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dprive                                                          H. Zhang
Internet-Draft                                                   P. Aras
Intended status: Standards Track                              Salesforce
Expires: September 12, 2019                                    W. Toorop
                                                              NLnet Labs
                                                            S. Dickinson
                                                              Sinodun IT
                                                               A. Mankin
                                                              Salesforce
                                                          March 11, 2019


                       DNS Zone Transfer over TLS
                   draft-hzpa-dprive-xfr-over-tls-01

Abstract

   DNS zone transfers are transmitted in clear text, which gives
   attackers the opportunity to collect the content of a zone by
   eavesdropping on network connections.  The DNS Transaction Signature
   (TSIG) mechanism is specified to restrict direct zone transfer to
   authorized clients only, but it does not add confidentiality.  This
   document specifies use of DNS-over-TLS to prevent zone contents
   collection via passive monitoring of zone transfers.

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 http://datatracker.ietf.org/drafts/current/.

   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 September 12, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   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 . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Zone Transfer Confidentiality Overview  . . . . . . . . . . .   4
   4.  Zone Transfer with DoT - Authentication . . . . . . . . . . .   4
     4.1.  TSIG  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  Mutual TLS  . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Session Establishment and Closing . . . . . . . . . . . . . .   4
     5.1.  AXFR Sessions . . . . . . . . . . . . . . . . . . . . . .   4
     5.2.  IXFR Sessions . . . . . . . . . . . . . . . . . . . . . .   4
     5.3.  Policies for Both AXFR and IXFR . . . . . . . . . . . . .   5
     5.4.  Next Steps  . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  Performance Considerations  . . . . . . . . . . . . . . . . .   5
   7.  Implementation Considerations . . . . . . . . . . . . . . . .   5
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   11. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   6
   12. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   13. Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   DNS has a number of privacy vulnerabilities, as discussed in detail
   in [I-D.bortzmeyer-dprive-rfc7626-bis].  Stub client to recursive
   resolver query privacy has received the most attention to date.
   There are now standards track documents for three encryption
   capabilities for stub to recursive queries and more work going on to
   guide deployment of specifically DNS-over-TLS (DoT) [RFC7858] and
   DNS-over-HTTPS (DoH) [RFC8484].

   [I-D.bortzmeyer-dprive-rfc7626-bis] established that stub client DNS
   query transactions are not public and needed protection, but on zone
   transfer [RFC1995] [RFC5936] it says only:





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   "Privacy risks for the holder of a zone (the risk that someone gets
   the data) are discussed in [RFC5936] and [RFC5155]."

   In what way is exposing the full contents of a zone a privacy risk?
   The contents of the zone could include information such as names of
   persons used in names of hosts.  Best practice is not to use personal
   information for domain names, but many such domain names exist.
   There may also be regulatory, policy or other reasons why the zone
   contents in full must be treated as private.

   Neither of the RFCs mentioned in [I-D.bortzmeyer-dprive-rfc7626-bis]
   contemplates the risk that someone gets the data through
   eavesdropping on network connections, only via enumeration or
   unauthorised transfer as described in the following paragraphs.

   [RFC5155] specifies NSEC3 to prevent zone enumeration, which is when
   queries for the authenticated denial of existences records of DNSSEC
   allow a client to walk through the entire zone.  Note that the need
   for this protection also motivates NSEC5 [I-D.vcelak-nsec5]; zone
   walking is now possible with NSEC3 due to crypto-breaking advances,
   and NSEC5 is a response to this problem.

   [RFC5155] does not address data obtained outside zone enumeration
   (nor does [I-D.vcelak-nsec5]).  Preventing eavesdropping of zone
   transfers (this draft) is orthogonal to preventing zone enumeration,
   though they aim to protect the same information.

   [RFC5936] specifies using TSIG [RFC2845] for authorization of the
   clients of a zone transfer and for data integrity, but does not
   express any need for confidentiality, and TSIG does not offer
   encryption.  Some operators use SSH tunnelling or IPSec to encrypt
   the transfer data.

   Because the AXFR zone transfer is typically carried out over TCP from
   authoritative DNS protocol implementations, encrypting AXFR using
   DNS-over-TLS [RFC7858] seems like a simple step forward.  This
   document specifies how to use DoT to prevent zone collection from
   zone transfers, including discussion of approaches for IXFR, which
   uses UDP or TCP.

   Next steps: Work on open questions at DNS table during IETF 104
   Hackathon, expand this draft, then solicit discussion on the DPRIVE
   mailing list.








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2.  Terminology

   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 BCP
   14 [RFC2119] and [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Privacy terminology is as described in Section 3 of [RFC6973].

   DNS terminology is as described in [RFC8499].

   DoT: DNS-over-TLS as specified in [RFC7858]

   DoH: DNS-over-HTTPS as specified in [RFC8484]

3.  Zone Transfer Confidentiality Overview

4.  Zone Transfer with DoT - Authentication

4.1.  TSIG

4.2.  Mutual TLS

5.  Session Establishment and Closing

5.1.  AXFR Sessions

   The connection flow in AXFR is a NOTIFY from the primary server to
   the secondary server, and then an AXFR request from the secondary to
   the primary after which the data flows.

   The connection for AXFR via DoT SHOULD be established using port 853,
   as specified in [RFC7858].  If there is no response on port 853, the
   connection MAY be attempted using port 443 in case the server offers
   DoT service on this port.

   TODO: diagram of connection flow for AXFR, without and with DoT

5.2.  IXFR Sessions

   [RFC1995] specifies that Incremental Transfer may use UDP if the
   entire IXFR response can be contained in a single DNS packet,
   otherwise, TCP is used.

   QUESTION: Given this, how should confidentiality of IXFR be provided?

   To discuss:



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   o  should IXFR have a mode in which TCP is mandatory?

   o  should IXFR have a mode in which TLS is mandatory?

   o  In workloads where there are frequent IXFRs, is the persistent
      connection mode that TCP-Mode would enable (as well as the
      retries) a benefit?

5.3.  Policies for Both AXFR and IXFR

   In order to assure the confidentiality of the zone information,
   entire group of servers involved in XFR (the primary and all
   secondaries) MUST have a consistent policy of requiring
   confidentiality.  If any do not, this is a weak link for attackers to
   exploit.  How to do this is TBD.

   Additionally, the entire group of servers involved in XFR (the
   primary and all secondaries) MUST have a consistent policy of
   requiring Strict or Opportunistic DoT [RFC8310].  How to do this is
   TBD.

5.4.  Next Steps

   Upcoming IETF Hackathon experiments will feed into this Session
   Establishment and Closing section, as much about this needs
   exploration as well as discussion on the mailing list.

6.  Performance Considerations

   The details in [RFC7858] and [RFC8310] about e.g. using persistent
   connections and TLS Session Resumption [RFC5077] are fully applicable
   to DNS Zone Transfer over DoT as well.

7.  Implementation Considerations

   TBA

8.  IANA Considerations

   TBD

9.  Security Considerations

   This document specifies a security measure against a DNS risk: the
   risk that an attacker collects entire DNS zones through eavesdropping
   on clear text DNS zone transfers.  It presents a new Security
   Consideration for DNS.  Some questions to discuss are: should DoT in




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   this new case be required to use only TLS 1.3 and higher to avoid
   residual exposure?  How should padding be used (if it should)?

10.  Acknowledgements

   The authors thank Benno Overeinder, Shumon Huque and Tim Wicinski for
   review and discussions.

11.  Contributors

   The following contributed significantly to the document:

12.  Changelog

   draft-hzpa-dprive-xfr-over-tls-01

   o  Editorial changes, updates to references.

   draft-hzpa-dprive-xfr-over-tls-00

   o  Initial commit

13.  Normative References

   [I-D.bortzmeyer-dprive-rfc7626-bis]
              Bortzmeyer, S. and S. Dickinson, "DNS Privacy
              Considerations", draft-bortzmeyer-dprive-rfc7626-bis-02
              (work in progress), January 2019.

   [I-D.vcelak-nsec5]
              Vcelak, J., Goldberg, S., Papadopoulos, D., Huque, S., and
              D. Lawrence, "NSEC5, DNSSEC Authenticated Denial of
              Existence", draft-vcelak-nsec5-08 (work in progress),
              December 2018.

   [RFC1995]  Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
              DOI 10.17487/RFC1995, August 1996, <https://www.rfc-
              editor.org/info/rfc1995>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
              editor.org/info/rfc2119>.

   [RFC2845]  Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
              Wellington, "Secret Key Transaction Authentication for DNS
              (TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000,
              <https://www.rfc-editor.org/info/rfc2845>.



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   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
              January 2008, <https://www.rfc-editor.org/info/rfc5077>.

   [RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
              Security (DNSSEC) Hashed Authenticated Denial of
              Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
              <https://www.rfc-editor.org/info/rfc5155>.

   [RFC5936]  Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
              (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,
              <https://www.rfc-editor.org/info/rfc5936>.

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013, <https://www.rfc-
              editor.org/info/rfc6973>.

   [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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8310]  Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
              for DNS over TLS and DNS over DTLS", RFC 8310,
              DOI 10.17487/RFC8310, March 2018, <https://www.rfc-
              editor.org/info/rfc8310>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/info/rfc8484>.

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <https://www.rfc-editor.org/info/rfc8499>.

Authors' Addresses








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   Han Zhang
   Salesforce
   San Francisco, CA
   United States

   Email: hzhang@salesforce.com


   Pallavi Aras
   Salesforce
   Herndon, VA
   United States

   Email: paras@salesforce.com


   Willem Toorop
   NLnet Labs
   Science Park 400
   Amsterdam  1098 XH
   The Netherlands

   Email: willem@nlnetlabs.nl


   Sara Dickinson
   Sinodun IT
   Magdalen Centre
   Oxford Science Park
   Oxford  OX4 4GA
   United Kingdom

   Email: sara@sinodun.com


   Allison Mankin
   Salesforce
   Herndon, VA
   United States

   Email: allison.mankin@gmail.com










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