Internet Area P. Vixie
DNSIND Working Group
<draft-ietf-dnsind-notify-00.txt> 30 November 1994
Notify: a mechanism for prompt notification of authority zone changes
Status of this Memo
This document is an Internet-Draft. 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-Drafts.
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.'
To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet- Drafts
Shadow Directories on ds.internic.net (US East Coast), nic.nordu.net
(Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific
Rim).
Abstract
This draft describes a new DNS opcode, NOTIFY, by which a master
server advises a set of slave servers that the master's data has been
changed and that a query should be initiated to discover the new
data.
0. Rationale and Scope
Slow propagation of new and changed data in a DNS zone can be due to
a zone's relatively long refresh times. Longer refresh times are
beneficial in that they reduce load on the master servers, but that
benefit comes at the cost of new data not becoming available to DNS
clients until a sometimes inconvenient interval has elapsed.
The Notify DNS message allows master servers to inform slave servers
when data have changed, an interrupt as opposed to poll model, which
it is hoped will reduce propagation delay while not unduly increasing
the masters' load.
This document defines a new DNS opcode called NOTIFY whose numeric
value is four (4). All fields not otherwise specified must contain
binary zero, and implementations are free to ignore any request or
Paul Vixie [Page 1]
<draft-vixie-dns-notify-00.txt>DNS NOTIFY 30 November 1994
response packets where this is not the case. The intent of this
requirement is to permit future use specifications to be backward
compatible with implementations conforming only to the initial use
specification.
This document intentionally gives more definition to the roles of DNS
master and slave servers, their enumeration in NS RRs, and the SOA
MNAME field. In that sense, this document can be considered an
addendum to RFC 1035.
1. NOTIFY Message
When a master has updated one or more RRs in which slave servers may
be interested, the master may send the changed RR's name and type to
each known slave server using a best efforts protocol based on the
NOTIFY opcode.
One section (query) and one RR type (SOA) are defined at this time.
Use of other sections or of other RR types is not defined by this
document, and implementations are free to ignore packets containing
such values unless they also implement some future specification(s)
which specify their use.
NOTIFY is similar to QUERY in that it has an initiator packet with QR
``set'' and a response packet with QR ``clear''. The response packet
contains no useful information, but its reception by the master is a
hint that the slave has received the NOTIFY and that the master can
remove the slave from any retry queue for this NOTIFY event.
A master repeatedly sends NOTIFY to a slave until either too many
copies have been sent (a ``timeout'') or until a NOTIFY-QR is
received from the slave with a matching query ID, QNAME, and IP
source address. The interval between retransmissions, and the total
number of retransmissions, should be operational parameters
specifiable by the name server administrator, perhaps on a per-zone
basis. Reasonable defaults are a 60 second interval and 5 attempts.
It is also reasonable to use additive or exponential backoff for the
retry interval.
A NOTIFY packet has QCOUNT>0, ANCOUNT=AUCOUNT=ADCOUNT=0. In the
future, it is expected that this specification will be amended such
that ANCOUNT, AUCOUNT, and/or ADCOUNT may be allowed to be nonzero,
to indicate that the new data is contained within the NOTIFY packet,
possibly along with authentication data to validate this update. For
now, NOTIFY is merely a hint that the slave server should query the
master for a new copy of the RR(s) specified in the query section.
As a result of this query, the slave server may decide to take some
action, such as initiating a zone transfer.
Paul Vixie [Page 2]
<draft-vixie-dns-notify-00.txt>DNS NOTIFY 30 November 1994
Note that there is at this time no specification for incremental
updates; the slave servers are NOT free to overlay a previous AXFR's
data with data from a QUERY, even if that QUERY occurred as a result
of a NOTIFY and the response to the QUERY is authoritative. NOTIFY
may be the basis on which incremental updates are specified, but at
this time it is only an ``update hint.''
If a slave receives a NOTIFY request from a host which is not listed
in the slave's static list of masters for the zone containing the
QNAME, it must ignore the request and may log an error in its
operations log. This is necessary for security reasons.
Implementations are also free to ignore NOTIFY requests that come
from a UDP port other than the DNS port (53), as these are by
definition not from another name server.
The only useful hint at this time is that the SOA RR has changed.
Upon receipt of a NOTIFY hint for QTYPE=SOA, the slave should behave
as though the zone given in the QNAME had reached its REFRESH
interval, i.e., it should query the master that sent the NOTIFY
request, asking for the same QTYPE and QNAME as were given in the
NOTIFY request. If an answer comes, and the SOA RR has a newer
serial number than the slave's current copy of the zone, then a zone
transfer should be initiated.
2. Some Definitions and Two Requirements
Definition: a Master Server is any authoritative server configured
to be the source of AXFR from one or more slave servers. It is
named in an NS RR for the zone.
Definition: a Slave Server is a possibly-authoritative server which
uses AXFR to retrieve the zone. All slave servers are named in
the NS RRs for the zone. Slaves which use AXFR to retrieve a
zone, and which respect the SOA timeouts, but which are not
listed in the zone's NS RR set, are ``unregistered''.
Unregistered slaves are sometimes used to hot-wire a cache, and
are outside the scope of the DNS prototols, but NOTIFY defines
optional support for them. Note that a server is not, in the
strict RFC 1035 sense of the term, ``authoritative'' for a zones
it loads via AXFR, unless it is listed in the zone's NS RR set.
The question of whether such servers should set the AA bit on
responses they generate from such data, remains open.
Definition: the Primary Master Server is the master server at the
root of the AXFR dependency graph. The primary master is named
in the zone's SOA MNAME field and by an NS RR. The source of
the primary master's zone data is external to the DNS and is not
a formal concern of this document.
Paul Vixie [Page 3]
<draft-vixie-dns-notify-00.txt>DNS NOTIFY 30 November 1994
Requirement: for a zone to make use of the NOTIFY protocol, its
servers must be organized into a dependency graph such that
there is a primary master, and all other servers must use AXFR
either from the primary master or from some slave which is also
a master. A slave which is also a master is referred to later
in this document as a ``slave-master''. No loops are permitted
in the AXFR dependency graph.
Requirement: for a zone to make use of the NOTIFY protocol, all
servers named in the zone's NS RR set (under the delegation
point) must use AXFR to do zone updates, or, if some other
protocol is used (e.g., FTP or NFS), it must simulate the retry
and refresh semantics of SOA/AXFR.
3. Semantic Details
Master servers should maintain a list of slaves which have queried
the SOA of the zone within the last SOA REFRESH interval. On a best
efforts basis, NOTIFY requests should be sent to each slave server
address whose last successful query for the changed RR's name and
type was within that interval. Note that queries from UDP ports
other than the DNS service port (53) are not subject to this
requirement, since they cannot (by definition) be from other name
servers.
In a deep tree where some slaves AXFR new zones from other slaves, it
can happen that some slaves will receive multiple NOTIFYs of the same
RR change: one from the primary master, and one from each slave-
master from which it has requested this RR's name and type within the
last SOA REFRESH interval. The protocol supports this multiplicity
by requiring that NOTIFY be sent by a slave-master only AFTER it has
updated the RR. With an SOA NOTIFY, the RR can only change after a
subsequent AXFR. Thus, barring delivery reordering, the last NOTIFY
any slave receives will be the one indicating the latest change.
Since a slave always requests SOAs and AXFRs only from its locally
designated masters, it will have an opportunity to retry its SOA
query after its masters have completed each zone update.
A zone transfer resulting from an SOA NOTIFY should be deferred for a
random period of time so that a large number of slaves will not
simultaneously request a zone transfer when the serial number
changes. It is reasonable that the delay will occur before the slave
even bothers to ask for the new SOA, but it is not reasonable for the
master to insert the delay before sending the SOA NOTIFY to any
slaves. The delay should be chosen to be between 10 and 60 seconds,
unless these limits are overridden by the name server administrator.
The rest of this section is concerned only with SOA NOTIFY.
Paul Vixie [Page 4]
<draft-vixie-dns-notify-00.txt>DNS NOTIFY 30 November 1994
3.a. Zone has Updated on Primary Master
Primary master sends a NOTIFY request to all servers named in the NS
RR, except the one that is also named in the SOA MNAME, and
optionally to all name servers which have queried for this SOA within
the last SOA REFRESH interval. The NOTIFY has the following
characteristics:
query ID: (new)
op: NOTIFY
resp: NOERROR
flags: AA
qcount: 1
qname: (zone name)
qclass: C_IN
qtype: T_SOA
ancount, aucount, adcount: 0
Note that setting any flag other than AA should cause slave servers
to ignore this query. Only AA is defined, the others all must
contain binary zero.
3.a.1. Zone has Updated on Slave-Master
As above in 3.a, except that only those authoritative name servers
(i.e., those listed in the zone's NS RR set) which have queried for
this name and type within the SOA REFRESH interval need to be
notified. Optionally, the slave-master may send to all servers which
have sent such recent queries, without regard to whether they are
listed in the zone's NS RR set.
3.b. Slave Receives a NOTIFY Packet from a Master
When a slave server receives a NOTIFY request from one of its locally
designated masters for the zone enclosing the given QNAME, with
QTYPE=SOA and !QR, it should enter the state it would if the zone's
refresh timer had expired. It will also send a NOTIFY response back
to the NOTIFY request's source, with the following characteristics:
query ID: (same)
op: NOTIFY
resp: NOERROR
flags: QR AA
qcount: 1
qname: (zone name)
qclass: C_IN
qtype: T_SOA
ancount, aucount, adcount: 0
Paul Vixie [Page 5]
<draft-vixie-dns-notify-00.txt>DNS NOTIFY 30 November 1994
Note that this is intentionally identical to the NOTIFY request,
except that the QR bit is also set. Note, also, that the query ID
must be the same as was received in the NOTIFY request.
3.c. Master Receives a NOTIFY-QR Packet from Slave
When a master server receives a NOTIFY packet (with QR), it deletes
this query from the retry queue, thus completing the ``notification
process'' of ``this'' RR change to ``that'' server.
Security Considerations
DNS security is being considered overall by the DNSSEC working group.
We believe that the NOTIFY operation's only security considerations
are (A) that a previous SOA query can optionally cause a master to
NOTIFY a false slave, and (B) that a NOTIFY request with a forged
IP/UDP source address can cause a slave to send spurious SOA queries
to its masters, leading to the possibility of a benign denial of
service attack if the forged requests are received very often.
Author's Address
Paul Vixie
Vixie Enterprises
Star Route Box 159A
Woodside, CA 94062
Phone: +1 415 747 0204
E-Mail: paul@vix.com
Paul Vixie [Page 6]
