EDNS EXPIRE OPTION
Internet Systems Consortium
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Mark_Andrews@isc.org
Provide 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
expiry 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 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
or 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 <TBD>. The EDNS EXPIRE option MAY be 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
network 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 the EXPIRE
timer, in network 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 its 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 a AXFR or
IXFR query and there is a 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.
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.
DOMAIN NAMES - CONCEPTS AND FACILITIES
DOMAIN NAMES - IMPLEMENTATION AND SPECIFICATION
Key words for use in RFCs to Indicate Requirement Levels
Extension Mechanisms for DNS (EDNS0)