How does a Client Verify if the DNS server is Alive or down

[Reindl Harald]

Am 20.10.2015 um 13:26 schrieb Steven Carr:
> On 20 October 2015 at 10:41, Phil Mayers <p.mayers at imperial.ac.uk> wrote:
>> On 20/10/15 07:26, Harshith Mulky wrote:
>>>
>>> How can a Client verify if the DNS Server is Running(named service is
>>> Running) or Down?
>>
>> By the presence or absence of a reply to a query.
>
> That doesn't always verify if the server is running or not, your
> client might not be allowed to query it, in which case you may get a
> refused response, or it may just not respond to you at all.

and the client can distinct NXDOMAIN, REFUSED, no connection at all

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