What would be happen if one of two dns was down?

[MontyRee]

sorry for non-txt based previous e-mail. sending again.



So thanks for kind and concrete answers.

and addtional questions are...


-. others can use other resolvers like windows based or other bind version.
    so this program works well as you said without exception?


-. in the point of high-availability of service,
   what it better two authorative dns servers or two master dns servers using L4 switch?



So thanks again.


Regards.



> Subject: RE: What would be happen if one of two dns was down?
> From: chris_cox at stercomm.com
> To: bind-users at isc.org
> Date: Tue, 12 Aug 2008 10:44:02 -0500
>
> On Tue, 2008-08-12 at 06:42 +0000, MontyRee wrote:
>> 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.
>
> The slave will answer queries for the zone until the zone TTL expires
> in which case if cannot contact the master, the zone will go effectively
> dead.
>
> I think I used some bad "terms" in my explanation. Basically
> there is an expiration ttl for which a slave will consider its
> data to be good. After that, it will need to hit the master.
>
> (I trip up on using the right words)
>
> The value is often set to 2 weeks or more. But if the master is
> down for a LONG time... you'll lose it all eventually (the slave
> won't answer for that zone anymore).
>
> If you're seeing this problem after a short period of time, that's
> likely NOT the cause unless somebody set the expiry in the SOA
> to something really small.
>
> Caching in DNS is a wonderful thing, but can cause scenarios where
> resolution is working for one and not for another (because of
> the various Time To Live values and the time of last query/cache).
>
> Can you give us a feel for the amount of time between the failure
> and the problem? Is it almost immediate? If so, then it's some
> other kind of configuration issue (unless, as I said the zone was
> just totally miconfigured). Can you post the SOA for the zone?
>
>>
>>
>> It means, dns failover doesn't works well?
>> and some resolver or some bind version, insist querying for the downed dns server?
>
> Usually the client resolver is looking to query multiple nameservers, if
> the first one is down, it moves onto the next and so on. Failover works
> fine in this style (normally). Of course, a client might NOT be aware
> of more than one nameserver... in which case there is no failover (duh).
>
>
> ...
>>
>> So thanks for your help again..
>
> Did I explain it better this time?
>
>


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