What does "deleted from unreachable cache" mean?

Peter Olsson pol at leissner.se
Thu Aug 2 17:25:27 UTC 2012

On Thu, Aug 02, 2012 at 03:26:08PM +0100, Cathy Almond wrote:
> 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]
Excellent information, thanks!
However, it is worrying that the master sometimes is unreachable.
Is there some way I can make the slave server log, with timestamp,
what zone it was trying to refresh when it failed?


Peter Olsson                    pol at leissner.se

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