| Internet-Draft | Dynamic DNS Update Leases | October 2022 |
| Cheshire & Lemon | Expires 10 April 2023 | [Page] |
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¶
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 working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
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."¶
This Internet-Draft will expire on 10 April 2023.¶
Copyright (c) 2022 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 (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
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").¶
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 "Key words for use in RFCs to Indicate Requirement Levels", when, and only when, they appear in all capitals, as shown here [RFC2119] [RFC8174].¶
The EDNS(0) Update Lease option is included in a standard DNS Update message [RFC2136] within an EDNS(0) OPT pseudo-RR [RFC6891].¶
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 ([RFC2845]) 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
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.¶
DNS Update requestors SHOULD 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 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.¶
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.¶
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 updates records from being garbage collected.¶
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, if the state produced by the previous update or Refresh is still in effect, fail. The update SHOULD NOT be constructed to fail in the case that the state produced by the previous update or Refresh has for some reason been garbage collected.¶
An update message that changes the server state resulting from a previous Refresh or update is an update, not a Refresh.¶
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.¶
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.¶
Requestors SHOULD Refresh resource records after 75% of the original lease has elapsed. If the requestor uses UDP and does not receive a response from the server, the requestor SHOULD retry after 2 seconds. The requestor SHOULD continue to retry, doubling the length of time between each retry, or retry using TCP.¶
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.¶
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.¶
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, either multiple messages or TCP SHOULD be used.¶
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.¶
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.¶
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.¶
When DNS Update is enabled on an authoritative server, the Security Considerations of that specification [RFC2136] should be considered.¶
The addition of a record lifetime to facilitate automated garbage collection does not itself add any significant new security concerns.¶
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, retaining the value '2', changing the name 'UL' to 'Update Lease', changing the status to 'Standard' and changing the reference to refer to the final version of this document published by the RFC Editor.¶
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.¶