Network Working Group A. Mayrhofer
Internet-Draft nic.at GmbH
Intended status: Standards Track November 24, 2015
Expires: May 27, 2016
The EDNS(0) Padding Option
draft-ietf-dprive-edns0-padding-01
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
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This Internet-Draft will expire on May 27, 2016.
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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. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4
8. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
8.1. draft-ietf-dprive-edns0-padding-01 . . . . . . . . . . . 4
8.2. draft-ieft-dprive-edns0-padding-00 . . . . . . . . . . . 5
8.3. draft-mayrhofer-edns0-padding-01 . . . . . . . . . . . . 5
8.4. draft-mayrhofer-edns0-padding-00 . . . . . . . . . . . . 5
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
9.1. Normative References . . . . . . . . . . . . . . . . . . 5
9.2. Informative References . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
The Domain Name System (DNS) [RFC1035] was specified to transport DNS
packets in clear text 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 WG). Encrypting the DNS transport is
considered as one of the options to improve the situation.
However, even if both DNS query and response packets were encrypted,
meta data of these packets could be used to correlate such packets
with well known unencrypted packets, 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 to artificially increase the size of a
DNS message by a variable number of bytes, significantly hampering
size-based correlation of the encrypted packet.
2. Terminology
The terms "Requestor", "Responder" are to be interpreted as specified
in [RFC6891].
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].
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3. The 'Padding' Option
The EDNS(0) [RFC6891] 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 pad DNS packets by a variable number of bytes. The 'Padding'
option MUST occur at most once per OPT meta-RR.
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. Application developers who
are concerned about misguided lower layer compression MAY instead
fill the PADDING octets with the output of a cryptographic random
number generator. Responders MUST NOT reject messages containing
non-0x00 PADDING octets.
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 Section 6.2.3 and 6.2.4 of [RFC6891]).
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.
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Responders MUST NOT pad DNS responses when the respective DNS query
did not indicate EDNS(0).
5. IANA Considerations
IANA has assigned EDNS Option Code 12 for Padding.
IANA is requested to update the respective registration record by
changing the Reference field to [[THISRFC]] and the Status field to
'Standard'.
6. Security Considerations
Padding DNS packets obviously increases their size, and will
therefore lead to increased traffic, can lead to increased number of
truncated packets when used over UDP-based transport.
The use of the EDNS(0) Padding provides only a benefit when DNS
packets are not transported in clear text. Implementations therefore
SHOULD avoid using 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 [RFC7525]), implementations
and deployments SHOULD disable TLS-level compression.
The payload of the 'Padding' option could (like many other fields in
the DNS protocol) be used as a covert channel.
7. Acknowledgements
This document was inspired by a discussion with Daniel Kahn Gillmor
during IETF93, as an alternative to the proposed padding on the TLS
layer. Allison Mankin and Christian Huitema suggested text for this
document.
8. Changes
8.1. draft-ietf-dprive-edns0-padding-01
Fixed 'octects' typo. Changed 'covert channel' text to align with
allowing non-0x00 padding. changed IANA considerations - assigned
option code is 12. Changed field definitions to allow for non-0x00
padding, removed FORMERR requirement. referenced rfc7525 in security
considerations. added acknowledgements.
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8.2. draft-ieft-dprive-edns0-padding-00
Adopted by WG. Changed text about message size limit based on
feedback.
8.3. draft-mayrhofer-edns0-padding-01
Changed minimum padding size to 0, rewrote Usage Considerations
section, extended Security considerations section
8.4. draft-mayrhofer-edns0-padding-00
Initial version
9. References
9.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC6891, April 2013,
<http://www.rfc-editor.org/info/rfc6891>.
9.2. Informative References
[RFC7525] 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/RFC7525, May
2015, <http://www.rfc-editor.org/info/rfc7525>.
Author's Address
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Alexander Mayrhofer
nic.at GmbH
Karlsplatz 1/2/9
Vienna 1010
Austria
Email: alex.mayrhofer.ietf@gmail.com
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