draft-ietf-dprive-padding-policy-01.txt   draft-ietf-dprive-padding-policy-02.txt 
Network Working Group A. Mayrhofer Network Working Group A. Mayrhofer
Internet-Draft nic.at GmbH Internet-Draft nic.at GmbH
Intended status: Standards Track July 3, 2017 Intended status: Experimental September 27, 2017
Expires: January 4, 2018 Expires: March 31, 2018
Padding Policy for EDNS(0) Padding Policy for EDNS(0)
draft-ietf-dprive-padding-policy-01 draft-ietf-dprive-padding-policy-02
Abstract Abstract
RFC 7830 specifies the EDNS0 'Padding' option, but does not specify RFC 7830 specifies the EDNS0 'Padding' option, but does not specify
the length of padding to be used in specific applications. This memo the actual padding length for specific applications. This memo lists
lists the possible options ("Padding Policies"), discusses the the possible options ("Padding Policies"), discusses the implications
implications of each of these options, and provides a recommended of each of these options, and provides a recommended (experimental)
option. option.
Status of This Memo Status of This Memo
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provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 4, 2018. This Internet-Draft will expire on March 31, 2018.
Copyright Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. General Guidance . . . . . . . . . . . . . . . . . . . . . . 3 3. General Guidance . . . . . . . . . . . . . . . . . . . . . . 3
4. Padding Strategies . . . . . . . . . . . . . . . . . . . . . 3 4. Padding Strategies . . . . . . . . . . . . . . . . . . . . . 3
4.1. No Padding . . . . . . . . . . . . . . . . . . . . . . . 3 4.1. No Padding . . . . . . . . . . . . . . . . . . . . . . . 3
4.2. Fixed Length Padding . . . . . . . . . . . . . . . . . . 3 4.2. Fixed Length Padding . . . . . . . . . . . . . . . . . . 3
4.3. Block Length Padding . . . . . . . . . . . . . . . . . . 4 4.3. Block Length Padding . . . . . . . . . . . . . . . . . . 4
4.4. Random Length Padding . . . . . . . . . . . . . . . . . . 4 4.4. Maximal Lenth Padding ('The Full Monty') . . . . . . . . 5
4.5. Random Block Length Padding . . . . . . . . . . . . . . . 5 4.5. Random Length Padding . . . . . . . . . . . . . . . . . . 5
5. Recommended Strategy . . . . . . . . . . . . . . . . . . . . 5 4.6. Random Block Length Padding . . . . . . . . . . . . . . . 5
5. Recommended Strategy . . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 6 8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 9. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. draft-ietf-dprive-padding-policy-01 . . . . . . . . . . . 6 9.1. draft-ietf-dprive-padding-policy-02 . . . . . . . . . . . 7
9.2. draft-ietf-dprive-padding-policy-00 . . . . . . . . . . . 6 9.2. draft-ietf-dprive-padding-policy-01 . . . . . . . . . . . 7
9.3. draft-mayrhofer-dprive-padding-profiles-00 . . . . . . . 6 9.3. draft-ietf-dprive-padding-policy-00 . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 9.4. draft-mayrhofer-dprive-padding-profiles-00 . . . . . . . 7
10.1. Normative References . . . . . . . . . . . . . . . . . . 6 10. Normative References . . . . . . . . . . . . . . . . . . . . 7
10.2. Informative References . . . . . . . . . . . . . . . . . 7 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction 1. Introduction
RFC 7830 [RFC7830] specifies the Extensions Mechanisms for DNS RFC 7830 [RFC7830] specifies the Extensions Mechanisms for DNS
(EDNS(0)) "Padding" option, which allows DNS clients and servers to (EDNS(0)) "Padding" option, which allows DNS clients and servers to
artificially increase the size of a DNS message by a variable number artificially increase the size of a DNS message by a variable number
of bytes, hampering size-based correlation of encrypted DNS messages. of bytes, hampering size-based correlation of encrypted DNS messages.
However, RFC 7830 deliberately does not specify the actual length of However, RFC 7830 deliberately does not specify the actual length of
padding to be used. This memo discusses options regarding the actual padding to be used. This memo discusses options regarding the actual
size of padding, lists advantages and disadvantages of each of these size of padding, lists advantages and disadvantages of each of these
"Padding Strategies", and provides a recommended strategy (TODO "Padding Strategies", and provides a recommended (experimental)
pending concensus of the working group!) strategy.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in "OPTIONAL" in this document are to be interpreted as described in
[RFC2119]. [RFC2119].
3. General Guidance 3. General Guidance
Padding DNS messages does not have any semantic impact on the DNS Padding DNS messages does not have any semantic impact on the DNS
protocol. However, the length of (possible) padding does depend on protocol. However, the length of (possible) padding does depend on
the circumstances under which a DNS message is created, specifically the circumstances under which a DNS message is created, specifically
the maximum message length as dictated by protocol negotiations. the maximum message length as dictated by protocol negotiations.
Therefore, in order to not impact the possibility to add other EDNS Since padding may frustrate the message space available to other EDNS
options, "Padding" MUST be the last ENDS option applied before a DNS options, "Padding" MUST be the last EDNS option applied before a DNS
message is sent. message is sent.
Especially in situations with scarce computing and networking Especially in situations with scarce computing and networking
resources such as long-life battery powered devices, the tradeoff resources such as long-life battery powered devices, the tradeoff
between significantly increasing the size of DNS messages by generous between significantly increasing the size of DNS messages by generous
padding and the corresponding gain in confidentiality must be padding and the corresponding gain in confidentiality must be
carefully considered. carefully considered.
4. Padding Strategies 4. Padding Strategies
This section is a non-exhaustive list of possible strategies to This section is a non-exhaustive list of possible strategies in
choosing padding length choosing padding length
4.1. No Padding 4.1. No Padding
In the "No Padding" policy, the EDNS0 Padding option is not used, and In the "No Padding" policy, the EDNS0 Padding option is not used, and
the size of the final (actually, "non-padded") message obviously the size of the final (actually, "non-padded") message obviously
matches exactly the size of the unpadded messages. Even though this exactly matches the size of the unpadded message. Even though this
"non-policy" seems redundant in this list, its properties must be "non-policy" seems redundant in this list, its properties must be
considered for cases where only of the parties (client or server) considered for cases where just one of the parties (client or server)
applies padding. applies padding.
Note that employing this "policy" is required also when the message Also, this "policy" is required when the remaining message size of
size of the unpadded message does not allow for the Padding option to the unpadded message does not allow for the Padding option to be
be included (less than 4 octets message space left). included (less than 4 octets left).
Advantages: The only advantage of this approach is that this "policy" Advantages: This "policy" requires no additional resources on client,
requires no additional resources on client, server and network side. server and network side.
Disadvantages: The original size of the message remains unchanged, Disadvantages: The original size of the message remains unchanged,
hence this approach provides no additional confidentiality. hence this approach provides no additional confidentiality.
TODO: Recommend that this policy MUST NOT be used unless message size "No Padding" MUST NOT be used unless message size disallows the use
disallows the use of Padding. of Padding.
4.2. Fixed Length Padding 4.2. Fixed Length Padding
In fixed length padding, a sender chooses to pad each message with a In fixed length padding, a sender chooses to pad each message with a
padding of constant length. padding of constant length.
Options: Actual length of padding Options: Actual length of padding
Advantages: Since the padding is constant in length, this policy is Advantages: Since the padding is constant in length, this policy is
very easy to implement, and at least ensures that the message length 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 diverges from the length of the original packet (even only by a fixed
value) value)
Disadvantage: Obviously, the amount of padding easily discoverable Disadvantage: Obviously, the amount of padding easily discoverable
from a single unencrypted message, or by observing message patterns. from a single unencrypted message, or by observing message patterns.
When a public DNS server applies this policy, the length of the When a public DNS server applies this policy, the length of the
padding hence must be assumed to be public knowledge. Therefore, padding hence must be assumed to be public knowledge. Therefore,
this policy is equally useless "No Padding" option described above. 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 4.3. Block Length Padding
In Block Length Padding, a sender pads each message so that its 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 padded length is a multiple of a chosen block length. This creates a
greatly reduced variety of message lengths. An implementor needs to greatly reduced variety of message lengths. An implementor needs to
consider that even the zero-length EDNS0 Padding Option increases the consider that even the zero-length EDNS0 Padding Option increases the
length of the packet by 4 octets. length of the packet by 4 octets.
Options: Block Length - values between 16 and 128 (TODO Discuss!) Options: Block Length - values between 16 and 128 octets for the
octets seem reasonable 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 Advantages: This policy is reasonably easy to implement, reduces the
variety of message ("fingerprint") sizes significantly, and does not variety of message ("fingerprint") sizes significantly, and does not
require a source of (pseudo) random numbers, since the amount of require a source of (pseudo) random numbers, since the padding length
padding can be derived from the actual (unpadded) message. required can be derived from the actual (unpadded) message.
Disadvantage: Given an unpadded message and the block size of the Disadvantage: Given an unpadded message and the block size of the
padding (which is assumed to be public knowledge once a server is padding (which is assumed to be public knowledge once a server is
reachable), the size of a message can be predicted. Therefore, the reachable), the size of a padded message can be predicted.
minimum and maximum length of the unpadded message is known. Therefore, minimum and maximum length of the unpadded message are
known.
TODO: Recommended policy? Block Length Padding is the currently RECOMMENDED strategy (see
Section 5).
4.4. Random Length Padding 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 When using Random Length Padding, a sender pads each message with a
random amount of padding. Due to the size of the EDNS0 Padding random amount of padding. Due to the size of the EDNS0 Padding
Option itself, each message size is hence increased by at least 4 Option itself, each message size is hence increased by at least 4
octets. The upper limit for pading is the maximum message size. octets. The upper limit for pading is the maximum message size.
However, a client or server may choose to impose a lower maximum However, a client or server may choose to impose a lower maximum
padding length. padding length.
Alternatively, pad a certain percentage of "remaining space"?
Options: Maximum (and eventually minimum) padding length. Options: Maximum (and eventually minimum) padding length.
Advantages: This policy should create the best "distribution" of Advantages: Theoretically, this policy should create a natural
message sizes "distribution" of message sizes
Disadvantage: This policy requires a good source of (pseudo) random Disadvantage: This policy requires a good source of (pseudo) keeping
numbers which keeps up with the required message rates. Especially up with the required message rates. Especially on busy servers, this
on busy servers, this could be a significant hindrance. may be a significant hindrance.
TODO: Recommendation - this is (at first glance) the best policy, but TODO: Recommendation - this is (at first glance) the best policy, but
requires significant effort requires significant effort
4.5. Random Block Length Padding 4.6. Random Block Length Padding
This policy combines Block Length Padding with a random component. This policy combines Block Length Padding with a random component.
Specifically, a sender randomly chooses between a few block lenght'es Specifically, a sender randomly chooses between a few block lenght'es
and then applies Block Length Padding based on the chosen block and then applies Block Length Padding based on the chosen block
length. The random selection of block lenght might even be length. The random selection of block lenght might even be
reasonably based on a "weak" source of randomness, such as the reasonably based on a "weak" source of randomness, such as the
transction ID of the message. transction ID of the message.
Options: Number of size of the set of Block Lengths, source of Options: Number of size of the set of Block Lengths, source of
"randomness" "randomness"
Advantages: Compared to Block Length Padding, this creates more Advantages: Compared to Block Length Padding, this creates more
variety in the resulting message sizes for a certain individual variety in the resulting message sizes for a certain individual
original message length. Also, compared to "Random Length Padding", original message length. Also, compared to "Random Length Padding",
it might not require a "full blown" random number source. it might not require a "full blown" random number source.
Disadvantage: Requires more implementation effort compared to simple Disadvantage: Requires more implementation effort compared to simple
Block Length Padding Block Length Padding
TODO: Recommend over simple Block Length Padding? Random Block Length Padding (as other combinations of padding
strategies) require further empirical study.
5. Recommended Strategy 5. Recommended Strategy
Based on empirical research performed by Daniel K. Gillmor Based on empirical research performed by Daniel K. Gillmor
[dkg-padding-ndss], EDNS Padding SHOULD be performed as follows: [dkg-padding-ndss], EDNS Padding SHOULD be performed as follows:
(1) Clients should pad queries to the closest multiple of 128 (1) Clients SHOULD pad queries to the closest multiple of 128
octets. octets.
(2) If a Server sees padding in a query, it should pad its response (2) If a Server sees padding in a query, it SHOULD pad the
to a multiple of 468 octects. corresponding response to a multiple of 468 octects.
(3) TODO: recommend to not pad when query was unpadded? 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 6. Acknowledgements
Daniel K. Gillmor performed empirical research out of which the Daniel K. Gillmor performed empirical research out of which the
"Recommended Strategy" was copied. "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 7. IANA Considerations
This document has no considerations for IANA. This document has no considerations for IANA.
8. Security Considerations 8. Security Considerations
The choice of the right padding policy (and the right parameters for The choice of the right padding policy (and the right parameters for
the chose policy) has a significant impact on the resilience of the chose policy) has a significant impact on the resilience of
encrypted DNS against size-based correlation attacks. Therefore, any encrypted DNS against size-based correlation attacks. Therefore, any
skipping to change at page 6, line 30 skipping to change at page 7, line 25
policy and its parameters. policy and its parameters.
A clients carefully chosen Padding policy may be without effect if A clients carefully chosen Padding policy may be without effect if
the corresponding server does apply an inffective (or no) Padding the corresponding server does apply an inffective (or no) Padding
policy on the response packets. Therefore, a client applying 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 may want to chose a DNS server which does apply at least an equally
effective Padding policy on responses. effective Padding policy on responses.
9. Changes 9. Changes
9.1. draft-ietf-dprive-padding-policy-01 [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" Some (mostly editorial) changes to text. Added "Recommendation"
section based on dkg's research. section based on dkg's research.
9.2. draft-ietf-dprive-padding-policy-00 9.3. draft-ietf-dprive-padding-policy-00
Initial (mostly unmodified) WG version. Changed "Profile" to Initial (mostly unmodified) WG version. Changed "Profile" to
"Policy" to avoid confusion with the (D)TLS profiles document. "Policy" to avoid confusion with the (D)TLS profiles document.
9.3. draft-mayrhofer-dprive-padding-profiles-00 9.4. draft-mayrhofer-dprive-padding-profiles-00
Initial version Initial version
10. References 10. Normative References
10.1. Normative References [dkg-padding-ndss]
Gillmor, D., "Empirical DNS Padding Policy", March 2017,
<https://dns.cmrg.net/
ndss2017-dprive-empirical-DNS-traffic-size.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC7830] Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830, [RFC7830] Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830,
DOI 10.17487/RFC7830, May 2016, DOI 10.17487/RFC7830, May 2016,
<http://www.rfc-editor.org/info/rfc7830>. <https://www.rfc-editor.org/info/rfc7830>.
10.2. Informative References
[dkg-padding-ndss]
Gillmor, D., "Empirical DNS Padding Policy", March 2017,
<https://dns.cmrg.net/ndss2017-dprive-empirical-DNS-
traffic-size.pdf>.
Author's Address Author's Address
Alexander Mayrhofer Alexander Mayrhofer
nic.at GmbH nic.at GmbH
Karlsplatz 1/2/9 Karlsplatz 1/2/9
Vienna 1010 Vienna 1010
Austria Austria
Email: alex.mayrhofer.ietf@gmail.com Email: alex.mayrhofer.ietf@gmail.com
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