// Copyright (c) 2012 The Chromium OS Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef PATCHPANEL_DNS_DNS_RESPONSE_H_
#define PATCHPANEL_DNS_DNS_RESPONSE_H_

#include <stddef.h>
#include <stdint.h>

#include <string>

#include "base/macros.h"
#include "base/memory/ref_counted.h"
#include "base/strings/string_piece.h"
#include "base/time/time.h"

#include "patchpanel/dns/io_buffer.h"
#include "patchpanel/dns/net_export.h"

namespace net {

class AddressList;
class DnsQuery;
class IOBufferWithSize;

namespace dns_protocol {
struct Header;
}

// Structure representing a Resource Record as specified in RFC 1035, Section
// 4.1.3.
struct NET_EXPORT_PRIVATE DnsResourceRecord {
  DnsResourceRecord();
  ~DnsResourceRecord();

  std::string name;  // in dotted form
  uint16_t type;
  uint16_t klass;
  uint32_t ttl;
  base::StringPiece rdata;  // points to the original response buffer
};

// Iterator to walk over resource records of the DNS response packet.
class NET_EXPORT_PRIVATE DnsRecordParser {
 public:
  // Construct an uninitialized iterator.
  DnsRecordParser();

  // Construct an iterator to process the |packet| of given |length|.
  // |offset| points to the beginning of the answer section.
  DnsRecordParser(const void* packet, size_t length, size_t offset);

  // Returns |true| if initialized.
  bool IsValid() const { return packet_ != nullptr; }

  // Returns |true| if no more bytes remain in the packet.
  bool AtEnd() const { return cur_ == packet_ + length_; }

  // Returns current offset into the packet.
  size_t GetOffset() const { return cur_ - packet_; }

  // Parses a (possibly compressed) DNS name from the packet starting at
  // |pos|. Stores output (even partial) in |out| unless |out| is NULL. |out|
  // is stored in the dotted form, e.g., "example.com". Returns number of bytes
  // consumed or 0 on failure.
  // This is exposed to allow parsing compressed names within RRDATA for TYPEs
  // such as NS, CNAME, PTR, MX, SOA.
  // See RFC 1035 section 4.1.4.
  unsigned ReadName(const void* pos, std::string* out) const;

  // Parses the next resource record into |record|. Returns true if succeeded.
  bool ReadRecord(DnsResourceRecord* record);

  // Skip a question section, returns true if succeeded.
  bool SkipQuestion();

 private:
  const char* packet_;
  size_t length_;
  // Current offset within the packet.
  const char* cur_;
};

// Buffer-holder for the DNS response allowing easy access to the header fields
// and resource records. After reading into |io_buffer| must call InitParse to
// position the RR parser.
class NET_EXPORT_PRIVATE DnsResponse {
 public:
  // Possible results from ParseToAddressList.
  enum Result {
    DNS_PARSE_OK = 0,
    DNS_MALFORMED_RESPONSE,    // DnsRecordParser failed before the end of
                               // packet.
    DNS_MALFORMED_CNAME,       // Could not parse CNAME out of RRDATA.
    DNS_NAME_MISMATCH,         // Got an address but no ordered chain of CNAMEs
                               // leads there.
    DNS_SIZE_MISMATCH,         // Got an address but size does not match.
    DNS_CNAME_AFTER_ADDRESS,   // Found CNAME after an address record.
    DNS_ADDRESS_TTL_MISMATCH,  // OBSOLETE. No longer used.
    DNS_NO_ADDRESSES,          // OBSOLETE. No longer used.
    // Only add new values here.
    DNS_PARSE_RESULT_MAX,  // Bounding value for histograms.
  };

  // Constructs a response buffer large enough to store one byte more than
  // largest possible response, to detect malformed responses.
  DnsResponse();

  // Constructs a response buffer of given length. Used for TCP transactions.
  explicit DnsResponse(size_t length);

  // Constructs a response from |data|. Used for testing purposes only!
  DnsResponse(const void* data, size_t length, size_t answer_offset);

  ~DnsResponse();

  // Internal buffer accessor into which actual bytes of response will be
  // read.
  IOBufferWithSize* io_buffer() { return io_buffer_.get(); }

  // Assuming the internal buffer holds |nbytes| bytes, initialize the parser
  // without matching it against an existing query.
  bool InitParseWithoutQuery(int nbytes);

  // Returns true if response is valid, that is, after successful InitParse.
  bool IsValid() const;

  // All of the methods below are valid only if the response is valid.

  // Accessors for the header.
  uint16_t flags() const;  // excluding rcode
  uint8_t rcode() const;

  unsigned answer_count() const;
  unsigned additional_answer_count() const;

  // Accessors to the question. The qname is unparsed.
  base::StringPiece qname() const;
  uint16_t qtype() const;

  // Returns an iterator to the resource records in the answer section.
  // The iterator is valid only in the scope of the DnsResponse.
  // This operation is idempotent.
  DnsRecordParser Parser() const;

 private:
  // Convenience for header access.
  const dns_protocol::Header* header() const;

  // Buffer into which response bytes are read.
  scoped_refptr<IOBufferWithSize> io_buffer_;

  // Iterator constructed after InitParse positioned at the answer section.
  // It is never updated afterwards, so can be used in accessors.
  DnsRecordParser parser_;

  DISALLOW_COPY_AND_ASSIGN(DnsResponse);
};

}  // namespace net

#endif  // PATCHPANEL_DNS_DNS_RESPONSE_H_