What does "deleted from unreachable cache" mean?

Cathy Almond cathya at isc.org
Thu Aug 2 14:26:08 UTC 2012

On 19/07/12 00:49, Peter Olsson wrote:
> Hello!
> After my latest bind upgrade our slave server started
> occasionally writing these messages to the log:
> master 2a02:xxxx:xxxx:xxxx::2#53 (source ::#0) deleted from unreachable cache
> master 62.xxx.xxx.2#53 (source deleted from unreachable cache
> DNS seems to work fine anyway, and all zonefiles in the slave
> seem to update like they should, so everything seems ok. But I
> would like to be certain that there is nothing to worry about,
> so I wonder what these messages mean. (I didn't find anything
> interesting in the list archives or in Google.)
> Both master and slave are FreeBSD, running port bind97-
> Thanks!

There'll be a new KB FAQ published on this early next week
(https://kb.isc.org/article/AA-00765).  Preview is that it will say
something like this:

What does named log message "deleted from unreachable cache" mean?

An example of the messages being logged is:

02-Aug-2012 07:58:20.601 general: info: master (source deleted from unreachable cache

BIND maintains a cache of unreachable masters to which it refers when
handling a zone refresh. If a zone refresh fails with a specific master
(either during the query for the SOA or after querying and while
attempting a subsequent zone transfer), then this master is cached as
'unreachable' for 10 minutes.

As of versions 9.6-ESV-R6, 9.7.5, 9.8.2 and 9.9.0 onwards, the change
below implements an earlier removal of a master server from the
unreachable cache if a notify is received from it.

Note that receipt of a notify (which is a UDP packet travelling from
master to slave) doesn't guarantee that the master will be reachable
from the slave, but it does ensure quicker recovery in the situation
where a master was temporarily unavailable, for example for a reboot.

This is the relevant info from the Release Notes:

Master servers that had previously been marked as unreachable because of
failed zone transfer attempts will now be removed from the "unreachable"
list (i.e. considered reachable again) if the slave receives a NOTIFY
message from them. [RT #25960]

In the CHANGES file, it is described thus:
3204. [bug]   When a master server that has been marked as
unreachable sends a NOTIFY, mark it reachable again. [RT #25960]

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