[dns-operations] Increasing the number of search path entries in a stub resolver

Florian Weimer fweimer at redhat.com
Wed May 4 08:05:25 UTC 2016

On 05/04/2016 09:39 AM, Nico CARTRON wrote:
> Hi Florian,
>> On 04 May 2016, at 09:05, Florian Weimer <fweimer at redhat.com> wrote:
>> Our stub resolver currently has a hard limit for 6 search domains that can be specified in /etc/resolv.conf.  We are considering lifting that limit.  Apparently, this is desirable for deployments which migrate from NIS host name lookups to DNS because NIS supports a larger number of default domain names (or something equivalent to that).
>>  From a larger ecosystem perspective, do you think that longer search paths would increase resolver load in an unacceptable way?  For example, host name lookups with a 10-element search path could easily require 20 queries.
> How many search domains are you thinking of?
> 20? 50? No limit?

No limit.  We cannot simply change a constant to increase the limit, so 
removing the limit altogether is no extra effort.

> I've seen Windows clients with long lists (15+); while this didn't really affect resolution times seriously, there was a collateral effect: the cache was much bigger with of course a lot of NXDOMAIN for all the tested search domains.

Interesting observation.  We will likely recommend to run a local stub 
resolver to reduce load somewhat, but the cache size impact will still 
be there.


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