DNS - use Internal or External (performance)

Hi
Can someone help me gauge whether or not its best to use internal DNS servers or use say the ISP DNS servers?
We are using internal, 4x DCs with DNS on over two sites recently performance of internet has dropped.  Not sure if this is o365 related or not either, however when a DC goes offline it causes much issue and takes a lot of time for logons to work etc.
Wondered if pointing DNS externally will help with the above or anything else?
thanks
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CHI-LTDAsked:
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CompProbSolvConnect With a Mentor Commented:
Are you asking if local workstations should use the local DCs or your ISP DNS servers for DNS?  Definitely the local DCs.  The primary reason is that your ISPs DNS servers won't know how to resolve local addresses which will cause you no end of problems.

You can configure the DNS servers in your local DCs to use the ISP DNS servers for forwarding if those servers seem reasonable.
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Niten KumarPrincipal Systems AdministratorCommented:
Logon problems means issues with your dc's.  What kind of problems do you have when a particular dc goes offline? How many sites do you have and how are the dc's distributed amongst the sites. Are sites properly defined and have you checked dc and replication health.
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CHI-LTDAuthor Commented:
ok will leave clients pointing to local dcs.
will give server dns changes some more thought.

So if one DC was offline for maintenance other servers (not sure about clients) would say there are no logon servers to process account (or similar) but there is still one other DC available.  

I have 4x DCs.  2x at site a and 2 at site b.

The clients point to site a and also have the site b in the tcpip settings, but down the list.

dcdiag reports fine
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Niten KumarPrincipal Systems AdministratorCommented:
Can you type set at command prompt on the machines that cause logon problems and check what is displayed under logon server
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Niten KumarConnect With a Mentor Principal Systems AdministratorCommented:
The computers at site b should output the one of the dcs at site b as the logon server. Likewise for site a.
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CHI-LTDAuthor Commented:
let using local, but may look at pointing internal DNS servers to Google.
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