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Versions: (draft-hzpa-dprive-xfr-over-tls) 00 01

dprive                                                          H. Zhang
Internet-Draft                                                   P. Aras
Updates: 1995 (if approved)                                   Salesforce
Intended status: Standards Track                               W. Toorop
Expires: November 21, 2020                                    NLnet Labs
                                                            S. Dickinson
                                                              Sinodun IT
                                                               A. Mankin
                                                            May 20, 2020

                       DNS Zone Transfer-over-TLS


   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 November 21, 2020.

Copyright Notice

   Copyright (c) 2020 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Use Cases for XFR-over-TLS  . . . . . . . . . . . . . . . . .   4
   4.  Connection and Data Flows in Existing XFR Mechanisms  . . . .   5
     4.1.  AXFR Mechanism  . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  IXFR Mechanism  . . . . . . . . . . . . . . . . . . . . .   6
     4.3.  Data Leakage of NOTIFY and SOA Message Exchanges  . . . .   7
       4.3.1.  NOTIFY  . . . . . . . . . . . . . . . . . . . . . . .   7
       4.3.2.  SOA . . . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Connection and Data Flows in XoT  . . . . . . . . . . . . . .   8
     5.1.  Performance Considerations  . . . . . . . . . . . . . . .   8
     5.2.  TLS versions  . . . . . . . . . . . . . . . . . . . . . .   8
     5.3.  AXoT mechanism  . . . . . . . . . . . . . . . . . . . . .   8
     5.4.  IXoT mechanism  . . . . . . . . . . . . . . . . . . . . .   9
       5.4.1.  Fallback to AXFR  . . . . . . . . . . . . . . . . . .  10
   6.  Zone Transfer with DoT - Authentication . . . . . . . . . . .  10
     6.1.  TSIG  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     6.2.  SIG(0)  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     6.3.  TLS . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
       6.3.1.  Opportunistic . . . . . . . . . . . . . . . . . . . .  11
       6.3.2.  Strict  . . . . . . . . . . . . . . . . . . . . . . .  11
       6.3.3.  Mutual  . . . . . . . . . . . . . . . . . . . . . . .  11
     6.4.  IP Based ACL on the Primary . . . . . . . . . . . . . . .  11
     6.5.  ZONEMD  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     6.6.  Comparison of Authentication Methods  . . . . . . . . . .  12
   7.  Policies for Both AXFR and IXFR . . . . . . . . . . . . . . .  13
   8.  Multi-primary Configurations  . . . . . . . . . . . . . . . .  14
   9.  Implementation Considerations . . . . . . . . . . . . . . . .  14
   10. Implementation Status . . . . . . . . . . . . . . . . . . . .  14
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  15
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   14. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  15
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     15.2.  Informative References . . . . . . . . . . . . . . . . .  17

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     15.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   DNS has a number of privacy vulnerabilities, as discussed in detail
   in [I-D.ietf-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)

   [I-D.ietf-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:

   "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.ietf-dprive-rfc7626-bis]
   contemplates the risk that someone gets the data through
   eavesdropping on network connections, only via enumeration or
   unauthorized 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

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   encryption.  Some operators use SSH tunneling 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.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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].

   Note that in this document we choose to use the terms 'primary' and
   'secondary' for two servers engaged in zone transfers.

   DNS terminology is as described in [RFC8499].

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

   XoT: Generic XFR-over-TLS mechanisms as specified in this document

   AXoT: AXFR-over-TLS

   IXoT: IXFR over-TLS

3.  Use Cases for XFR-over-TLS

   o  Confidentiality.  Clearly using an encrypted transport for zone
      transfers will defeat zone content leakage that can occur via
      passive surveillance.

   o  Authentication.  Use of single or mutual TLS authentication (in
      combination with ACLs) can complement and potentially be an
      alternative to TSIG.

   o  Performance.  Existing AXFR and IXFR mechanisms have the burden of
      backwards compatibility with older implementations based on the
      original specifications in [RFC1034] and [RFC1035].  For example,
      some older AXFR servers don't support using a TCP connection for
      multiple AXFR sessions or XFRs of different zones because they

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      have not been updated to follow the guidance in [RFC5936].  Any
      implementation of XFR-over-TLS would obviously be required to
      implement optimized and interoperable transfers as described in
      [RFC5936] e.g. transfer of multiple zones over one connection.

   o  Performance.  Current usage of TCP for IXFR is sub-optimal in some
      cases i.e.  connections are frequently closed after a single IXFR.

4.  Connection and Data Flows in Existing XFR Mechanisms

   The original specification for zone transfers in [RFC1034] and
   [RFC1035] was based on a polling mechanism: a secondary performed a
   periodic SOA query (based on the refresh timer) to determine if an
   AXFR was required.

   [RFC1995] and [RFC1996] introduced the concepts of IXFR and NOTIFY
   respectively, to provide for prompt propagation of zone updates.
   This has largely replaced AXFR where possible, particularly for
   dynamically updated zones.

   [RFC5936] subsequently redefined the specification of AXFR to improve
   performance and interoperability.

   In this document we use the phrase "XFR mechanism" to describe the
   entire set of message exchanges between a secondary and a primary
   that concludes in a successful AXFR or IXFR request/response.  This
   set may or may not include

   o  NOTIFY messages

   o  SOA queries

   o  Fallback from IXFR to AXFR

   o  Fallback from IXFR-over-UDP to IXFR-over-TCP

   The term is used to encompasses the range of permutations that are
   possible and is useful to distinguish the 'XFR mechanism' from a
   single XFR request/response exchange.

4.1.  AXFR Mechanism

   The figure below provides an outline of an AXFR mechanism including

   Figure 1.  AXFR Mechanism [1]

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   1.  An AXFR is often (but not always) preceded by a NOTIFY (over UDP)
       from the primary to the secondary.  A secondary may also initiate
       an AXFR based on a refresh timer or scheduled/triggered zone

   2.  The secondary will normally (but not always) make a SOA query to
       the primary to obtain the serial number of the zone held by the

   3.  If the primary serial is higher than the secondaries serial
       (using Serial Number Arithmetic [RFC1982]), the secondary makes
       an AXFR request (over TCP) to the primary after which the AXFR
       data flows in one or more AXFR responses on the TCP connection.

   [RFC5936] specifies that AXFR must use TCP as the transport protocol
   but details that there is no restriction in the protocol that a
   single TCP session must be used only for a single AXFR exchange, or
   even solely for XFRs.  For example, it outlines that the SOA query
   can also happen on this connection.  However, this can cause
   interoperability problems with older implementations that support
   only the trivial case of one AXFR per connection.

   Further details of the limitations in existing AXFR implementations
   are outlined in [RFC5936].

   It is noted that unless the NOTIFY is sent over a trusted
   communication channel and/or signed by TSIG is can be spoofed causing
   unnecessary zone transfer attempts.

   Similarly unless the SOA query is sent over a trusted communication
   channel and/or signed by TSIG the response can, in principle, be
   spoofed causing a secondary to incorrectly believe its version of the
   zone is update to date.  Repeated successful attacks on the SOA could
   result in a secondary serving stale zone data.

4.2.  IXFR Mechanism

   The figure below provides an outline of the IXFR mechanism including

   Figure 1.  IXFR Mechanism [2]

   1.  An IXFR is normally (but not always) preceded by a NOTIFY (over
       UDP) from the primary to the secondary.  A secondary may also
       initiate an IXFR based on a refresh timer or scheduled/triggered
       zone maintenance.

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   2.  The secondary will normally (but not always) make a SOA query to
       the primary to obtain the serial number of the zone held by the

   3.  If the primary serial is higher than the secondaries serial
       (using Serial Number Arithmetic [RFC1982]), the secondary makes
       an IXFR request to the primary after the primary sends an IXFR

   [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.  In fact is says in non-normative language:

   "Thus, a client should first make an IXFR query using UDP."

   So there may be a forth step above where the client falls back to
   IXFR-over-TCP.  There may also be a forth step where the secondary
   must fall back to AXFR because e.g. the primary does not support

   However it is noted that at least two widely used open source
   authoritative nameserver implementations (BIND [3] and NSD [4]) do
   IXFR using TCP by default in their latest releases.  For BIND TCP
   connections are sometimes used for SOA queries but in general they
   are not used persistently and close after an IXFR is completed.

   It is noted that the specification for IXFR was published well before
   TCP was considered a first class transport for DNS.  This document
   therefore updates [RFC1995] to state that DNS implementations that
   support IXFR-over-TCP MUST use [RFC7766] to optimise the use of TCP
   connections and SHOULD use [RFC7858] to manage persistent

4.3.  Data Leakage of NOTIFY and SOA Message Exchanges

   This section attempts to presents a rationale for also encrypting the
   other messages in the XFR mechanism.

   Since the SOA of the published zone can be trivially discovered by
   simply querying the publicly available authoritative servers leakage
   of this RR is not discussed in the following sections.

4.3.1.  NOTIFY

   Unencrypted NOTIFY messages identify configured secondaries on the

   [RFC1996] also states:

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   "If ANCOUNT>0, then the answer section represents an unsecure hint at
   the new RRset for this .

   But since the only supported QTYPE for NOTIFY is SOA, this does not
   pose a potential leak.

4.3.2.  SOA

   For hidden primaries or secondaries the SOA response leaks the degree
   of lag of any downstream secondary.

5.  Connection and Data Flows in XoT

5.1.  Performance Considerations

   The details in [RFC7766], [RFC7858] and [RFC8310] about e.g. using
   persistent connections and TLS Session Resumption [RFC5077] are fully
   applicable to XFR-over-TLS as well.

   It is RECOMMENDED that clients and servers that support XoT also
   implement EDNS0 Keepalive [RFC7828].

   It is useful to note that in these mechanisms it is the secondary
   that initiates the TLS connection to the primary for a XFR request,
   so that in terms of connectivity the secondary is the TLS client and
   the primary the TLS server.

5.2.  TLS versions

   For improved security all implementations of this specification MUST
   use only TLS 1.3 [RFC8446] or later.

5.3.  AXoT mechanism

   The figure below provides an outline of the AXoT mechanism including

   Figure 3: AXoT mechanism [5]

   The connection for AXFR-over-TLS SHOULD be established using port
   853, as specified in [RFC7858], unless there is mutual agreement
   between the secondary and primary to use a port other than port 853
   for XFR-over-TLS.

   All implementations that support XoT MUST fully implement [RFC5953]
   behavior on TLS connections.

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   Sections 4.1, 4.1.1 and 4.1.2 of [RFC5936] describe guidance for AXFR
   clients and servers with regard to re-use of sessions for multiple
   AXFRs, AXFRs of different zones and using TCP session for other
   queries including SOA.

   For clarity we restate here that an AXoT client MAY use an already
   opened TLS connection to send a AXFR request.  Using an existing open
   connection is RECOMMENDED over opening a new connection.  (Non-AXoT
   session traffic can also use an open connection.)

   For clarity we additionally state here that an AXoT client MAY use an
   already opened TLS connection to send a SOA request.  Using an
   existing open connection is RECOMMENDED over opening a new

   QUESTION: Should there be a requirement that the SOA is always done
   on a TLS connection if the XFR is?  For the case when no transfer is
   required this could be unnecessary overhead.

5.4.  IXoT mechanism

   The figure below provides an outline of the IXoT mechanism including

   Figure 4: IXoT mechanism [6]

   The connection for IXFR-over-TLS SHOULD be established using port
   853, as specified in [RFC7858], unless there is mutual agreement
   between the secondary and primary to use a port other than port 853
   for XFR-over-TLS.

   [RFC1995] says nothing with respect to optimizing IXFRs over TCP or
   re-using already open TCP connections to perform IXFRs or other
   queries.  We provide guidance here that aligns with the guidance in
   [RFC5936] for AXFR and with that for performant TCP/TLS usage in
   [RFC7766] and [RFC7858].

   An IXoT client MAY use an already opened TLS connection to send a
   IXFR request.  Using an existing open connection is RECOMMENDED over
   opening a new connection.  (Non-IXoT session traffic can also use an
   open connection.)

   An IXoT client MAY use an already open TLS connection to send an SOA
   query.  Using an existing open connection is RECOMMENDED over opening
   a new connection.

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   An IXoT server MUST be able to handle multiple IXoT requests on a
   single TLS connection, as well as to handle other query/response
   transactions over it.

   An IXoT client MAY keep an existing TLS session open in the
   expectation it is likely to need to perform an IXFR in the near
   future.  The client may use the frequency of recent IXFRs to
   calculate an average update rate and then use EDNS0 Keepalive to
   request an appropriate timeout from the server (if the server
   supports EDNS0 Keepalive).  If the server does not support EDNS0
   Keepalive the client MAY keep the connection open for a few seconds
   ([RFC7766] recommends that servers use timeouts of at least a few

   An IXoT client MAY pipeline IXFR requests for different zones on a
   single TLS connection.  AN IXoT server MAY respond to those requests
   out of order.

   QUESTION: Since this is a new specification should there be a
   requirement that IXoT servers are RECOMMENDED to condense responses
   as described in Section 6 of [RFC1995].  [RFC1995] document says this
   is optional and MAY be done but it can significantly reduce the size
   of responses and may have implications for padding?

5.4.1.  Fallback to AXFR

   Fallback to AXFR can happen, for example, if the server is not able
   to provide an IXFR for the requested SOA.  Implementations differ in
   how long they store zone deltas and how many may be stored at any one

   After a failed IXFR a IXoT client SHOULD request the AXFR on the
   already open TLS connection.

6.  Zone Transfer with DoT - Authentication

6.1.  TSIG

   TSIG [RFC2845] provides a mechanism for two parties to exchange
   secret keys which can then be used to create a message digest to
   protect individual DNS messages.  This allows each party to
   authenticate that a request or response (and the data in it) came
   from the other party, even if it was transmitted-over-an unsecured
   channel or via a proxy.  It provides party-to-party data
   authentication, but not hop-to-hop channel authentication or

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6.2.  SIG(0)


6.3.  TLS

6.3.1.  Opportunistic

   Opportunistic TLS [RFC8310] provides a defence against passive
   surveillance, providing on-the-wire confidentiality.

6.3.2.  Strict

   Strict TLS [RFC8310] requires that a client is configured with an
   authentication domain name (and/or SPKI pinset) that should be used
   to authenticate the TLS handshake with the server.  This additionally
   provides a defense for the client against active surveillance,
   providing client-to-server authentication and end-to-end channel

6.3.3.  Mutual

   This is an extension to Strict TLS [RFC8310] which requires that a
   client is configured with an authentication domain name (and/or SPKI
   pinset) and a client certificate.  The client offers the certificate
   for authentication by the server and the client can authentic the
   server the same way as in Strict TLS.  This provides a defense for
   both parties against active surveillance, providing bi-directional
   authentication and end-to-end channel confidentiality.

6.4.  IP Based ACL on the Primary

   Most DNS server implementations offer an option to configure an IP
   based Access Control List (ACL), which is often used in combination
   with TSIG based ACLs to restrict access to zone transfers on primary

   This is also possible with XoT but it must be noted that as with TCP
   the implementation of such an ACL cannot be enforced on the primary
   until a XFR request is received on an established connection.

   If control were to be any more fine-grained than this then a
   separate, dedicated port would need to be agreed between primary and
   secondary for XoT such that implementations would be able to refuse
   connections on that port to all clients except those configured as

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6.5.  ZONEMD

   Message Digest for DNS Zones (ZONEMD)
   [I-D.ietf-dnsop-dns-zone-digest] digest is a mechanism that can be
   used to verify the content of a standalone zone.  It is designed to
   be independent of the transmission channel or mechanism, allowing a
   general consumer of a zone to do origin authentication of the entire
   zone contents.  Note that the current version of
   [I-D.ietf-dnsop-dns-zone-digest] states:

   "As specified at this time, ZONEMD is not designed for use in large,
   dynamic zones due to the time and resources required for digest
   calculation.  The ZONEMD record described in this document includes
   fields reserved for future work to support large, dynamic zones."

   It is complementary the above mechanisms and can be used in
   conjunction with XFR-over-TLS but is not considered further.

6.6.  Comparison of Authentication Methods

   The Table below compares the properties of each of the above methods
   in terms of what protection they provide to the secondary and primary
   servers during XoT in terms of:

   o  'Data Auth': Authentication that the DNS message data is signed by
      the party with whom credentials were shared (the signing party may
      or may not be party operating the far end of a TCP/TLS connection
      in a 'proxy' scenario).  For the primary the TSIG on the XFR
      request confirms that the requesting party is authorized to
      request zone data, for the secondary it authenticates the zone
      data that is received.

   o  'Channel Conf': Confidentiality of the communication channel
      between the client and server (i.e. the two end points of a TCP/
      TLS connection).

   o  Channel Auth: Authentication of the identity of party to whom a
      TCP/TLS connection is made (this might not be a direct connection
      between the primary and secondary in a proxy scenario).

   It is noted that zone transfer scenarios can vary from a simple
   single primary/secondary relationship where both servers are under
   the control of a single operator to a complex hierarchical structure
   which includes proxies and multiple operators.  Each deployment
   scenario will require specific analysis to determine which
   authentication methods are best suited to the deployment model in

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   Table 1: Properties of Authentication methods for XoT [7]

   Based on this analysis it can be seen that:

   o  A combination of Opportunistic TLS and TSIG provides both data
      authentication and channel confidentiality for both parties.
      However this does not stop a MitM attack on the channel which
      could be used to gather zone data.

   o  Using just mutual TLS can be considered a standalone solution if
      the secondary has reason to place equivalent trust in channel
      authentication as data authentication e.g. the same operator runs
      both the primary and secondary.

   o  Using TSIG, Strict TLS and an ACL on the primary provides all 3
      properties for both parties with probably the lowest operational

7.  Policies for Both AXFR and IXFR

   We call the entire group of servers involved in XFR (all the
   primaries and all the secondaries) the 'transfer group'.

   Within any transfer group both AXFRs and IXFRs for a zone SHOULD all
   use the same policy e.g. if AXFRs use AXoT all IXFRs SHOULD use IXoT.

   In order to assure the confidentiality of the zone information, the
   entire transfer group MUST have a consistent policy of requiring
   confidentiality.  If any do not, this is a weak link for attackers to

   A XoT policy should specify

   o  If TSIG is required

   o  What kind of TLS is required (Opportunistic, Strict or mTLS)

   o  If IP based ACLs should also be used.

   Since this may require configuration of a number of servers who may
   be under the control of different operators the desired consistency
   could be hard to enforce and audit in practice.

   Certain aspects of the Policies can be relatively easily tested
   independently e.g. by requesting zone transfers without TSIG, from
   unauthorized IP addresses or over cleartext DNS.  Other aspects such
   as if a secondary will accept data without a TSIG digest or if

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   secondaries are using Strict as opposed to Opportunistic TLS are more

   NOTE: The authors request feedback on this challenge and welcome
   suggestions of how to practically manage this.

8.  Multi-primary Configurations

   Also known as multi-master configurations this model can provide
   flexibility and redundancy particularly for IXFR.  A secondary will
   receive one or more NOTIFY messages and can send an SOA to all of the
   configured primaries.  It can then choose to send an IXFR request to
   the primary with the highest SOA (or other criteria e.g.  RTT).

   When using persistent connections the secondary may have a TLS
   connection already open to one or more primaries.  Should a secondary
   preferentially request an IXFR from a primary to which it already has
   an open TLS connection or the one with the highest SOA (assuming it
   doesn't have a connection open to it already)?

   Two extremes can be envisaged here.  In the first case the secondary
   continues to use one persistent connection to a single primary until
   it has reason not to.  Reasons not to might include the primary
   repeatedly closing the connection, long RTTs on transfers or the SOA
   of the primary being an unacceptable lag behind the SOA of an
   alternative primary.

   At the other extreme a primary could keep multiple persistent
   connections open to all available primaries and only request IXFRs
   from the primary with the highest serial number.  Since normally the
   number of secondaries and primaries in direct contact in a transfer
   group is reasonably low this might be feasible if latency is the most
   significant concern.

9.  Implementation Considerations


10.  Implementation Status

   The 1.9.2 version of Unbound [8] includes an option to perform AXFR-
   over-TLS (instead of TCP).  This requires the client (secondary) to
   authenticate the server (primary) using a configured authentication
   domain name.

   It is noted that use of a TLS proxy in front of the primary server is
   a simple deployment solution that can enable server side XoT.

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11.  IANA Considerations


12.  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:

   o  How should padding be used in IXFR?

   o  Should there be an option to 'pad' an AXFR response (i.e. a set of
      AXFR responses on a given connection) to hide the zone size?

13.  Acknowledgements

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

14.  Changelog


   o  Minor editorial updates

   o  Add requirement for TLS 1.3. or later


   o  Rename after adoption and reference update.

   o  Add placeholder for SIG(0) discussion

   o  Update section on ZONEMD


   o  Substantial re-work of the document.


   o  Editorial changes, updates to references.


   o  Initial commit

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15.  References

15.1.  Normative References

              Bortzmeyer, S. and S. Dickinson, "DNS Privacy
              Considerations", draft-ietf-dprive-rfc7626-bis-05 (work in
              progress), May 2020.

              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-

   [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-

   [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,

   [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,

   [RFC5936]  Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
              (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,

   [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-

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   [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-

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,

   [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>.

15.2.  Informative References

              Wessels, D., Barber, P., Weinberg, M., Kumari, W., and W.
              Hardaker, "Message Digest for DNS Zones", draft-ietf-
              dnsop-dns-zone-digest-07 (work in progress), April 2020.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC1982]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
              DOI 10.17487/RFC1982, August 1996, <https://www.rfc-

   [RFC1996]  Vixie, P., "A Mechanism for Prompt Notification of Zone
              Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996,
              August 1996, <https://www.rfc-editor.org/info/rfc1996>.

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   [RFC5953]  Hardaker, W., "Transport Layer Security (TLS) Transport
              Model for the Simple Network Management Protocol (SNMP)",
              RFC 5953, DOI 10.17487/RFC5953, August 2010,

   [RFC7766]  Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
              D. Wessels, "DNS Transport over TCP - Implementation
              Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016,

15.3.  URIs

   [1] https://github.com/hanzhang0116/hzpa-dprive-xfr-over-tls/

   [2] https://github.com/hanzhang0116/hzpa-dprive-xfr-over-tls/

   [3] https://www.isc.org/bind/

   [4] https://www.nlnetlabs.nl/projects/nsd/about/

   [5] https://github.com/hanzhang0116/hzpa-dprive-xfr-over-tls/

   [6] https://github.com/hanzhang0116/hzpa-dprive-xfr-over-tls/

   [7] https://github.com/hanzhang0116/hzpa-dprive-xfr-over-tls/

   [8] https://github.com/NLnetLabs/unbound/blob/release-1.9.2/doc/

Authors' Addresses

   Han Zhang
   San Francisco, CA
   United States

   Email: hzhang@salesforce.com

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   Pallavi Aras
   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
   Herndon, VA
   United States

   Email: allison.mankin@gmail.com

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