Forum Discussion
Deploying Domain Controllers with an Availablility Group
I'm not sure what you mean by 'servers in an availability groups all use the same DNS settings'.
A VM NIC in Azure, by default, inherits its DNS settings from the virtual network. Open the VM nic resource in the Azure portal - DNS servers and you'll see two options:
'Inherit from virtual network' - this is the default setting for a NIC
'Custom' - allows you to use custom DNS settings for that VM NIC
An Availability set works differently than individual VM's. All VM's in an availability set use the same DNS settings. When you change one NIC DNS settings, that setting will propagate to all servers in the Availability Set. If you try to manually change the DNS setting, say in the VM, when the VM reboots, it will inherit the DNS from the Availability set.
This creates an issue specific to DC's where at least one DC will have it's primary DNS pointing to itself, which in not best practice.
I've just run up 2 VMs in an availability set and can change the DNS settings on a single NIC on a VM in the availability set.
Here are a couple resources. This has been a known issue for a while, but there hasn't been much direction from Microsoft regarding it.
https://social.msdn.microsoft.com/Forums/en-US/ac870e43-730c-4e2a-bd1e-b2bab1754cdf/individual-dns-settings-for-domain-controllers-in-azure-in-availability-sets
A quick search on uservoice brough this up: https://feedback.azure.com/forums/281804-azure-resource-manager/suggestions/18463720-delink-availability-set-from-dns-settings
For your test, change the DNS of one of the NICs in the Availablity set in the Portal, not using PowerBI, it will post a message stating that all servers will be rebooted to inherit the changes.
