draft-ietf-dprive-xfr-over-tls-02.txt

dprive                                                         W. Toorop
Internet-Draft                                                NLnet Labs
Updates: 1995, 7766 (if approved)                           S. Dickinson
Intended status: Standards Track                              Sinodun IT
Expires: January 14, 2021                                       S. Sahib
                                                                 P. Aras
                                                               A. Mankin
                                                              Salesforce
                                                           July 13, 2020


                       DNS Zone Transfer-over-TLS
                   draft-ietf-dprive-xfr-over-tls-02

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 TLS, rather then clear text, 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 January 14, 2021.

Copyright Notice

   Copyright (c) 2020 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



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   (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  . . . . . . . . . . . . . . . . .   5
   4.  Connection and Data Flows in Existing XFR Mechanisms  . . . .   5
     4.1.  AXFR Mechanism  . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  IXFR Mechanism  . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Data Leakage of NOTIFY and SOA Message Exchanges  . . . .   8
       4.3.1.  NOTIFY  . . . . . . . . . . . . . . . . . . . . . . .   8
       4.3.2.  SOA . . . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Connections and Data Flows in XoT . . . . . . . . . . . . . .   8
     5.1.  TLS versions  . . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  Connection usage  . . . . . . . . . . . . . . . . . . . .   8
       5.2.1.  High level XoT descriptions . . . . . . . . . . . . .   9
       5.2.2.  Previous specifications . . . . . . . . . . . . . . .   9
     5.3.  Update to RFC7766 . . . . . . . . . . . . . . . . . . . .  10
     5.4.  Connection Establishment  . . . . . . . . . . . . . . . .  10
       5.4.1.  Draft Version Identification  . . . . . . . . . . . .  11
     5.5.  Port selection  . . . . . . . . . . . . . . . . . . . . .  11
     5.6.  AXoT mechanism  . . . . . . . . . . . . . . . . . . . . .  11
       5.6.1.  Coverage and relationship to RFC5936  . . . . . . . .  12
       5.6.2.  AXoT connection and message handling  . . . . . . . .  12
       5.6.3.  Padding AXoT responses  . . . . . . . . . . . . . . .  14
     5.7.  IXoT mechanism  . . . . . . . . . . . . . . . . . . . . .  15
       5.7.1.  Coverage and relationship to RFC1995  . . . . . . . .  15
       5.7.2.  IXoT connection and message handling  . . . . . . . .  15
       5.7.3.  Condensation of responses . . . . . . . . . . . . . .  16
       5.7.4.  Fallback to AXFR  . . . . . . . . . . . . . . . . . .  16
       5.7.5.  Padding of IXoT responses . . . . . . . . . . . . . .  16
   6.  Multi-primary Configurations  . . . . . . . . . . . . . . . .  16
   7.  Zone Transfer with DoT - Authentication . . . . . . . . . . .  17
     7.1.  TSIG  . . . . . . . . . . . . . . . . . . . . . . . . . .  17
     7.2.  SIG(0)  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     7.3.  TLS . . . . . . . . . . . . . . . . . . . . . . . . . . .  18
       7.3.1.  Opportunistic . . . . . . . . . . . . . . . . . . . .  18
       7.3.2.  Strict  . . . . . . . . . . . . . . . . . . . . . . .  18
       7.3.3.  Mutual  . . . . . . . . . . . . . . . . . . . . . . .  18
     7.4.  IP Based ACL on the Primary . . . . . . . . . . . . . . .  18
     7.5.  ZONEMD  . . . . . . . . . . . . . . . . . . . . . . . . .  19



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     7.6.  Comparison of Authentication Methods  . . . . . . . . . .  19
   8.  Policies for Both AXFR and IXFR . . . . . . . . . . . . . . .  20
   9.  Implementation Considerations . . . . . . . . . . . . . . . .  21
   10. Implementation Status . . . . . . . . . . . . . . . . . . . .  21
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
     11.1.  Registration of XoT Identification String  . . . . . . .  21
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  21
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  22
   14. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  22
   15. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  22
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     16.2.  Informative References . . . . . . . . . . . . . . . . .  24
     16.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

1.  Introduction

   DNS has a number of privacy vulnerabilities, as discussed in detail
   in [RFC7626].  Stub client to recursive resolver query privacy has
   received the most attention to date, with standards track documents
   for both DNS-over-TLS (DoT) [RFC7858] and DNS-over-HTTPS (DoH)
   [RFC8484], and a proposal for DNS-over-QUIC
   [I-D.ietf-dprive-dnsoquic].  There is ongoing work on DNS privacy
   requirements for exchanges between recursive resolvers and
   authoritative servers [I-D.ietf-dprive-phase2-requirements] and some
   suggestions for how signaling of DoT support by authoritatives might
   work, e.g., [I-D.vandijk-dprive-ds-dot-signal-and-pin].  However
   there is currently no RFC that specifically defines authoritative
   support for DNS-over-TLS.

   [RFC7626] 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.  The
   contents of the zone could also include references to locations that
   allow inference about location information of the individuals
   associated with the zone's organization.  It could also include
   references to other organizations.  Examples of this could be:

   o  Person-laptop.example.org



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   o  MX-for-Location.example.org

   o  Service-tenant-from-another-org.example.org

   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 [RFC7626] 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
   encryption.  Some operators use SSH tunneling or IPSec to encrypt the
   transfer data.

   Because both AXFR and IXFR zone transfers are typically carried out
   over TCP from authoritative DNS protocol implementations, encrypting
   zone transfers using TLS, based closely on DoT [RFC7858], seems like
   a simple step forward.  This document specifies how to use TLS as a
   transport to prevent zone collection from zone transfers.

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].

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



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   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
      have not been updated to follow the guidance in [RFC5936].  Any
      implementation of XFR-over-TLS (XoT) 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.




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   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
   NOTIFYs.

   Figure 1.  AXFR Mechanism [1]

   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
       maintenance.

   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
       primary.

   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 connection 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].



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4.2.  IXFR Mechanism

   The figure below provides an outline of the IXFR mechanism including
   NOTIFYs.

   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.

   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
       primary.

   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
       response.

   [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
   IXFR.

   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 optimize the use of TCP
   connections and SHOULD use [RFC7858] to manage persistent
   connections.







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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
   primary.

   [RFC1996] also states:

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

   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.  Connections and Data Flows in XoT

5.1.  TLS versions

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

5.2.  Connection usage

   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.

   The details in [RFC7766], [RFC7858] and [RFC8310] about, e.g.,
   persistent connection and message handling are fully applicable to
   XoT as well.  However any behavior specified here takes precedence
   for XoT.






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5.2.1.  High level XoT descriptions

   The figure below provides an outline of the AXoT mechanism including
   NOTIFYs.

   Figure 3: AXoT mechanism [5]

   The figure below provides an outline of the IXoT mechanism including
   NOTIFYs.

   Figure 4: IXoT mechanism [6]

5.2.2.  Previous specifications

   We note that whilst [RFC5936] already recommends re-using open TCP
   connections, it does state:

      "Non-AXFR session traffic can also use an open TCP connection."

   when discussing AXFR-over-TCP.  It defines an AXFR session as an AXFR
   query message and the sequence of AXFR response messages returned for
   it.  Note that this excludes any SOA queries issued as part of the
   overall AXFR mechanism.  This requirement needs to be re-evaluated
   when considering applying the same model to XoT since

   o  There is no guarantee that a XoT server (which is very likely, but
      not necessarily, a purely authoritative server) will also support
      DoT for regular queries.  Requiring a purely authoritative server
      to also respond to any query over a TLS connection would be
      equivalent to defining a form of authoritative DoT.  We consider
      this to be out of scope for this document, which is focussed
      purely on zone transfers.

   o  It would, however, be optimal for XoT to include the capability to
      send SOA queries over an already open TLS connection.

   Moreover, it is worth noting that [RFC7766] made general
   implementation recommendations with regard to TCP/TLS connection
   handling:












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      "To mitigate the risk of unintentional server overload, DNS
      clients MUST take care to minimize the number of concurrent TCP
      connections made to any individual server. It is RECOMMENDED
      that for any given client/server interaction there SHOULD be no
      more than one connection for regular queries, one for zone
      transfers, and one for each protocol that is being used on top
      of TCP (for example, if the resolver was using TLS). However,
      it is noted that certain primary/ secondary configurations with
      many busy zones might need to use more than one TCP connection
      for zone transfers for operational reasons (for example, to
      support concurrent transfers of multiple zones)."

   Whilst this recommends a particular behavior for the clients using
   TCP, it does not relax the requirement for servers to handle 'mixed'
   traffic (regular queries and zone transfers) on any open TCP/TLS
   connection.  It also overlooks the potential that other transports
   might want to take the same approach with regard to using separate
   connections for different purposes.

5.3.  Update to RFC7766

   This specification for XoT updates the guidance in [RFC7766] to
   provide the same separation of connection purpose (regular queries
   and zone transfers) for all transports being used on top of TCP.
   Therefore, it is RECOMMENDED that for each protocol used on top of
   TCP in any given client/server interaction there SHOULD be no more
   than one connection for regular queries and one for zone transfers.
   We provide specific details in the following sections of reasons
   where more than one connection might be required for zone transfers.

5.4.  Connection Establishment

   This specification additionally limits the scope of XoT as defined
   here to be the use of dedicated TLS connections (XoT connections) to
   exchange only traffic specific to enabling zone transfers.  The set
   of transactions supported on such connections is limited to:

   o  AXFR

   o  IXFR

   o  SOA

   and is collectively referred to hereafter as 'XoT traffic'.

   Such connections MUST use an ALPN token of 'xot' during the TLS
   handshake (see Section 11).




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   In the absence of DNS specific capability signaling mechanisms this
   greatly simplifies the implementation of XoT such that a XoT exchange
   can occur between any primary and secondary regardless of the role of
   each (e.g. purely authoritative, recursive resolver also
   authoritatively hosting zones, stub) or of other DNS transport
   capability each may have.  It also clearly makes XoT support
   orthogonal to any set of zone transfer authentication mechanisms
   chosen by the two parties.

   XoT clients MUST only send XoT traffic on XoT connections.  If a XoT
   server receives traffic other than XoT traffic on a XoT connection it
   MUST respond with the extended DNS error code 21 - Not Supported
   [I-D.ietf-dnsop-extended-error].  It SHOULD treat this as protocol
   error and close the connection.

   With the update to [RFC7766] guidance above, clients are free to open
   separate connections to the server to make any other queries they may
   need over either TLS, TCP or UDP.  A specification for connections
   that support both XoT traffic and non-XoT traffic may be the subject
   of a future work.

5.4.1.  Draft Version Identification

   _RFC Editor's Note:_ Please remove this section prior to publication
   of a final version of this document.

   Only implementations of the final, published RFC can identify
   themselves as "xot".  Until such an RFC exists, implementations MUST
   NOT identify themselves using this string.

   Implementations of draft versions of the protocol MUST add the string
   "-" and the corresponding draft number to the identifier.  For
   example, draft-ietf-dprive-xfr-over-tls-02 is identified using the
   string "xot-02".

5.5.  Port selection

   The connection for XoT 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 XoT.
   There MAY be agreement to use different ports for AXoT and IXoT.

5.6.  AXoT mechanism








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5.6.1.  Coverage and relationship to RFC5936

   [RFC5936] re-specified AXFR providing additional guidance beyond that
   provided in [RFC1034] and [RFC1035].  For example, sections 4.1,
   4.1.1 and 4.1.2 of [RFC5936] provide improved guidance for AXFR
   clients and servers with regard to re-use of connections for multiple
   AXFRs and AXFRs of different zones.  However [RFC5936] was
   constrained by having to be backwards compatible with some very early
   basic implementations of AXFR.

   Here we specify some optimized behaviors for AXoT, based closely on
   those in [RFC5936], but without the constraint of backwards
   compatibility since it is expected that all implementations of AXoT
   fully implement the behavior described here.

   Where any behavior is not explicitly described here, the behavior
   specified in [RFC5936] MUST be followed.  Any behavior specified here
   takes precedence for AXoT implementations over that in [RFC5936].

5.6.2.  AXoT connection and message handling

   The first paragraph of Section 4.1.1 of [RFC5936] says that clients
   SHOULD close the connection when there is no 'apparent need' to use
   the connection for some time period.

   For AXoT this requirement is updated: AXoT clients and servers SHOULD
   use EDNS0 Keepalive [RFC7828] to establish the connection timeouts to
   be used.  The client SHOULD send the EDNS0 Keepalive option on every
   AXoT request sent so that the server has every opportunity to update
   the Keepalive timeout.  The AXoT server may use the frequency of
   recent AXFRs to calculate an average update rate as input to the
   decision of what EDNS0 Keepalive timeout to use.  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 seconds).

   Whilst the specification for EDNS0 [RFC6891]  does not specifically
   mention AXFRs, it does say

       "If an OPT record is present in a received request, compliant
       responders MUST include an OPT record in their respective
       responses."

   We clarify here that if an OPT record is present in a received AXoT
   request, compliant responders MUST include an OPT record in each of
   the subsequent AXoT responses.  Note that this requirement, combined
   with the use of EDNS0 Keepalive, enables AXoT servers to signal the
   desire to close a connection due to low resources by sending an EDNS0



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   Keepalive option with a timeout of 0 on any AXoT response (in the
   absence of another way to signal the abort of a AXoT transfer).

   An AXoT server MUST be able to handle multiple AXFR requests on a
   single XoT connection (for the same and different zones).

   [RFC5936] says:

       "An AXFR client MAY use an already opened TCP connection to
       start an AXFR session. Using an existing open connection is
       RECOMMENDED over opening a new connection. (Non-AXFR session
       traffic can also use an open connection.)"

   For AXoT this requirement is updated: AXoT clients SHOULD re-use an
   existing open XoT connection when starting any new AXoT session to
   the same primary, and for issuing SOA queries, instead of opening a
   new connection.  The number of XoT connections between a secondary
   and primary SHOULD be minimized.

   Valid reasons for not re-using existing connections might include:

   o  reaching a configured limit for the number of outstanding queries
      allowed on a single XoT connection

   o  the message ID pool has already been exhausted on an open
      connection

   o  a large number of timeouts or slow responses have occurred on an
      open connection

   o  an EDNS0 Keepalive option with a timeout of 0 has been received
      from the server and the client is in the process of closing the
      connection

   If no XoT connections are currently open, AXoT clients MAY send SOA
   queries over UDP, TCP or TLS.

   [RFC5936] says:

      "Some old AXFR clients expect each response message to contain
      only a single RR. To interoperate with such clients, the server
      MAY restrict response messages to a single RR."

   This is opposed to the normal behavior of containing a sufficient
   number of RRs to reasonably amortize the per-message overhead.  We
   clarify here that AXoT clients MUST be able to handle responses that
   include multiple RRs, up to the largest number that will fit within a
   DNS message (taking the required content of the other sections into



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   account, as described here and in [RFC5936]).  This removes any
   burden on AXoT servers of having to accommodate a configuration
   option or support for restricting responses to containing only a
   single RR.

   An AXoT client SHOULD pipeline AXFR requests for different zones on a
   single XoT connection.  An AXoT server SHOULD respond to those
   requests as soon as the response is available i.e. potentially out of
   order.

5.6.3.  Padding AXoT responses

   The goal of padding AXoT responses would be two fold:

   o  to obfuscate the actual size of the transferred zone to minimize
      information leakage about the entire contents of the zone.

   o  to obfuscate the incremental changes to the zone between SOA
      updates to minimize information leakage about zone update activity
      and growth.

   Note that the re-use of XoT connections for transfers of multiple
   different zones complicates any attempt to analyze the traffic size
   and timing to extract information.

   We note here that any requirement to obfuscate the total zone size is
   likely to require a server to create 'empty' AXoT responses.  That
   is, AXoT responses that contain no RR's apart from an OPT RR
   containing the EDNS(0) option for padding.  However, as with existing
   AXFR, the last AXoT response message sent MUST contain the same SOA
   that was in the first message of the AXoT response series in order to
   signal the conclusion of the zone transfer.

   [RFC5936] says:

      "Each AXFR response message SHOULD contain a sufficient number
      of RRs to reasonably amortize the per-message overhead, up to
      the largest number that will fit within a DNS message (taking
      the required content of the other sections into account, as
      described below)."

   'Empty' AXoT responses generated in order to meet a padding
   requirement will be exceptions to the above statement.  In order to
   guarantee support for future padding policies, we state here that
   secondary implementations MUST be resilient to receiving padded AXoT
   responses, including 'empty' AXoT responses that contain only an OPT
   RR containing the EDNS(0) option for padding.




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   Recommendation of specific policies for padding AXoT responses are
   out of scope for this specification.  Detailed considerations of such
   policies and the trade-offs involved are expected to be the subject
   of future work.

5.7.  IXoT mechanism

5.7.1.  Coverage and relationship to RFC1995

   [RFC1995] says nothing with respect to optimizing IXFRs over TCP or
   re-using already open TCP connections to perform IXFRs or other
   queries.  Therefore, there arguably is an implicit assumption
   (probably unintentional) that a TCP connection is used for one and
   only one IXFR request.  Indeed, several open source implementations
   currently take this approach.

   We provide new guidance here specific to IXoT that aligns with the
   guidance in [RFC5936] for AXFR, that in section Section 5.6 for AXoT,
   and with that for performant TCP/TLS usage in [RFC7766] and
   [RFC7858].

   Where any behavior is not explicitly described here, the behavior
   specified in [RFC1995] MUST be followed.  Any behavior specified here
   takes precedence for IXoT implementations over that in [RFC1995].

5.7.2.  IXoT connection and message handling

   In a manner entirely analogous to that described in paragraph 2 of
   Section 5.6.2 IXoT clients and servers SHOULD use EDNS0 Keepalive
   [RFC7828] to establish the connection timeouts to be used.

   An IXoT server MUST be able to handle multiple IXoT requests on a
   single XoT connection (for the same and different zones).

   IXoT clients SHOULD re-use an existing open XoT connection when
   making any new IXoT request to the same primary, and for issuing SOA
   queries, instead of opening a new connection.  The number of XoT
   connections between a secondary and primary SHOULD be minimized.

   Valid reasons for not re-using existing connections are the same as
   those described in Section 5.6.2

   If no XoT connections are currently open, IXoT clients MAY send SOA
   queries over UDP, TCP or TLS.

   An IXoT client SHOULD pipeline IXFR requests for different zones on a
   single XoT connection.  An IXoT server SHOULD respond to those




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   requests as soon as the response is available i.e. potentially out of
   order.

5.7.3.  Condensation of responses

   [RFC1995] says condensation of responses is optional and MAY be done.
   Whilst it does add complexity to generating responses it can
   significantly reduce the size of responses.  However any such
   reduction might be offset by increased message size due to padding.
   This specification does not update the optionality of condensation.

5.7.4.  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
   time.

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

5.7.5.  Padding of IXoT responses

   The goal of padding IXoT responses would be to obfuscate the
   incremental changes to the zone between SOA updates to minimize
   information leakage about zone update activity and growth.  Both the
   size and timing of the IXoT responses could reveal information.

   IXFR responses can vary in size greatly from the order of 100 bytes
   for one or two record updates, to tens of thousands of bytes for
   large dynamic DNSSEC signed zones.  The frequency of IXFR responses
   can also depend greatly on if and how the zone is DNSSEC signed.

   In order to guarantee support for future padding policies, we state
   here that secondary implementations MUST be resilient to receiving
   padded IXoT responses.

   Recommendation of specific policies for padding IXoT responses are
   out of scope for this specification.  Detailed considerations of such
   policies and the trade-offs involved are expected to be the subject
   of future work.

6.  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




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   configured primaries.  It can then choose to send an XFR request to
   the primary with the highest SOA (or other criteria, e.g., RTT).

   When using persistent connections the secondary may have a XoT
   connection already open to one or more primaries.  Should a secondary
   preferentially request an XFR from a primary to which it already has
   an open XoT 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.  The first one can be considered
   a 'preferred primary connection' model.  In this 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.

   The other extreme can be considered a 'parallel primary connection'
   model.  Here a secondary could keep multiple persistent connections
   open to all available primaries and only request XFRs 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.

   Recommendation of a particular scheme is out of scope of this
   document but implementations are encouraged to provide configuration
   options that allow operators to make choices about this behavior.

7.  Zone Transfer with DoT - Authentication

7.1.  TSIG

   TSIG [RFC2845] provides a mechanism for two or more parties to use
   shared 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
   confidentiality.

7.2.  SIG(0)

   SIG(0) [RFC2535] similarly also provides a mechanism to digitally
   sign a DNS message but uses public key authentication, where the
   public keys are stored in DNS as KEY RRs and a private key is stored




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   at the signer.  It also provides party-to-party data authentication,
   but not hop-to-hop channel authentication or confidentiality.

7.3.  TLS

7.3.1.  Opportunistic

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

7.3.2.  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
   confidentiality.

7.3.3.  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.

7.4.  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
   servers.

   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
   secondaries.






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7.5.  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 XoT but is not considered further.

7.6.  Comparison of Authentication Methods

   The Table below compares the properties of a selection 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
   question.




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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
      overhead.

8.  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
   exploit.

   A XoT policy should specify

   o  If TSIG or SIG(0) 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
   challenging.

   The mechanics of co-ordinating or enforcing such policies are out of
   the scope of this document but may be the subject of future
   operational guidance.

9.  Implementation Considerations

   TBD

10.  Implementation Status

   The 1.9.2 version of Unbound [8] includes an option to perform AXoT
   (instead of AXFR-over-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.

11.  IANA Considerations

11.1.  Registration of XoT Identification String

   This document creates a new registration for the identification of
   XoT in the "Application Layer Protocol Negotiation (ALPN) Protocol
   IDs" registry [RFC7301].

   The "xot" string identifies XoT:

   Protocol: XoT

   Identification Sequence: 0x64 0x6F 0x72 ("xot")

   Specification: This document

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.

   This does not mitigate:

   o  the risk that some level of zone activity might be inferred by
      observing zone transfer sizes and timing on encrypted connections




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      (even with padding applied), in combination with obtaining SOA
      records by directly querying authoritative servers.

   o  the risk that hidden primaries might be inferred or identified via
      observation of encrypted connections.

   o  the risk of zone contents being obtained via zone enumeration
      techniques.

   Security concerns of DoT are outlined in [RFC7858] and [RFC8310].

13.  Acknowledgements

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

14.  Contributors

   Significant contributions to the document were made by:

   Han Zhang
   Salesforce
   San Francisco, CA
   United States

   Email: hzhang@salesforce.com

15.  Changelog

   draft-ietf-dprive-xfr-over-tls-02

   o  Significantly update descriptions for both AXoT and IXoT for
      message and connection handling taking into account previous
      specifications in more detail

   o  Add use of APLN and limitations on traffic on XoT connections.

   o  Add new discussions of padding for both AXoT and IXoT

   o  Add text on SIG(0)

   o  Update security considerations

   o  Move multi-primary considerations to earlier as they are related
      to connection handling

   draft-ietf-dprive-xfr-over-tls-01




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   o  Minor editorial updates

   o  Add requirement for TLS 1.3. or later

   draft-ietf-dprive-xfr-over-tls-00

   o  Rename after adoption and reference update.

   o  Add placeholder for SIG(0) discussion

   o  Update section on ZONEMD

   draft-hzpa-dprive-xfr-over-tls-02

   o  Substantial re-work of the document.

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

   o  Editorial changes, updates to references.

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

   o  Initial commit

16.  References

16.1.  Normative References

   [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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   [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>.

   [RFC7626]  Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
              DOI 10.17487/RFC7626, August 2015, <https://www.rfc-
              editor.org/info/rfc7626>.

   [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>.

16.2.  Informative References

   [I-D.ietf-dnsop-dns-zone-digest]
              Wessels, D., Barber, P., Weinberg, M., Kumari, W., and W.
              Hardaker, "Message Digest for DNS Zones", draft-ietf-
              dnsop-dns-zone-digest-08 (work in progress), June 2020.




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   [I-D.ietf-dnsop-extended-error]
              Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D.
              Lawrence, "Extended DNS Errors", draft-ietf-dnsop-
              extended-error-16 (work in progress), May 2020.

   [I-D.ietf-dprive-dnsoquic]
              Huitema, C., Mankin, A., and S. Dickinson, "Specification
              of DNS over Dedicated QUIC Connections", draft-ietf-
              dprive-dnsoquic-00 (work in progress), April 2020.

   [I-D.ietf-dprive-phase2-requirements]
              Livingood, J., Mayrhofer, A., and B. Overeinder, "DNS
              Privacy Requirements for Exchanges between Recursive
              Resolvers and Authoritative Servers", draft-ietf-dprive-
              phase2-requirements-01 (work in progress), June 2020.

   [I-D.vandijk-dprive-ds-dot-signal-and-pin]
              Dijk, P., Geuze, R., and E. Bretelle, "Signalling
              Authoritative DoT support in DS records, with key
              pinning", draft-vandijk-dprive-ds-dot-signal-and-pin-00
              (work in progress), May 2020.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/info/rfc1034>.

   [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-
              editor.org/info/rfc1982>.

   [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>.

   [RFC2535]  Eastlake 3rd, D., "Domain Name System Security
              Extensions", RFC 2535, DOI 10.17487/RFC2535, March 1999,
              <https://www.rfc-editor.org/info/rfc2535>.

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013, <https://www.rfc-
              editor.org/info/rfc6891>.





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   [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,
              <https://www.rfc-editor.org/info/rfc7766>.

16.3.  URIs

   [1] https://github.com/hanzhang0116/hzpa-dprive-xfr-over-tls/blob/
       master/02-draft-dprive-svg/AXFR_mechanism.svg

   [2] https://github.com/hanzhang0116/hzpa-dprive-xfr-over-tls/blob/
       master/02-draft-dprive-svg/IXFR_mechanism.svg

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

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

   [5] https://github.com/hanzhang0116/hzpa-dprive-xfr-over-tls/blob/
       master/02-draft-dprive-svg/AXoT_mechanism.svg

   [6] https://github.com/hanzhang0116/hzpa-dprive-xfr-over-tls/blob/
       master/02-draft-dprive-svg/IXoT_mechanism.svg

   [7] https://github.com/hanzhang0116/hzpa-dprive-xfr-over-tls/
       blob/02_updates/02-draft-svg/
       Properties_of_Authentication_methods_for_XoT.svg

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

Authors' Addresses

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

   Email: willem@nlnetlabs.nl












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

   Email: sara@sinodun.com


   Shivan Sahib
   Salesforce
   Vancouver, BC
   Canada

   Email: ssahib@salesforce.com


   Pallavi Aras
   Salesforce
   Herndon, VA
   United States

   Email: paras@salesforce.com


   Allison Mankin
   Salesforce
   Herndon, VA
   United States

   Email: allison.mankin@gmail.com



















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