So thanks for kind answer.


Additional questions below.


>> Hello, all.
>>
>>
>> I have operated two dns(primary and secondary) for one domain like below.
>>
>>
>> example.com IN NS ns1.example.com
>> example.com IN NS ns2.example.com
>>
>>
>> and there was a event that ns1.example.com dns was down.
>> As I know, if ns1 dns is down, all requests go to the ns2.example.com.

>
> Depending on what 'down' means, it could take some time before
> the request is sent to ns2. So there will likely be a delay, even
> if not much (it will feel like forever to some users).



my 'down' means that system down so can't ping to server.


>> But when ns1.example.com dns was down, actually some people can't lookup the domain.

>
> Sounds like a configuration issue. However realize there is a zone
> cache and if ns2 is slaving zones of ns2 (typical bind master slave
> scenario) and the zone cache expires, then ns2 will refuse to
> trust the slaved zone it had... and thus nothing works.



Sorry, I can't understand what you said.
actually the master dns server(system) down time was just a hour and slave dns
works well without any problem, but at that time some can connect but some said that
they can't resolve the domain at all.


It means, dns failover doesn't works well?
and some resolver or some bind version, insist querying for the downed dns server?


>> What's the problem and how can I solve this problem?
>>
>> If I adding some dns server like ns3,ns4.ns5, it would be helpful to solve this problem?

>
> No if it's the cache issue... but that's if we're talking about
> authoritative zones that you're serving.
>
> If you're just needing some nameservers that answer recursively
> for generic requests and many are of dubious quality, then adding
> extra 'might' help.... but I'd suspect something else is wrong
> before I'd do that.
>
>



So thanks for your help again..




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