draft-ietf-dprive-padding-policy-05.txt   draft-ietf-dprive-padding-policy-06.txt 
Network Working Group A. Mayrhofer Network Working Group A. Mayrhofer
Internet-Draft nic.at GmbH Internet-Draft nic.at GmbH
Intended status: Experimental April 12, 2018 Intended status: Experimental July 19, 2018
Expires: October 14, 2018 Expires: January 20, 2019
Padding Policy for EDNS(0) Padding Policy for EDNS(0)
draft-ietf-dprive-padding-policy-05 draft-ietf-dprive-padding-policy-06
Abstract Abstract
RFC 7830 specifies the EDNS(0) 'Padding' option, but does not specify RFC 7830 specifies the EDNS(0) 'Padding' option, but does not specify
the actual padding length for specific applications. This memo lists the actual padding length for specific applications. This memo lists
the possible options ("Padding Policies"), discusses implications of the possible options ("Padding Policies"), discusses implications of
each of these options, and provides a recommended (experimental) each of these options, and provides a recommended (experimental)
option. option.
Status of This Memo Status of This Memo
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 14, 2018. This Internet-Draft will expire on January 20, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. General Guidance . . . . . . . . . . . . . . . . . . . . . . 3 3. General Guidance . . . . . . . . . . . . . . . . . . . . . . 3
4. Padding Strategies . . . . . . . . . . . . . . . . . . . . . 3 4. Padding Strategies . . . . . . . . . . . . . . . . . . . . . 3
4.1. Block Length Padding - Recommended Strategy . . . . . . . 3 4.1. Block Length Padding - Recommended Strategy . . . . . . . 3
4.2. Other Sensible Strategies . . . . . . . . . . . . . . . . 5 4.2. Other Strategies . . . . . . . . . . . . . . . . . . . . 5
4.2.1. Maximal Length Padding . . . . . . . . . . . . . . . 5 4.2.1. Maximal Length Padding . . . . . . . . . . . . . . . 5
4.2.2. Random Length Padding . . . . . . . . . . . . . . . . 5 4.2.2. Random Length Padding . . . . . . . . . . . . . . . . 5
4.2.3. Random Block Length Padding . . . . . . . . . . . . . 6 4.2.3. Random Block Length Padding . . . . . . . . . . . . . 6
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 8. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. draft-ietf-dprive-padding-policy-05 . . . . . . . . . . . 7 8.1. draft-ietf-dprive-padding-policy-06 . . . . . . . . . . . 7
8.2. draft-ietf-dprive-padding-policy-04 . . . . . . . . . . . 7 8.2. draft-ietf-dprive-padding-policy-05 . . . . . . . . . . . 7
8.3. draft-ietf-dprive-padding-policy-03 . . . . . . . . . . . 8 8.3. draft-ietf-dprive-padding-policy-04 . . . . . . . . . . . 8
8.4. draft-ietf-dprive-padding-policy-02 . . . . . . . . . . . 8 8.4. draft-ietf-dprive-padding-policy-03 . . . . . . . . . . . 8
8.5. draft-ietf-dprive-padding-policy-01 . . . . . . . . . . . 8 8.5. draft-ietf-dprive-padding-policy-02 . . . . . . . . . . . 8
8.6. draft-ietf-dprive-padding-policy-00 . . . . . . . . . . . 8 8.6. draft-ietf-dprive-padding-policy-01 . . . . . . . . . . . 8
8.7. draft-mayrhofer-dprive-padding-profiles-00 . . . . . . . 8 8.7. draft-ietf-dprive-padding-policy-00 . . . . . . . . . . . 8
8.8. draft-mayrhofer-dprive-padding-profiles-00 . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8 9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 9 9.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Non-sensible Padding Policies . . . . . . . . . . . 9 Appendix A. Non-sensible Padding Policies . . . . . . . . . . . 9
A.1. No Padding . . . . . . . . . . . . . . . . . . . . . . . 9 A.1. No Padding . . . . . . . . . . . . . . . . . . . . . . . 9
A.2. Fixed Length Padding . . . . . . . . . . . . . . . . . . 9 A.2. Fixed Length Padding . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction 1. Introduction
[RFC7830] specifies the Extensions Mechanisms for DNS (EDNS(0)) [RFC7830] specifies the Extensions Mechanisms for DNS (EDNS(0))
"Padding" option, which allows DNS clients and servers to "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
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Padding DNS messages is useful only when transport is encrypted, Padding DNS messages is useful only when transport is encrypted,
using protocols such as DNS over Transport Layer Security [RFC7858], using protocols such as DNS over Transport Layer Security [RFC7858],
DNS over Datagram Transport Layer Security [RFC8094] or other DNS over Datagram Transport Layer Security [RFC8094] or other
encrypted DNS transports specified in the future. encrypted DNS transports specified in the future.
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 BCP
[RFC2119]. 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. General Guidance 3. General Guidance
EDNS(0) options space: The maximum message length as dictated by EDNS(0) options space: The maximum message length as dictated by the
protocol limitation limits the space for EDNS(0) options. Since protocol limits the space for EDNS(0) options. Since padding will
padding will reduce the message space available to other EDNS(0) reduce the message space available to other EDNS(0) options,
options, "Padding" MUST be the last EDNS(0) option applied before a "Padding" MUST be the last EDNS(0) option applied before a DNS
DNS message is sent. message is sent.
Resource Conservation: Especially in situations where networking and Resource Conservation: Especially in situations where networking and
processing resources are scarce (e.g. battery powered long-life processing resources are scarce (e.g. battery powered long-life
devices, low bandwidth or high cost links), the tradeoff between devices, low bandwidth or high cost links), the tradeoff between
increased size of padded DNS messages and the corresponding gain in increased size of padded DNS messages and the corresponding gain in
confidentiality must be carefully considered. confidentiality must be carefully considered.
Transport Protocol Independence: The message size used as input to Transport Protocol Independence: The message size used as input to
the various padding strategies MUST be calculated excluding the the various padding strategies MUST be calculated excluding the
potential extra 2-octet length field used in TCP transport. potential extra 2-octet length field used in TCP transport.
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"Block Length Padding" strategy as follows: "Block Length Padding" strategy 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 receives a query that includes the EDNS(0) Padding (2) If a Server receives a query that includes the EDNS(0) Padding
Option, it MUST pad the corresponding response (See Section 4 of Option, it MUST pad the corresponding response (See Section 4 of
RFC7830) and SHOULD pad the corresponding response to a multiple RFC7830) and SHOULD pad the corresponding response to a multiple
of 468 octets (see below). of 468 octets (see below).
Note that the recommendation above applies only if DNS transport is Note that the recommendation above applies only if the DNS transport
encrypted (See Section 6 of RFC 7830). is encrypted (See Section 6 of RFC 7830).
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 EDNS(0) Padding Option increases consider that even the zero-length EDNS(0) Padding Option increases
the length of the packet by 4 octets. the length of the packet by 4 octets.
Options: Block Length - values between 16 and 128 octets for the Options: Block Length - for queries, values between 16 and 128 octets
queries seem reasonable, responses will require larger block sizes were discussed before empiric research was performed. Responses will
(see [dkg-padding-ndss] and above for a discussion). require larger block sizes (see [dkg-padding-ndss] and above for a
discussion).
Very large block lengths will have confidentiality properties similar Very large block lengths will have confidentiality properties similar
to the "Maximal Length Padding" strategy (Section 4.2.1), since to the "Maximal Length Padding" strategy (Section 4.2.1), since
almost all messages will fit into a single block. In that case, almost all messages will fit into a single block. Such "very large
reasonable values may be 288 bytes for the query (the maximum size of 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 a one-question query over TCP, without any EDNS(0) options), and the
EDNS(0) buffer size of the server for the responses. EDNS(0) 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 padding length require a source of (pseudo) random numbers, since the padding length
required 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 padded message can be predicted. reachable), the size range of a padded message can be predicted.
Therefore, minimum and maximum length of the unpadded message are Therefore, the minimum length of the unpadded message can be infered.
known.
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
Length Padding may cause unneccessary fragmentation for UDP based
delivery. Also, chosing a block length larger than the MTU of course
forces to always fragment.
The empirical research cited above performed a simulation of padding, The empirical research cited above performed a simulation of padding,
based on real-world DNS traffic captured on busy recursive resolvers based on real-world DNS traffic captured on busy recursive resolvers
of a research network. The evaluation of the performance of of a research network. The evaluation of the performance of
individual padding policies was based on a "cost to attacker" and individual padding policies was based on a "cost to attacker" and
"cost to defender" function, where the "cost to attacker" was defined "cost to defender" function, where the "cost to attacker" was defined
as the percentage of query/response pairs falling into the same size as the percentage of query/response pairs falling into the same size
bucket, and "cost to defender" as the size factor between padded and bucket, and "cost to defender" as the size factor between padded and
unpadded messages. Padding with a block size of 128 bytes on the unpadded messages. Padding with a block size of 128 bytes on the
query side, and 468 bytes on the response side was considered the query side, and 468 bytes on the response side was considered the
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length values that are a large fraction of the MTU, unless the block 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 length is chosen so that a multiple just fits into the MTU, Block
Padding may cause unneccessary fragmentation for UDP based delivery. Padding may cause unneccessary fragmentation for UDP based delivery.
Also, chosing a block length larger than the MTU of course always Also, chosing a block length larger than the MTU of course always
forces to always fragment. forces to always fragment.
Note: Once DNSSEC validating clients become more prevalent, observed Note: Once DNSSEC validating clients become more prevalent, observed
size patterns are expected to change significantly. In such case, size patterns are expected to change significantly. In such case,
the recommended strategy might need to be revisited. the recommended strategy might need to be revisited.
4.2. Other Sensible Strategies 4.2. Other Strategies
4.2.1. Maximal Length Padding 4.2.1. Maximal Length Padding
In Maximal Length Padding the sender pads every message to the In Maximal Length Padding the sender pads every message to the
maximum size as allowed by protocol negotiations. maximum size as allowed by protocol negotiations.
Advantages: Maximal Length Padding, when combined with encrypted Advantages: Maximal Length Padding, when combined with encrypted
transport, provides the highest possible level of message size transport, provides the highest possible level of message size
confidentiality. confidentiality.
Disadvantages: Maximal Length Padding is wasteful, and requires Disadvantages: Maximal Length Padding is wasteful, and requires
resources on the client, all intervening network and equipment, and resources on the client, all intervening network and equipment, and
the server. Depending on the negotiated size, this strategy will the server. Depending on the negotiated size, this strategy will
commonly exceed the MTU, and then result in a consistent number of commonly exceed the MTU, and then result in a consistent number of
fragments reducing delivery probability when datagram based transport fragments reducing delivery probability when datagram based transport
(such as UDP) is used. (such as UDP) is used.
Maximal Length Padding is NOT RECOMMENDED. Due to resource consumption, Maximal Length Padding is NOT
RECOMMENDED.
4.2.2. Random Length Padding 4.2.2. 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 EDNS(0) Padding 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 Option itself, each message size is hence increased by at least 4
octets. The upper limit for padding is the maximum message size. octets. The upper limit for padding 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.
Options: Maximum and minimum padding length. Options: Maximum and minimum padding length.
Advantages: Theoretically, this policy should create a natural Advantages: Theoretically, this policy should create a natural
"distribution" of message sizes. "distribution" of message sizes.
Disadvantage: This policy requires a good source of (pseudo) random Disadvantage: Random Length padding allows an attacker who can
numbers which can keep up with the required message rates. observe a large number of requests to infer the length of the
Especially on busy servers, this may be a hindrance. original value by observing the distribution of total lengths.
According to the limited empirical data available, Random Length According to the limited empirical data available, Random Length
Padding performs slightly worse than Block Length Padding. 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 4.2.3. 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 length Specifically, a sender randomly chooses between a few block length
values and then applies Block Length Padding based on the chosen values and then applies Block Length Padding based on the chosen
block length. The random selection of block length might even be block length. The random selection of block length 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
transaction ID of the message. transaction ID of the message.
Options: Number of and the values for the set of Block Lengths, Options: Number of and the values for the set of Block Lengths,
source of "randomness" source of "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.
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
Random Block Length Padding (as other combinations of padding Random Block Length Padding (as other combinations of padding
strategies) requires further empirical study. strategies) requires further empirical study.
5. Acknowledgements 5. Acknowledgements
Daniel K. Gillmor performed empirical research out of which the Daniel K. Gillmor performed empirical research out of which the
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responses by a certain amount of jitter. Such strategies are out of responses by a certain amount of jitter. Such strategies are out of
scope of this document. Additionally, there is neither enough scope of this document. Additionally, there is neither enough
theoretic analysis nor experimental data available to recommend any theoretic analysis nor experimental data available to recommend any
such countermeasures. such countermeasures.
8. Changes 8. Changes
[Note to RFC Editors: This whole section is to be removed before [Note to RFC Editors: This whole section is to be removed before
publication] publication]
8.1. draft-ietf-dprive-padding-policy-05 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 Changes based on outcomes of IETF-wide LC + various reviews: Meral
Shirazipour (Gen-ART), Charlie Kaufmann (SECDIR), Joe Clarke (OPSDIR Shirazipour (Gen-ART), Charlie Kaufmann (SECDIR), Joe Clarke (OPSDIR
- changed document flow based on comments), - changed document flow based on comments),
8.2. draft-ietf-dprive-padding-policy-04 8.3. draft-ietf-dprive-padding-policy-04
Changes based on WGLC: Changed implementor consideration text in Changes based on WGLC: Changed implementor consideration text in
Security Con section (Sara), moved "No Padding" and "Fixed Length Security Con section (Sara), moved "No Padding" and "Fixed Length
Padding" to appendix (Stephane, Paul), Changed TODO in Random Padding Padding" to appendix (Stephane, Paul), Changed TODO in Random Padding
to info from empirical study (Stephen), Added note to pad only if to info from empirical study (Stephen), Added note to pad only if
transport encrypted (Stephen), added intro text referencing to transport encrypted (Stephen), added intro text referencing to
DNSoTLS and DNSoDTLS (Stephane), added text about timing/jitter to DNSoTLS and DNSoDTLS (Stephane), added text about timing/jitter to
security considerations. security considerations.
8.3. draft-ietf-dprive-padding-policy-03 8.4. draft-ietf-dprive-padding-policy-03
Editorial changes in various spots. Added text about excluding TCP Editorial changes in various spots. Added text about excluding TCP
length field, more security considerations, addressing Sara's other length field, more security considerations, addressing Sara's other
feedback to -02. feedback to -02.
8.4. draft-ietf-dprive-padding-policy-02 8.5. draft-ietf-dprive-padding-policy-02
Changed Document Status to Experimental, added "maximum length" Changed Document Status to Experimental, added "maximum length"
padding policy, reworded "block length" policy, some editorial padding policy, reworded "block length" policy, some editorial
changes. changes.
8.5. draft-ietf-dprive-padding-policy-01 8.6. 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.
8.6. draft-ietf-dprive-padding-policy-00 8.7. 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.
8.7. draft-mayrhofer-dprive-padding-profiles-00 8.8. draft-mayrhofer-dprive-padding-profiles-00
Initial version Initial version
9. References 9. References
9.1. Normative References 9.1. Normative References
[dkg-padding-ndss] [dkg-padding-ndss]
Gillmor, D., "Empirical DNS Padding Policy", March 2017, Gillmor, D., "Empirical DNS Padding Policy", March 2017,
<https://dns.cmrg.net/ <https://dns.cmrg.net/
skipping to change at page 9, line 5 skipping to change at page 9, line 14
[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,
<https://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,
<https://www.rfc-editor.org/info/rfc7830>. <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 9.2. Informative References
[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>. 2016, <https://www.rfc-editor.org/info/rfc7858>.
[RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram [RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
Transport Layer Security (DTLS)", RFC 8094, Transport Layer Security (DTLS)", RFC 8094,
DOI 10.17487/RFC8094, February 2017, DOI 10.17487/RFC8094, February 2017,
skipping to change at page 10, line 4 skipping to change at page 10, line 16
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 (almost) as useless as the "No Padding" option this policy is (almost) as useless as the "No Padding" option
described above. described above.
"Fixed Length Padding" MUST NOT be used except for experimental "Fixed Length Padding" MUST NOT be used except for test applications.
applications.
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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