Internet Area P. Vixie DNSIND Working Group 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] 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] 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] 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] 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] 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]