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Versions: 00

dprive                                                          H. Zhang
Internet-Draft                                                   P. Aras
Intended status: Standards Track                              Salesforce
Expires: January 9, 2020                                       W. Toorop
                                                              NLnet Labs
                                                            S. Dickinson
                                                              Sinodun IT
                                                               A. Mankin
                                                              Salesforce
                                                            July 8, 2019


            DNS Zone Transfer using DNS Stateful Operations
                  draft-zatda-dprive-xfr-using-dso-00

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.  This document specifies use of
   DNS Stateful Operations to enable a subscribe/publish mechanism for
   zone transfers reducing the over head introduced by NOTITY/SOA
   interactions prior to zone transfer request.  This additionally
   prevents zone contents collection via passive monitoring of zone
   transfers by restricting XFR using DSO to require TLS.

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 9, 2020.

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 . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Use Cases for XFR-using-DSO . . . . . . . . . . . . . . . . .   4
   4.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Transport . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  State Considerations  . . . . . . . . . . . . . . . . . . . .   5
   7.  Protocol Operation  . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  XuD SUBSCRIBE-XFR . . . . . . . . . . . . . . . . . . . .   6
       7.1.1.  SUBSCRIBE-XFR Request . . . . . . . . . . . . . . . .   7
       7.1.2.  SUBSCRIBE-XFR Response  . . . . . . . . . . . . . . .   9
     7.2.  XuD Notifications . . . . . . . . . . . . . . . . . . . .  12
       7.2.1.  DSO-IXFR Message  . . . . . . . . . . . . . . . . . .  13
       7.2.2.  Fallback to AXFR  . . . . . . . . . . . . . . . . . .  14
     7.3.  XuD UNSUBSCRIBE-XFR . . . . . . . . . . . . . . . . . . .  15
       7.3.1.  UNSUBSCRIBE-XFR Message . . . . . . . . . . . . . . .  15
     7.4.  Authentication  . . . . . . . . . . . . . . . . . . . . .  17
     7.5.  Multi-primary configurations  . . . . . . . . . . . . . .  17
     7.6.  DNS Stateful Operations TLV Context Summary . . . . . . .  17
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   9.  Implementation Considerations . . . . . . . . . . . . . . . .  18
   10. Implementation Status . . . . . . . . . . . . . . . . . . . .  18
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  19
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19
   13. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  19
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     14.2.  Informative References . . . . . . . . . . . . . . . . .  20
     14.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   [I-D.hzpa-dprive-xfr-over-tls] enumerates the existing issues with
   clear text XFR mechanisms, outlines some use cases for using
   encrypted channels for zone transfer and also describes using TLS for
   zone transfers.  It additionally discusses the various authentication



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   mechanisms that can be used to provide data and channel
   authentication, and channel confidentiality.

   This draft describes the use of a DSO [RFC8490] based protocol to
   perform zone transfers.  This mechanism is heavily based on an
   existing use of DSO where DNS clients can subscribe to receive
   asynchronous notifications of changes to RRSets of interest: DNS PUSH
   Notifications [I-D.ietf-dnssd-push].  That specification was
   developed with DNS Service Discovery in mind, this document describes
   an analogous protocol (XFR-using-DSO) where DNS clients can subscribe
   to receive asynchronous notifications of changes to zones of
   interest, it is developed with efficient and confidential zone
   transfers between primaries and secondaries in mind.

   In the XFR-using-DSO model, a DSO connection is first opened between
   the client and server, the client can then subscribe to one or more
   zones to be notified of changes and the server can publish changes to
   the zone over the connection.  Clients can choose to unsubscribe from
   zone updates at any time.

   Servers could also use the DSO session to send command-style messages
   to the client, for example, to instruct a client to stop serving a
   zone or delete a zone.  No such commands are defined in this version
   of the specification, but will likely be added in a future version.

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].

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

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

   XuD: XFR-using-DOS mechanisms as specified in this document








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3.  Use Cases for XFR-using-DSO

   This section includes additional use cases in addition to those
   specified in [I-D.hzpa-dprive-xfr-over-tls] that XuD can offer.

   o  Confidentiality.  Since this mechanism could, in principle,
      eliminate the need for NOTIFY and SOA queries it can provide
      complete confidentiality for the entire zone transfer mechanism.

   o  Security.  For some network configurations it is not desirable to
      have port 53 on the secondary open to an untrusted network for the
      sole purpose of receiving NOTIFYs.  NOTIFYs can also be trivially
      spoofed unless secured with TSIG.  For the DSO case, secondaries
      could initiate DSO connections to the primary and following that
      server-initiated DSO NOTIFY messages could be sent on that
      connection which could simultaneously be used for SOA and IXFR
      requests.  This would allow a firewall to be restricted to just
      allowing outgoing connections from secondary to primary.  Note
      that a similar but more constrained mechanism exists for IXFR
      whereby a short refresh period can be configured which triggers
      periodic SOA/IXFR requests from the secondary.  TODO: Look at the
      details of the NSD implementation.

   o  Performance.  For the DSO case, a new subscribe/publish mechanism
      could be envisaged that greatly reducing the number of messages
      required to perform one transfer.

   o  Improved error handling and retries.  In the DSO case new explicit
      error codes could be defined that allow a server to indicate the
      reason for a failed or aborted XFR request.  Also a new client
      initiated message could be used to gracefully cancel AXFRs.

   o  New command channel.  For the DSO case it would be possible to
      include new server-initiated 'control' commands e.g. 'stop serving
      this zone', 'delete this zone'.

   QUESTION: Is there any case where the primary might want to initiate
   the DSO connection to the secondary?

4.  Overview

   The figure below provides an outline of the XuD protocol.

   Figure 1: XuD protocol [1]

   A DNS XuD client subscribes for zone notifications for a particular
   zone by connecting to the appropriate authoritative server for that
   zone, and sending DSO message(s) indicating the zone(s) of interest.



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   When the client loses interest in receiving further updates to these
   zones, it unsubscribes.

   The authoritative server for a DNS zone is any server capable of
   generating the correct change notifications for a zone.  It may be a
   primary, secondary, or stealth name server [RFC7719].

   Standard DNS Queries MAY be sent over a XuD (i.e., DSO) session.  For
   any zone for which the server is authoritative, it MUST respond
   authoritatively for queries on names falling within that zone both
   for normal DNS queries and for XuD subscriptions.  For names for
   which the server is acting as a recursive resolver, e.g. when the
   server is the local recursive resolver, for any query for which it
   supports XuD subscriptions, it MUST also support standard queries.

   XuD imposes less load on the responding server than rapid polling
   would, but XuD notifications do still have a cost, so XuD clients
   MUST only create XuD subscriptions for zones they are authorised to
   transfer.

   Generally, as described in the DNS Stateful Operations specification
   [RFC8490], a client must not keep a session to a server open
   indefinitely if it has no subscriptions (or other operations) active
   on that session.  A client MAY close a session as soon as it becomes
   idle, and then if needed in the future, open a new session when
   required.  Alternatively, a client MAY speculatively keep an idle
   session open for some time, subject to the constraint that it MUST
   NOT keep a session open that has been idle for more than the
   session's idle timeout (15 seconds by default) [RFC8490].

5.  Transport

   XuD clients MUST use DNS Stateful Operations [RFC8490] running over
   TLS over TCP [RFC7858].

   The connection for XuD 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 XuD.

   QUESTION: Is there a use case to allow XuD over TCP where
   confidentiality is not an issue e.g when the zone contents are
   already publicly available?

6.  State Considerations

   Each XuD server is capable of handling some finite number of XuD
   subscriptions.  This number will vary from server to server and is
   based on physical machine characteristics, network bandwidth, and



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   operating system resource allocation.  After a client establishes a
   session to a DNS server, each subscription is individually accepted
   or rejected.  Servers may employ various techniques to limit
   subscriptions to a manageable level.  Correspondingly, the client is
   free to establish simultaneous sessions to alternate DNS servers that
   support XuDs for the zone and distribute subscriptions at the
   client's discretion.  In this way, both clients and servers can react
   to resource constraints.

7.  Protocol Operation

   The XuD protocol is a session-oriented protocol, and makes use of DNS
   Stateful Operations (DSO) [RFC8490].

   For details of the DSO message format refer to the DNS Stateful
   Operations specification [RFC8490].  Those details are not repeated
   here.

   XuD clients and servers MUST support DSO.  A single server can
   support DNS Queries, DNS Updates, and XuD (using DSO) on the same TCP
   port.

   A XuD exchange begins with the client making a TLS/TCP connection to
   the appropriate server.

   A typical XuD client will immediately issue a DSO Keepalive operation
   to request a session timeout and/or keepalive interval longer than
   the the 15-second default values, but this is not required.  A XuD
   client MAY issue other requests on the session first, and only issue
   a DSO Keepalive operation later if it determines that to be
   necessary.  Sending either a DSO Keepalive operation or a XuD
   subscription over the TLS/TCP connection to the server signals the
   client's support of DSO and serves to establish a DSO session.

   In accordance with the current set of active subscriptions, the
   server sends relevant asynchronous XuD notifications to the client.
   Note that a client MUST be prepared to receive (and silently ignore)
   XuD notifications for subscriptions it has previously removed, since
   there is no way to prevent the situation where a XuD notification is
   in flight from server to client while the client's unsubscribe
   message cancelling that subscription is simultaneously in flight from
   client to server.

7.1.  XuD SUBSCRIBE-XFR

   After connecting, and requesting a longer idle timeout and/or
   keepalive interval if necessary, a XuD client then indicates its
   desire to receive XuD notifications for a given zone by sending a



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   SUBSCRIBE-XFR request to the server.  A SUBSCRIBE-XFR request is
   encoded in a DSO message [RFC8490].  This specification defines a
   primary DSO TLV for XuD SUBSCRIBE-XFR Requests (tentatively DSO Type
   Code 0x50).

   DSO messages with the SUBSCRIBE-XFR TLV as the Primary TLV are not
   permitted in early data.

   The entity that initiates a SUBSCRIBE-XFR request is by definition
   the client.  A server MUST NOT send a SUBSCRIBE-XFR request over an
   existing session from a client.  If a server does send a SUBSCRIBE-
   XFR request over a DSO session initiated by a client, this is a fatal
   error and the client should immediately abort the connection with a
   TLS close_notify alert.  See Section 6.1 of [RFC8446].

   TODO: Need to define a DSO version of TSIG to cover the SUBSCRIBE-XFR
   and DSO-XFR responses, since the Additional section count in DSO
   message MUST be zero.  Note the client only needs to use TSIG in the
   SUBSCRIBE-XFR message to prove it is authorised to request zone
   transfers, but all DSO-XFR messages should be signed if primary TSIG
   is required for the authentication model in use.

7.1.1.  SUBSCRIBE-XFR Request

   A SUBSCRIBE-XFR request begins with the standard DSO 12-byte header
   [RFC8490], followed by the SUBSCRIBE-XFR primary TLV.  A SUBSCRIBE-
   XFR request message is illustrated in Figure 2.

   The MESSAGE ID field MUST be set to a unique value, that the client
   is not using for any other active operation on this DSO session.  For
   the purposes here, a MESSAGE ID is in use on this session if the
   client has used it in a request for which it has not yet received a
   response, or if the client has used it for a subscription which it
   has not yet cancelled using UNSUBSCRIBE-XFR.  In the SUBSCRIBE-XFR
   response the server MUST echo back the MESSAGE ID value unchanged.

   The other header fields MUST be set as described in the DSO
   specification [RFC8490].  The DNS OPCODE field contains the OPCODE
   value for DNS Stateful Operations (6).  The four count fields MUST be
   zero, and the corresponding four sections MUST be empty (i.e.,
   absent).

   The DSO-TYPE is SUBSCRIBE-XFR (tentatively 0x50).

   The DSO-LENGTH is the length of the DSO-DATA that follows, which
   specifies the name and class of the zone and optionally the SOA value
   of the client's version of the zone.




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   If the client has no copy of the zone it MUST omit the SOA value to
   indicate to the server that a DSO-AXFR is required in response (see
   the next section).

                                      1  1  1  1  1  1
        0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  \
      |                  MESSAGE ID                   |   \
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
      |QR| OPCODE(6) |         Z          |   RCODE   |    |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
      |             QDCOUNT (MUST BE ZERO)            |    |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+     > HEADER
      |             ANCOUNT (MUST BE ZERO)            |    |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
      |             NSCOUNT (MUST BE ZERO)            |    |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
      |             ARCOUNT (MUST BE ZERO)            |   /
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  /
      | DSO-TYPE = SUBSCRIBE-XFR (tentatively 0x50)   |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
      |   DSO-LENGTH (number of octets in DSO-DATA)   |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  \
      |                                               |   \
      \                     NAME                      \    |
      \                                               \    |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+     > DSO-DATA
      |                     CLASS                     |    |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
      |                   SOA value                   |   /
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  /

   Figure 2: SUBSCRIBE-XFR Request

   The DSO-DATA for a SUBSCRIBE-XFR request MUST contain exactly one
   NAME, CLASS and SOA value.  Since SUBSCRIBE-XFR requests are sent
   over TCP, multiple SUBSCRIBE-XFR DSO request messages can be
   concatenated in a single TCP stream and packed efficiently into TCP
   segments.

   If accepted, the subscription will stay in effect until the client
   cancels the subscription using UNSUBSCRIBE-XFR or until the DSO
   session between the client and the server is closed.

   SUBSCRIBE-XFR requests on a given session MUST be unique.  A client
   MUST NOT send a SUBSCRIBE-XFR message that duplicates the NAME, CLASS
   and SOA value of an existing active subscription on that DSO session.
   For the purpose of this matching, the established DNS case-



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   insensitivity for US-ASCII letters applies (e.g., "example.com" and
   "Example.com" are the same).  If a server receives such a duplicate
   SUBSCRIBE-XFR message this is an error and the server MUST
   immediately terminate the connection with a TLS close_notify alert.

   QUESTION: Is there a use case where a client may want to signal that
   the version of the zone it holds has been updated via another
   mechanism and the zone transfer should restart from a different SOA
   than that currently exchanged between client and server?

   DNS wildcarding is not supported.  SUBSCRIBE-XFR requests received
   for zones containing wildcards are considered an error (see below).

   A CLASS of 'ANY' (255) is not supported.

7.1.2.  SUBSCRIBE-XFR Response

   Each SUBSCRIBE-XFR request generates exactly one SUBSCRIBE-XFR
   response from the server.  A SUBSCRIBE-XFR request message is
   illustrated in Figure 3.

   A SUBSCRIBE-XFR response begins with the standard DSO 12-byte header
   [RFC8490].  The QR bit in the header is set indicating it is a
   response.  The header MAY be followed by one or more optional TLVs,
   such as a Retry Delay TLV.

   The MESSAGE ID field MUST echo the value given in the Message ID
   field of the SUBSCRIBE-XFR request.  This is how the client knows
   which request is being responded to.

   A SUBSCRIBE-XFR response message MUST NOT include a SUBSCRIBE-XFR
   TLV.  If a client receives a SUBSCRIBE-XFR response message
   containing a SUBSCRIBE-XFR TLV then the response message is processed
   but the SUBSCRIBE-XFR TLV MUST be silently ignored.

















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                                       1  1  1  1  1  1
         0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  \
       |                  MESSAGE ID                   |   \
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
       |QR| OPCODE(6) |         Z          |   RCODE   |    |
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
       |             QDCOUNT (MUST BE ZERO)            |    |
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+     > HEADER
       |             ANCOUNT (MUST BE ZERO)            |    |
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
       |             NSCOUNT (MUST BE ZERO)            |    |
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
       |             ARCOUNT (MUST BE ZERO)            |   /
       +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  /

   Figure 3: SUBSCRIBE-XFR Response Message

   In the SUBSCRIBE-XFR response the RCODE indicates whether or not the
   subscription was accepted.  Supported RCODEs are as follows:

   +-----------+-------+-----------------------------------------------+
   | Mnemonic  | Value | Description                                   |
   +-----------+-------+-----------------------------------------------+
   | NOERROR   | 0     | SUBSCRIBE-XFR successful.                     |
   | FORMERR   | 1     | Server failed to process request due to a     |
   |           |       | malformed request.                            |
   | SERVFAIL  | 2     | Server failed to process request due to a     |
   |           |       | problem with the server.                      |
   | NOTIMP    | 4     | Server does not implement DSO.                |
   | REFUSED   | 5     | Server refuses to process request for policy  |
   |           |       | or security reasons.                          |
   | NOTAUTH   | 9     | Server is not authoritative for the requested |
   |           |       | name.                                         |
   | DSOTYPENI | 11    | SUBSCRIBE-XFR operation not supported.        |
   +-----------+-------+-----------------------------------------------+

   Table 1: SUBSCRIBE-XFR Response codes

   This document specifies only these RCODE values for SUBSCRIBE-XFR
   Responses.  Servers sending SUBSCRIBE-XFR Responses SHOULD use one of
   these values.  Note that NXDOMAIN is not a valid RCODE in response to
   a SUBSCRIBE-XFR Request.  However, future circumstances may create
   situations where other RCODE values are appropriate in SUBSCRIBE-XFR
   Responses, so clients MUST be prepared to accept SUBSCRIBE-XFR
   Responses with any other RCODE value.





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   If the server sends a nonzero RCODE in the SUBSCRIBE-XFR response,
   that means:

   a  the client is (at least partially) misconfigured,

   b  the server resources are exhausted, or

   c  there is some other unknown failure on the server.

   In any case, the client shouldn't retry the subscription to this
   server right away.  If a client has other authoritative servers
   configured for a given zone an alternative server can be tried
   immediately.

   If the client has other successful subscriptions to this server,
   these subscriptions remain even though additional subscriptions may
   be refused.  Neither the client nor the server are required to close
   the connection, although, either end may choose to do so.

   If the server sends a nonzero RCODE then it SHOULD append a Retry
   Delay TLV [RFC8490] to the response specifying a delay before the
   client attempts this operation again.  Recommended values for the
   delay for different RCODE values are given below.  These recommended
   values apply both to the default values a server should place in the
   Retry Delay TLV, and the default values a client should assume if the
   server provides no Retry Delay TLV.

   For RCODE = 1 (FORMERR) the delay may be any value selected by the
   implementer.  A value of five minutes is RECOMMENDED, to reduce the
   risk of high load from defective clients.

   For RCODE = 2 (SERVFAIL) the delay should be chosen according to the
   level of server overload and the anticipated duration of that
   overload.  By default, a value of one minute is RECOMMENDED.  If a
   more serious server failure occurs, the delay may be longer in
   accordance with the specific problem encountered.

   For RCODE = 4 (NOTIMP), which occurs on a server that doesn't
   implement DNS Stateful Operations [RFC8490], it is unlikely that the
   server will begin supporting DSO in the next few minutes, so the
   retry delay SHOULD be one hour.  Note that in such a case, a server
   that doesn't implement DSO is unlikely to place a Retry Delay TLV in
   its response, so this recommended value in particular applies to what
   a client should assume by default.

   For RCODE = 5 (REFUSED), which occurs on a server that implements
   XuDs, but is currently configured to disallow XuDs, the retry delay
   may be any value selected by the implementer and/or configured by the



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   operator.  Since it is possible that the misconfiguration may be
   repaired at any time, the retry delay should not be set too high.  By
   default, a value of 5 minutes is RECOMMENDED.

   For RCODE = 9 (NOTAUTH), which occurs on a server that implements
   XuDs, but is not configured to be authoritative for the requested
   name, the retry delay may be any value selected by the implementer
   and/or configured by the operator.  Since it is possible that the
   misconfiguration may be repaired at any time, the retry delay should
   not be set too high.  By default, a value of 5 minutes is
   RECOMMENDED.

   For RCODE = 11 (DSOTYPENI), which occurs on a server that implements
   DSO but doesn't implement XuD, it is unlikely that the server will
   begin supporting XuD in the next few minutes, so the retry delay
   SHOULD be one hour.

   For other RCODE values, the retry delay should be set by the server
   as appropriate for that error condition.  By default, a value of 5
   minutes is RECOMMENDED.

   For RCODE = 9 (NOTAUTH), the time delay applies to requests for other
   names falling within the same zone.  Requests for names falling
   within other zones are not subject to the delay.  For all other
   RCODEs the time delay applies to all subsequent requests to this
   server.

   After sending an error response the server MAY allow the session to
   remain open, or MAY send a Retry Delay Operation TLV instructing the
   client to close the session, as described in the DSO specification
   [RFC8490].  Clients MUST correctly handle both cases.

7.2.  XuD Notifications

   Once a subscription has been successfully established, the server
   generates DSO-IXFR messages to send to the client as appropriate.  In
   the case that the server could not provide a DSO-IXFR message based
   on the SOA received from the client an initial DSO-AXFR message will
   be sent immediately following the SUBSCRIBE-XFR Response.  Subsequent
   changes to the zone are then communicated to the client in subsequent
   DSO-IXFR messages.

   Until an UNSUBSCRIBE-XFR message is received the server MUST assume
   that the client is updating the client's version of the zone with the
   notifications sent and can therefore hold state on the SOA version
   the client holds.  It MUST use this to generate the DSO-IXFR messages
   sent on a XuD session.




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7.2.1.  DSO-IXFR Message

   A DSO-IXFR unidirectional message begins with the standard DSO
   12-byte header [RFC8490], followed by the DSO-IXFR primary TLV.  A
   DSO-IXFR message is illustrated in Figure 4.

   In accordance with the definition of DSO unidirectional messages, the
   MESSAGE ID field MUST be zero.  There is no client response to a DSO-
   IXFR message.

   The other header fields MUST be set as described in the DSO
   specification [RFC8490].  The DNS OPCODE field contains the OPCODE
   value for DNS Stateful Operations (6).  The four count fields MUST be
   zero, and the corresponding four sections MUST be empty (i.e.,
   absent).

   The DSO-TYPE is DSO-IXFR (tentatively 0x51).

   The DSO-LENGTH is the length of the DSO-DATA that follows, which
   specifies the changes being communicated.

   The DSO-DATA contains one or more change notifications.  A DSO-IXFR
   Message MUST contain at least one change notification.  If a DSO-IXFR
   Message is received that contains no change notifications, this is a
   fatal error, and the receiver MUST immediately terminate the
   connection with a TLS close_notify alert.

























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                                      1  1  1  1  1  1
        0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  \
      |           MESSAGE ID (MUST BE ZERO)           |   \
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
      |QR| OPCODE(6) |         Z          |   RCODE   |    |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
      |             QDCOUNT (MUST BE ZERO)            |    |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+     > HEADER
      |             ANCOUNT (MUST BE ZERO)            |    |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
      |             NSCOUNT (MUST BE ZERO)            |    |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
      |             ARCOUNT (MUST BE ZERO)            |   /
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  /
      |      DSO-TYPE = DSO-IXFR (tentatively 0x51)   |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
      |   DSO-LENGTH (number of octets in DSO-DATA)   |
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  \
      |                                               |   \
      |                                               |    |
      |            IXFR BODY AS PER RFC1995           |     > DSO-DATA
      |                                               |    |
      |                                               |   /
      +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  /

   Figure 4: DSO-IXFR Message

   The DSO-DATA in a DSO-IXFR message is identical to the contents of a
   [RFC1995] IXFR message that would be sent to communicate the same
   zone incremental zone transfer over UDP or TCP i.e. the set of one or
   more difference sequences that follow the DNS Header in an IXFR
   message.

   When processing the records received in a DSO-IXFR Message, the
   receiving client MUST validate that the zone being updated correspond
   with at least one currently active subscription on that session.
   Specifically, the SOA name and CLASS MUST match the SOA name and
   CLASS given in a SUBSCRIBE-XFR request, subject to the usual
   established DNS case-insensitivity for US-ASCII letters.

7.2.2.  Fallback to AXFR

   The format of the DSO-AXFR message is a standard DSO header with DSO-
   TYPE of DSO-AXFR (tentatively DSO Type Code 0x52) and the body is
   identical to a [RFC5936] AXFR response body.

   TODO: More detail here.



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   If the SUBSCRIBE-XFR message contained no SOA value, the server MUST
   send a DSO-AXFR message as its first message on the connection.

   Alternatively if incremental zone transfer is not available, the
   entire zone MAY be returned in a DSO-AXFR message.

   QUESTION: Should we bother with a separate DSO-AXFR message or just
   allow full zone transfer inside the DSO-IXFR message as with
   [RFC1995] IXFR?  A separate message type makes is more explicit and
   IXFR was constrained by having to respond to a IXFR request.

7.3.  XuD UNSUBSCRIBE-XFR

   To cancel an individual subscription without closing the entire DSO
   session, the client sends an UNSUBSCRIBE-XFR message over the
   established DSO session to the server.  The UNSUBSCRIBE-XFR message
   is encoded as a DSO unidirectional message [RFC8490].  This
   specification defines a primary unidirectional DSO TLV for XuD
   UNSUBSCRIBE-XFR Messages (tentatively DSO Type Code 0x53).

   A server MUST NOT initiate an UNSUBSCRIBE-XFR message.  If a server
   does send an UNSUBSCRIBE-XFR message over a DSO session initiated by
   a client, this is a fatal error and the client should immediately
   abort the connection with a TLS close_notify alert.

7.3.1.  UNSUBSCRIBE-XFR Message

   An UNSUBSCRIBE-XFR unidirectional message begins with the standard
   DSO 12-byte header [RFC8490], followed by the UNSUBSCRIBE-XFR primary
   TLV.  An UNSUBSCRIBE-XFR message is illustrated in Figure 5.

   In accordance with the definition of DSO unidirectional messages, the
   MESSAGE ID field MUST be zero.  There is no server response to an
   UNSUBSCRIBE-XFR message.

   The other header fields MUST be set as described in the DSO
   specification [RFC8490].  The DNS OPCODE field contains the OPCODE
   value for DNS Stateful Operations (6).  The four count fields MUST be
   zero, and the corresponding four sections MUST be empty (i.e.,
   absent).

   The DSO-TYPE is UNSUBSCRIBE-XFR (tentatively 0x53).

   The DSO-LENGTH field contains the value 2, the length of the 2-octet
   MESSAGE ID contained in the DSO-DATA.

   The DSO-DATA contains the value given in the MESSAGE ID field of an
   active SUBSCRIBE-XFR request.  This is how the server knows which



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   SUBSCRIBE-XFR request is being cancelled.  After receipt of the
   UNSUBSCRIBE-XFR message, the SUBSCRIBE-XFR request is no longer
   active.

   It is allowable for the client to issue an UNSUBSCRIBE-XFR message
   for a previous SUBSCRIBE-XFR request for which the client has not yet
   received a SUBSCRIBE-XFR response.  This is to allow for the case
   where a client starts and stops a subscription in less than the
   round-trip time to the server.  The client is NOT required to wait
   for the SUBSCRIBE-XFR response before issuing the UNSUBSCRIBE-XFR
   message.

   Consequently, it is possible for a server to receive an UNSUBSCRIBE-
   XFR message that does not match any currently active subscription.
   This can occur when a client sends a SUBSCRIBE-XFR request, which
   subsequently fails and returns an error code, but the client sent an
   UNSUBSCRIBE-XFR message before it became aware that the SUBSCRIBE-XFR
   request had failed.  Because of this, servers MUST silently ignore
   UNSUBSCRIBE-XFR messages that do not match any currently active
   subscription.

                                     1  1  1  1  1  1
       0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  \
     |           MESSAGE ID (MUST BE ZERO)           |   \
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
     |QR| OPCODE(6) |         Z          |   RCODE   |    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
     |             QDCOUNT (MUST BE ZERO)            |    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+     > HEADER
     |             ANCOUNT (MUST BE ZERO)            |    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
     |             NSCOUNT (MUST BE ZERO)            |    |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |
     |             ARCOUNT (MUST BE ZERO)            |   /
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  /
     | DSO-TYPE = UNSUBSCRIBE-XFR (tentatively 0x53) |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
     |                DSO-LENGTH (2)                 |
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  \
     |              SUBSCRIBE-XFR MESSAGE ID             |   > DSO-DATA
     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+  /

   Figure 5: UNSUBSCRIBE-XFR Message

   QUESTION: Do we need the equivalent of a RECONFIRM message from DNS
   PUSH Notifications [I-D.ietf-dnssd-push]?




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7.4.  Authentication

   The authentication considerations are largely the same as those
   presented in [I-D.hzpa-dprive-xfr-over-tls].

7.5.  Multi-primary configurations

   The multi-primary considerations share some of the same issues as
   those presented in [I-D.hzpa-dprive-xfr-over-tls] but are different
   because the client is not performing SOA queries.

   TODO: More detail required here.

7.6.  DNS Stateful Operations TLV Context Summary

   This document defines four new DSO TLVs.  As suggested in Section 8.2
   of the DNS Stateful Operations specification [RFC8490], the valid
   contexts of these new TLV types are summarized below.

   The client TLV contexts are:

   C-P: Client request message, primary TLV

   C-U: Client unidirectional message, primary TLV

   C-A: Client request or unidirectional message, additional TLV

   CRP: Response back to client, primary TLV

   CRA: Response back to client, additional TLV

             +-----------------+-----+-----+-----+-----+-----+
             | TLV Type        | C-P | C-U | C-A | CRP | CRA |
             +-----------------+-----+-----+-----+-----+-----+
             | SUBSCRIBE-XFR   | X   |     |     |     |     |
             | DSO-IXFR        |     |     |     |     |     |
             | DSO-AXFR        |     |     |     |     |     |
             | UNSUBSCRIBE-XFR |     | X   |     |     |     |
             +-----------------+-----+-----+-----+-----+-----+

   Table 2: DSO TLV Client Context Summary

   The server TLV contexts are:

   S-P: Server request message, primary TLV

   S-U: Server unidirectional message, primary TLV




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   S-A: Server request or unidirectional message, additional TLV

   SRP: Response back to server, primary TLV

   SRA: Response back to server, additional TLV

             +-----------------+-----+-----+-----+-----+-----+
             | TLV Type        | S-P | S-U | S-A | SRP | SRA |
             +-----------------+-----+-----+-----+-----+-----+
             | SUBSCRIBE-XFR   |     |     |     |     |     |
             | DSO-IXFR        |     |  X  |     |     |     |
             | DSO-AXFR        |     |  X  |     |     |     |
             | UNSUBSCRIBE-XFR |     |     |     |     |     |
             +-----------------+-----+-----+-----+-----+-----+

   Table 3: DSO TLV Server Context Summary

8.  IANA Considerations

   This document also defines four new DNS Stateful Operation TLV types
   to be recorded in the IANA DSO Type Code Registry.

   +-----------------+----------+----------+--------------+------------+
   |       Name      |  Value   |  Early   |    Status    | Definition |
   |                 |          |   Data   |              |            |
   +-----------------+----------+----------+--------------+------------+
   |  SUBSCRIBE-XFR  |   TBA    |    NO    |  Standards   |  Section   |
   |                 |  (0x50)  |          |    Track     |    7.1     |
   |     DSO-IXFR    |   TBA    |    NA    |  Standards   |  Section   |
   |                 |  (0x51)  |          |    Track     |    7.1     |
   |     DSO-AXFR    |   TBA    |    NA    |  Standards   |  Section   |
   |                 |  (0x51)  |          |    Track     |    7.2     |
   | UNSUBSCRIBE-XFR |   TBA    |    NA    |  Standards   |  Section   |
   |                 |  (0x52)  |          |    Track     |    7.2     |
   +-----------------+----------+----------+--------------+------------+

   Table 5: IANA DSO TLV Type Code Assignment

9.  Implementation Considerations

   TBD

10.  Implementation Status

   TBD






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11.  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  Should DoT in this new case be required to use only TLS 1.3 and
      higher to avoid residual exposure?

   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?

12.  Acknowledgements

13.  Changelog

   draft-zatda-dprive-xfr-using-dso-00

   o  Initial commit

14.  References

14.1.  Normative References

   [I-D.hzpa-dprive-xfr-over-tls]
              Zhang, H., Aras, P., Toorop, W., Dickinson, S., and A.
              Mankin, "DNS Zone Transfer over TLS", draft-hzpa-dprive-
              xfr-over-tls-01 (work in progress), March 2019.

   [I-D.ietf-dnssd-push]
              Pusateri, T. and S. Cheshire, "DNS Push Notifications",
              draft-ietf-dnssd-push-21 (work in progress), July 2019.

   [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>.

   [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>.





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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>.

   [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>.

14.2.  Informative References

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

   [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>.

   [RFC8490]  Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
              Lemon, T., and T. Pusateri, "DNS Stateful Operations",
              RFC 8490, DOI 10.17487/RFC8490, March 2019,
              <https://www.rfc-editor.org/info/rfc8490>.

14.3.  URIs

   [1] https://github.com/Sinodun/draft-xfr-using-dso/blob/master/draft-
       01-svg/XuD_Protocol.svg

Authors' Addresses

   Han Zhang
   Salesforce
   San Francisco, CA
   United States

   Email: hzhang@salesforce.com










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   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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