Network Working Group A. Mayrhofer
Internet-Draft nic.at GmbH
Intended status: Experimental September 27, 2017
Expires: March 31, 2018
Padding Policy for EDNS(0)
draft-ietf-dprive-padding-policy-02
Abstract
RFC 7830 specifies the EDNS0 'Padding' option, but does not specify
the actual padding length for specific applications. This memo lists
the possible options ("Padding Policies"), discusses the 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
working documents as Internet-Drafts. The list of current Internet-
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 March 31, 2018.
Copyright Notice
Copyright (c) 2017 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
(https://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.
Mayrhofer Expires March 31, 2018 [Page 1]
Internet-Draft draft-ietf-dprive-padding-policy September 2017
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 . . . . . . . . . . . . . . . . . . 3
4.3. Block Length Padding . . . . . . . . . . . . . . . . . . 4
4.4. Maximal Lenth Padding ('The Full Monty') . . . . . . . . 5
4.5. Random Length Padding . . . . . . . . . . . . . . . . . . 5
4.6. Random Block Length Padding . . . . . . . . . . . . . . . 5
5. Recommended Strategy . . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. draft-ietf-dprive-padding-policy-02 . . . . . . . . . . . 7
9.2. draft-ietf-dprive-padding-policy-01 . . . . . . . . . . . 7
9.3. draft-ietf-dprive-padding-policy-00 . . . . . . . . . . . 7
9.4. draft-mayrhofer-dprive-padding-profiles-00 . . . . . . . 7
10. Normative References . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
RFC 7830 [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].
Mayrhofer Expires March 31, 2018 [Page 2]
Internet-Draft draft-ietf-dprive-padding-policy September 2017
3. General Guidance
Padding DNS messages does not have any semantic impact on the DNS
protocol. However, the length of (possible) padding does depend on
the circumstances under which a DNS message is created, specifically
the maximum message length as dictated by protocol negotiations.
Since padding may frustrate the message space available to other EDNS
options, "Padding" MUST be the last EDNS option applied before a DNS
message is sent.
Especially in situations with scarce computing and networking
resources such as long-life battery powered devices, the tradeoff
between significantly increasing the size of DNS messages by generous
padding and the corresponding gain in confidentiality must be
carefully considered.
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 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.
4.2. Fixed Length Padding
In fixed length padding, a sender chooses to pad each message with a
padding of constant length.
Mayrhofer Expires March 31, 2018 [Page 3]
Internet-Draft draft-ietf-dprive-padding-policy September 2017
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 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 EDNS0 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 EDNS0 options), and the EDNS
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 of a padded message can be predicted.
Therefore, minimum and maximum length of the unpadded message are
known.
Mayrhofer Expires March 31, 2018 [Page 4]
Internet-Draft draft-ietf-dprive-padding-policy September 2017
Block Length Padding is the currently RECOMMENDED strategy (see
Section 5).
4.4. Maximal Lenth 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 EDNS0 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 eventually minimum) padding length.
Advantages: Theoretically, this policy should create a natural
"distribution" of message sizes
Disadvantage: This policy requires a good source of (pseudo) keeping
up with the required message rates. Especially on busy servers, this
may be a significant hindrance.
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 lenght'es
and then applies Block Length Padding based on the chosen block
length. The random selection of block lenght might even be
reasonably based on a "weak" source of randomness, such as the
transction ID of the message.
Mayrhofer Expires March 31, 2018 [Page 5]
Internet-Draft draft-ietf-dprive-padding-policy September 2017
Options: Number of size of 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) require further empirical study.
5. Recommended Strategy
Based on empirical research performed by Daniel K. Gillmor
[dkg-padding-ndss], EDNS Padding SHOULD be performed as follows:
(1) Clients SHOULD pad queries to the closest multiple of 128
octets.
(2) If a Server sees padding in a query, it SHOULD pad the
corresponding 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
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.
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.
Mayrhofer Expires March 31, 2018 [Page 6]
Internet-Draft draft-ietf-dprive-padding-policy September 2017
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 chose policy) has a significant impact on the resilience of
encrypted DNS against size-based correlation attacks. Therefore, any
implementor of EDNS0 Padding must carefully consider the chosen
policy and its parameters.
A clients carefully chosen Padding policy may be without effect if
the corresponding server does apply an inffective (or no) Padding
policy on the response packets. Therefore, a client applying Padding
may want to chose a DNS server which does apply at least an equally
effective Padding policy on responses.
9. Changes
[Note to RFC Editors: This whole section is to be removed before
publication]
9.1. draft-ietf-dprive-padding-policy-02
Changed Document Status to Experimental, added "maximum length"
padding policy, reworded "block length" policy, some editorial
changes.
9.2. draft-ietf-dprive-padding-policy-01
Some (mostly editorial) changes to text. Added "Recommendation"
section based on dkg's research.
9.3. draft-ietf-dprive-padding-policy-00
Initial (mostly unmodified) WG version. Changed "Profile" to
"Policy" to avoid confusion with the (D)TLS profiles document.
9.4. draft-mayrhofer-dprive-padding-profiles-00
Initial version
10. Normative References
Mayrhofer Expires March 31, 2018 [Page 7]
Internet-Draft draft-ietf-dprive-padding-policy September 2017
[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
Alexander Mayrhofer
nic.at GmbH
Karlsplatz 1/2/9
Vienna 1010
Austria
Email: alex.mayrhofer.ietf@gmail.com
Mayrhofer Expires March 31, 2018 [Page 8]
