Add var.dns_prefix_private_cluster

[lonegunmanb]

Describe your changes

Issue number

#568

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[zioproto]

Warning dns_prefix or dns_prefix_private_cluster are exclusive.

We do have a prefix variable that is not nullable:

variable "prefix" {
  type        = string
  default     = ""
  description = "(Optional) The prefix for the resources created in the specified Azure Resource Group. Omitting this variable requires both `var.cluster_log_analytics_workspace_name` and `var.cluster_name` have been set."
  nullable    = false
}

Would it be okay to have dns_prefix as empty string "" and dns_prefix_private_cluster as a string ?

[lonegunmanb]

Thanks for your review @zioproto , quite good catch! I've removed nullable=false from var.prefix now we can set it to null explicitly.

An empty string for var.prefix and non-empty string for var.dns_prefix_private_cluster would trigger a schema validation error so I think it's enough for those two variables. WDYT?

[zioproto]

During the testing I figure out we need more code. I pushed 2 commits with more preconditions.

https://learn.microsoft.com/en-us/azure/aks/private-clusters?tabs=azure-portal#configure-a-private-dns-zone

There is another problem.

the private dns zone name should be in either of these formats:

• private.eastus.azmk8s.io
• privatelink.eastus.azmk8s.io
• [a-zA-Z0-9-]{1,32}.private.eastus.azmk8s.io
• [a-zA-Z0-9-]{1,32}.privatelink.eastus.azmk8s.io

Should we also create the necessary role assignments ?

The docs say:
"You also need a user-assigned identity or service principal with the Private DNS Zone Contributor and Network Contributor roles."
I enforced the identities with the precondition but I did not create the assignments yet

[zioproto]

Another problem. The default value for var.prefix is "". In my tests I had to pass an explicit var.prefix = null. At one point we could introduce a braking change making the default value for var.prefix = null ?

[zioproto]

the match [a-z] should be changed into [a-z0-9] to match region names with numbers like eastus2

[zioproto]

the match [a-z] should be changed into [a-z0-9] to match region names with numbers like eastus2

[zioproto]

Suggested change

	"private\\.[a-z]+\\.azmk8s\\.io",
	"private\\.[a-z0-9]+\\.azmk8s\\.io",