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AWSTemplateFormatVersion: '2010-09-09'

Description: This example template describes an Amazon EC2 Systems Manager document.
  It contains a sample script that can be used to join an EC2 instance to an Active
  Directory domain and add to an existing AD security group. 
  This document uses the domain user name and password that is stored in the SSM parameter. 

Metadata:
  "AWS::CloudFormation::Interface":
    ParameterGroups:

      - Label:
          default: Active Directory Settings
        Parameters:
          - DirectoryNameParameter
          - ADUserNameParameter
          - ADUserPasswordParameter
          - ADDNSIPAddressesParameter
      - Label:
          default: gMSA Settings
        Parameters:
          - gMSAADSecurityGroup

Parameters:

  DirectoryNameParameter:
    Description: 'SSM Parameter name to use for Active Directory Name:'
    MaxLength: '1024'
    MinLength: '1'
    Type: AWS::SSM::Parameter::Value<String>

  ADUserNameParameter:
    Description: 'SSM Parameter name to use for AD User name:'
    MaxLength: '1024'
    MinLength: '1'
    Type: AWS::SSM::Parameter::Value<String>

  ADUserPasswordParameter:
    AllowedPattern: (?!^([aA][wW][sS]|[sS][sS][mM]))(?=^[-a-zA-Z0-9_.]*$).*
    ConstraintDescription: please specify a valid Parameter Store name.
    Description: 'SSM Parameter name to use for AD User password:'
    MaxLength: '1024'
    MinLength: '1'
    Type: String

  ADDNSIPAddressesParameter:
    Description: 'SSM Parameter name to use for AD DNS IP server addresses:'
    MaxLength: '1024'
    MinLength: '1'
    Type: AWS::SSM::Parameter::Value<String>

  gMSAADSecurityGroup:
    Type: String
    Description: 'Active Directory security group to join the instance, that controls access to the gMSA password.'

Resources:

  Document:
    Type: AWS::SSM::Document

    Properties:

      DocumentType: Command
      Content:
        schemaVersion: '2.2'
        description: Run a PowerShell script to domain join a Windows instance securely.

        parameters:

          DirectoryName:
            type: String
            default: !Ref DirectoryNameParameter

          ADUserName:
            type: String
            default: !Ref ADUserNameParameter

          ADUserPasswordParam:
            type: String
            default: !Ref ADUserPasswordParameter

          ADDNSIPAddresses:
            type: String
            default: !Ref ADDNSIPAddressesParameter

          ADSecurityGroup:
            type: String
            default: !Ref gMSAADSecurityGroup

        mainSteps:

        - action: aws:runPowerShellScript
          name: GenerateDomainJoinSSMDoc
          precondition:
            StringEquals:
             - platformType
             - Windows
          inputs:
            runCommand:
            - "Fn::Sub": |
                # Example PowerShell script to domain join a Windows instance securely
                $ErrorActionPreference = "Stop"
                try {
                    # Retrieve configuration values from parameters
                    $domain = "{{DirectoryName}}"
                    $username = "{0}\{1}" -f $domain, "{{ADUserName}}"
                    $password = (Get-SSMParameterValue -Name "{{ADUserPasswordParam}}" -WithDecryption $True).Parameters[0].Value | ConvertTo-SecureString -asPlainText -Force
                    $ipdns = "{{ADDNSIPAddresses}}"

                    $gMSASecurityGroup = "{{ADSecurityGroup}}"

                    # Create a System.Management.Automation.PSCredential object
                    $credential = New-Object System.Management.Automation.PSCredential($username, $password)

                    # Get the VPC DNS server.
                    $dnsclient = Get-DnsClientServerAddress -AddressFamily IPv4 | Where-Object {$_.ServerAddresses.Count -gt 0} | Select-Object -First 1

                    # During retry, we should avoid adding duplicate DNS servers, if it was already added in the previous attempt.
                    # VPC DNS server is the last one in the list.
                    $vpcdns = $dnsclient.ServerAddresses | select -last 1
                
                    # Set up the IPv4 address of the AD DNS server as the first DNS server on this machine
                    $dnsserverstoupdate = $("{0},{1}" -f $ipdns, $vpcdns)
                    Write-Output ("Adding AD DNS server addresses :{0} to the IPV4 interface Index:{1}." -f $dnsserverstoupdate, $dnsclient.InterfaceIndex)
                    Set-DnsClientServerAddress -InterfaceIndex $dnsclient.InterfaceIndex -ServerAddresses $dnsserverstoupdate

                    # Join the domain
                    Add-Computer -DomainName $domain -Credential $credential

                    # Install AD Module
                    Install-WindowsFeature RSAT-AD-PowerShell

                    Write-Output ("Checking the AD security group :{0}" -f $gMSASecurityGroup)
                    $group = Get-ADGroup -Filter "Name -eq `"$gMSASecurityGroup`""

                    if($group) {
                      Write-Output "AD security group exists. Proceeding to add the computer to the AD security group."
                      $hostname = Invoke-Expression -Command "Hostname"
                      $computer = Get-ADComputer -Filter "Name -eq `"$hostname`"" -Credential $credential
                      Write-Output ("Adding host :{0} to the AD security group :{1}" -f $computer.DistinguishedName, $gMSASecurityGroup)
                      Add-ADGroupMember -Identity $group.ObjectGUID -Members $computer.DistinguishedName -Credential $credential
                    } else {
                      Write-Output ("AD security group :{0} don't exist. You should create and then add the computer to that group." -f $gMSASecurityGroup)
                    }

                    Restart-Computer -Force
                } catch [Exception] {
                    Write-Output $_.Exception.ToString()
                    Write-Output "Command execution failed."
                    $host.SetShouldExit(1)
                }

Outputs:
  DocumentName:
    Description: The name of the document.
    Export:
      Name: !Sub '${AWS::StackName}-Document'
    Value: !Ref 'Document'