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DNSIND Working Group                          Brian Wellington (TISLabs)
INTERNET-DRAFT                              Olafur Gudmundsson (TISLabs)
                                                              April 1999


Updates: RFC 2136

      Deferred Dynamic Domain Name System (DNS) Delete Operations

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   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.''

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at


   This document proposes a mechanism for notifying a dynamic DNS server
   that a delete operation should be performed at a certain point in the
   future.  This works within the framework of the current DNS dynamic
   update protocol, and provides needed functionality for clients with
   leased dynamic addresses.

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1 - Introduction

Dynamic update operations for the Domain Name System [RFC1034, RFC1035]
are defined in [RFC2136], but there is no automated method of specifying
that records should have a fixed lifetime, or lease.

1.1 - Overview of DNS Dynamic Update

DNS dynamic update defines a new DNS opcode and a new interpretation of
the DNS message if that opcode is used.  An update can specify
insertions or deletions of data, along with prerequisites necessary for
the updates to occur.  All tests and changes for a DNS update request
are restricted to a single zone, and are performed at the primary server
for the zone.  The primary server for a dynamic zone must increment the
zone SOA serial number when an update occurs or before the next
retrieval of the SOA.

1.2 - Overview of DHCP leases

DHCP [RFC2131] provides a means for a host to obtain a network address
from a DHCP server.  The server may ``lease'' this address to the host,
meaning that it is valid only for the period of time specified in the
lease.  The host may may extend its lease with subsequent requests, or
may issue a message to release the address back to the server when it is
no longer needed.

2 - Background

When a host receives dynamic addresses with associated dynamic DNS
records, the records can be updated by either the host or the DHCP
server.  In many cases, update by the server is recommended, since the
server maintains lease information for each address.  In some cases,
though, the server cannot update some or all of the DNS records.  This
happens when the DNS and DHCP server are under different administration,
for example.

A host can easily update its own DNS records when receiving information
from the DHCP server.  It can also delete its records when shutting
down.  If the host unexpectedly goes down, though, it cannot delete the
records.  When the DHCP lease on the address expires and is not renewed,
the DHCP server may reassign the address.  The DNS records now point to
an assigned address, but not the correct address.  Until the host
updates its records again, DNS will contain bad information.

Since the DHCP and DNS servers are often not co-located with the
clients, the possibility of a host unexpectedly going down and not
communicating with the servers is non-trivial.

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If the host could set a lease on the DNS records similar to that on its
address, the DNS records would lose validity at the same time as the
address.  This would prevent bad information from remaining in DNS.  DNS
has no such provision for leases, though, since this would require
storing a lease time along with each record (or each record in a dynamic

An alternative method is suggested.  A ``delete'' update is sent along
with the ``add'' update, but the delete is marked in such a way that it
will not be exectuted immediately.  Instead, it will be stored for the
specified amount of time before being applied.  If the host wishes to
extend or shorten the lifetime of the DNS record(s), it can replace the
``deferred delete'' record, which will reset the lease time of the
record(s).  The ``deferred delete'' record would, of course, also be
removed if a normal delete update was received.

3 - Protocol changes

When doing a delete update operation as defined in [RFC2136] (deleting
an RR, an RRset, or all RRset from a name), the TTL field MUST be
specified as 0.  An [RFC2136] compliant server will silently ignore (*)
an update record with a non-zero TTL.  This document overloads the TTL
field.  If TTL is non-zero, the value represents the number of seconds
(a 32 bit unsigned integer) before which the delete will be applied to
the zone.  Thus, the delete operation will be deferred for that number
of seconds, where the number of seconds indicates the lease time.  A 32
bit integer provides for a lease time of over 136 years, which should be
long enough for most uses.

3.1 - Storage and execution

Deferred delete records are stored, persistently, by the name server.
The name server SHOULD attempt to evaluate the deletes in a timely
manner.  If multiple deferred deletes are sent in the same DNS message
with the same TTL value, they MUST be processed atomically if processed
as planned (that is, none of the deferred deletes are updated or

3.2 - Processing of deferred deletes

When a deferred delete is received, the server must check to see if it
matches an existing deferred delete records, where matching indicates
the same name, type, class, and rdata.  If a match is found, the new
deferred delete MUST replace the old one.  If the deferred delete does
not refer to any record in the server, it should fail as a normal delete

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3.3 - Processing of normal deletes

When a normal delete is received and accepted, the server SHOULD purge
any matching deferred delete records.

3.4 - Processing of cancellations

The value 0xFFFFFFFF (the largest unsigned 32 bit integer) in the TTL
field has a special meaning.  If a delete containing this lease time is
received, the server will unconditionally remove any matching deferred
deletes.  If no deferred delete matches, this request will be silently

3.5 - Processing of adds

When data is added through a dynamic update which matches a deferred
delete, there is no additional processing done.

4 - TTL handling

Any record that may be deleted SHOULD have a short TTL compared to its
lease time, to prevent deleted data from being cached past its

When the time until an RR is deleted becomes low enough, the server MAY
modify the TTL of the RRset.  Whenever the TTL is automatically reduced
by this process, the zone will be considered ``changed'' for the purpose
of automatic SOA SERIAL increment (as in [RFC2136]) and real time zone
slave notification [RFC1996].  As these operations can potentially be
expensive (more so if DNSSEC [RFC2535] signatures must be regenerated),
the specific limits and effects are left to the implementation.

If the TTL is modified by the server, it is not reset if the lease is
renewed.  Therefore, the original RR SHOULD be sent with the lease
renewal if the client expects that the server has modified the TTL.

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5 - Usage

Normally, a deferred delete update will initially be sent along with an
add, although this is not required.  Further updates to the deferred
delete may be sent independently, although the add should be sent again.
If the deferred delete is associated with a leased address, the lease
time of the update SHOULD be approximately equal to the lease time of
the address.

6 - Protocol robustness

This protocol has no inherent protection against replayed messages,
which can either originate from an attack or faulty hardware.  To
prevent this problem, prerequisites should be used in the update
message, such as a test for the existence of a TXT record describing the
lease, which would be added along with the other records (see [RFC2136],
section 5).

7 - Security considerations

This addition to the dynamic DNS protocol does not affect the security
of the protocol.  If security is desired, TSIG [TSIG] and/or DNSSEC
[RFC2535] authentication should be used, as specified in [simple-update]
or [RFC2137, update2].  The authors strongly recommend using security
along with this protocol.

If a DNSSEC signed-zone is modified with deferred deletes, the server
must resign any affected records when the delete is executed.  No
special processing is required when the delete is received.

8 - IANA Considerations


9 - References

[RFC1034]  P. Mockapetris, ``Domain Names - Concepts and Facilities,''
           RFC 1034, ISI, November 1987.

[RFC1035]  P. Mockapetris, ``Domain Names - Implementation and
           Specification,'' RFC 1035, ISI, November 1987.

[RFC1996]  P. Vixie ``A Mechanism for Prompt Notification of Zone
           Changes (DNS NOTIFY),'' RFC 1996, ISC, August 1996.

[RFC2136]  P. Vixie (Ed.), S. Thomson, Y. Rekhter, J. Bound ``Dynamic
           Updates in the Domain Name System,'' RFC 2136, ISC & Bellcore
           & Cisco & DEC, April 1997.

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[RFC2137]  D. Eastlake ``Secure Domain Name System Dynamic Update,'' RFC
           2137, CyberCash, April 1997.

[RFC2535]  D. Eastlake ``Domain Name System Security Extensions,'' RFC
           2535, IBM, March 1999.

[TSIG]     P. Vixie (ed), O. Gudmundsson, D. Eastlake, B. Wellington
           ``Secret Key Transaction Signatures for DNS (TSIG),'' draft-
           ietf-dnsind-tsig-08.txt, ISC & TISLabs & IBM & TISLabs,
           February 1999.

           B. Wellington ``Simple Secure Domain Name System (DNS)
           Dynamic Update,'' draft-ietf-dnssec-simple-update-00.txt,
           TISLabs, November 1998.

[update2]  D. Eastlake ``Secure Domain Name System (DNS) Dynamic
           Update,'' draft-ietf-dnssec-update2-00.txt, Transfinite
           Systems Company, August 1998.

8 - Author's Address

   Brian Wellington                          Olafur Gudmundsson
       TISLabs at Network Associates             TISLabs at Network Associates
       3060 Washington Road, Route 97            3060 Washington Road, Route 97
       Glenwood, MD 21738                      Glenwood, MD 21738
       +1 443 259 2369                           +1 443 259 2389
       <bwelling@tislabs.com>                    <ogud@tislabs.com>

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