Need of caching on bind server

Mukund Sivaraman muks at
Thu Aug 25 05:36:52 UTC 2016

Hi Harshith

On Thu, Aug 25, 2016 at 04:47:03AM +0000, Harshith Mulky wrote:
> Hello,
> I am trying to understand why caching is required on the bind server,
> when the client receiving the responses would be caching based on TTL
> values.
> So,
> Is caching required on the server, if the client is not able to cache
> such responses? Isn't it a overhead on both the client and server
> systems to cache the same responses at respective ends

Applications usually do not cache DNS records. They usually *use* DNS
data such as addresses immediately and discard the data afterwards. Of
course, there are exceptions where applications do cache DNS data but
this is not the norm.

A DNS resolver is also a DNS client that resolves answers to questions
and caches them. It is a server to a different kind of DNS client called
a stub resolver that an application uses.

> What are the possible Use cases of caching the responses at the
> Server?
> What if there is a dynamic updates of Records on Server and Server
> still sends the cached Responses?

DNS resolution is a time-consuming operation. Local resolver caches help
improve performance and also reduce load on remote nameservers. Caches
also build resiliency into DNS against short disruptive conditions.

The TTL field (and some other fields) influence how long a DNS record is
valid in the cache before it expires. DNS data is weakly coherent and
data in a cache may be out of sync with changes to the corresponding
zone on an authoritative server until this data's TTL expires. This is a
normal condition.

(Similarly, DNS data on secondary nameservers may be out of sync with
changes to a zone on a primary nameserver for a period of time.)

For answers to these questions, I suggest reading RFC 1034 which
explains concepts of DNS.

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