NODATA results. http://crbug.com/115051
    328 
    329   // getcanonname in eglibc returns the first owner name of an A or AAAA RR.
    330   // If the response passed all the checks so far, then |expected_name| is it.
    331   *addr_list = AddressList::CreateFromIPAddressList(ip_addresses,
    332                                                     expected_name);
    333   *ttl = base::TimeDelta::FromSeconds(ttl_sec);
    334   return DNS_PARSE_OK;
    335 }
    336 
    337 }  // namespace net
    338