# Update or create a DNS CNAME record
<a name="step4-configure-dns-cname"></a>

After you properly configure SPNs for your file system, you can cut over to Amazon FSx by replacing each DNS record that resolved to the original file system with a DNS record that resolves to the default DNS name of the Amazon FSx file system.

The `dnsserver` and `activedirectory` Windows modules are required to run the commands presented in this section.

**To install the required PowerShell modules**

1. Log on to a Windows instance joined to the same Active Directory that your Amazon FSx file system is joined to as a user that is a member of a group that has DNS administration permissions (**Amazon Delegated Domain Name System Administrators** in Amazon Managed Microsoft AD, and **Domain Admins** or another group to which you've delegated DNS administration permissions in your self-managed Active Directory). 

   For more information, see [Connecting to Your Windows Instance](https://docs.amazonaws.cn/AWSEC2/latest/WindowsGuide/connecting_to_windows_instance.html) in the *Amazon EC2 User Guide*.

1. Open PowerShell as administrator.

1. The PowerShell DNS Server module is required to perform the instructions in this procedure. Install it using the following command.

   ```
   Install-WindowsFeature RSAT-DNS-Server
   ```

**To update or create a custom DNS name to your Amazon FSx file system**

1. Connect to your Amazon EC2 instance as a user that is a member of a group that has DNS administration permissions (**Amazon Delegated Domain Name System Administrators** in Amazon Managed Active Directory, and **Domain Admins** or another group to which you've delegated DNS administration permissions in your self-managed Active Directory).

   For more information, see [Connecting to Your Windows Instance](https://docs.amazonaws.cn/AWSEC2/latest/WindowsGuide/connecting_to_windows_instance.html) in the *Amazon EC2 User Guide*.

1. At the command prompt, run the following script. This script migrates any existing DNS CNAME records to your Amazon FSx file system. If none are found, it creates a new DNS CNAME record for the DNS alias `alias_fqdn` that resolves to the default DNS name for your Amazon FSx file system. 

   To run the script:
   + Replace `alias_fqdn` with the DNS alias that you associated with the file system.
   + Replace `file_system_DNS_name` with the DNS name Amazon FSx has assigned to the file system.

   ```
   $Alias="alias_fqdn"
   $FSxDnsName="file_system_dns_name"
   $AliasHost=$Alias.Split('.')[0]
   $ZoneName=((Get-WmiObject Win32_ComputerSystem).Domain)
   $DnsServerComputerName = (Resolve-DnsName $ZoneName -Type NS | Where Type -eq 'A' | Select -ExpandProperty Name) | Select -First 1
   Add-DnsServerResourceRecordCName -Name $AliasHost -ComputerName $DnsServerComputerName -HostNameAlias $FSxDnsName -ZoneName $ZoneName
   ```

1. Repeat the previous step for each DNS alias that you associated with the file system in [Step 1](step1-assign-dns-alias.md).

You've now added a DNS CNAME value for your Amazon FSx file system with the DNS alias. You can now use the DNS alias to access your data.

**Note**  
When updating a DNS CNAME record to point to an Amazon FSx file system previously pointed to another file system, clients might not be able to connect with file system for a brief period of time. When the client DNS cache refreshes, they should be able to connect using the DNS alias. For more information, see [Can't access the file system using a DNS alias](unable-to-access.md#cant-connect-using-dns-alias).