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Versions: (draft-mayrhofer-dprive-padding-profile) 00 01 02 03 04 05 06 RFC 8467

Network Working Group                                       A. Mayrhofer
Internet-Draft                                               nic.at GmbH
Intended status: Experimental                           January 17, 2018
Expires: July 21, 2018


                       Padding Policy for EDNS(0)
                  draft-ietf-dprive-padding-policy-03

Abstract

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

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
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   Drafts is at https://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 July 21, 2018.

Copyright Notice

   Copyright (c) 2018 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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  General Guidance  . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Padding Strategies  . . . . . . . . . . . . . . . . . . . . .   3
     4.1.  No Padding  . . . . . . . . . . . . . . . . . . . . . . .   3
     4.2.  Fixed Length Padding  . . . . . . . . . . . . . . . . . .   4
     4.3.  Block Length Padding  . . . . . . . . . . . . . . . . . .   4
     4.4.  Maximal Length Padding ('The Full Monty') . . . . . . . .   5
     4.5.  Random Length Padding . . . . . . . . . . . . . . . . . .   5
     4.6.  Random Block Length Padding . . . . . . . . . . . . . . .   6
   5.  Recommended Strategy  . . . . . . . . . . . . . . . . . . . .   6
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  Changes . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  draft-ietf-dprive-padding-policy-03 . . . . . . . . . . .   8
     9.2.  draft-ietf-dprive-padding-policy-02 . . . . . . . . . . .   8
     9.3.  draft-ietf-dprive-padding-policy-01 . . . . . . . . . . .   8
     9.4.  draft-ietf-dprive-padding-policy-00 . . . . . . . . . . .   8
     9.5.  draft-mayrhofer-dprive-padding-profiles-00  . . . . . . .   8
   10. Normative References  . . . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   [RFC7830] specifies the Extensions 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.

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
   [RFC2119].







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3.  General Guidance

   EDNS(0) options space: The maximum message length as dictated by
   protocol limitation limits the space for EDNS(0) options.  Since
   padding will reduce the message space available to other EDNS(0)
   options, "Padding" 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 (eg. battery powered long-life
   devices, low bandwidth or high cost links), the tradeoff 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
   signifcantly 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
   was transported over TCP, and the padding strategy would consider the
   extra 2 octects 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 is a non-exhaustive list of possible strategies in
   choosing padding length.

4.1.  No Padding

   In the "No Padding" policy, the EDNS(0) 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 where 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 (less than 4 octets left).

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



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

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

4.2.  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 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 must be assumed to be public knowledge.  Therefore, this
   policy is (almost) as useless as the "No Padding" option described
   above.

   "Fixed Length Padding" MUST NOT be used except for experimental
   applications.

4.3.  Block Length Padding

   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 EDNS(0) Padding Option increases
   the length of the packet by 4 octets.

   Options: Block Length - values between 16 and 128 octets for the
   queries seem reasonable, responses will require larger block sizes
   (see [dkg-padding-ndss] and Section 5 for a discussion).

   Very large block lengths will have confidentiality properties similar
   to the "Maximum Length Padding" strategy (Section 4.4), since almost
   all messages will fit into a single block.  In that case, reasonable
   values may be 288 bytes for the query (the maximum size of a one-
   question query over TCP, without any EDNS(0) options), and the
   EDNS(0) buffer size of the server for the responses.





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   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 of a padded message can be predicted.
   Therefore, minimum and maximum length of the unpadded message are
   known.

   Block Length Padding is the currently RECOMMENDED strategy (see
   Section 5).

4.4.  Maximal Length Padding ('The Full Monty')

   In Maximal Length Padding the sender pads every message to the
   maximum size as 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 network and equipment, and
   the server.

   Maximal Length Padding is NOT RECOMMENDED.

4.5.  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 EDNS(0) Padding
   Option itself, each message size is hence increased by at least 4
   octets.  The upper limit for pading 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: This policy requires a good source of (pseudo) which
   can keep up with the required message rates.  Especially on busy
   servers, this may be a significant hindrance.





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   TODO: Recommendation - this is (at first glance) the best policy, but
   requires significant effort

4.6.  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
   transction 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.  Also, compared to "Random Length Padding",
   it might not require a "full blown" random number source.

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

   Random Block Length Padding (as other combinations of padding
   strategies) requires further empirical study.

5.  Recommended Strategy

   Based on empirical research performed by Daniel K.  Gillmor
   [dkg-padding-ndss], EDNS(0) Padding SHOULD be performed 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
        [RFC7830]) and SHOULD pad the response to a multiple of 468
        octects.

   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" 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



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   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 MTU values.

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

6.  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
   performed reviews and provided substantial comments.

7.  IANA Considerations

   This document has no considerations for IANA.

8.  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 EDNS(0) Padding must carefully consider the chosen
   policy and its parameters.

   No matter how carefully a client selects their Padding policy, this
   effort can be jeopardized if the server chooses to apply an
   inffective Padding policy to the corresponding response packets.
   Therefore, a client applying Padding may want to choose a DNS server
   which does apply at least an equally effective Padding policy on
   responses.

   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 leak via other side channels (particularly timing
   information).

9.  Changes

   [Note to RFC Editors: This whole section is to be removed before
   publication]








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9.1.  draft-ietf-dprive-padding-policy-03

   Editorial changes in various spots.  Added text about excluding TCP
   length field, more security considerations, addressing Sara's other
   feedback to -02.

9.2.  draft-ietf-dprive-padding-policy-02

   Changed Document Status to Experimental, added "maximum length"
   padding policy, reworded "block length" policy, some editorial
   changes.

9.3.  draft-ietf-dprive-padding-policy-01

   Some (mostly editorial) changes to text.  Added "Recommendation"
   section based on dkg's research.

9.4.  draft-ietf-dprive-padding-policy-00

   Initial (mostly unmodified) WG version.  Changed "Profile" to
   "Policy" to avoid confusion with the (D)TLS profiles document.

9.5.  draft-mayrhofer-dprive-padding-profiles-00

   Initial version

10.  Normative References

   [dkg-padding-ndss]
              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>.

Author's Address








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

   Email: alex.mayrhofer.ietf@gmail.com












































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