Internet Engineering Task Force (IETF) A. Mayrhofer
Request for Comments: 7830 nic.at GmbH
Category: Standards Track May 2016
ISSN: 2070-1721
The EDNS(0) Padding Option
Abstract
This document specifies the EDNS(0) "Padding" option, which allows
DNS clients and servers to pad request and response messages by a
variable number of octets.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 7830.
Copyright Notice
Copyright (c) 2016 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
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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.
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RFC 7830 EDNS(0) Padding May 2016
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. The "Padding" Option . . . . . . . . . . . . . . . . . . . . 3
4. Usage Considerations . . . . . . . . . . . . . . . . . . . . 3
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . 4
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
7.1. Normative References . . . . . . . . . . . . . . . . . . 4
7.2. Informative References . . . . . . . . . . . . . . . . . 5
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
The Domain Name System (DNS) [RFC 1035] was specified to transport DNS
messages in cleartext form. Since this can expose significant
amounts of information about the Internet activities of an end user,
the IETF has undertaken work to provide confidentiality to DNS
transactions (see the DPRIVE working group). Encrypting the DNS
transport is considered one of the options to improve the situation.
However, even if both DNS query and response messages were encrypted,
metadata could still be used to correlate such messages with well-
known unencrypted messages, hence jeopardizing some of the
confidentiality gained by encryption. One such property is the
message size.
This document 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 the encrypted message.
2. Terminology
The terms "Requestor" and "Responder" are to be interpreted as
specified in [RFC 6891].
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
[RFC 2119].
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RFC 7830 EDNS(0) Padding May 2016
3. The "Padding" Option
The EDNS(0) [RFC 6891] specifies a mechanism to include new options in
DNS packets, contained in the RDATA of the OPT meta-RR. This
document specifies the "Padding" option in order to allow clients and
servers to pad DNS packets by a variable number of bytes. The
"Padding" option MUST occur at most, once per OPT meta-RR (and hence,
at most once per message).
The figure below specifies the structure of the option in the RDATA
of the OPT RR:
0 8 16
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| OPTION-CODE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| OPTION-LENGTH |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| (PADDING) ... (PADDING) ... /
+- - - - - - - - - - - - - - - -
Figure 1
The OPTION-CODE for the "Padding" option is 12.
The OPTION-LENGTH for the "Padding" option is the size (in octets) of
the PADDING. The minimum number of PADDING octets is 0.
The PADDING octets SHOULD be set to 0x00. Other values MAY be used,
for example, in cases where there is a concern that the padded
message could be subject to compression before encryption. PADDING
octets of any value MUST be accepted in the messages received.
4. Usage Considerations
This document does not specify the actual amount of padding to be
used, since this depends on the situation in which the option is
used. However, padded DNS messages MUST NOT exceed the number of
octets specified in the Requestor's Payload Size field encoded in the
RR Class Field (see Sections 6.2.3 and 6.2.4 of [RFC 6891]).
Responders MUST pad DNS responses when the respective DNS query
included the "Padding" option, unless doing so would violate the
maximum UDP payload size.
Responders MAY pad DNS responses when the respective DNS query
indicated EDNS(0) support of the Requestor and the "Padding" option
was not included.
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RFC 7830 EDNS(0) Padding May 2016
Responders MUST NOT pad DNS responses when the respective DNS query
did not indicate EDNS(0) support.
5. IANA Considerations
IANA has assigned Option Code 12 for "Padding" in the "DNS EDNS0
Option Codes (OPT)" registry.
IANA has updated the respective registration record by changing the
Reference field to RFC 7830 and the Status field to "Standard".
6. Security Considerations
Padding DNS packets obviously increases their size, and will
therefore lead to increased traffic.
The use of the EDNS(0) padding only provides a benefit when DNS
packets are not transported in cleartext. Further, it is possible
that EDNS(0) padding may make DNS amplification attacks easier.
Therefore, implementations MUST NOT use this option if the DNS
transport is not encrypted.
Padding length might be affected by lower-level compression.
Therefore (as described in Section 3.3 of [RFC 7525]), implementations
and deployments SHOULD disable compression at the Transport Layer
Security (TLS) level.
The payload of the "Padding" option could (like many other fields in
the DNS protocol) be used as a covert channel.
7. References
7.1. Normative References
[RFC 1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC 1035,
November 1987, <http://www.rfc-editor.org/info/RFC 1035>.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC 6891, April 2013,
<http://www.rfc-editor.org/info/RFC 6891>.
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7.2. Informative References
[RFC 7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC 7525, May
2015, <http://www.rfc-editor.org/info/RFC 7525>.
Acknowledgements
This document was inspired by a discussion with Daniel Kahn Gillmor
during IETF 93, as an alternative to the proposed padding on the TLS
layer. Allison Mankin, Andreas Gustafsson, Christian Huitema, Jinmei
Tatuya, and Shane Kerr suggested text for this document.
Author's Address
Alexander Mayrhofer
nic.at GmbH
Karlsplatz 1/2/9
Vienna 1010
Austria
Email: alex.mayrhofer.ietf@gmail.com
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