An EDNS(0) option to negotiate Leases on DNS Updates
draft-ietf-dnssd-update-lease-04

Internet Engineering Task Force                              S. Cheshire
Internet-Draft                                                Apple Inc.
Intended status: Standards Track                                T. Lemon
Expires: 3 August 2023                                         Apple Inc
                                                         30 January 2023

          An EDNS(0) option to negotiate Leases on DNS Updates
                    draft-ietf-dnssd-update-lease-04

Abstract

   This document describes an EDNS(0) option that can be used by DNS
   Update requestors and DNS servers to include a lease lifetime in a
   DNS Update or response, allowing a server to garbage collect stale
   resource records that have been added by DNS Updates

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
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   This Internet-Draft will expire on 3 August 2023.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Terminology Used in this Document . . . . . .   3
     2.1.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Mechanisms  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Update Message Format . . . . . . . . . . . . . . . . . . . .   3
     4.1.  Requestor Behavior  . . . . . . . . . . . . . . . . . . .   4
     4.2.  Server Behavior . . . . . . . . . . . . . . . . . . . . .   5
   5.  Refresh Messages  . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Refresh Message Format  . . . . . . . . . . . . . . . . .   5
     5.2.  Requestor Behavior  . . . . . . . . . . . . . . . . . . .   6
       5.2.1.  Coalescing Refresh Messages . . . . . . . . . . . . .   6
     5.3.  Server Behavior . . . . . . . . . . . . . . . . . . . . .   7
   6.  Retransmission Strategy . . . . . . . . . . . . . . . . . . .   7
   7.  Garbage Collection  . . . . . . . . . . . . . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   11. Normative References  . . . . . . . . . . . . . . . . . . . .   9
   12. Informative References  . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   Dynamic DNS Update [RFC2136] allows for a mapping from a persistent
   hostname to a dynamic IP address.  This capability is particularly
   beneficial to mobile hosts, whose IP address may frequently change
   with location.  However, the mobile nature of such hosts often means
   that dynamically updated resource records are not properly deleted.
   Consider, for instance, a mobile user who publishes address records
   via dynamic update.  If this user moves their laptop out of range of
   the Wi-Fi access point, the address record containing stale
   information may remain on the server indefinitely.  An extension to
   Dynamic Update is thus required to tell the server to automatically
   delete resource records if they are not refreshed after a period of
   time.

   Note that overloading the resource record TTL [RFC1035] is not
   appropriate for purposes of garbage collection.  Data that is
   susceptible to frequent change or invalidation, thus requiring a
   garbage collection mechanism, needs a relatively short resource
   record TTL to avoid polluting intermediate DNS caches with stale
   data.  Using this TTL, short enough to minimize stale cached data, as
   a garbage collection lease lifetime would result in an unacceptable
   amount of network traffic due to refreshes (see Section 5 "Refresh
   Messages").

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2.  Conventions and Terminology Used in this Document

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

2.1.  Abbreviations

   DNS-SD  DNS-based service discovery [RFC6763]

   EDNS(0)  Extension Mechanism for DNS, version 0 [RFC6891]

3.  Mechanisms

   The EDNS(0) Update Lease option is included in a standard DNS Update
   message [RFC2136] within an EDNS(0) OPT pseudo-RR [RFC6891].

4.  Update Message Format

   Dynamic DNS Update Leases Requests and Responses are formatted as
   standard DNS Dynamic Update messages [RFC2136].  This update MUST
   include the EDNS(0) OPT RR, as described in [RFC6891].  This OPT RR
   MUST include an EDNS(0) Option as shown below.  Note that if a TSIG
   resource record ([RFC8945]) is included to authenticate the update,
   the TSIG RR MUST appear _after_ the OPT RR, allowing the message
   digest in the TSIG to cover the OPT RR.

   The Update Lease EDNS(0) option is formatted as follows:

    Field Name       Field Type   Description
    ----------------------------------------------------------------
    OPTION-CODE      u_int16_t    UPDATE-LEASE (2)
    OPTION-LENGTH    u_int16_t    4 or 8
    LEASE            u_int32_t    desired lease (request) or
                                  granted lease (response), in seconds
    KEY-LEASE        u_int32_t    optional desired (or granted)
                                  lease for KEY records, in seconds

                                  Figure 1

   Update Requests contain, in the LEASE field of the OPT RDATA, an
   unsigned 32-bit integer indicating the lease lifetime, in seconds,
   desired by the requestor, represented in network (big-endian) byte
   order.  In Update Responses, this field contains the actual lease
   granted by the server.  The lease granted by the server may be less
   than, greater than, or equal to the value requested by the requestor.

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   There are two variants of the EDNS(0) UPDATE-LEASE option, the basic
   (4-byte) variant and the extended (8-byte) variant.

   In the basic (4-byte) variant, the LEASE indicated in the Update
   Lease option applies to all resource records in the Update section.

   In the extended (8-byte) variant, the Update Lease communicates two
   lease lifetimes.  The LEASE indicated in the Update Lease option
   applies to all resource records in the Update section *except* for
   KEY records.  The KEY-LEASE indicated in the Update Lease option
   applies to KEY records in the Update section.

   The reason the KEY record can be given a special lease time is that
   this record is used in the DNS-SD Service Registration Protocol
   [I-D.ietf-dnssd-srp] to reserve a name (or names) when the service is
   not present.

4.1.  Requestor Behavior

   DNS Update requestors MUST send an Update Lease option with any DNS
   Update that is not intended to be present indefinitely.  The Update
   Lease option SHOULD specify a time interval that is no shorter than
   30 minutes (1800 seconds).  Requestors MAY specify a shorter lease if
   they anticipate that the records being updated will change sooner
   than 30 minutes.  Requestors that expect the updated records to be
   relatively static MAY request appropriately longer leases.

   If the DNS response received by the requestor does not include an
   Update Lease option, this is an indication that the DNS server does
   not support the Update Lease option.  The requestor SHOULD in this
   case continue sending Refresh messages (see below) as if the server
   had returned an identical update lease option in its response.

   If the DNS response does include an Update Lease option, the
   requestor MUST use the interval(s) returned in this option when
   determining when to send Refresh messages.  This is true both if the
   interval(s) returned by the server are shorter and if they are
   longer.

   When sending an initial update, the requestor MUST delay the initial
   transmission by a random amount of time across the range of 0-3000
   milliseconds, with a granularity of no more than 10 milliseconds.
   This prevents synchronization of multiple devices of the same type at
   a site upon recovery from a power failure.  This requirement applies
   only to the initial update on startup: since renews include a random
   factor as well, any synchronization that occurs after such an event
   should quickly normalize.

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4.2.  Server Behavior

   DNS Servers implementing the Update Lease option MUST include an
   Update Lease option in response to any successful DNS Update
   (RCODE=0) that includes an Update Lease option.  Servers MAY return
   different lease interval(s) than specified by the requestor, granting
   relatively longer or shorter leases to reduce network traffic due to
   Refreshes, or reduce stale data, respectively.

   Note that both the 4-byte and 8-byte variant are valid on both
   clients and servers.  If a server receives a 4-byte variant, it MUST
   respond with a 4-byte variant.  If a client sends an 8-byte variant,
   it MUST accept either an 8-byte variant or a 4-byte variant in the
   response.  If it receives a 4-byte variant, it MUST assume that both
   the key lease and update lease values are the same on the server.

5.  Refresh Messages

   A Refresh message is a DNS Update message that is sent to the server
   after an initial DNS Update has been sent, in order to prevent the
   update's records from being garbage collected.

5.1.  Refresh Message Format

   Refresh messages are formatted like Dynamic Update Leases Requests
   and Responses (see Section 4 "Update Message Format").  The Refresh
   message is constructed with the assumption that the result of the
   previous update or Refresh is still in effect.  The Refresh message
   will, in the case that the records added in a previous update were
   for some reason garbage collected, result in those records being
   added again.

   The Refresh message SHOULD NOT include any update prerequisites that
   would fail if the requestor's previous update or Refresh is still in
   effect.  It also SHOULD NOT include prerequisites that would fail if
   the records affected the previous update or Refresh are no longer
   present--that is, the Refresh should also work as an initial update.
   There may be cases where this is not possible, in which case the
   response from the server can be used to determine how to proceed when
   the update fails.

   An update message that changes the server state resulting from a
   previous Refresh or update is an update, not a Refresh.

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   The Update Lease option in a Refresh contains the desired new lease
   on Requests, and the actual granted lease on Responses.  The LEASE
   interval indicated in the Update Lease option applies to all resource
   records in the Update section of the Refresh request, except that if
   a KEY-LEASE interval is included as well, that interval applies to
   any KEY records included in the Update section.

5.2.  Requestor Behavior

   A requestor that intends that its records from a previous update,
   whether an initial update or a Refresh, remain active, MUST send a
   Refresh message before the lease elapses, or else the records will be
   removed by the server.

   In order to prevent records expiring, requestors MUST Refresh
   resource records after approximately 75% of the original lease has
   elapsed.  This interval MUST be adjusted randomly by +/-10% so that
   after a network restart, subsequent Renews do not remain synchronized
   across devices affected by the power failure.

   For Refresh messages, the server is expected to return an Update
   Lease option, if supported, just as with the initial update.  As with
   the initial update, the requestor MUST use the interval(s) specified
   by the server when determining when to send the next Refresh message.

   When sending Refresh messages, the requestor MUST include an Update
   Lease option, as it did for the initial Update.  The Update Lease
   option MAY either specify the same intervals as in the initial
   Update, or MAY use the values returned by the server in the previous
   Update, whether it was an initial Update or a Refresh.  As with
   Update responses, the requestor MUST use the intervals returned by
   the server in the response when determining when to send the next
   refresh message.

5.2.1.  Coalescing Refresh Messages

   If the requestor has performed multiple successful updates with a
   single server, the requestor MAY include Refreshes for all such
   updates to that server in a single message.  This effectively places
   all records for a requestor on the same expiration schedule, reducing
   network traffic due to Refreshes.

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   In doing so, the requestor includes in the Refresh message all
   existing updates to the server, including those not yet close to
   expiration, so long as at least one resource record in the message
   has elapsed at least 75% of its original lease.  If the requestor
   uses UDP, the requestor MUST NOT coalesce Refresh messages if doing
   so would cause truncation of the message; in this case, the requestor
   should either send multiple messages should use TCP to send the
   entire update at once.

   Requestors SHOULD NOT send a Refresh messages when all of the records
   in the Refresh have more than 50% of their lease interval remaining
   before expiry.  However, there may be cases where the requestor needs
   to send an early refresh, and it MAY do so.  For example, a power-
   constrained device may need to send an update when the radio is
   powered so as to avoid having to power it up later.

   Another case where this may be needed is if the lease interval
   registered with the server is no longer appropriate and the Requestor
   wishes to negotiate a different lease interval.  However, in this
   case, if the server does not honor the requested interval in its
   response, the requestor MUST NOT retry this negotiation.

5.3.  Server Behavior

   Upon receiving a valid Refresh Request, the server MUST send an
   acknowledgment.  This acknowledgment is identical to the Update
   Response format described in Section 4 "Update Message Format", and
   contains the new lease of the resource records being Refreshed.  The
   server MUST NOT increment the SOA serial number of a zone as the
   result of a Refresh.

   However, the server's state may not match what the client expects.
   In this case, a Refresh may actually appear to be an Update from the
   server's perspective.  In this case, if the Update changes the
   contents of the zone, the server MUST update the zone serial number.

6.  Retransmission Strategy

   The DNS protocol, including DNS updates, can operate over UDP or TCP.
   When using UDP, reliable transmission must be guaranteed by
   retransmitting if a DNS UDP message is not acknowledged in a
   reasonable interval.  Section 4.2.1 of [RFC1035] provides some
   guidance on this topic, as does Section 1 of [RFC1536].

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7.  Garbage Collection

   If the Update Lease of a resource record elapses without being
   refreshed, the server MUST NOT return the expired record in answers
   to queries.  The server MAY delete the record from its database.  The
   lease interval(s) returned by the server to the requestor are used in
   determining when the lease on a resource record has expired.

   For all resource records other than a KEY record included in an
   update, the Update Lease is the LEASE value in the Update Lease
   option.  For KEY records, if the optional KEY-LEASE value was
   included, this interval is used rather than the interval specified in
   LEASE.  If KEY-LEASE was not specified, the interval specified in
   LEASE is used.

8.  Security Considerations

   Section 8 of [RFC2136] describes problems that can occur around DNS
   updates.  Servers implementing this specification should follow these
   recommendations.

   Several additional issues can arise when relying on Update Lease.
   First, a too-long lease time is not much different than no lease
   time: the records associated with this lease time will effectively
   never be cleaned up.  Servers implementing Update Lease should have a
   default upper bound on the maximum acceptable value both for the
   LEASE and KEY-LEASE values sent by the client.  Servers MAY provide a
   way for the operator to change this upper limit.  We recommend that
   the default values for these limits should be 24 hours and 7 days,
   respectively.

   The second issue is that a too-short lease can result in increased
   server load and a failure to persist data subject to such a short
   lease.  Servers implementing Update Lease MUST have a default minimum
   lease interval that is acceptable.  We recommend a minimum of 30
   seconds for both the LEASE and KEY-LEASE intervals.

   There may be some cost associated with renewing leases.  A malicious
   (or buggy) client could renew at a high rate in order to overload the
   server more than it would be overloaded by query traffic.  This risk
   is present for regular DNS update as well.  Servers MUST have a
   minimum interval before which they will not accept a Refresh for a
   recently updated record.  Such messages MUST be silently ignored.

   Some authentication strategy should be used when accepting DNS
   updates.  Shared secret [RFC8945] or public key signing should be
   required.  Keys should have limited authority: compromise of a key
   should not result in compromise of the entire contents of one or more

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   zones managed by the server.  Key management strategy is out of scope
   for this document, however.  An example of a key management strategy
   can be found in [I-D.ietf-dnssd-srp], which uses "first come, first-
   served naming" rather than an explicit trust establishment process to
   conver update permission to a set of records.

9.  IANA Considerations

   The EDNS(0) OPTION CODE 2 has already been assigned for this DNS
   extension.  This document appears in the registry with the name 'UL'
   and the status 'On-hold,' and a document reference to an older
   version of this document.  When this document has been approved, the
   IANA is asked to update the registry as follows:

       OLD:
           Value: 2
           Name: UL
           Status: On-hold
           Reference: http://files.dns-sd.org/draft-sekar-dns-ul.txt

       NEW:
           Value: 2
           Name: Update Lease
           Status: Standard
           Reference: [this document]

10.  Acknowledgments

   Thanks to Marc Krochmal and Kiren Sekar for their work in 2006 on the
   precursor to this document.  Thanks also to Roger Pantos and Chris
   Sharp for their contributions.  Thanks to Chris Box, Esko Dijk,
   Jonathan Hui, Peter van Dijk, Abtin Keshvarzian, Nathan Dyck, Steve
   Hanna, Gabriel Montenegro, and Kangping Dong for their reviews of
   this document.

11.  Normative References

   [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>.

   [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>.

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   [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, DOI 10.17487/RFC2136, April 1997,
              <https://www.rfc-editor.org/info/rfc2136>.

   [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>.

   [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>.

12.  Informative References

   [RFC1536]  Kumar, A., Postel, J., Neuman, C., Danzig, P., and S.
              Miller, "Common DNS Implementation Errors and Suggested
              Fixes", RFC 1536, DOI 10.17487/RFC1536, October 1993,
              <https://www.rfc-editor.org/info/rfc1536>.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://www.rfc-editor.org/info/rfc6763>.

   [RFC8945]  Dupont, F., Morris, S., Vixie, P., Eastlake 3rd, D.,
              Gudmundsson, O., and B. Wellington, "Secret Key
              Transaction Authentication for DNS (TSIG)", STD 93,
              RFC 8945, DOI 10.17487/RFC8945, November 2020,
              <https://www.rfc-editor.org/info/rfc8945>.

   [I-D.ietf-dnssd-srp]
              Lemon, T. and S. Cheshire, "Service Registration Protocol
              for DNS-Based Service Discovery", Work in Progress,
              Internet-Draft, draft-ietf-dnssd-srp-18, 9 January 2023,
              <https://www.ietf.org/archive/id/draft-ietf-dnssd-srp-
              18.txt>.

Authors' Addresses

   Stuart Cheshire
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014
   United States of America
   Phone: +1 408 974 3207
   Email: cheshire@apple.com

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   Ted Lemon
   Apple Inc
   P.O. Box 958
   Brattleboro, Vermont 05302
   United States of America
   Email: mellon@fugue.com

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