Explanation
Box 1: Register-CustomDnsServer
Resolving external DNS names from Azure Stack Hub
To resolve DNS names for endpoints outside Azure Stack Hub (for example: www.bing.com), you need to provide DNS servers that Azure Stack Hub can use to forward DNS requests for which Azure Stack Hub isn't authoritative. For deployment, DNS servers that Azure Stack Hub forwards requests to are required in the Deployment Worksheet (in the DNS Forwarder field). Provide at least two servers in this field for fault tolerance. Without these values, Azure Stack Hub deployment fails.
Configure conditional DNS forwarding
Important
This only applies to an AD FS deployment.
To enable name resolution with your existing DNS infrastructure, configure conditional forwarding.
To add a conditional forwarder, you must use the privileged endpoint.
For this procedure, use a computer in your datacenter network that can communicate with the privileged endpoint in Azure Stack Hub.
1. Open an elevated Windows PowerShell session (run as administrator), and connect to the IP address of the privileged endpoint. Use the credentials for CloudAdmin authentication.
$cred=Get-Credential
Enter-PSSession -ComputerName <IP Address of ERCS> -ConfigurationName PrivilegedEndpoint
-Credential $cred
2. After you connect to the privileged endpoint, run the following PowerShell command. Substitute the sample values provided with your domain name and IP addresses of the DNS servers you want to use.
Register-CustomDnsServer -CustomDomainName "contoso.com" -CustomDnsIPAddresses
"192.168.1.1","192.168.1.2"
Box 2: fabrikam.com
The external DNS zone.
Box 3: "192.160.20.1", "192.160.20.2",
The IP addresses for the DNS servers in External DNS zone.
Reference:
https://learn.microsoft.com/en-us/azure-stack/operator/azure-stack-integrate-dns