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

Hongyi Zhao hongyi.zhao at gmail.com
Mon Feb 23 04:39:43 UTC 2015

Thanks a lot.  Got it.


2015-02-23 11:51 GMT+08:00 Dave Warren <davew at hireahit.com>:

> On 2015-02-22 18:21, Hongyi Zhao wrote:
>> 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 following
>>     notes on this option:
>>         prefetch: <yes or no>
>>               If yes, message cache elements are prefetched before they
>> expire
>>               to  keep  the  cache  up to date.  Default is no.  Turning
>> it on
>>               gives about 10 percent more traffic and load on the
>> machine, but
>>               popular items do not expire from the cache.
>> 2- From the following webpage: https://calomel.org/unbound_dns.html,
>>     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?
> To be completely honest, I don't see a conflict between the two
> explanations.
> The description in the man unbound.conf is less comprehensive as it only
> mentions that this feature keeps the cache up to date, whereas the
> description from the webpage quoted explains the mechanism that is used to
> determine which records need to be prefetched.
> If you're unclear, just read #2 above, it's more specific and therefore
> probably more useful to understand how the feature actually works, but be
> aware that it does increase the outbound queries slightly since, from an
> external perspective, it drops all TTLs about 10%, while internally it
> respects TTLs in a consistent and reliable fashion. I found this feature
> made a significant difference when we relocated and were stuck on the end
> of a high latency 3Mb line for some months, but it makes little
> noticeable-to-end-user difference now that we're back on a high bandwidth,
> low latency connection.
> --
> Dave Warren
> http://www.hireahit.com/
> http://ca.linkedin.com/in/davejwarren
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Hongyi Zhao <hongyi.zhao at gmail.com>
Xinjiang Technical Institute of Physics and Chemistry
Chinese Academy of Sciences
GnuPG DSA: 0xD108493
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