EDNS Padding Policies: Padding Policies for Extension Mechanisms for DNS (EDNS(0))

Internet Engineering Task Force (IETF)                      A. Mayrhofer
Request for Comments: 8467                                   nic.at GmbH
Category: Experimental                                      October 2018
ISSN: 2070-1721


      Padding Policies for Extension Mechanisms for DNS (EDNS(0))

Abstract

   RFC 7830 specifies the "Padding" option for Extension Mechanisms for
   DNS (EDNS(0)) but does not specify the actual padding length for
   specific applications.  This memo lists the possible options
   ("padding policies"), discusses the implications of each option, and
   provides a recommended (experimental) option.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are candidates for any level of
   Internet Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8467.

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Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................2
   3. General Guidance ................................................3
   4. Padding Strategies ..............................................3
      4.1. Recommended Strategy: Block-Length Padding .................3
      4.2. Other Strategies ...........................................5
           4.2.1. Maximal-Length Padding ..............................5
           4.2.2. Random-Length Padding ...............................5
           4.2.3. Random-Block-Length Padding .........................6
   5. IANA Considerations .............................................6
   6. Security Considerations .........................................6
   7. References ......................................................7
      7.1. Normative References .......................................7
      7.2. Informative References .....................................7
   Appendix A.  Padding Policies That Are Not Sensible ................8
     A.1.  No Padding .................................................8
     A.2.  Fixed-Length Padding .......................................8
   Acknowledgements ...................................................9
   Author's Address ...................................................9

1.  Introduction

   [RFC7830] specifies the Extension Mechanisms for DNS (EDNS(0))
   "Padding" option, which allows DNS clients and servers to
   artificially increase the size of a DNS message by a variable number
   of bytes, hampering size-based correlation of encrypted DNS messages.

   However, RFC 7830 deliberately does not specify the actual length of
   padding to be used.  This memo discusses options regarding the actual
   size of padding, lists advantages and disadvantages of each of these
   "padding strategies", and provides a recommended (experimental)
   strategy.

   Padding DNS messages is useful only when transport is encrypted using
   protocols such as DNS over Transport Layer Security [RFC7858], DNS
   over Datagram Transport Layer Security [RFC8094], or other encrypted
   DNS transports specified in the future.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  General Guidance

   EDNS(0) options space: The maximum message length, as dictated by the
   protocol, limits the space for EDNS(0) options.  Since padding will
   reduce the message space available to other EDNS(0) options, the
   "Padding" option MUST be the last EDNS(0) option applied before a DNS
   message is sent.

   Resource Conservation: Especially in situations where networking and
   processing resources are scarce (e.g., battery-powered long-life
   devices, low bandwidth, or high-cost links), the trade-off between
   increased size of padded DNS messages and the corresponding gain in
   confidentiality must be carefully considered.

   Transport Protocol Independence: The message size used as input to
   the various padding strategies MUST be calculated excluding the
   potential extra 2-octet length field used in TCP transport.
   Otherwise, the padded (observable) size of the DNS packets could
   significantly change between different transport protocols and reveal
   an indication of the original (unpadded) length.  For example, given
   a Block-Length Padding strategy with a block length of 32 octets and
   a DNS message with a size of 59 octets, the message would be padded
   to 64 octets when transported over UDP.  If that same message were
   transported over TCP and the padding strategy considered the extra 2
   octets of the length field (61 octets in total), the padded message
   would be 96 octets long (as the minimum length of the "Padding"
   option is 4 octets).

4.  Padding Strategies

   This section contains a recommended strategy, as well as a
   non-exhaustive list of other sensible strategies, for choosing
   padding length.  Note that, for completeness, Appendix A contains two
   more strategies that are not sensible.

4.1.  Recommended Strategy: Block-Length Padding

   Based on empirical research performed by Daniel K. Gillmor
   [NDSS-PADDING], padding SHOULD be performed following the Block-
   Length Padding strategy as follows:

   (1)  Clients SHOULD pad queries to the closest multiple of 128
        octets.

   (2)  If a server receives a query that includes the EDNS(0) "Padding"
        option, it MUST pad the corresponding response (see Section 4 of
        RFC 7830) and SHOULD pad the corresponding response to a
        multiple of 468 octets (see below).

   Note that the recommendation above only applies if the DNS transport
   is encrypted (see Section 6 of RFC 7830).

   In Block-Length Padding, a sender pads each message so that its
   padded length is a multiple of a chosen block length.  This creates a
   greatly reduced variety of message lengths.  An implementor needs to
   consider that even the zero-length "Padding" option increases the
   length of the packet by 4 octets.

   Options: Block length.  For queries, values between 16 and 128 octets
   were discussed before empiric research was performed.  Responses will
   require larger block sizes (see [NDSS-PADDING] and above for a
   discussion).

   Very large block lengths will have confidentiality properties similar
   to the Maximal-Length Padding strategy (Section 4.2.1), since almost
   all messages will fit into a single block.  Such "very large block
   length" values are:

   o  288 bytes for the query (the maximum size of a one-question query
      over TCP, without any EDNS(0) options) and

   o  the EDNS(0) buffer size of the server for the responses.

   Advantages: This policy is reasonably easy to implement, reduces the
   variety of message ("fingerprint") sizes significantly, and does not
   require a source of (pseudo) random numbers, since the padding length
   required can be derived from the actual (unpadded) message.

   Disadvantage: Given an unpadded message and the block size of the
   padding (which is assumed to be public knowledge once a server is
   reachable), the size range of a padded message can be predicted.
   Therefore, the minimum length of the unpadded message can be
   inferred.

   The empirical research cited above performed a simulation of padding,
   based on real-world DNS traffic captured on busy recursive resolvers
   of a research network.  The evaluation of the performance of
   individual padding policies was based on a "cost to attacker" and
   "cost to defender" function, where the "cost to attacker" was defined
   as the percentage of query/response pairs falling into the same size
   bucket and "cost to defender" was defined as the size factor between
   padded and unpadded messages.  Padding with a block size of 128 bytes
   on the query side and 468 bytes on the response side was considered
   the optimum trade-off between defender and attacker cost.  The
   response block size of 468 was chosen so that 3 blocks of 468 octets
   would still comfortably fit into typical Maximum Transmission Unit
   (MTU) size values.

   The block size will interact with the MTU size.  Especially for
   length values that are a large fraction of the MTU, unless the block
   length is chosen so that a multiple just fits into the MTU, Block-
   Length Padding may cause unnecessary fragmentation for UDP-based
   delivery.  Of course, choosing a block length larger than the MTU
   always forces fragmentation.

   Note: Once DNSSEC-validating clients become more prevalent, observed
   size patterns are expected to change significantly.  In that case,
   the recommended strategy might need to be revisited.

4.2.  Other Strategies

4.2.1.  Maximal-Length Padding

   In Maximal-Length Padding, the sender pads every message to the
   maximum size allowed by protocol negotiations.

   Advantages: Maximal-Length Padding, when combined with encrypted
   transport, provides the highest possible level of message-size
   confidentiality.

   Disadvantages: Maximal-Length Padding is wasteful and requires
   resources on the client, all intervening networks and equipment, and
   the server.  Depending on the negotiated size, this strategy will
   commonly exceed the MTU and result in a consistent number of
   fragments, reducing delivery probability when datagram-based
   transport (such as UDP) is used.

   Due to resource consumption, Maximal-Length Padding is NOT
   RECOMMENDED.

4.2.2.  Random-Length Padding

   When using Random-Length Padding, a sender pads each message with a
   random amount of padding.  Due to the size of the "Padding" option
   itself, each message size is increased by at least 4 octets.  The
   upper limit for padding is the maximum message size.  However, a
   client or server may choose to impose a lower maximum padding length.

   Options: Maximum and minimum padding length.

   Advantages: Theoretically, this policy should create a natural
   distribution of message sizes.

   Disadvantage: Random-Length Padding allows an attacker who can
   observe a large number of requests to infer the length of the
   original value by observing the distribution of total lengths.

   According to the limited empirical data available, Random-Length
   Padding exposes slightly more entropy to an attacker than Block-
   Length Padding.  Because of that, and the risk outlined above,
   Random-Length Padding is NOT RECOMMENDED.

4.2.3.  Random-Block-Length Padding

   This policy combines Block-Length Padding with a random component.
   Specifically, a sender randomly chooses between a few block length
   values and then applies Block-Length Padding based on the chosen
   block length.  The random selection of block length might even be
   reasonably based on a "weak" source of randomness, such as the
   transaction ID of the message.

   Options: Number of and the values for the set of block lengths;
   source of randomness

   Advantages: Compared to Block-Length Padding, this creates more
   variety in the resulting message sizes for a certain individual
   original message length.

   Disadvantage: Requires more implementation effort compared to simple
   Block-Length Padding.

   Random-Block-Length Padding requires further empirical study, as do
   other combinations of padding strategies.

5.  IANA Considerations

   This document has no IANA actions.

6.  Security Considerations

   The choice of the right padding policy (and the right parameters for
   the chosen policy) has a significant impact on the resilience of
   encrypted DNS against size-based correlation attacks.  Therefore, any
   implementor of the "Padding" option must carefully consider which
   policies to implement, the default policy chosen, which parameters to
   make configurable, and the default parameter values.

   No matter how carefully a client selects their padding policy, this
   effort can be jeopardized if the server chooses to apply an
   ineffective padding policy to the corresponding response packets.
   Therefore, a client applying the "Padding" option may want to choose
   a DNS server that applies a padding policy on responses that is at
   least equally effective.

   Note that even with encryption and padding, it might be trivial to
   identify that the observed traffic is DNS.  Also, padding does not
   prevent information leaks via other side channels (particularly
   timing information and number of query/response pairs).
   Countermeasures against such side channels could include injecting
   artificial "cover traffic" into the stream of DNS messages or
   delaying DNS responses by a certain amount of jitter.  Such
   strategies are out of the scope of this document.  Additionally,
   there is not enough theoretic analysis or experimental data available
   to recommend any such countermeasures.

7.  References

7.1.  Normative References

   [NDSS-PADDING]
              Gillmor, D., "Empirical DNS Padding Policy", March 2017,
              <https://dns.cmrg.net/
              ndss2017-dprive-empirical-DNS-traffic-size.pdf>.

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

   [RFC7830]  Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830,
              DOI 10.17487/RFC7830, May 2016,
              <https://www.rfc-editor.org/info/rfc7830>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

7.2.  Informative References

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <https://www.rfc-editor.org/info/rfc7858>.

   [RFC8094]  Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
              Transport Layer Security (DTLS)", RFC 8094,
              DOI 10.17487/RFC8094, February 2017,
              <https://www.rfc-editor.org/info/rfc8094>.

Appendix A.  Padding Policies That Are Not Sensible

A.1.  No Padding

   In the No Padding policy, the "Padding" option is not used, and the
   size of the final (actually, "non-padded") message obviously exactly
   matches the size of the unpadded message.  Even though this
   "non-policy" seems redundant in this list, its properties must be
   considered for cases in which just one of the parties (client or
   server) applies padding.

   Also, this policy is required when the remaining message size of the
   unpadded message does not allow for the "Padding" option to be
   included -- i.e., there are fewer than 4 octets left.

   Advantages: This policy requires no additional resources on the
   client, server, and network side.

   Disadvantages: The original size of the message remains unchanged;
   hence, this approach provides no additional confidentiality.

   The No Padding policy MUST NOT be used unless message size disallows
   the use of the "Padding" option.

A.2.  Fixed-Length Padding

   In Fixed-Length Padding, a sender chooses to pad each message with a
   padding of constant length.

   Options: Actual length of padding

   Advantages: Since the padding is constant in length, this policy is
   very easy to implement and at least ensures that the message length
   diverges from the length of the original packet (even if only by a
   fixed value).

   Disadvantage: Obviously, the amount of padding is easily discoverable
   from a single unencrypted message or by observing message patterns.
   When a public DNS server applies this policy, the length of the
   padding hence must be assumed to be public knowledge.  Therefore,
   this policy is (almost) as useless as the No Padding policy described
   above.

   The Fixed-Length Padding policy MUST NOT be used except for test
   applications.

Acknowledgements

   Daniel K. Gillmor performed empirical research out of which the
   "Recommended Strategy" was copied.  Stephane Bortzmeyer and Hugo
   Connery provided text.  Shane Kerr, Sara Dickinson, Paul Hoffman,
   Magnus Westerlund, Charlie Kaufman, Joe Clarke, and Meral Shirazipour
   performed reviews or provided substantial comments.

Author's Address

   Alexander Mayrhofer
   nic.at GmbH
   Karlsplatz 1/2/9
   Vienna  1010
   Austria

   Email: alex.mayrhofer.ietf@gmail.com
   URI:   http://edns0-padding.org/