Fixing conflict between internal and external domain with same name via internal DNS entries

On newly deployed Windows Server 2012 R2, followed the advice given on another question on naming the internal domain the same as the external domain for the sake of username consistency for using with Office365.

I wondered at how this would affect DNS, but when you are revamping your deployment plan in mid-deployment...

So of course this has led to the external website not resolving properly internally as well as issues with any of the external domains DNS records being recognized internally.

I beleive that this can be fixed by the appropriate entries in the DC's DNS table, however, that is beyond my (admittedly basic) DNS knowledge.

I believe that I probably need to recreate all external DNS entriesas static entries only.

I am unsure of many things though:

#1. Should I be adding these entries to the existing internal domain lookup zone or creating a new zone?

#2. If a new zone, what kind of new zone should it be? (Primary, Secondary, Sub)

#3. If a new zone, do I need to change any of its default properties?
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Joshua GrantomSenior Systems AdministratorCommented:
It will be in the existing zone if your internal domain is the exact same name as your external.

Once you create the a, cname, and mx records to mirror external dns, everything should be accessible after that.

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Joshua GrantomSenior Systems AdministratorCommented:
do you have any external sub-domains?
MJCSAuthor Commented:
Should propagation be immediate?
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Joshua GrantomSenior Systems AdministratorCommented:
clients may need to flush dns cache

Run this from a command line.
ipconfig /flushdns
MJCSAuthor Commented:
Do I need to worry about spf/txt records?
Joshua GrantomSenior Systems AdministratorCommented:
I wouldn't worry about spf records because you are using Office 365. Only the external spf will be read by other mail servers.
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