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@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@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 <dan...@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/ >>> >> ------------------------------------------------------------------- >> Unsubscribe: https://cool.haxx.se/list/listinfo/curl-library >> Etiquette: https://curl.haxx.se/mail/etiquette.html > > > > -- > Saludos / Regards, > David Requena > ------------------------------------------------------------------- > Unsubscribe: https://cool.haxx.se/list/listinfo/curl-library > Etiquette: https://curl.haxx.se/mail/etiquette.html
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