The EDNS(0) Padding Option
A DNS option that lets clients and servers add filler bytes so encrypted messages reveal less through their size.
Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830, DOI 10.17487/RFC7830, May 2016, <https://www.rfc-editor.org/info/rfc7830>.
@misc{rfc7830,
series = {Request for Comments},
number = 7830,
howpublished = {RFC 7830},
publisher = {RFC Editor},
doi = {10.17487/RFC7830},
url = {https://www.rfc-editor.org/info/rfc7830},
author = {A. Mayrhofer},
title = {{The EDNS(0) Padding Option}},
pagetotal = 5,
year = 2016,
month = may,
} RFC text
Contents
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/rfc7830.
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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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.
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) [RFC1035] 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 [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].
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 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 [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 and the "Padding" option
was not included.
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 [RFC7525]), 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
[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>.
7.2. 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>.
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
AI ELI5
On this page
§1Why Pad Encrypted DNS
DNS was specified to carry messages in cleartext, which can expose a lot about an end user's Internet activity. The IETF has worked on confidentiality for DNS transactions, and encrypting the transport is one option.
RFC 7830 points out that even if queries and responses were both encrypted, metadata could still be used to correlate them with well-known unencrypted messages. One such property is message size. The RFC defines the EDNS(0) Padding option so that clients and servers can artificially increase a DNS message by a variable number of bytes, which hampers size-based correlation.
Even a sealed letter shows how big it is. Adding filler makes every letter look about the same size.
§3The Padding Option
EDNS(0) (RFC 6891) lets new options ride in the RDATA of the OPT meta-RR. The Padding option consists of an OPTION-CODE of 12, an OPTION-LENGTH that is the size in octets of the padding, and then the padding octets. The minimum number of padding octets is 0. The option MUST occur at most once per OPT meta-RR, and therefore at most once per message.
The padding octets SHOULD be set to 0x00. Other values MAY be used, for example when there is concern that the padded message could be compressed before encryption. Receivers MUST accept padding octets of any value.
The option is a small label saying 'filler here' and how much of it there is. The filler is usually just blank space.
§4When to Pad
The RFC does not say how much to pad, because that depends on the situation. A padded message MUST NOT exceed the number of octets in the Requestor's Payload Size field, which is encoded in the RR Class field.
- Responders MUST pad a response when the query included the Padding option, unless that would violate the maximum UDP payload size.
- Responders MAY pad a response when the query showed EDNS(0) support but did not include the option.
- Responders MUST NOT pad a response when the query did not indicate EDNS(0) support.
If you ask for a padded answer you get one, if you only seem able to handle it you might get one, and if you cannot handle it you never do.
§6Security Considerations
Padding makes packets bigger, so it leads to more traffic. It gives a benefit only when DNS packets are not sent in cleartext, and it may make DNS amplification attacks easier. Therefore implementations MUST NOT use the option if the DNS transport is not encrypted.
Padding length might be changed by lower-level compression, so implementations and deployments SHOULD disable compression at the Transport Layer Security (TLS) level, as described in Section 3.3 of RFC 7525. The RFC also notes that the padding payload could be used as a covert channel, like many other fields in DNS. IANA has assigned Option Code 12 for Padding.
Filler only helps if the letter is sealed, it makes the mail heavier, and someone could hide a secret note in the filler.