Re: DNS data not refreshed after CURLOPT_DNS_CACHE_TIMEOUT

Thanks Daniel and David for the explanations. I think now I
fairly understand the concept of DNS cache timeout.
I will do some tests and get back here if I have any questions.

On Tue, Sep 1, 2020 at 1:35 PM David Requena via curl-library <
curl-library_at_cool.haxx.se> wrote:

> Dear Narasimha,
>
> It seems to me you're missing the point here if I'm understanding your
> intended use case.
>
> Your intended workflow of connecting and then keep hitting DNS to check
> connection validity does not really make any sense.
> You see, as soon as the host you're connected to changes its ip address
> under your feet, the connection becomes invalid.
> In this circumstance, you need to reconnect to the given hostname, at
> which point, DNS *will* be checked if cache timeout has expired.
>
> In short: "curl doesn't resolve the host name AT ALL when it reuses an
> existing connection. It doesn't need to so it avoids it"
> And rightly so.
> By the time you see the change in DNS, you've already been unable to talk
> to the old ip address for some time.
>
> This way, the only timeout you ought to be concerned with should be TCP
> timeout, if any.
> DNS has nothing to do with a *live* connection.
>
> A much saner workflow would be simply to connect, keep using this
> connection until it becomes invalid, then reconnecting.
>
> Hope this helps clearing things out.
>
> Regards
>
> El mar., 1 sept. 2020 a las 9:07, Narasimha M via curl-library (<
> curl-library_at_cool.haxx.se>) escribió:
>
>> OK. So in the given example from man page, if I add a sleep between two
>> easy performs, should the application resolve the name?
>>
>> ret = curl_easy_perform(curl);
>> /* in this second request, the cache will not be used if more than
>> two seconds have passed since the previous name resolve */
>> sleep(5); /* add a sleep of five seconds, so as to force a fresh name
>> resolve */
>> ret = curl_easy_perform(curl);
>>
>> Thanks,
>> Narasimha
>>
>> On Tue, Sep 1, 2020 at 12:11 PM Daniel Stenberg <daniel_at_haxx.se> wrote:
>>
>>> On Tue, 1 Sep 2020, Narasimha M wrote:
>>>
>>> > Hence, I want to keep the connection and handle for a long time.
>>> However
>>> > during this time, there can be changes to the DNS server/data. I would
>>> like
>>> > the application to be agnostic to the changes to the DNS. This would
>>> have
>>> > been possible if CURL queried the DNS periodically and updated its
>>> cache for
>>> > long running handles.
>>>
>>> Sure it *could*. It's just code. It could do *anything* if we just add
>>> the
>>> code for it. This feature you're asking for has however not been
>>> implemented.
>>>
>>> > I am still not able to understand the use of CURLOPT_DNS_CACHE_TIMEOUT
>>> in
>>> > the context of a given connection which runs for a long time.
>>>
>>> curl doesn't resolve the host name AT ALL when it reuses an existing
>>> connection. It doesn't need to so it avoids it. Saves time and resources.
>>>
>>> The DNS cache is only used when a host name is resolved. If no resolving
>>> is
>>> done, the DNS cache is not used and therefore the timeout also has no
>>> meaning.
>>> The entry is since long removed from the DNS cache but the connection is
>>> still
>>> there - and working.
>>>
>>> > Could you please point me to any example which can illustrate
>>> > CURLOPT_DNS_CACHE_TIMEOUT in context of a given curl handle?
>>>
>>> How would a cache timeout even get visualized in an example? There's an
>>> example in the man page for the option:
>>> https://curl.haxx.se/libcurl/c/CURLOPT_DNS_CACHE_TIMEOUT.html
>>>
>>> --
>>>
>>> / daniel.haxx.se | Commercial curl support up to 24x7 is available!
>>> | Private help, bug fixes, support, ports, new
>>> features
>>> | https://www.wolfssl.com/contact/
>>>
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>
>
>
> --
> Saludos / Regards,
> David Requena
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Received on 2020-09-01