Internet DRAFT - draft-ietf-dprive-padding-policy
draft-ietf-dprive-padding-policy
Network Working Group A. Mayrhofer
Internet-Draft nic.at GmbH
Intended status: Experimental July 19, 2018
Expires: January 20, 2019
Padding Policy for EDNS(0)
draft-ietf-dprive-padding-policy-06
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
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This Internet-Draft will expire on January 20, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. General Guidance . . . . . . . . . . . . . . . . . . . . . . 3
4. Padding Strategies . . . . . . . . . . . . . . . . . . . . . 3
4.1. Block Length Padding - Recommended Strategy . . . . . . . 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. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. draft-ietf-dprive-padding-policy-06 . . . . . . . . . . . 7
8.2. draft-ietf-dprive-padding-policy-05 . . . . . . . . . . . 7
8.3. draft-ietf-dprive-padding-policy-04 . . . . . . . . . . . 8
8.4. draft-ietf-dprive-padding-policy-03 . . . . . . . . . . . 8
8.5. draft-ietf-dprive-padding-policy-02 . . . . . . . . . . . 8
8.6. draft-ietf-dprive-padding-policy-01 . . . . . . . . . . . 8
8.7. draft-ietf-dprive-padding-policy-00 . . . . . . . . . . . 8
8.8. draft-mayrhofer-dprive-padding-profiles-00 . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Non-sensible Padding Policies . . . . . . . . . . . 9
A.1. No Padding . . . . . . . . . . . . . . . . . . . . . . . 9
A.2. Fixed Length Padding . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
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.
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.
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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,
"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 (e.g. 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
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 was transported over TCP, and the padding strategy would
consider 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 in choosing padding
length. Note that, for completeness, Appendix A contains two more
(non-sensible) strategies.
4.1. Block Length Padding - Recommended Strategy
Based on empirical research performed by Daniel K. Gillmor
[dkg-padding-ndss], EDNS Padding SHOULD be performed following the
"Block Length Padding" strategy as follows:
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(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 corresponding response to a multiple
of 468 octets (see below).
Note that the recommendation above applies only 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 EDNS(0) 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 [dkg-padding-ndss] 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 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.
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 infered.
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 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
Padding may cause unneccessary fragmentation for UDP based delivery.
Also, chosing a block length larger than the MTU of course always
forces to always fragment.
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.
4.2. Other Strategies
4.2.1. Maximal Length Padding
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. Depending on the negotiated size, this strategy will
commonly exceed the MTU, and then 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 EDNS(0) Padding
Option itself, each message size is hence 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.
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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. Due to 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 (as other combinations of padding
strategies) requires further empirical study.
5. 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.
6. IANA Considerations
This document has no considerations for IANA.
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7. 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 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 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 and number of query/response pairs). Counter-measures
against such other 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
scope of this document. Additionally, there is neither enough
theoretic analysis nor experimental data available to recommend any
such countermeasures.
8. Changes
[Note to RFC Editors: This whole section is to be removed before
publication]
8.1. draft-ietf-dprive-padding-policy-06
Changes based on IESG evaluation: Removed duplicate paragraph about
MTU impact, switched Terminology boilerplate to RFC8174, changed text
regarding Random Padding, changed text regarding very large block
paddings, some minor edits.
8.2. draft-ietf-dprive-padding-policy-05
Changes based on outcomes of IETF-wide LC + various reviews: Meral
Shirazipour (Gen-ART), Charlie Kaufmann (SECDIR), Joe Clarke (OPSDIR
- changed document flow based on comments),
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8.3. draft-ietf-dprive-padding-policy-04
Changes based on WGLC: Changed implementor consideration text in
Security Con section (Sara), moved "No Padding" and "Fixed Length
Padding" to appendix (Stephane, Paul), Changed TODO in Random Padding
to info from empirical study (Stephen), Added note to pad only if
transport encrypted (Stephen), added intro text referencing to
DNSoTLS and DNSoDTLS (Stephane), added text about timing/jitter to
security considerations.
8.4. 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.
8.5. draft-ietf-dprive-padding-policy-02
Changed Document Status to Experimental, added "maximum length"
padding policy, reworded "block length" policy, some editorial
changes.
8.6. draft-ietf-dprive-padding-policy-01
Some (mostly editorial) changes to text. Added "Recommendation"
section based on dkg's research.
8.7. draft-ietf-dprive-padding-policy-00
Initial (mostly unmodified) WG version. Changed "Profile" to
"Policy" to avoid confusion with the (D)TLS profiles document.
8.8. draft-mayrhofer-dprive-padding-profiles-00
Initial version
9. References
9.1. Normative References
[dkg-padding-ndss]
Gillmor, D., "Empirical DNS Padding Policy", March 2017,
<https://dns.cmrg.net/
ndss2017-dprive-empirical-DNS-traffic-size.pdf>.
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[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>.
9.2. 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. Non-sensible Padding Policies
A.1. No Padding
In the "No Padding" policy, the EDNS0 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.
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.
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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 only by a fixed
value)
Disadvantage: Obviously, the amount of padding 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" option
described above.
"Fixed Length Padding" MUST NOT be used except for test applications.
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/
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