Script to Find Missing Subnets in Active Directory

by Jeremy Saunders on January 21, 2014

This PowerShell script will collect all Netlogon.log files from the Domain Controllers, export the last x lines and combine it into one file of unique IP Addresses in CSV format. This easily and simply allows you to then identify any missing subnets that need to be added and associated to an Active Directory Site.

Yes, there are a couple of good examples of this type of script already available on the Internet. The trouble with them is that they would not produce reliable results, especially across environments where Domain Controllers were not all at the same Windows Server versions. Not a great practice, but it does happen in the larger environments where migrations are completed in phases. I’ve previously blogged about the change to the fields in the Netlogon.log file. I also found that other scripts were quite inefficient when reading and collecting the Netlogon.log files over WAN connections. The Report the AD Missing Subnets from the NETLOGON.log script by Francois-Xavier CAT was the best available. So I used it as a base to help derive the results I was after.

The only variable to adjust is $LogsLines. You set this to the last number of lines to read from each Netlogon.log file.

There is an optional TrustedDomain parameter you can use to pass a different domain name to the script, which will allow you to run this script against trusted domains. You must use the fully qualified domain name. Refer to the example in the script comments.

The screen shot below shows the output from an environment with 16 Domain Controllers.

  • The script connects to each Domain Controller, copies the Netlogon.log file back to an Output folder where the script is located, exports the NO_CLIENT_SITE messages from the last x lines, based on the $LogsLines variable, and writes them to a text file.
  • It then reads all text files, outputting it into 1 large file.
  • From there is does a sort of all unique lines based on the IP Address field and exports the results to a CSV file. So whilst you may have hundreds or thousands of NO_CLIENT_SITE messages across all your logs, sorting in order by uniqueness makes this a less overwhelming task to complete.
  • You’ll notice that if the script will output errors to the screen if it’s unable to connect to a Domain Controller via the admin$ share to copy the Netlogon.log file, or is simply unable to ping the Domain Controller.


The screen shot below shows the output of unique IP Addresses in the CSV file. As the IP Addresses are ordered, it’s easy to group them together into missing subnets. Note how depending on the country/language, the date column may be incorrectly formatted in Excel. You can correct the cell formatting, or just ignore it.


As per the following screen shot, if you open the CSV in Notepad, you’ll see the actual date format.


Here is the Find_missing_subnets_in_ActiveDirectory.ps1 script:

  This script will get all the missing subnets from the NETLOGON.LOG file from each
  Domain Controller in the Domain. It does this by copying all the NETLOGON.LOG files
  locally and then parsing them all to create a CSV output of unique IP Addresses.
  The CSV file is sorted by IP Address to make it easy to group them into subnets.

  Script Name: Find_missing_subnets_in_ActiveDirectory.ps1
  Release 1.2
  Modified by 23/01/2014
  Written by 02/01/2014

  Syntax examples:

  - To execute the script in the current Domain:

  - To execute the script in a trusted Domain:
      Find_missing_subnets_in_ActiveDirectory.ps1 -TrustedDomain

  This script was derived from the AD-Find_missing_subnets_in_ActiveDirectory.ps1
  script written by Francois-Xavier CAT.
   - Report the AD Missing Subnets from the NETLOGON.log

  - Stripped down the code to remove the e-mail functionality. This is a nice to
    have feature and can be added back in for a future release. I felt that it was
    more important to focus on ensuring the core functionality of the script was
    working correctly and efficiently.

  - Reordered the Netlogon.log collection to make it more efficient.
  - Implemented a fix to deal with the changes to the fields in the Netlogon.log
    file from Windows 2012 and above:
  - Tidied up the way it writes the CSV file.
  - Changed the write-verbose and write-warning messages to write-host to vary the
    message colors and improve screen output.
  - Added a "replay" feature so that you have the ability to re-create the CSV
    from collected log files.

# Set this to the last number of lines to read from each NETLOGON.log file.
# This allows the report to contain the most recent and relevant errors.
[Int]$LogsLines = "200"

# Set this to $True to remove txt and log files from the output folder.
$Cleanup = $True

# Set this to $True if you have not removed the log files and want to replay
# them to create a CSV.
$ReplayLogFiles = $False


# PATH Information
$ScriptPath = (Split-Path -Path ((Get-Variable -Name MyInvocation).Value).MyCommand.Path)
$ScriptPathOutput = $ScriptPath + "\Output"

# Date and Time Information
$DateFormat = Get-Date -Format "yyyyMMdd_HHmmss"

$OutputFile = "$scriptPathOutput\$DateFormat-AD-Sites-MissingSubnets.csv"

$CombineAndProcess = $False

IF ($ReplayLogFiles -eq $False)
  IF (-not(Test-Path -Path $ScriptPathOutput))
    Write-Host -ForegroundColor green "Creating the Output Folder: $ScriptPathOutput"
    New-Item -Path $ScriptPathOutput -ItemType Directory | Out-Null

  if ([String]::IsNullOrEmpty($TrustedDomain)) {
    # Get the Current Domain Information
    $domain = [System.DirectoryServices.ActiveDirectory.Domain]::GetCurrentDomain()
  } else {
    $context = new-object System.DirectoryServices.ActiveDirectory.DirectoryContext("domain",$TrustedDomain)
    Try {
      $domain = [System.DirectoryServices.ActiveDirectory.Domain]::GetDomain($context)
    Catch [exception] {
      write-host -ForegroundColor red $_.Exception.Message

  Write-Host -ForegroundColor green "Domain: $domain"

  # Get the names of all the Domain Contollers in $domain
  Write-Host -ForegroundColor green "Getting all Domain Controllers from $domain ..."
  $DomainControllers = $domain | ForEach-Object -Process { $_.DomainControllers } | Select-Object -Property Name

  # Gathering the NETLOGON.LOG for each Domain Controller
  Write-Host -ForegroundColor green "Processing each Domain controller..."
  FOREACH ($dc in $DomainControllers)
    $DCName = $($dc.Name)

    # Get the Current Domain Controller in the Loop
    Write-Host -ForegroundColor green "Gathering the log from $DCName..."

    IF (Test-Connection -Cn $DCName -BufferSize 16 -Count 1 -ea 0 -quiet) {

      # NETLOGON.LOG path for the current Domain Controller
      $path = "\\$DCName\admin`$\debug\netlogon.log"

      # Testing the $path
      IF ((Test-Path -Path $path) -and ((Get-Item -Path $path).Length -ne $null))
        # Copy the NETLOGON.log locally for the current DC
        Write-Host -ForegroundColor green "- Copying the $path file..."
        $TotalTime = measure-command {Copy-Item -Path $path -Destination $ScriptPathOutput\$($dc.Name)-$DateFormat-netlogon.log}
        $TotalSeconds = $TotalTime.TotalSeconds
        Write-Host -ForegroundColor green "- Copy completed in $TotalSeconds seconds."

        IF ((Get-Content -Path $path | Measure-Object -Line).lines -gt 0)
          # Export the $LogsLines last lines of the NETLOGON.log and send it to a file
          ((Get-Content -Path $ScriptPathOutput\$DCName-$DateFormat-netlogon.log -ErrorAction Continue)[-$LogsLines .. -1]) |
            Foreach-Object {$_ -replace "\[\d{1,5}\] ", ""} |
            Out-File -FilePath "$ScriptPathOutput\$DCName.txt" -ErrorAction 'Continue' -ErrorVariable ErrorOutFileNetLogon
          Write-Host -ForegroundColor green "- Exported the last $LogsLines lines to $ScriptPathOutput\$DCName.txt."
        ELSE {Write-Host -ForegroundColor green "- File Empty."}

      } ELSE {Write-Host -ForegroundColor red "- $DCName is not reachable via the $path path."}

    } ELSE {Write-Host -ForegroundColor red "- $DCName is not reachable or offline."}

    $CombineAndProcess = $True


} ELSE {

  Write-Host -ForegroundColor green "Replaying the log files..."
  IF (Test-Path -Path $ScriptPathOutput)
    IF ((Get-ChildItem $scriptpathoutput\*.log | Measure-Object).Count -gt 0)
      $LogFiles = Get-ChildItem $scriptpathoutput\*.log

      ForEach ($LogFile in $LogFiles)
        $DCName = $LogFile.Name -Replace("-\d{7,8}_\d{6}-netlogon.log")
        Write-Host -ForegroundColor green "Processing the log from $DCName..."
        IF ((Get-Content -Path "$ScriptPathOutput\$($LogFile.Name)" | Measure-Object -Line).lines -gt 0)
          # Export the $LogsLines last lines of the NETLOGON.log and send it to a file
          ((Get-Content -Path "$ScriptPathOutput\$($LogFile.Name)" -ErrorAction Continue)[-$LogsLines .. -1]) |
                    Foreach-Object {$_ -replace "\[\d{1,5}\] ", ""} |
                    Out-File -FilePath "$ScriptPathOutput\$DCName.txt" -ErrorAction 'Continue' -ErrorVariable ErrorOutFileNetLogon
          Write-Host -ForegroundColor green "- Exported the last $LogsLines lines to $ScriptPathOutput\$DCName.txt."
        } ELSE {Write-Host -ForegroundColor green "- File Empty."}
        $CombineAndProcess = $True
    } ELSE {Write-Host -ForegroundColor red "There are no log files to process."}
  } ELSE {Write-Host -ForegroundColor red "The $ScriptpathOutput folder is missing."}

IF ($CombineAndProcess)

  # Combine all the TXT file in one
  $FilesToCombine = Get-Content -Path "$ScriptPathOutput\*.txt" -Exclude "*All_Export.txt" -ErrorAction SilentlyContinue |
    Foreach-Object {$_ -replace "\[\d{1,5}\] ", ""}

  if ($FilesToCombine)
    $FilesToCombine | Out-File -FilePath $ScriptPathOutput\$dateformat-All_Export.txt

    # Convert the TXT file to a CSV format
    Write-Host -ForegroundColor green "Importing exported data to a CSV format..."
    $importString = Import-Csv -Path $scriptpathOutput\$dateformat-All_Export.txt -Delimiter ' ' -Header Date,Time,Domain,Error,Name,IPAddress

    # Get Only the entries for the Missing Subnets
    $MissingSubnets = $importString | Where-Object {$_.Error -like "*NO_CLIENT_SITE*"}
    Write-Host -ForegroundColor green "Total of NO_CLIENT_SITE errors found within the last $LogsLines lines across all log files: $($MissingSubnets.count)"
    # Get the other errors from the log
    $OtherErrors = Get-Content $scriptpathOutput\$dateformat-All_Export.txt | Where-Object {$_ -notlike "*NO_CLIENT_SITE*"} | Sort-Object -Unique
    Write-Host -ForegroundColor green "Total of other Error(s) found within the last $LogsLines lines across all log files: $($OtherErrors.count)"

    # Export to a CSV File
    $UniqueIPAddresses = $importString | Select-Object -Property Date, Name, IPAddress, Domain, Error |
    Sort-Object -Property IPAddress -Unique
    $UniqueIPAddresses | Export-Csv -notype -path "$OutputFile"
    # Remove the quotes
    (get-content "$OutputFile") |% {$_ -replace '"',""} | out-file "$OutputFile" -Fo -En ascii
    Write-Host -ForegroundColor green "$($UniqueIPAddresses.count) unique IP Addresses exported to $OutputFile."

  }#IF File to Combine
  ELSE {Write-Host -ForegroundColor red "No .txt files to process."}

  IF ($Cleanup)
    Write-Host -ForegroundColor green "Removing the .txt and .log files..."
    Remove-item -Path $ScriptpathOutput\*.txt -force
    Remove-Item -Path $ScriptPathOutput\*.log -force


Write-Host -ForegroundColor green "Script Completed."


Jeremy Saunders

Jeremy Saunders

Independent Consultant | Contractor | Microsoft & Citrix Specialist | Desktop Virtualization Specialist at J House Consulting
Jeremy is a highly respected, IT Professional, with over 30 years’ experience in the industry. He is an independent IT consultant providing expertise to enterprise, corporate, higher education and government clients. His skill set, high ethical standards, integrity, morals and attention to detail, coupled with his friendly nature and exceptional design and problem solving skills, makes him one of the most highly respected and sought after Microsoft and Citrix technical resources in Australia. His alignment with industry and vendor best practices puts him amongst the leaders of his field.
Jeremy Saunders
Jeremy Saunders
Jeremy Saunders
  • thank you for the script its very helpful. Would it be possible to have a switch to have the domain as a input to the script. I have multiple domains I support and being able to just input the domain would allow me to use one script, I could always have mutliple copies of the script and just hard code the domain 🙂

    • Hi Jason,
      Not a problem. I’ve modified the script and will run up some tests before updating this article with a new version. Will let you know when it’s ready.

    • All done and tested Jason. Let me know if you have any issues.

  • Francois-Xavier Cat

    Nice work, thanks for the mention 😉

  • CarlWebster


    Ran the script on a large domain with many DCs and it worked flawlessly.



  • Pingback: Active Directory Health Check, Audit and Remediation Scripts()

  • Tri Trinh

    Great script!

  • Qualified Expert

    Thanks for the sharing once again Jeremy.
    So I assume there is no outage involve in creating those missing subnets in AD sites & services.

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