Extension Mechanisms for DNS (EDNS) EXPIRE Option

The expire field of a DNS zone's SOA record is supposed to indicate when a secondary server shall discard the contents of the zone when it has been unable to contact the primary . Current practice only works when all the secondaries contact the primary directly to perform refresh queries and zone transfers. While secondaries are expected to be able to, and often are configured to, transfer from other secondaries for robustness reasons as well as reachability constraints, there is no mechanism provided to preserve the expiry behaviour when using a secondary. Instead, secondaries have to know whether they are talking directly to the primary or another secondary and use that to decide whether or not to update the expire timer. This, however, fails to take into account delays in transferring from one secondary to another. There are also zone-transfer graphs in which the secondary never talks to the primary, so the effective expiry period becomes multiplied by the length of the zone-transfer graph, which is infinite when it contains loops. This document provides a mechanism to preserve the expiry behaviour regardless of what zone-transfer graph is constructed and whether the secondary is talking to the primary or another secondary.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .

The EDNS EXPIRE option has the value <9>. The EDNS EXPIRE option MAY be included on any QUERY, though usually this is only done on SOA, AXFR, and IXFR queries involved in zone maintenance. This is done by adding a zero-length EDNS EXPIRE option to the options field of the OPT record when the query is made.

When the query is directed to the primary server for the zone, the response will be an EDNS EXPIRE option of length 4 containing the value of the SOA EXPIRE field, in seconds and network byte order.

When the query is directed to a secondary server for the zone, then the response will be an EDNS EXPIRE option of length 4 containing the value of the expire timer on that server, in seconds and network byte order.

If an EDNS EXPIRE option is sent to a server that is not authoritative for the zone, it MUST NOT add an EDNS EXPIRE option to the response.

When a secondary server performs a zone-transfer request or a zone-refresh query, it SHALL add an EDNS EXPIRE option to the query message. If a secondary receives an EDNS EXPIRE option in a response to an SOA query, it SHALL update its expire timer to be the maximum of the value returned in the EDNS EXPIRE option and the current timer value. Similarly, if a secondary receives an EDNS EXPIRE option in its response to an IXFR query that indicated the secondary is up to date (serial matches current serial), the secondary SHALL update the expire timer to be the maximum of the value returned in the EDNS EXPIRE option and the current timer value. If the zone is transferred or updated as the result of an AXFR or IXFR query and there is an EDNS EXPIRE option with the response, then the value of the EDNS EXPIRE option SHOULD be used instead of the value of the SOA EXPIRE field to initialise the expire timer. In all cases, if the value of the SOA EXPIRE field is less than the value of the EDNS EXPIRE option, then the value of the SOA EXPIRE field MUST be used and MUST be treated as a maximum when updating or initialising the expire timer.

IANA has assigned an EDNS option code point for the EDNS EXPIRE option specified in with "Optional" status in the "DNS EDNS0 Option Codes (OPT)" registry.

The method described in this document ensures that servers that no longer have a connection to the primary server, direct or indirectly, cease serving the zone content when SOA EXPIRE timer is reached. This prevents stale data from being served indefinitely. The EDNS EXPIRE option exposes how long the secondaries have been out of communication with the primary server. This is not believed to be a problem and may provide some benefit to monitoring systems.