Wildcard and Naked CNAME records in Azure DNS

问题

I'm migrating all my DNS services to Azure DNS. Using Powershell I've managed to successfully create the CNAME record to get www.something.com to resolve to an Azure website-

$rs = New-AzureRmDnsRecordSet -Name "www" -RecordType "CNAME" -ZoneName "something.com" -ResourceGroupName "DNSRecords" -Ttl 60 -Overwrite -Force
Add-AzureRmDnsRecordConfig -RecordSet $rs -Cname "MySomethingAzureWebsite.azurewebsites.net"
Set-AzureRmDnsRecordSet -RecordSet $rs -Overwrite

But how to I create CNAME records for wildcard and naked addresses like

*.something.com or something.com

I'm assuming it's something (no pun intended) to do with

New-AzureRmDnsRecordSet -Name "www"

But through all the documentation and Internet examples I can't seem to find the right verbiage.

回答1:

The problem you're having is an issue with the rules of DNS, which forbid a CNAME record where another record exists. The naked address (or Apex) example.com already has two records (the SOA and NS) so a CNAME is not allowed.

If a CNAME RR is present at a node, no other data should be present; this ensures that the data for a canonical name and its aliases cannot be different. This rule also insures that a cached CNAME can be used without checking with an authoritative server for other RR types.

In order to create an Apex record, i.e. example.com you need to use an A record, which means it needs to point to the IP of your Azure website. Once you have that you can then create a CNAME from www to example.com. (this is the supported method - your IP Address for Azure websites is static)

the command you are looking for, would be something like

$rs = New-AzureRmDnsRecordSet -Name "@" -RecordType A `
       -ZoneName example.com -ResourceGroupName $RG `
       -Ttl $ttl -Force -Overwrite
Add-AzureRmDnsRecordConfig -RecordSet $rs -Ipv4Address $IPAddress
Set-AzureRmDnsRecordSet -RecordSet $rs -Overwrite

then

$rs = New-AzureRmDnsRecordSet -Name "www" -RecordType "CNAME" `
      -ZoneName "something.com" -ResourceGroupName "DNSRecords" `
      -Ttl 60 -Overwrite -Force
Add-AzureRmDnsRecordConfig -RecordSet $rs -Cname "example.com"
Set-AzureRmDnsRecordSet -RecordSet $rs -Overwrite

This will create your zone apex record and point the www cname to it making example.com and www.example.com point to the same place.

For a Wildcard you replace the Apex "@" with an asterix "*" so something like

$rs = New-AzureRmDnsRecordSet -Name "*" -RecordType A `
       -ZoneName example.com -ResourceGroupName $RG `
       -Ttl $ttl -Force -Overwrite
#You know the rest

回答2:

As the previous answer stated, you can't create a CNAME at the zone apex (name = "@"). This is because of the RFC constraints, and the correct approach is to use other record types e.g. A records at the zone apex instead, as the previous respondent explained.

However, you can create a wildcard CNAME at the zone apex. Since wildcards do not match against empty strings in DNS, they do not fall foul of the RFC constraints. For example, in PowerShell:

$z = New-AzureRmDnsZone -Name myzone.com -ResourceGroupName MyResourceGroup
$rs = New-AzureRmDnsRecordSet -Name "*" -Zone $z -Ttl 3600 -RecordType CNAME
Add-AzureRmDnsRecordConfig -RecordSet $rs -Cname cnametarget.com
Set-AzureRmDnsRecordSet -RecordSet $rs

This will resolve for queries to e.g. foo.myzone.com but not for myzone.com.

Wikipedia has a good page to learn more about wildcard DNS.

来源：https://stackoverflow.com/questions/34585964/wildcard-and-naked-cname-records-in-azure-dns