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

Introduction Dynamic DNS Update 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 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 "Refresh Messages").

Conventions and Terminology Used in this Document 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 BCP 14 when, and only when, they appear in all capitals, as shown here.

Abbreviations

DNS-SD: DNS-based service discovery
EDNS(0): Extension Mechanism for DNS, version 0

Mechanisms The EDNS(0) Update Lease option is included in a standard DNS Update message within an EDNS(0) OPT pseudo-RR .

Update Message Format Dynamic DNS Update Leases Requests and Responses are formatted as standard DNS Dynamic Update messages . This update MUST include the EDNS(0) OPT RR, as described in . This OPT RR MUST include an EDNS(0) Option as shown below. Note that if a TSIG resource record () 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:

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 to reserve a name (or names) when the service is not present.

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 30 minutes (1800 seconds). 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 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. When sending an initial update, 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 update on startup: since renews include a random factor as well, any synchronization that occurs after such an event should quickly normalize.

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. 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.

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.

Refresh Message Format Refresh messages are formatted like Dynamic Update Leases Requests and Responses (see "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 fail if the requestor's previous update or Refresh is still in effect. It also SHOULD NOT include prerequisites that would fail if the records affected the previous update or Refresh are no longer present--that is, the Refresh should also work as an initial update. 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 update fails. 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.

Requestor Behavior 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. In order to prevent records expiring, requestors MUST Refresh resource records after approximately 75% of the original lease has elapsed. This interval MUST be adjusted randomly by +/-10% so that after a network restart, subsequent Renews do not remain synchronized across devices affected by the power failure. 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.

Coalescing Refresh Messages 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, the requestor should either send multiple messages 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 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.

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 "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.

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. provides some guidance on this topic, as does .

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 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.

describes problems that can occur around DNS updates. Servers implementing this specification should follow these recommendations. Several additional issues can arise when relying on Update Lease. 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. We recommend that the default values for these limits should be 24 hours and 7 days, respectively. The second issue is that a too-short lease can result in increased server load and a failure to persist data subject to such a short lease. Servers implementing Update Lease MUST have a default minimum lease interval that is acceptable. We recommend a minimum of 30 seconds for both the LEASE and KEY-LEASE intervals. 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. Servers MUST have a minimum interval before which they will not accept a Refresh for a recently updated record. Such messages MUST be silently ignored. Some authentication strategy should be used when accepting DNS updates. Shared secret or public key signing 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, however. An example of a key management strategy can be found in , which uses "first come, first-served naming" rather than an explicit trust establishment process to conver update permission to a set of records.

IANA Considerations 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 as follows: OLD: Value: 2 Name: UL Status: On-hold Reference: http://files.dns-sd.org/draft-sekar-dns-ul.txt NEW: Value: 2 Name: Update Lease Status: Standard Reference: [this document]

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, and Kangping Dong for their reviews of this document.