Network Working Group A. Mayrhofer
Internet-Draft GmbH
Intended status: Standards Track November 4, 2015
Expires: May 7, 2016

The EDNS(0) Padding Option


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 Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on May 7, 2016.

Copyright Notice

Copyright (c) 2015 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 ( 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

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].

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 [[TODO-IANA]].

The OPTION-LENGTH for the 'Padding' option is the size (in octects) of the PADDING. The minimum number of padding octects is 0.

The PADDING octects MUST be set to 0x00. If a Responder detects non-0x00 octects in the padding of a query, a FORMERR (RCODE=1) MUST be returned.

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.

Responders MUST NOT pad DNS responses when the respective DNS query did not indicate EDNS(0).

5. IANA Considerations

IANA is requested to assign an EDNS Option Code (as described in Section 9 of [RFC6891]) for the 'Padding' option specified in this document.

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.

The payload of the 'Padding' option could be used as a covert channel. In order to prevent this, padding octets are required to be set to 0x00. It shall be noted that variations in the OPTION-SIZE itself could still be abused for expensive and low-bandwith covert communication.

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.

8. Changes

8.1. draft-ieft-dprive-edns0-padding-00

Adopted by WG. Changed text about message size limit based on feedback.

8.2. draft-mayrhofer-edns0-padding-01

Changed minimum padding size to 0, rewrote Usage Considerations section, extended Security considerations section

8.3. draft-mayrhofer-edns0-padding-00

Initial version

9. Normative References

[RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC6891] Damas, J., Graff, M. and P. Vixie, "Extension Mechanisms for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/RFC6891, April 2013.

Author's Address

Alexander Mayrhofer GmbH Karlsplatz 1/2/9 Vienna, 1010 Austria EMail: