We are using D365 v8.0 CRM and provisioned D365 Portals as well. We have planned for upgrade for CRM and during this CRM upgrade portals also wont be accessible for end users. So,we want to display custom message during CRM and portals down.
Any ideas please
Since the data for a Portal is stored in CRM, if the CRM is unavailable, so will the portal. So what I recommend (assuming you're using a different domain that the OOTB something.microsoftcrmportals.com) is to setup another website elsewhere (like an Azure Web App perhaps) that displays the message you want, and then update the DNS entry that typically points to your Portal to point to that temporary site. That way when users try to access the Portal, they'll actually be accessing your temporary site with the message you want.
I'd also recommend keeping the TTL on your DNS entry low so that once it does come back up (and you've pointed the DNS entry back at the actual Portal), they'll get an updated entry quickly.
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