EDNS EXPIRE OPTION
Internet Systems Consortium
950 Charter Street
Redwood City
CA
94063
US
marka@isc.org
This document specifies a method for secondary DNS servers to
honour the SOA EXPIRE field as if they were always transferring
from the primary, even when using other secondaries to perform
indirect transfers and refresh queries.
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 was no
mechanism provided to preserve the expiry behaviour when
using a secondary. Secondaries instead have to know whether
they were talking directly to the primary or another
secondary, 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 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
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 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 <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 primary server for the
zone, the response will be a 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 performs 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 a 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 which
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 EXPIRE EDNS 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
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 with "Optional" status.
This 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 prevent stale
data 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.
DOMAIN NAMES - CONCEPTS AND FACILITIES
DOMAIN NAMES - IMPLEMENTATION AND SPECIFICATION
Key words for use in RFCs to Indicate Requirement Levels
Extension Mechanisms for DNS (EDNS(0))