Internet Systems Consortium

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This document specifies a method for slave DNS servers to honour the SOA EXPIRE field as if they were always transferring from the master, even when using other slaves to perform indirect transfers and refresh queries.

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

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 <TBA>. The EDNS EXPIRE option MAY included set 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 master server for the zone, the response will be a EDNS EXPIRE option of length 4 containing the current value of the SOA EXPIRE field, in seconds and network byte order.

When the query is directed to a slave 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 then there will be no EDNS EXPIRE option added to the response.

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

IANA is requested to assign a EDNS option code point (Registry Name: DNS EDNS0 Options) for the EDNS EXPIRE option specified in .

The EDNS EXPIRE option exposes how long the slaves have been out of communication with the master server. This is not believed to be a problem and may provide some benefit to monitoring systems.