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An EDNS(0) option to negotiate Leases on DNS Updates

Document Type Active Internet-Draft (dnssd WG)
Authors Stuart Cheshire , Ted Lemon
Last updated 2024-07-12 (Latest revision 2023-07-07)
Replaces draft-sekar-dns-ul
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
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Stream WG state Submitted to IESG for Publication
Document shepherd Chris Box
Shepherd write-up Show Last changed 2023-03-26
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Internet Engineering Task Force                              S. Cheshire
Internet-Draft                                                Apple Inc.
Intended status: Standards Track                                T. Lemon
Expires: 8 January 2024                                        Apple Inc
                                                             7 July 2023

          An EDNS(0) option to negotiate Leases on DNS Updates


   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

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at https://dnssd-
   lease.html.  Status information for this document may be found at

   Discussion of this document takes place on the DNSSD Working Group
   mailing list (, which is archived at  Subscribe at

   Source for this draft and an issue tracker can be found at

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
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   Drafts is at

   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."

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   This Internet-Draft will expire on 8 January 2024.

Copyright Notice

   Copyright (c) 2023 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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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

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.  Types of DNS Update Request messages  . . . . . . . . . .   4
     4.2.  Requestor Behavior  . . . . . . . . . . . . . . . . . . .   5
     4.3.  Server Behavior . . . . . . . . . . . . . . . . . . . . .   6
   5.  Refresh Messages  . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Refresh Message Format  . . . . . . . . . . . . . . . . .   6
     5.2.  Requestor Behavior  . . . . . . . . . . . . . . . . . . .   7
       5.2.1.  Coalescing Refresh Messages . . . . . . . . . . . . .   8
     5.3.  Server Behavior . . . . . . . . . . . . . . . . . . . . .   8
   6.  Retransmission Strategy . . . . . . . . . . . . . . . . . . .   9
   7.  Garbage Collection  . . . . . . . . . . . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  11
   12. Informative References  . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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

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   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

2.  Conventions and Terminology Used in this Document

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

   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

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   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.

   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.

   In the case of a KEY record and some other record, obviously the KEY
   LEASE applies to the key, and the LEASE applies to the other record.
   If more than one record that is not a KEY record is added by the
   update, the LEASE (not the KEY LEASE) is applied to all such records.
   Records that are removed are permanently removed.

4.1.  Types of DNS Update Request messages

   This document describes two types of updates: Registrations and
   Refreshes.  A Registration is a DNS Update Request that is intended
   to add information not already present on the DNS server.  A Refresh
   is intended simply to renew the lease on a previous Registration
   without changing anything.  Both messages are DNS Update messages, so
   the term "DNS Update message" is to specify behavior that is the same
   for both types of DNS Update message.

   In some cases it may be necessary to add new information without
   removing old information.  For the purpose of this document, such
   messages are referred to as Registrations, although in effect they
   may also refresh whatever information is unchanged from a previous

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4.2.  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
   1800 seconds (30 minutes).  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 SHOULD 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

   When sending a Registration, 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 Registration on startup: since Refreshes include
   a random factor as well, any synchronization that occurs after such
   an event should quickly randomize.

   Note: the requirement for 10ms granularity is a scheduling
   requirement intended to result in an even spread of requests, so that
   every request doesn't come an exact number of seconds after startup.
   This requirement should not be construed as requiring anything of the
   link layer on which the packet is transmitted: the link layer may
   well impose its own constraints on the timing at which a message is
   sent, and this document does not claim to override such constraints.

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   Note: the reason for the 3000ms (three second) random interval as
   opposed to some other random interval is to allow for enough time to
   meaningfully spread the load when many devices renew at once, without
   delaying so long that the delay in discovery of devices becomes
   obvious to an end user.  A 3-second random delay means that if there
   are for example 100 devices, and the random number generator spread
   is even, we would have one renewal every 30ms.  In practice, on
   relatively constrained devices acting as SRP servers, we are seeing
   the processing time for an SRP registration taking on the order of
   7ms, so this seems reasonable.

4.3.  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, but clients and servers may exist that do not
   support the newer 8-byte variant.  Therefore, clients and servers
   that do support this variant must account for the possibility that
   the server with which they are communicating does not.

   A client that receives a 4-byte variant from a server when it sent an
   8-byte variant MUST treat the 4-byte variant as specifying both the
   lease time and the key lease time.  A server that supports the 8-byte
   variant MUST treat the 4-byte variant as specifying both the lease
   time and the key lease time.  When a server receives a 4-byte
   variant, it MUST respond with a 4-byte variant.  In this case the key
   and the other records expire at the same time.

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 Registration 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

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   added again.

   The Refresh message SHOULD NOT include any update prerequisites that
   would fail if the requestor's previous Registration or Refresh is
   still in effect.  It also SHOULD NOT include prerequisites that would
   fail if the records affected by the previous Registration or Refresh
   are no longer present--that is, the Refresh should also work as a
   Registration.  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 Refresh fails.

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

   The Update Lease option in a Refresh message contains the desired new
   lease for Requests, and the actual granted lease for 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 a Registration 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 before they expire.  At the time of registration,
   the client computes an interval that is 80% of the lease time plus a
   random offset between 0 and 5% of the lease time.  The random offset
   is to prevent refreshes from being synchronized.  When this interval
   has expired, the client MUST refresh the message if the data in the
   initial Registration should continue to be advertised.

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

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   When sending Refresh messages, the requestor MUST include an Update
   Lease option, as it did for the initial Registration.  The Update
   Lease option MAY either specify the same intervals as in the initial
   Registration, or MAY use the values returned by the server in the
   previous Update Response, whether it was a response to a Registration
   a Refresh.  As with responses to Registrations, 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 Registrations with
   a single server, the requestor MAY include Refreshes for all such
   Registrations 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.

   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 or 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 (sleepy) 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 serial number of a zone as the result
   of a Refresh.

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   However, the server's state may not match what the client expects.
   In this case, a Refresh may actually appear to be a Registration,
   from the server's perspective.  If the Refresh 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].
   Section 3.1.3 of [RFC8085] also provides useful guidance that is
   particularly relevant to DNS.

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 a DNS
   Update request, 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

   Several additional issues can arise when relying on the Update Lease
   option.  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.
   Default values for these limits of 24 hours and 7 days, respectively,

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   The second issue is that a too-short lease can result in increased
   server load as requestors rapidly renew the lease.  A delay in
   renewing could result in the data being removed prematurely.  Servers
   implementing Update Lease MUST have a default minimum lease interval
   that avoids this issue.  We RECOMMEND a minimum of 30 seconds for
   both the LEASE and KEY-LEASE intervals.  However, in most cases, much
   longer lease times (for example, an hour) are RECOMMENDED.

   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.  The server MUST enforce a
   minimum interval between updates.  After a Refresh or Registration
   has been successfully processed and acknowledged, another Update of
   either type from the client during that interval MUST be silently
   ignored by the server.

   Some authentication strategy should be used when accepting DNS
   updates.  Shared secret [RFC8945] or public key signing (e.g.  SIG(0)
   [RFC2931]) 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 zones managed by the server.  Key management
   strategy is out of scope for this document.  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 confer update permission to a set of

9.  IANA Considerations

   The EDNS(0) OPTION CODE 2 has already been assigned for this DNS
   extension.  This document appears in the DNS EDNS0 Option Codes (OPT)
   registry [EDNS0Codes] 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:

           Value: 2
           Name: UL
           Status: On-hold

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

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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, Kangping Dong, and Tim Wicinski for their
   working group reviews of this document.  Thanks to David Dong, Olafur
   Gudmundsson, Brian Trammel, and Shivan Sahib for their directorate
   reviews and IANA reviews.

11.  Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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,

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

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,

   [RFC2931]  Eastlake 3rd, D., "DNS Request and Transaction Signatures
              ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September
              2000, <>.

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   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <>.

   [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,

              Lemon, T. and S. Cheshire, "Service Registration Protocol
              for DNS-Based Service Discovery", Work in Progress,
              Internet-Draft, draft-ietf-dnssd-srp-20, 29 May 2023,

              "DNS EDNS0 Option Codes (OPT)", April 2023,

Authors' Addresses

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

   Ted Lemon
   Apple Inc
   P.O. Box 958
   Brattleboro, Vermont 05302
   United States of America

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