Add simple non-response-based question parsing for mDNS passive listening

net/dns/dns_response.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/dns/dns_response.h"
 
 #include "base/string_util.h"
 #include "base/sys_byteorder.h"
 #include "net/base/address_list.h"
 #include "net/base/big_endian.h"
@@ skipping 109 matching lines @@
       reader.ReadU16(&out->klass) &&
       reader.ReadU32(&out->ttl) &&
       reader.ReadU16(&rdlen) &&
       reader.ReadPiece(&out->rdata, rdlen)) {
     cur_ = reader.ptr();
     return true;
   }
   return false;
 }
 
+bool DnsRecordParser::SkipQuestion() {
+  size_t consumed = ReadName(cur_, NULL);
+  if (!consumed)
+    return false;
+
+  const char* next = cur_ + consumed + 2 * sizeof(uint16);  // QTYPE + QCLASS
+  if (next > packet_ + length_)
+    return false;
+
+  cur_ = next;
+
+  return true;
+}
+
 DnsResponse::DnsResponse()
     : io_buffer_(new IOBufferWithSize(dns_protocol::kMaxUDPSize + 1)) {
 }
 
 DnsResponse::DnsResponse(size_t length)
     : io_buffer_(new IOBufferWithSize(length)) {
 }
 
 DnsResponse::DnsResponse(const void* data,
                          size_t length,
@@ skipping 28 matching lines @@
     return false;
   }
 
   // Construct the parser.
   parser_ = DnsRecordParser(io_buffer_->data(),
                             nbytes,
                             hdr_size + question.size());
   return true;
 }
 
+bool DnsResponse::InitParseWithoutQuery(int nbytes) {
+  if (nbytes >= io_buffer_->size())
+    return false;
+
+  size_t hdr_size = sizeof(dns_protocol::Header);
+  parser_ = DnsRecordParser(
+      io_buffer_->data(), nbytes, hdr_size);
+
+  unsigned qdcount = base::NetToHost16(header()->qdcount);
+  for (unsigned i = 0; i < qdcount; ++i) {
+    if (!parser_.SkipQuestion()) {
+      parser_ = DnsRecordParser();  // Make parser invalid again.
+      return false;
+    }
+  }
+
+  return true;
+}
+
 bool DnsResponse::IsValid() const {
   return parser_.IsValid();
 }
 
 uint16 DnsResponse::flags() const {
   DCHECK(parser_.IsValid());
   return base::NetToHost16(header()->flags) & ~(dns_protocol::kRcodeMask);
 }
 
 uint8 DnsResponse::rcode() const {
@@ skipping 98 matching lines @@
 
   // getcanonname in eglibc returns the first owner name of an A or AAAA RR.
   // If the response passed all the checks so far, then |expected_name| is it.
   *addr_list = AddressList::CreateFromIPAddressList(ip_addresses,
                                                     expected_name);
   *ttl = base::TimeDelta::FromSeconds(ttl_sec);
   return DNS_PARSE_OK;
 }
 
 }  // namespace net