Maintained by: NLnet Labs

[Unbound-users] About the prefetch switch used in the unbound.conf.

Hongyi Zhao
Mon Feb 23 03:21:50 CET 2015

Hi all,

Currently, I use the latest release of unbound 1.5.2 compilled by myself on
the Debian wheezy.  I have some confusion of the using the the "prefetch"
option in the unbound.conf file.   Let me described it as follows:

1- From the manual of unbound.conf, i.e., `man unbound.conf', I can get the
    notes on this option:

        prefetch: <yes or no>
              If yes, message cache elements are prefetched before they
              to  keep  the  cache  up to date.  Default is no.  Turning it
              gives about 10 percent more traffic and load on the machine,
              popular items do not expire from the cache.

2- From the following webpage:,
    I can get the explanation on this option as follows:

  # perform prefetching of close to expired message cache entries.  If a client
  # requests the dns lookup and the TTL of the cached hostname is going to
  # expire in less than 10% of its TTL, unbound will (1st) return the ip of the
  # host to the client and (2nd) pre-fetch the dns request from the remote dns
  # server. This method has been shown to increase the amount of cached hits by
  # local clients by 10% on average.
   prefetch: yes

As you can see, the above two explanations on the mechanism of
prefetching in unbound are not so

well the same.  Could someone please give me some hints on the this thing?


Hongyi Zhao <hongyi.zhao at>
Xinjiang Technical Institute of Physics and Chemistry
Chinese Academy of Sciences
GnuPG DSA: 0xD108493
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <>