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Versions: 00 draft-ietf-dnsop-dns-capture-format

dnsop                                                       J. Dickinson
Internet-Draft                                                  J. Hague
Intended status: Standards Track                            S. Dickinson
Expires: May 4, 2017                                          Sinodun IT
                                                            T. Manderson
                                                                 J. Bond
                                                                   ICANN
                                                        October 31, 2016


                   C-DNS: A DNS Packet Capture Format
              draft-dickinson-dnsop-dns-capture-format-00

Abstract

   This document describes a data representation for collections of DNS
   messages.  The format is designed for efficient storage of large
   packet captures of DNS traffic; it attempts to minimize the size of
   such packet capture files but retain the full DNS message contents
   along with the most useful transport meta data.  It is intended to
   assist with the development of DNS traffic monitoring applications
   and provide a more efficient data exchange format than alternatives
   such as PCAP files.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 4, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents



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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Terminology  . . . . . . . . . . . . . . . . . .   4
   3.  Data Collection Use Cases . . . . . . . . . . . . . . . . . .   4
   4.  Design Considerations . . . . . . . . . . . . . . . . . . . .   6
   5.  C-DNS conceptual overview . . . . . . . . . . . . . . . . . .   7
   6.  Choice of CBOR  . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  C-DNS CBOR format . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  CDDL definition . . . . . . . . . . . . . . . . . . . . .   8
     7.2.  Format overview . . . . . . . . . . . . . . . . . . . . .   8
     7.3.  File header contents  . . . . . . . . . . . . . . . . . .   9
     7.4.  File preamble contents  . . . . . . . . . . . . . . . . .   9
     7.5.  Configuration contents  . . . . . . . . . . . . . . . . .  10
     7.6.  Block contents  . . . . . . . . . . . . . . . . . . . . .  11
     7.7.  Block preamble map  . . . . . . . . . . . . . . . . . . .  12
     7.8.  Block table map . . . . . . . . . . . . . . . . . . . . .  12
     7.9.  IP address table  . . . . . . . . . . . . . . . . . . . .  13
     7.10. Class/Type table  . . . . . . . . . . . . . . . . . . . .  13
     7.11. Name/RDATA table  . . . . . . . . . . . . . . . . . . . .  14
     7.12. Query Signature table . . . . . . . . . . . . . . . . . .  14
     7.13. Question table  . . . . . . . . . . . . . . . . . . . . .  16
     7.14. Resource Record (RR) table  . . . . . . . . . . . . . . .  16
     7.15. Question list table . . . . . . . . . . . . . . . . . . .  17
     7.16. Resource Record list table  . . . . . . . . . . . . . . .  17
     7.17. Query/Response data . . . . . . . . . . . . . . . . . . .  17
     7.18. Address Event counts  . . . . . . . . . . . . . . . . . .  20
   8.  C-DNS to PCAP . . . . . . . . . . . . . . . . . . . . . . . .  21
     8.1.  Name Compression  . . . . . . . . . . . . . . . . . . . .  22
   9.  Data Collection . . . . . . . . . . . . . . . . . . . . . . .  23
     9.1.  Matching algorithm  . . . . . . . . . . . . . . . . . . .  23
     9.2.  Message identifiers . . . . . . . . . . . . . . . . . . .  24
       9.2.1.  Primary ID (required) . . . . . . . . . . . . . . . .  24
       9.2.2.  Secondary ID (optional) . . . . . . . . . . . . . . .  24
     9.3.  Algorithm Parameters  . . . . . . . . . . . . . . . . . .  24
     9.4.  Algorithm Requirements  . . . . . . . . . . . . . . . . .  24
     9.5.  Algorithm Limitations . . . . . . . . . . . . . . . . . .  25
     9.6.  Workspace . . . . . . . . . . . . . . . . . . . . . . . .  25
     9.7.  Output  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     9.8.  Post Processing . . . . . . . . . . . . . . . . . . . . .  25



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   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  25
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  25
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  25
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     13.2.  Informative References . . . . . . . . . . . . . . . . .  26
     13.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  27
   Appendix A.  CDDL . . . . . . . . . . . . . . . . . . . . . . . .  27
   Appendix B.  DNS Name compression example . . . . . . . . . . . .  33
     B.1.  NSD compression algorithm . . . . . . . . . . . . . . . .  34
     B.2.  Knot Authoritative compression algorithm  . . . . . . . .  34
     B.3.  Observed differences  . . . . . . . . . . . . . . . . . .  35
   Appendix C.  Comparison of Binary Formats . . . . . . . . . . . .  35
   Appendix D.  Sample data on the C-DNS format  . . . . . . . . . .  35
     D.1.  Comparison to full PCAPS  . . . . . . . . . . . . . . . .  35
     D.2.  Block size choices  . . . . . . . . . . . . . . . . . . .  35
     D.3.  Blocking vs more simple output  . . . . . . . . . . . . .  36
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  37

1.  Introduction

   There has long been a need to collect DNS queries and responses on
   authoritative and recursive name servers for monitoring and analysis.
   This data is used in a number of ways including traffic monitoring,
   analyzing network attacks and DITL [ditl].

   A wide variety of tools already exist to facilitate the collection of
   DNS traffic data.  DSC [dsc], packetq [packetq], dnscap [dnscap] and
   dnstap [dnstap].  However, there is no standard exchange format for
   large DNS packet captures and PCAP [pcap] or PCAP-NG [pcapng] are
   typically used in practice.

   There has also been work on using other text based formats to
   describe DNS packets [I-D.daley-dnsxml], [I-D.hoffman-dns-in-json]
   but these are largely aimed at producing convenient representations
   of single messages.

   Many DNS operators may receive 100's of thousands of queries per
   second on a single name server instance so a mechanism to minimize
   the storage size (and therefore upload overhead) of the data
   collected is highly desirable.

   This documents focusses on the problem of capturing and storing large
   packet capture files of DNS traffic.

   This document contains





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   o  A discussion of the some common use cases in which such DNS data
      is collected.  See Section 3.

   o  A discussion of the major design considerations in developing an
      efficient data representation for collections of DNS messages.
      See Section 4.

   o  A definition of a CBOR [RFC7049] representation of a collection of
      DNS messages.  This will be referred to as the C-DNS format
      (Compacted-DNS).  See Section 7.

   o  Notes on converting C-DNS back to PCAP format.  See Section 8.

   o  Some high level implementation considerations for applications
      designed to produce C-DNS, e.g. a query response matching
      algorithm.  See Section 9.

2.  Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Data Collection Use Cases

   In an ideal world it would be optimal to collect full packet captures
   of all packets going in or out of a name server.  However, there are
   several design choices or other limitations that are common to many
   DNS installations and operators.

   o  Servers are hosted in a variety of situations

      *  Operator self hosted servers

      *  Third party hosting (including multiple third parties)

      *  Third party hardware (including multiple third parties)

   o  Data is collected under different conditions

      *  On well provisioned servers running in a steady state.

      *  On heavily loaded servers

      *  On virtualized servers

      *  On servers that are under attack




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      *  On servers that are unwitting intermediaries in attacks

   o  Traffic can be collected via a variety of mechanisms

      *  On the same hardware as the name server itself

      *  Using a network tap to listen in from another server

      *  Using port mirroring to listen in from another server

   o  The capabilities of data collection (and upload) networks vary

      *  Out-of-band networks with the same capacity as the in-band
         network

      *  Out-of-band networks with less capacity than the in-band
         network

      *  Everything on the in-band network

   Clearly, there is a wide range of use cases from very limited data
   collection environments (third party hardware, servers that are under
   attack, packet capture on the name server itself and no out-of-band
   network) to 'limitless' environments (self hosted, well provisioned
   servers, using a network tap or port mirroring with an out-of-band
   networks with the same capacity as the in-band network).  In the
   former, it is unfeasible to reliably collect full PCAPS especially if
   the server is under attack.  In the latter case, collection of full
   PCAPs may be reasonable.

   As a result of these restrictions the data format discussed below was
   designed with the most limited use case in mind such that

   o  Data collection will occur on the same hardware as the name server
      itself.

   o  Collected data will be stored on the same hardware as the name
      server itself, at least temporarily.

   o  Collected data being returned to some central analysis system will
      use the same network interface as the DNS queries and responses.

   o  There are multiple third party servers involved.

   and therefore minimal storage size of the capture files is a major
   factor.





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   Another consideration for any application that records DNS traffic is
   that the running of the name server software and the transmission of
   DNS queries and responses is the most important job of a name server.
   Any data collection system co-located with the name server will need
   to be intelligent enough to carefully manage its CPU, disk, memory
   and network utilization.  Hence this use case benefits from a format
   that has a relatively low overhead to produce and minimizes the
   requirement for further potentially costly compression.

   However, it was also essential that interoperability with less
   restricted infrastructure was maintained.  In particular it is highly
   desirable that the resulting collection format should facilitate the
   re-creation of common formats (such as PCAPs) that are as close to
   the original as is realistic given the restrictions above.

4.  Design Considerations

   This section presents some of the major design considerations used in
   the development of the C-DNS format.

   o  The basic unit of data is a combined DNS Query and the associated
      Response (a 'Q/R data item').  The same structure will be used for
      unmatched queries and responses.  Queries without responses will
      be captured omitting the Response data.  Responses without queries
      will be captured omitting the Query data (but using the Query
      section from the Response, if present, as an identifying QNAME).

   Rationale: A Query and Response represents the basic level of a
   clients interaction with the server.  Also, combining the Query and
   Response into one item lowers storage requirements due to commonality
   in the data in most cases.

   o  Each Q/R data item will comprise a default Q/R data description
      and a set of optional sections.  Inclusion of optional sections
      shall be configurable.

   Rationale: Different users will have different requirements for data
   to be available for analysis.  Users with minimal requirements should
   not have to pay the cost of recording full data, however this will
   limit the ability to reconstruct PCAPS.  For example omitting the
   Resource Records from a Response will reduce the files size, and in
   principle responses can be synthesized if there is enough context.

   o  Multiple Q/R items will be collected into blocks in the format.
      Common data in a block will be abstracted and referenced from
      individual Q/R items by indexing.  The maximum number of Q/R items
      in a block will be configurable.




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   Rationale: This blocking and indexing provides a significant
   reduction in the volume of file data generated.  Whilst introducing
   complexity it provides compression of the data that makes use of
   knowledge of the DNS packet structure.

   [TODO: Further discussion on commonality between DNS packets e.g.

   o  common query signatures

   o  for the authoritative case there are a finite set of valid
      responses and much commonality in NXDOMAIN responses]

   It is anticipated that the files produced will be subject to further
   compression using general purpose compression tools.  Measurements
   show that blocking significantly reduces the CPU required to perform
   such strong compression.  See Appendix D.

   o  Meta-data about other packets received should also be included in
      each block.  For example counts of malformed DNS packets and non-
      DNS packets (e.g.  ICMP, TCP resets) sent to the server are of
      interest.

   It should be noted that any structured capture format that does not
   capture the DNS payload byte for byte will likely be limited to some
   extent in that it cannot represent 'malformed' DNS packets.  Only
   those packets that can be transformed reasonably into the structured
   format can be represented by it.  So if a query is malformed this
   will lead to the (well formed) DNS responses with error code FORMERR
   appearing as 'unmatched'.

   [TODO: Need further discussion of well-formed vs malformed packets
   and how name servers view this definition.]

   Packets such as those described above can be separately recorded in a
   PCAP file for later analysis.

5.  C-DNS conceptual overview

   The following figures show purely schematic representations of the
   C-DNS format to convey the high-level structure of the C-DNS format.
   Section 7 provides a detailed discussion of the CBOR representation
   and individual elements.

   Figure showing the C-DNS format (PNG) [1]

   Figure showing the C-DNS format (SVG) [2]

   Figure showing the Q/R data item and Block tables format (PNG) [3]



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   Figure showing the Q/R data item and Block tables format (SVG) [4]

6.  Choice of CBOR

   This document presents a detailed format description using CBOR, the
   Concise Binary Object Representation defined in [RFC7049].

   The choice of CBOR was made taking a number of factors into account.

   o  CBOR is a binary representation, and so economical in storage
      space.

   o  Other similar representations were investigated, and whilst all
      had attractive features, none had a significant advantage over
      CBOR.  See Appendix C and Appendix D - for some discussion of
      this.

   o  CBOR is an IETF Standard and familiar to IETF participants, and
      being based on the successful JSON text format, requires very
      little familiarization for those in the wider industry.

   o  CBOR can also be easily converted to JSON for debugging and other
      human inspection requirements.

   o  CBOR data schemas can be described using CDDL
      [I-D.greevenbosch-appsawg-cbor-cddl].

7.  C-DNS CBOR format

7.1.  CDDL definition

   The CDDL definition for the C-DNS format is given in Appendix A.

7.2.  Format overview

   A C-DNS file begins with a file header containing a file type
   identifier and preamble.  The preamble contains information on the
   collection settings.

   This is followed by a series of data blocks.

   A block consists of a block header, containing various tables of
   common data, and some statistics for the traffic received over the
   block.  The block header is then followed by a list of the Q/R pairs
   detailing the queries and responses received during the block.  The
   list of Q/R pairs is in turn followed by a list of per-client counts
   of particular IP events that occurred during collection of the block
   data.



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   The exact nature of the DNS data will affect what block size is the
   best fit, however sample data for a root server indicated that block
   sizes in the low 1000's give good results.  See Appendix D.2 for more
   details.

   If no field type is specified then the field is unsigned.

   In the following

   o  For all quantities that contain bit flags, bit 0 indicates the
      least significant bit.

   o  Items described as indexes are the index of the data item in the
      referenced table.  Indexes are 1-based.  An index value of 0 is
      reserved to mean not present.

7.3.  File header contents

   The file header contains the following:

   +-------------+---------------+-------------------------------------+
   | Field       | Type          | Description                         |
   +-------------+---------------+-------------------------------------+
   | File type   | Text string   | String identifying the file type    |
   | ID          |               |                                     |
   |             |               |                                     |
   | File        | Map of items  | Collection information for the      |
   | preamble    |               | whole file.                         |
   |             |               |                                     |
   | File Blocks | Array of      | The data blocks                     |
   |             | Blocks        |                                     |
   +-------------+---------------+-------------------------------------+

7.4.  File preamble contents

   The file preamble contains the following:















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   +---------------+----------+----------------------------------------+
   | Field         | Type     | Description                            |
   +---------------+----------+----------------------------------------+
   | Format        | Unsigned | Indicates version of format used in    |
   | version       |          | file.                                  |
   |               |          |                                        |
   | Configuration | Map of   | The collection configuration.          |
   |               | items    | Optional.                              |
   |               |          |                                        |
   | Generator ID  | Text     | String identifying the collection      |
   |               | string   | program. Optional.                     |
   |               |          |                                        |
   | Host ID       | Text     | String identifying the collecting      |
   |               | string   | host. Blank if converting an existing  |
   |               |          | PCAP file. Optional.                   |
   +---------------+----------+----------------------------------------+

7.5.  Configuration contents

   The collection configuration contains the following items.  All are
   optional.

   +-------------+----------+------------------------------------------+
   | Field       | Type     | Description                              |
   +-------------+----------+------------------------------------------+
   | Query       | Unsigned | To be matched with a query, a response   |
   | timeout     |          | must arrive within this number of        |
   |             |          | seconds.                                 |
   |             |          |                                          |
   | Skew        | Unsigned | The network stack may report a response  |
   | timeout     |          | before the corresponding query. A        |
   |             |          | response is not considered to be missing |
   |             |          | a query until after this many micro-     |
   |             |          | seconds.                                 |
   |             |          |                                          |
   | Snap length | Unsigned | Collect up to this many bytes per        |
   |             |          | packet.                                  |
   |             |          |                                          |
   | Promiscuous | Unsigned | 1 if promiscuous mode was enabled on the |
   | mode        |          | interface, 0 otherwise.                  |
   |             |          |                                          |
   | Interfaces  | Array of | Identifiers of the interfaces used for   |
   |             | text     | collection.                              |
   |             | strings  |                                          |
   |             |          |                                          |
   | VLAN IDs    | Array of | Identifiers of VLANs selected for        |
   |             | unsigned | collection.                              |
   |             |          |                                          |



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   | Filter      | Text     | "tcpdump" [pcap] style filter for input. |
   |             | string   |                                          |
   |             |          |                                          |
   | Query       | Unsigned | Bit flags indicating sections in Query   |
   | collection  |          | packets to be collected.                 |
   | options     |          |                                          |
   |             |          | Bit 0. Collect second and subsequent     |
   |             |          | question sections.                       |
   |             |          | Bit 1. Collect Answer sections.          |
   |             |          | Bit 2. Collect Authority sections.       |
   |             |          | Bit 3. Collection Additional sections.   |
   |             |          |                                          |
   | Response    | Unsigned | Bit flags indicating sections in         |
   | collection  |          | Response packets to be collected.        |
   | options     |          |                                          |
   |             |          | Bit 0. Collect second and subsequent     |
   |             |          | question sections.                       |
   |             |          | Bit 1. Collect Answer sections.          |
   |             |          | Bit 2. Collect Authority sections.       |
   |             |          | Bit 3. Collection Additional sections.   |
   |             |          |                                          |
   | Accept RR   | Array of | A set of RR type names [rrtypes]. If not |
   | types       | text     | empty, only the nominated RR types are   |
   |             | strings  | collected.                               |
   |             |          |                                          |
   | Ignore RR   | Array of | A set of RR type names [rrtypes]. If not |
   | types       | text     | empty, all RR types are collected except |
   |             | strings  | those listed. If present, this item must |
   |             |          | be empty if a non-empty list of Accept   |
   |             |          | RR types is present.                     |
   +-------------+----------+------------------------------------------+

7.6.  Block contents

   Each block contains the following:
















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   +-------------+------------------+----------------------------------+
   | Field       | Type             | Description                      |
   +-------------+------------------+----------------------------------+
   | Block       | Map of items     | Overall information for the      |
   | preamble    |                  | block.                           |
   |             |                  |                                  |
   | Block       | Map of           | Statistics about the block.      |
   | statistics  | statistics       |                                  |
   |             |                  |                                  |
   | Block       | Map of tables    | The tables containing data       |
   | tables      |                  | referenced by individual Q/R     |
   |             |                  | entries.                         |
   |             |                  |                                  |
   | Q/Rs        | Array of Q/Rs    | Details of individual Q/R pairs. |
   |             |                  |                                  |
   | Address     | Array of Address | Per client counts of ICMP        |
   | Event       | Event counts     | messages and TCP resets.         |
   | Counts      |                  |                                  |
   +-------------+------------------+----------------------------------+

7.7.  Block preamble map

   The block preamble map contains overall information for the block.

   +-----------+----------+--------------------------------------------+
   | Field     | Type     | Description                                |
   +-----------+----------+--------------------------------------------+
   | Timestamp | Array of | A timestamp for the earliest record in the |
   |           | unsigned | block. The timestamp is specified as a     |
   |           |          | CBOR array with two elements as in Posix   |
   |           |          | struct timeval. The first element is an    |
   |           |          | unsigned integer time_t and the second is  |
   |           |          | an unsigned integer number of              |
   |           |          | microseconds. The latter is always a value |
   |           |          | between 0 and 999,999.                     |
   +-----------+----------+--------------------------------------------+

7.8.  Block table map

   The block table map contains the block tables.  Each element, or
   table, is an array.  The following tables detail the contents of each
   block table.

   The Present column in the following tables indicates the
   circumstances when an optional field will be present.  A Q/R pair may
   be:

   o  A Query plus a Response.



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   o  A Query without a Response.

   o  A Response without a Query.

   Also:

   o  A Query and/or a Response may contain an OPT section.

   o  A Question may or may not be present.  If the Query is available,
      the Question section of the Query is used.  If no Query is
      available, the Question section of the Response is used.  Unless
      otherwise noted, a Question refers to the first Question in the
      Question section.

   So, for example, a field listed with a Present value of QUERY is
   present whenever the Q/R pair contains a Query.  If the pair contains
   a Response only, the field will not be present.

7.9.  IP address table

   This table holds all client and server IP addresses in the block.
   Each item in the table is a single IP address.

   +---------+--------+------------------------------------------------+
   | Field   | Type   | Description                                    |
   +---------+--------+------------------------------------------------+
   | Address | Byte   | The IP address, in network byte order. The     |
   |         | string | string is 4 bytes long for an IPv4 address, 16 |
   |         |        | bytes long for an IPv6 address.                |
   +---------+--------+------------------------------------------------+

7.10.  Class/Type table

   This table holds pairs of RR CLASS and TYPE values.  Each item in the
   table is a CBOR map.

                         +-------+--------------+
                         | Field | Description  |
                         +-------+--------------+
                         | Class | CLASS value. |
                         |       |              |
                         | Type  | TYPE value.  |
                         +-------+--------------+








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7.11.  Name/RDATA table

   This table holds the contents of all NAME or RDATA items in the
   block.  Each item in the table is the content of a single NAME or
   RDATA.

   +-------+--------+--------------------------------------------------+
   | Field | Type   | Description                                      |
   +-------+--------+--------------------------------------------------+
   | Data  | Byte   | The NAME or RDATA contents. NAMEs, and labels    |
   |       | string | within RDATA contents, are in uncompressed label |
   |       |        | format.                                          |
   +-------+--------+--------------------------------------------------+

7.12.  Query Signature table

   This table holds elements of the Q/R data that are often common to
   between different individual Q/R records.  Each item in the table is
   a CBOR map.  Each item in the map has an unsigned value and an
   unsigned key.

   The following abbreviations are used in the Present (P) column

   o  Q = QUERY

   o  A = Always

   o  QT = QUESTION

   o  QO = QUERY, OPT

   o  QR = QUERY & RESPONSE

   o  R = RESPONSE

   +------------+----+-------------------------------------------------+
   | Field      | P  | Description                                     |
   +------------+----+-------------------------------------------------+
   | Server     | A  | The index in the IP address table of the server |
   | address    |    | IP address.                                     |
   |            |    |                                                 |
   | Server     | A  | The server port.                                |
   | port       |    |                                                 |
   |            |    |                                                 |
   | Transport  | A  | Bit flags describing the protocol used to       |
   | flags      |    | service the query. Bit 0 is the least           |
   |            |    | significant bit.                                |
   |            |    | Bit 0. Transport type. 0 = UDP, 1 = TCP.        |



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   |            |    | Bit 1. IP type. 0 = IPv4, 1 = IPv6.             |
   |            |    |                                                 |
   | Q/R        | A  | Bit flags indicating information present in     |
   | signature  |    | this Q/R pair. Bit 0 is the least significant   |
   | flags      |    | bit.                                            |
   |            |    | Bit 0. 1 if a Query is present.                 |
   |            |    | Bit 1. 1 if a Response is present.              |
   |            |    | Bit 2. 1 if one or more Question is present.    |
   |            |    | Bit 3. 1 if a Query is present and it has an    |
   |            |    | OPT Resource Record.                            |
   |            |    | Bit 4. 1 if a Response is present and it has an |
   |            |    | OPT Resource Record.                            |
   |            |    | Bit 5. 1 if a Response is present but has no    |
   |            |    | Question.                                       |
   |            |    |                                                 |
   | Query      | Q  | Query OPCODE.                                   |
   | OPCODE     |    |                                                 |
   |            |    |                                                 |
   | Q/R DNS    | A  | Bit flags with values from the Query and        |
   | flags      |    | Response DNS flags. Bit 0 is the least          |
   |            |    | significant bit. Flag values are 0 if the Query |
   |            |    | or Response is not present.                     |
   |            |    | Bit 0. Query Checking Disabled (CD) flag.       |
   |            |    | Bit 1. Query Authenticated Data (AD) flag.      |
   |            |    | Bit 2. Query reserved (Z) flag.                 |
   |            |    | Bit 3. Query Recursion Available (RA) flag.     |
   |            |    | Bit 4. Query Recursion Desired (RD) flag.       |
   |            |    | Bit 5. Query TrunCation (TC) flag.              |
   |            |    | Bit 6. Query Authoritative Answer (AA) flag.    |
   |            |    | Bit 7. Query DNSSEC answer OK (D0) flag.        |
   |            |    | Bit 8. Response Checking Disabled (CD) flag.    |
   |            |    | Bit 9. Response Authenticated Data (AD) flag.   |
   |            |    | Bit 10. Response reserved (Z) flag.             |
   |            |    | Bit 11. Response Recursion Available (RA) flag. |
   |            |    | Bit 12. Response Recursion Desired (RD) flag.   |
   |            |    | Bit 13. Response TrunCation (TC) flag.          |
   |            |    | Bit 14. Response Authoritative Answer (AA)      |
   |            |    | flag.                                           |
   |            |    |                                                 |
   | Query      | Q  | Query RCODE. If the Query contains OPT, this    |
   | RCODE      |    | value incorporates any EXTENDED_RCODE_VALUE.    |
   |            |    |                                                 |
   | Question   | QT | The index in the Class/Type table of the CLASS  |
   | Class/Type |    | and TYPE of the first Question.                 |
   |            |    |                                                 |
   | Question   | QT | The QDCOUNT in the Query, or Response if no     |
   | QDCOUNT    |    | Query present.                                  |
   |            |    |                                                 |



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   | Query      | Q  | Query ANCOUNT.                                  |
   | ANCOUNT    |    |                                                 |
   |            |    |                                                 |
   | Query      | Q  | Query ARCOUNT.                                  |
   | ARCOUNT    |    |                                                 |
   |            |    |                                                 |
   | Query      | Q  | Query NSCOUNT.                                  |
   | NSCOUNT    |    |                                                 |
   |            |    |                                                 |
   | Query EDNS | QO | The Query EDNS version.                         |
   | version    |    |                                                 |
   |            |    |                                                 |
   | EDNS UDP   | QO | The Query EDNS sender's UDO payload size        |
   | size       |    |                                                 |
   |            |    |                                                 |
   | Query OPT  | QO | The index in the NAME/RDATA table of the OPT    |
   | RDATA      |    | RDATA.                                          |
   |            |    |                                                 |
   | Response   | R  | Response RCODE. If the Response contains OPT,   |
   | RCODE      |    | this value incorporates any                     |
   |            |    | EXTENDED_RCODE_VALUE.                           |
   +------------+----+-------------------------------------------------+

7.13.  Question table

   This table holds details on individual Questions in a Question
   section.  Each item in the table is a CBOR map containing a single
   Question.  Each item in the map has an unsigned value and an unsigned
   key.  This data is optionally collected.

   +------------+------------------------------------------------------+
   | Field      | Description                                          |
   +------------+------------------------------------------------------+
   | QNAME      | The index in the NAME/RDATA table of the QNAME.      |
   |            |                                                      |
   | Class/Type | The index in the Class/Type table of the CLASS and   |
   |            | TYPE of the Question.                                |
   +------------+------------------------------------------------------+

7.14.  Resource Record (RR) table

   This table holds details on individual Resource Records in RR
   sections.  Each item in the table is a CBOR map containing a single
   Resource Record.  This data is optionally collected.







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   +------------+------------------------------------------------------+
   | Field      | Description                                          |
   +------------+------------------------------------------------------+
   | NAME       | The index in the NAME/RDATA table of the NAME.       |
   |            |                                                      |
   | Class/Type | The index in the Class/Type table of the CLASS and   |
   |            | TYPE of the RR.                                      |
   |            |                                                      |
   | TTL        | The RR Time to Live.                                 |
   |            |                                                      |
   | RDATA      | The index in the NAME/RDATA table of the RR RDATA.   |
   +------------+------------------------------------------------------+

7.15.  Question list table

   This table holds a list of second and subsequent individual Questions
   in a Question section.  Each item in the table is a CBOR unsigned.
   This data is optionally collected.

   +----------+--------------------------------------------------------+
   | Field    | Description                                            |
   +----------+--------------------------------------------------------+
   | Question | The index in the Question table of the individual      |
   |          | Question.                                              |
   +----------+--------------------------------------------------------+

7.16.  Resource Record list table

   This table holds a list of individual Resource Records in a Answer,
   Authority or Additional section.  Each item in the table is a CBOR
   unsigned.  This data is optionally collected.

   +-------+-----------------------------------------------------------+
   | Field | Description                                               |
   +-------+-----------------------------------------------------------+
   | RR    | The index in the Resource Record table of the individual  |
   |       | Resource Record.                                          |
   +-------+-----------------------------------------------------------+

7.17.  Query/Response data

   The block Q/R data is a CBOR array of individual Q/R items.  Each
   item in the array is a CBOR map containing details on the individual
   Q/R pair.

   Note that there is no requirement that the elements of the Q/R array
   are presented in strict chronological order.




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   The following abbreviations are used in the Present (P) column

   o  Q = QUERY

   o  A = Always

   o  QT = QUESTION

   o  QO = QUERY, OPT

   o  QR = QUERY & RESPONSE

   o  R = RESPONSE

   Each item in the map has an unsigned value (with the exception of
   those listed below) and an unsigned key.

   o  Query extended information and Response extended information which
      are of Type Extended Information.

   o  Response delay which is an integer (This can be negative if the
      network stack/capture library returns them out of order.)





























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   +-------------+----+------------------------------------------------+
   | Field       | P  | Description                                    |
   +-------------+----+------------------------------------------------+
   | Time offset | A  | Q/R timestamp as an offset in microseconds     |
   |             |    | from the Block pre-amble Timestamp. The        |
   |             |    | timestamp is the timestamp of the Query, or    |
   |             |    | the Response if there is no Query.             |
   |             |    |                                                |
   | Client      | A  | The index in the IP address table of the       |
   | address     |    | client IP address.                             |
   |             |    |                                                |
   | Client port | A  | The client port.                               |
   |             |    |                                                |
   | Transaction | A  | DNS transaction identifier.                    |
   | ID          |    |                                                |
   |             |    |                                                |
   | Query       | A  | The index of the more information on the Q/R   |
   | signature   |    | in the Query Signature table.                  |
   |             |    |                                                |
   | Client      | Q  | The IPv4 TTL or IPv6 Hoplimit from the Query   |
   | hoplimit    |    | packet.                                        |
   |             |    |                                                |
   | Response    | QR | The time different between Query and Response, |
   | delay       |    | in microseconds.                               |
   |             |    |                                                |
   | Question    | QT | The index in the NAME/RDATA table of the QNAME |
   | NAME        |    | for the first Question.                        |
   |             |    |                                                |
   | Response    | R  | The size of the DNS message (not the packet    |
   | size        |    | containing the message, just the DNS message)  |
   |             |    | that forms the Response.                       |
   |             |    |                                                |
   | Query       | Q  | Extended Query information. This item is only  |
   | extended    |    | present if collection of extra Query           |
   | information |    | information is configured.                     |
   |             |    |                                                |
   | Response    | R  | Extended Response information. This item is    |
   | extended    |    | only present if collection of extra Query      |
   | information |    | information is configured.                     |
   +-------------+----+------------------------------------------------+

   The collector always collects basic Q/R information.  It may be
   configured to collect details on Question, Answer, Authority and
   Additional sections of the Query, the Response or both.  Note that
   only the second and subsequent Questions of any Question section are
   collected (the details of the first are in the basic information),
   and that OPT Records are not collected in the Additional section.




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   The Extended information is a CBOR map as follows.  Each item in the
   map is present only of collection of the relevant details is
   configured.  Each item in the map has an unsigned value and an
   unsigned key.

   +------------+------------------------------------------------------+
   | Field      | Description                                          |
   +------------+------------------------------------------------------+
   | Question   | The index in the Questions list table of the entry   |
   |            | listing the second and subsequent Question sections  |
   |            | for the Query or Response.                           |
   |            |                                                      |
   | Answer     | The index in the RR list table of the entry listing  |
   |            | the Answer Resource Record sections for the Query or |
   |            | Response.                                            |
   |            |                                                      |
   | Authority  | The index in the RR list table of the entry listing  |
   |            | the Authority Resource Record sections for the Query |
   |            | or Response.                                         |
   |            |                                                      |
   | Additional | The index in the RR list table of the entry listing  |
   |            | the Additional Resource Record sections for the      |
   |            | Query or Response.                                   |
   +------------+------------------------------------------------------+

7.18.  Address Event counts

   This table holds counts of various IP related events relating to
   traffic with individual client addresses.






















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   +----------+----------+---------------------------------------------+
   | Field    | Type     | Description                                 |
   +----------+----------+---------------------------------------------+
   | Event    | Unsigned | The type of event. The following events     |
   | type     |          | types are currently defined:                |
   |          |          | 0. TCP reset.                               |
   |          |          | 1. ICMP time exceeded.                      |
   |          |          | 2. ICMP destination unreachable.            |
   |          |          | 3. ICMPv6 time exceeded.                    |
   |          |          | 4. ICMPv6 destination unreachable.          |
   |          |          | 5. ICMPv6 packet too big.                   |
   |          |          |                                             |
   | Event    | Unsigned | A code relating to the event. Optional.     |
   | code     |          |                                             |
   |          |          |                                             |
   | Address  | Unsigned | The index in the IP address table of the    |
   | index    |          | client address.                             |
   |          |          |                                             |
   | Count    | Unsigned | The number of occurrences of this event     |
   |          |          | during the block collection period.         |
   +----------+----------+---------------------------------------------+

8.  C-DNS to PCAP

   It is possible to re-construct PCAP files from the C-DNS format.
   However this is a lossy process and some of the issues with
   reconstructing both the DNS payload and the full packet stream are
   outlined here.

   Firstly the reconstruction depends on whether or not all the optional
   sections of both the query and response were captured in the C-DNS
   file.  Clearly if they were not all captured the reconstruction is
   imperfect.

   Secondly, even if all sections of the response were captured name
   compression presents a challenge in reconstructing the DNS response
   payload byte for byte.  Section 8.1 discusses this is more detail.

   Thirdly, not all transport information is captured in the C-DNS
   format.  For example, the following aspects of the original packet
   stream cannot be re-constructed from the C-DNS format:

   o  IP Fragmentation

   o  TCP stream information:

      *  Multiple DNS messages may have been sent in a single TCP
         segment



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      *  A DNS payload may have be split across multiple TCP segments

      *  Multiple DNS messages may have be sent on a single TCP session

   o  Malformed DNS messages and non-DNS packets

   Simple assumptions can be made on the reconstruction - fragmented and
   DNS-over-TCP messages can be reconstructed into 'single' packets and
   a single TCP session can be constructed for each TCP packet.

   Additionally if the malformed and non-DNS packets are captured
   separately into PCAPs they can be merged with PCAPs reconstructed
   from C-DNS to produce a more complete packet stream.

8.1.  Name Compression

   All the names stored in the C-DNS format are full domain names; no
   DNS style name compression is used on the individual names within the
   format.  Therefore when reconstructing a packet name compression must
   be used in order to re-produce the on the wire representation of the
   packet.

   [RFC1035] name compression works by substituting trailing sections of
   a name with a reference back to the occurrence of those sections
   earlier in the packet.  Not all name server software uses the same
   algorithm when compressing domain names within the responses.  Some
   attempt maximum recompression at the expense of runtime resources,
   others use heuristics to balance compression and speed and others use
   different rules for what is a valid compression target.

   This means that responses to the same question from different name
   server software which match in terms of DNS payload content (header,
   counts, RRs with name compression removed) do not necessarily match
   byte for byte on the wire.

   From the C-DNS format it is not possible to ensure that the DNS
   response payload is reconstructed byte for byte.  However it can at
   least, in principle, be reconstructed to have the correct payload
   length (since the original response length is captured) if there is
   enough knowledge of the commonly implemented name compression
   algorithms.  For example, a simplistic approach would be to try each
   algorithm in turn to see if it reproduces the original length,
   stopping at the first match.  This would not guarantee the correct
   algorithm has been used as it is possible to match the length whilst
   still not matching the on the wire bytes but without further
   information added to the C-BOR this is the best that can be achieved.





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   Appendix B presents an example of two differing compression
   algorithms used by well known name server software.

9.  Data Collection

   This section describes a non-normative proposed algorithm for the
   processing of a captured stream of DNS queries and responses and
   matching queries/responses where possible.

   For the purposes of this discussion, it is assumed that the input has
   been pre-processed such that:

   1.  All IP fragmentation reassembly, TCP stream reassembly etc. has
       already been performed

   2.  Each message is associated with transport meta-data required to
       generate the Primary ID (see below)

   3.  Each message has a well-formed DNS header of 12 bytes and (if
       present) the first RR in the query section can be parsed to
       generate the Secondary ID (see below).

       *  As noted earlier, this requirement can result in a malformed
          query being removed in the pre-processing stage, but the
          correctly formed response with RCODE of FORMERR being present

   DNS messages are processed in the order they are delivered to the
   application.

   o  It should be noted that packet capture libraries do not necessary
      provide packets in strict chronological order.

   [TODO: Discuss the corner cases resulting from this in more detail.]

9.1.  Matching algorithm

   A schematic representation of the algorithm for matching Q/R pairs is
   shown in the following diagram:

   Figure showing the packet matching algorithm format (PNG) [5]

   Figure showing the packet matching algorithm format (SVG) [6]

   and further details of the algorithm are given in the following
   sections.






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9.2.  Message identifiers

9.2.1.  Primary ID (required)

   A Primary ID can be constructed for each message which is composed of
   the following data:

   1.  Source IP Address

   2.  Destination IP Address

   3.  Source Port

   4.  Destination Port

   5.  Transport

   6.  DNS Message ID

9.2.2.  Secondary ID (optional)

   If present, the first question in the Question section is used as a
   secondary ID for each message.  Note that there may be well formed
   DNS queries that have a QDCOUNT of 0, and some responses may have a
   QDCOUNT of 0 (for example, RCODE=FORMERR or NOTIMP)

9.3.  Algorithm Parameters

   1.  Configurable timeout

9.4.  Algorithm Requirements

   The algorithm is designed to handle the following input data:

   1.  Multiple queries with the same Primary ID (but different
       Secondary ID) arriving before any responses for these queries are
       seen.

   2.  Multiple queries with the same Primary and Secondary ID arriving
       before any responses for these queries are seen.

   3.  Queries for which no later response can be found within the
       specified timeout.

   4.  Responses for which no previous query can be found within the
       specified timeout.





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9.5.  Algorithm Limitations

   For cases 1 and 2 listed in the above requirements, it is not
   possible to unambiguously match queries with responses.  The solution
   to this employed in this algorithm is to match to the earliest query
   with the correct Primary and Secondary ID.

9.6.  Workspace

   A FIFO structure is used to hold the Q/R items during processing.

9.7.  Output

   The output is a list of Q/R data items.  Both the Query and Response
   elements are optional in these items, therefore Q/R items have one of
   three types of content:

   1.  Paired Q/R messages

   2.  A query message (no response)

   3.  A response message (no query)

   The timestamp of a list item is that of the query for cases 1 and 2
   and that of the response for case 3.

9.8.  Post Processing

   When ending capture, all remaining entries in the Q/R FIFO should be
   treated as timed out queries.

10.  IANA Considerations

   None

11.  Security Considerations

   Any control interface MUST perform authentication and encryption.

   Any data upload MUST be authenticated and encrypted.

12.  Acknowledgements

   The authors wish to thank CZ.NIC, in particular Tomas Gavenciak, for
   many useful discussions on binary formats, compression and packet
   matching.  Also Jan Vcelak and Wouter Wijngaards for discussions on
   name compression.




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   Also, Miek Gieben for mmark [7]

13.  References

13.1.  Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <http://www.rfc-editor.org/info/rfc1035>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <http://www.rfc-editor.org/info/rfc7049>.

13.2.  Informative References

   [ditl]     DNS-OARC, "DITL", 2016, <https://www.dns-
              oarc.net/oarc/data/ditl>.

   [dnscap]   DNS-OARC, "DNSCAP", 2016, <https://www.dns-oarc.net/tools/
              dnscap>.

   [dnstap]   dnstap.io, "dnstap", 2016, <http://dnstap.info/>.

   [dsc]      Wessels, D. and J. Lundstrom, "DSC", 2016,
              <https://www.dns-oarc.net/tools/dsc>.

   [I-D.daley-dnsxml]
              Daley, J., Morris, S., and J. Dickinson, "dnsxml - A
              standard XML representation of DNS data", draft-daley-
              dnsxml-00 (work in progress), July 2013.

   [I-D.greevenbosch-appsawg-cbor-cddl]
              Vigano, C. and H. Birkholz, "CBOR data definition language
              (CDDL): a notational convention to express CBOR data
              structures", draft-greevenbosch-appsawg-cbor-cddl-09 (work
              in progress), September 2016.

   [I-D.hoffman-dns-in-json]
              Hoffman, P., "Representing DNS Messages in JSON", draft-
              hoffman-dns-in-json-09 (work in progress), October 2016.





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   [packetq]  .SE - The Internet Infrastructure Foundation, "PacketQ",
              2014, <https://github.com/dotse/PacketQ>.

   [pcap]     tcpdump.org, "PCAP", 2016, <http://www.tcpdump.org/>.

   [pcapng]   Tuexen, M., Risso, F., Bongertz, J., Combs, G., and G.
              Harris, "pcap-ng", 2016, <https://github.com/pcapng/
              pcapng>.

   [rrtypes]  IANA, "RR types", 2016, <http://www.iana.org/assignments/
              dns-parameters/dns-parameters.xhtml#dns-parameters-4>.

13.3.  URIs

   [1] https://github.com/dns-stats/draft-dns-capture-
       format/blob/master/cdns_format.png

   [2] https://github.com/dns-stats/draft-dns-capture-
       format/blob/master/cdns_format.svg

   [3] https://github.com/dns-stats/draft-dns-capture-
       format/blob/master/qr_data_format.png

   [4] https://github.com/dns-stats/draft-dns-capture-
       format/blob/master/qr_data_format.svg

   [5] https://github.com/dns-stats/draft-dns-capture-
       format/blob/master/packet_matching.png

   [6] https://github.com/dns-stats/draft-dns-capture-
       format/blob/master/packet_matching.svg

   [7] https://github.com/miekg/mmark

   [8] https://www.nlnetlabs.nl/projects/nsd/

   [9] https://www.knot-dns.cz/

Appendix A.  CDDL

; CDDL specification of the file format for C-DNS,
; which describes a collection of DNS Query/Response pairs.

File = [
    file-type-id  : tstr,          ; "DNS-STAT"
    file-preamble : FilePreamble,
    file-blocks   : [* Block],
]



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FilePreamble = {
    format-version  => uint,
    ? configuration => Configuration,
    ? generator-id  => tstr,
    ? host-id       => tstr,
}

format-version = 0
configuration  = 1
generator-id   = 2
host-id        = 3

Configuration = {
    ? query-timeout    => uint,
    ? skew-timeout     => uint,
    ? snaplen          => uint,
    ? promisc          => uint,
    ? interfaces       => [* tstr],
    ? vlan-ids         => [* uint],
    ? filter           => tstr,
    ? query-options    => uint,    ; See below
    ? response-options => uint,
    ? accept-rr-types  => [* tstr],
    ? ignore-rr-types  => [* tstr],
}

; query-options and response-options are bitmasks. A bit set adds in the
; specified sections.
;
; second & subsequent question sections = 1
; answer sections = 2
; authority sections = 4
; additional sections = 8

query-timeout    = 0
skew-timeout     = 1
snaplen          = 2
promisc          = 3
interfaces       = 4
vlan-ids         = 5
filter           = 6
query-options    = 7
response-options = 8
accept-rr-types  = 9;
ignore-rr-types  = 10;


Block = {



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    preamble             => BlockPreamble,
    ? statistics         => BlockStatistics, ; Much of this could be derived
    tables               => BlockTables,
    queries              => [* QueryResponse],
    address-event-counts => [* AddressEventCount],
}

preamble             = 0
statistics           = 1
tables               = 2
queries              = 3
address-event-counts = 4

BlockPreamble = {
    start-time => Timeval
}

start-time = 1

Timeval = [
    seconds      : uint,
    microseconds : uint,
]

BlockStatistics = {
    ? total-packets        => uint,
    ? total-pairs          => uint,
    ? unmatched_queries    => uint,
    ? unmatched_responses  => uint,
    ? malformed-packets    => uint,
    ? non-dns-packets      => uint,
    ? out-of-order-packets => uint,
    ? missing-pairs        => uint,
    ? missing-packets      => uint,
    ? missing-non-dns      => uint,
}

total-packets        = 0
total-pairs          = 1
unmatched_queries    = 2
unmatched_responses  = 3
malformed-packets    = 4
non-dns-packets      = 5
out-of-order-packets = 6
missing-pairs        = 7
missing-packets      = 8
missing-non-dns      = 9




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BlockTables = {
    ip-address => [* bstr],
    classtype  => [* ClassType],
    name-rdata => [* bstr],            ; Holds both Name RDATA and RDATA
    query_sig  => [* QuerySignature]
    ? qlist    => [* QuestionList],
    ? qrr      => [* Question],
    ? rrlist   => [* RRList],
    ? rr       => [* RR],
}

ip-address = 0
classtype  = 1
name-rdata = 2
query_sig  = 3
qlist      = 4
qrr        = 5
rrlist     = 6
rr         = 7

QueryResponse = {
    time-useconds         => int,        ; Time offset from start of block
    client-address-index  => uint,
    client-port           => uint,
    transaction-id        => uint,
    query-signature-index => uint,
    ? client-hoplimit     => uint,
    ? delay-useconds      => int,        ; Times may be -ve at capture
    ? query-name-index    => uint,
    ? response-size       => uint,       ; DNS size of response
    ? query-extended      => QueryResponseExtended,
    ? response-extended   => QueryResponseExtended,
}

time-useconds         = 0
client-address-index  = 1
client-port           = 2
transaction-id        = 3
query-signature-index = 4
client-hoplimit       = 5
delay-useconds        = 6
query-name-index      = 7
response-size         = 8
query-extended        = 9
response-extended     = 10

ClassType = {
    type  => uint,



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    class => uint,
}

type  = 0
class = 1

QuerySignature = {
    server-address-index    => uint,
    server-port             => uint,
    transport-flags         => uint,
    qr-sig-flags            => uint,
    ? query-opcode          => uint,
    qr-dns-flags            => uint,
    ? query-rcode           => uint,
    ? query-classtype-index => uint,
    ? query-qd-count        => uint,
    ? query-an-count        => uint,
    ? query-ar-count        => uint,
    ? query-ns-count        => uint,
    ? edns-version          => uint,
    ? udp-buf-size          => uint,
    ? opt-rdata-index       => uint,
    ? response-rcode        => uint,
}

server-address-index  = 0
server-port           = 1
transport-flags       = 2
qr-sig-flags          = 3
query-opcode          = 4
qr-dns-flags          = 5
query-rcode           = 6
query-classtype-index = 7
query-qd-count        = 8
query-an-count        = 9
query-ar-count        = 10
query-ns-count        = 11
edns-version          = 12
udp-buf-size          = 13
opt-rdata-index       = 14
response-rcode        = 15

QuestionList = [
    * uint,                           ; Index of Question
]

Question = {                          ; Second and subsequent questions
    name-index      => uint,          ; Index to a name in the name-rdata table



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    classtype-index => uint,
}

name-index      = 0
classtype-index = 1

RRList = [
    * uint,                           ; Index of RR
]

RR = {
    name-index      => uint,          ; Index to a name in the name-rdata table
    classtype-index => uint,
    ttl             => uint,
    rdata-index     => uint,          ; Index to RDATA in the name-rdata table
}

ttl         = 2
rdata-index = 3

QueryResponseExtended = {
    ? question-index   => uint,       ; Index of QuestionList
    ? answer-index     => uint,       ; Index of RRList
    ? authority-index  => uint,
    ? additional-index => uint,
}

question-index   = 0
answer-index     = 1
authority-index  = 2
additional-index = 3

AddressEventCount = {
    ae-type          => &AddressEventType,
    ? ae-code        => uint,
    ae-address-index => uint,
    ae-count         => uint,
}

ae-type          = 0
ae-code          = 1
ae-address-index = 2
ae-count         = 3

AddressEventType = (
    tcp-reset: 0,
    icmp-time-exceeded     : 1,
    icmp-dest-unreachable  : 2,



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    icmpv6-time-exceeded   : 3,
    icmpv6-dest-unreachable: 4,
    icmpv6-packet-too-big  : 5,
)

Appendix B.  DNS Name compression example

   The basic algorithm which follows the guidance in [RFC1035] is simply
   to collect each name, and the offset in the packet at which it
   starts, during packet construction.  As each name is added, it is
   offered to each of the collected names in order of collection,
   starting from the first name.  If labels at the end of the name can
   be replaced with a reference back to part (or all) of the earlier
   name, and if the uncompressed part of the name is shorter than any
   compression already found, the earlier name is noted as the
   compression target for the name.

   The following tables illustrate the process.  In an example packet,
   the first name is example.com.

          +---+-------------+--------------+--------------------+
          | N | Name        | Uncompressed | Compression Target |
          +---+-------------+--------------+--------------------+
          | 1 | example.com |              |                    |
          +---+-------------+--------------+--------------------+

   The next name added is bar.com.  This is matched against example.com.
   The com part of this can be used as a compression target, with the
   remaining uncompressed part of the name being bar.

          +---+-------------+--------------+--------------------+
          | N | Name        | Uncompressed | Compression Target |
          +---+-------------+--------------+--------------------+
          | 1 | example.com |              |                    |
          | 2 | bar.com     | bar          | 1 + offset to com  |
          +---+-------------+--------------+--------------------+

   The third name added is www.bar.com.  This is first matched against
   example.com, and as before this is recorded as a compression target,
   with the remaining uncompressed part of the name being www.bar.  It
   is then matched against the second name, which again can be a
   compression target.  Because the remaining uncompressed part of the
   name is www, this is an improved compression, and so it is adopted.








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          +---+-------------+--------------+--------------------+
          | N | Name        | Uncompressed | Compression Target |
          +---+-------------+--------------+--------------------+
          | 1 | example.com |              |                    |
          | 2 | bar.com     | bar          | 1 + offset to com  |
          | 3 | www.bar.com | www          | 2                  |
          +---+-------------+--------------+--------------------+

   As an optimization, if a name is already perfectly compressed - in
   other words, the uncompressed part of the name is empty - no further
   names will be considered for compression.

B.1.  NSD compression algorithm

   Using the above basic algorithm the packet lengths of responses
   generated by NSD [8] can be matched almost exactly.  At the time of
   writing, a tiny number (<.01%) of the reconstructed packets had
   incorrect lengths.

B.2.  Knot Authoritative compression algorithm

   The Knot Authoritative [9] name server uses different compression
   behavior, which is the result of internal optimization designed to
   balance runtime speed with compression size gains.  In brief, and
   omitting complications, Knot Authoritative will only consider the
   QNAME and names in the immediately preceding RR section in an RRSET
   as compression targets.

   A set of smart heuristics as described below can be implemented to
   mimic this and while not perfect it produces output nearly, but not
   quite, as good a match as with NSD.  The heuristics are:

   1.  A match is only perfect if the name is completely compressed AND
       the TYPE of the section in which the name occurs matches the TYPE
       of the name used as the compression target.

   2.  If the name occurs in RDATA:

       a  If the compression target name is in a query, then only the
          first RR in an RRSET can use that name as a compression
          target.

       b  The compression target name MUST be in RDATA.

       c  The name section TYPE must match the compression target name
          section TYPE.





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       d  The compression target name MUST be in the immediately
          preceding RR in the RRSET.

   Using this algorithm less than 0.1% of the reconstructed packets had
   incorrect lengths.

B.3.  Observed differences

   In sample traffic collected on a root name server around 2-4% of
   responses generated by Knot had different packet lengths to those
   produced by NSD.

Appendix C.  Comparison of Binary Formats

   Several binary representations were considered in particular CBOR,
   Apache Avro and Protocol Buffers.

   Protocol Buffers and Avro both require a data schema, and validate
   data being stored against that schema.

   [TODO: Finish pros and cons of CBOR vs Avro vs Protocol buffers -
   tools, schema, adoption, etc.]

   The difference in file sizes were mostly minimal See Appendix D.3.

Appendix D.  Sample data on the C-DNS format

   This section presents some example figures for the output size of
   capture files when using different block sizes, data representations
   and binary formats.  The data is sample data for a root instance.

   [TODO: This section needs more work..]

D.1.  Comparison to full PCAPS

   As can be seen in more detail below for this sample data the
   compressed C-DNS files are around 30% the size of the full compressed
   PCAPs.  It should also be noted that experiments showed that
   compression of the C-DNS format required very roughly an order of
   magnitude less CPU resources than compression of full PCAPSs when
   using one core from a 3.5GHz i7 processor.

D.2.  Block size choices

   [TODO: Discuss trade-off of file block size vs memory consumption.]

   [TODO: Add graph that demonstrates block size of 5000 is optimal for
   the sample data used.]



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D.3.  Blocking vs more simple output

   Some experiments were conducted producing output in a very simple
   format involving a single record per Q/R data item (akin to a .csv
   representation).  The aim here was to examine whether the blocking
   mechanism (using a block size of 5000) was worth the complexity,
   particularly after compression of the output file using several
   general purpose compression tools.  The original PCAP file was
   325.79M and compressed using xz to 24.3Mb.

          +-------------+-------------+--------+--------+-------+
          | Format      | Output size |    lz4 |   gzip |    xz |
          +-------------+-------------+--------+--------+-------+
          | cbor-simple |      44.23M | 16.06M | 11.50M | 7.51M |
          | cbor-block  |      22.44M | 15.14M | 10.70M | 7.23M |
          +-------------+-------------+--------+--------+-------+

   It might be expected that blocking is exploiting commonality that a
   general purpose compression engine could also exploit, and the
   figures do indeed bear this out.  The more powerful (and resource-
   consuming) the compression, the closer the compressed simple file
   size gets to the compressed chunk file size.  With no compression,
   the blocked output size is typically half that of the simple output,
   but as greater degrees of compression are applied the gap shrinks.
   However, even with the stronger compressor, the chunked output
   remains roughly 5-10% smaller than the simple output.  This, and the
   higher gains at lower compression, might be significant, depending on
   the target environment.

   [TODO: Add data on reduction in CPU overhead of compressing blocked
   output vs simple output.]

   This was repeated using some other binary representations:

        +-----------------+-------------+--------+--------+-------+
        | Format          | Output size |    lz4 |   gzip |    xz |
        +-----------------+-------------+--------+--------+-------+
        | json-simple     |     189.85M | 25.59M | 16.03M | 9.74M |
        | avro-simple     |      43.31M | 16.07M | 11.92M | 7.99M |
        | avro-block      |      17.44M | 12.94M | 10.08M | 7.18M |
        | protobuf-simple |      46.02M | 15.79M | 11.59M | 7.94M |
        | protobuf-block  |      22.08M | 15.43M | 10.91M | 7.40M |
        +-----------------+-------------+--------+--------+-------+

   There's not a lot to choose between the three contenders with simple
   output.  Avro produces the smaller output, CBOR the next and Protocol
   Buffers the largest, but the different is under 10%. However, with
   blocking, while CBOR and Protocol Buffers are again within a few



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   percentage points of each other (though Protocol Buffers now has a
   slight advantage), Avro produces files in the region of 20% smaller,
   and holds a diminishing advantage through increased compression.

Authors' Addresses

   John Dickinson
   Sinodun IT
   Magdalen Centre
   Oxford Science Park
   Oxford  OX4 4GA

   Email: jad@sinodun.com


   Jim Hague
   Sinodun IT
   Magdalen Centre
   Oxford Science Park
   Oxford  OX4 4GA

   Email: jim@sinodun.com


   Sara Dickinson
   Sinodun IT
   Magdalen Centre
   Oxford Science Park
   Oxford  OX4 4GA

   Email: sara@sinodun.com


   Terry Manderson
   ICANN

   Email: terry.manderson@icann.org


   John Bond
   ICANN

   Email: john.bond@icann.org








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