draft-ietf-dprive-padding-policy-03.txt   draft-ietf-dprive-padding-policy-04.txt 
Network Working Group A. Mayrhofer Network Working Group A. Mayrhofer
Internet-Draft nic.at GmbH Internet-Draft nic.at GmbH
Intended status: Experimental January 17, 2018 Intended status: Experimental February 6, 2018
Expires: July 21, 2018 Expires: August 10, 2018
Padding Policy for EDNS(0) Padding Policy for EDNS(0)
draft-ietf-dprive-padding-policy-03 draft-ietf-dprive-padding-policy-04
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
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working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
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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 July 21, 2018. This Internet-Draft will expire on August 10, 2018.
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.
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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 . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. General Guidance . . . . . . . . . . . . . . . . . . . . . . 3 3. General Guidance . . . . . . . . . . . . . . . . . . . . . . 3
4. Padding Strategies . . . . . . . . . . . . . . . . . . . . . 3 4. Padding Strategies . . . . . . . . . . . . . . . . . . . . . 3
4.1. No Padding . . . . . . . . . . . . . . . . . . . . . . . 3 4.1. Block Length Padding . . . . . . . . . . . . . . . . . . 3
4.2. Fixed Length Padding . . . . . . . . . . . . . . . . . . 4 4.2. Maximal Length Padding ('The Full Monty') . . . . . . . . 4
4.3. Block Length Padding . . . . . . . . . . . . . . . . . . 4 4.3. Random Length Padding . . . . . . . . . . . . . . . . . . 4
4.4. Maximal Length Padding ('The Full Monty') . . . . . . . . 5 4.4. Random Block Length Padding . . . . . . . . . . . . . . . 5
4.5. Random Length Padding . . . . . . . . . . . . . . . . . . 5 5. Recommended Strategy . . . . . . . . . . . . . . . . . . . . 5
4.6. Random Block Length Padding . . . . . . . . . . . . . . . 6 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
5. Recommended Strategy . . . . . . . . . . . . . . . . . . . . 6 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7 8. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 9. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. draft-ietf-dprive-padding-policy-03 . . . . . . . . . . . 8 9.1. draft-ietf-dprive-padding-policy-04 . . . . . . . . . . . 7
9.2. draft-ietf-dprive-padding-policy-02 . . . . . . . . . . . 8 9.2. draft-ietf-dprive-padding-policy-03 . . . . . . . . . . . 7
9.3. draft-ietf-dprive-padding-policy-01 . . . . . . . . . . . 8 9.3. draft-ietf-dprive-padding-policy-02 . . . . . . . . . . . 7
9.4. draft-ietf-dprive-padding-policy-00 . . . . . . . . . . . 8 9.4. draft-ietf-dprive-padding-policy-01 . . . . . . . . . . . 7
9.5. draft-mayrhofer-dprive-padding-profiles-00 . . . . . . . 8 9.5. draft-ietf-dprive-padding-policy-00 . . . . . . . . . . . 7
10. Normative References . . . . . . . . . . . . . . . . . . . . 8 9.6. draft-mayrhofer-dprive-padding-profiles-00 . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Non-sensible Padding Policies . . . . . . . . . . . 8
A.1. No Padding . . . . . . . . . . . . . . . . . . . . . . . 8
A.2. Fixed Length Padding . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
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
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 (experimental) "Padding Strategies", and provides a recommended (experimental)
strategy. 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.
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
EDNS(0) options space: The maximum message length as dictated by EDNS(0) options space: The maximum message length as dictated by
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a "Block Length" padding strategy with a block length of 32 octets, 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 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 padded to 64 octets when transported over UDP. If that same message
was transported over TCP, and the padding strategy would consider the was transported over TCP, and the padding strategy would consider the
extra 2 octects of the length field (61 octets in total), the padded 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 message would be 96 octets long (as the minimum length of the Padding
option is 4 octets). option is 4 octets).
4. Padding Strategies 4. Padding Strategies
This section is a non-exhaustive list of possible strategies in This section is a non-exhaustive list of sensible strategies in
choosing padding length. choosing padding length. Note that, for completeness, Appendix A
contains two more (non-sensible) strategies.
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.
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 4.1. 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 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 - values between 16 and 128 octets for the
queries seem reasonable, responses will require larger block sizes queries seem reasonable, responses will require larger block sizes
(see [dkg-padding-ndss] and Section 5 for a discussion). (see [dkg-padding-ndss] and Section 5 for a discussion).
Very large block lengths will have confidentiality properties similar Very large block lengths will have confidentiality properties similar
to the "Maximum Length Padding" strategy (Section 4.4), since almost to the "Maximum Length Padding" strategy (Section 4.2), since almost
all messages will fit into a single block. In that case, reasonable 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- 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 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 of a padded message can be predicted.
Therefore, minimum and maximum length of the unpadded message are Therefore, minimum and maximum length of the unpadded message are
known. known.
Block Length Padding is the currently RECOMMENDED strategy (see Block Length Padding is the currently RECOMMENDED strategy (see
Section 5). Section 5).
4.4. Maximal Length Padding ('The Full Monty') 4.2. Maximal Length Padding ('The Full Monty')
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. the server.
Maximal Length Padding is NOT RECOMMENDED. Maximal Length Padding is NOT RECOMMENDED.
4.5. Random Length Padding 4.3. 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 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.
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) which Disadvantage: This policy requires a good source of (pseudo) which
can keep up with the required message rates. Especially on busy can keep up with the required message rates. Especially on busy
servers, this may be a significant hindrance. servers, this may be a hindrance.
TODO: Recommendation - this is (at first glance) the best policy, but According to the limited empirical data available, Random Length
requires significant effort Padding performs slightly worse than Block Length Padding.
4.6. Random Block Length Padding 4.4. 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
transction ID of the message. transction 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"
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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. 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(0) 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 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 response to a multiple of 468 RFC7830) and SHOULD pad the corresponding response to a multiple
octects. of 468 octects.
Note that the recommendation above does apply only if DNS transport
is encrypted (See Section 6 of RFC 7830).
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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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 chosen policy) has a significant impact on the resilience of the chosen 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
implementor of EDNS(0) Padding must carefully consider the chosen implementor of EDNS(0) Padding must carefully consider which policies
policy and its parameters. 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 No matter how carefully a client selects their Padding policy, this
effort can be jeopardized if the server chooses to apply an effort can be jeopardized if the server chooses to apply an
inffective Padding policy to the corresponding response packets. inffective Padding policy to the corresponding response packets.
Therefore, a client applying Padding may want to choose a DNS server Therefore, a client applying Padding may want to choose a DNS server
which does apply at least an equally effective Padding policy on which does apply at least an equally effective Padding policy on
responses. responses.
Note that even with encryption and padding, it might be trivial to Note that even with encryption and padding, it might be trivial to
identify that the observed traffic is DNS. Also, padding does not identify that the observed traffic is DNS. Also, padding does not
prevent information leak via other side channels (particularly timing prevent information leak via other side channels (particularly timing
information). 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.
9. Changes 9. 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]
9.1. draft-ietf-dprive-padding-policy-03 9.1. 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.
9.2. 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.
9.2. draft-ietf-dprive-padding-policy-02 9.3. 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.
9.3. draft-ietf-dprive-padding-policy-01 9.4. 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.
9.4. draft-ietf-dprive-padding-policy-00 9.5. 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.
9.5. draft-mayrhofer-dprive-padding-profiles-00 9.6. draft-mayrhofer-dprive-padding-profiles-00
Initial version Initial version
10. Normative References 10. References
10.1. 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/
ndss2017-dprive-empirical-DNS-traffic-size.pdf>. 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,
<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>.
10.2. 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.
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 experimental
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
URI: http://edns0-padding.org/
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