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
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This Internet-Draft will expire on July 21, 2018.
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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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