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• Download Main Script
• Script Code
• Another Version That's not Necessarily Dependent on DnsShell
• Download (alternative version)
• Script Code (alternative version)

The main script uses the PowerShell module DnsShell from Codeplex (NB! Dead link! DnsShell is no longer on Codeplex and I think even Codeplex itself has been discontinued), which you will need to download to use that script, and it finds duplicate PTR records on a Windows DNS server.

I also tossed in a version that can be adapted to not rely on DnsShell (comment added 2023-05-18: And it's a good thing I did! Because DnsShell no longer is easily available), but you will then somehow need to get the reverse zones into a file or variable in the correct format. It can also be used with DnsShell to produce slightly different-looking results from what the main script does.

Multiple host names for the same PTR record are displayed separated by a semicolon in the data.

You will need to supply the -DnsServer parameter.

[CmdletBinding()]
param([Parameter(Mandatory=$true)][string] $DnsServer)

# Copyright 2014, Svendsen Tech
# All rights reserved.
# Joakim Borger Svendsen
# 2014-08-28


Import-Module .\DnsShell # this has to exist ( http://dnsshell.codeplex.com/ )

$Zones = Get-DnsZone -Server $DnsServer

$ReverseZones = $Zones | Where-Object { $_.ZoneName -like '*.arpa' } |
    Select-Object -ExpandProperty ZoneName

# Example zone:
# $ReverseZones = @('12.10.in-addr.arpa')

$Dupes = @(foreach ($z in $ReverseZones) {
    
    Write-Verbose -Message "Processing zone: $z"
    
    $Records = Get-DnsRecord -ServerName $DnsServer -ZoneName $z -RecordType PTR
    $Records | ForEach-Object -Begin {
        $IpHash = @{}
        } -Process {
        $TempArray = $_.Name -replace '\.in-addr\.arpa$' -split '\.'
        $Ip = ($TempArray[-1..-($TempArray.Count)]) -join '.'
        if (-not $IpHash.ContainsKey($Ip)) {
            # New PTR. The Name property is the PTR. Create one-element array.
            $IpHash.$Ip = @($_.HostName)
        }
        else {
            # Duplicate/alias PTR, add to array.
            $IpHash.$Ip += $_.HostName
        }
            
    }
    $Duplicates = $IpHash.GetEnumerator() | Where-Object { $_.Value.Count -gt 1 }
    $Duplicates | ForEach-Object {
        New-Object psobject -Property @{
            IP = $_.Name
            Duplicates = $_.Value -join '; '
        }
    }
})

# Display in console.
$Dupes | Select IP, Duplicates | Format-Table -AutoSize

# Export CSV file (if any dupes found).
if ($Dupes.Count) {
    Write-Host -Fore Green 'Exporting CSV file PTRdupes.csv'
    $Dupes | Select IP, Duplicates | Export-Csv -Encoding UTF8 -NoTypeInformation -Path PTRdupes.csv -Delimiter ';'
}

> $Var = .\Get-DupePTR.ps1
...
> $Var | Format-Table # or format-list, export-csv, or whatever sensible you want to do

[CmdletBinding()]
param()
# Copyright (C) 2014, Svendsen Tech. All rights reserved.
# Joakim Borger Svendsen
# 2014-08-15
# edited: 2014-11-08


$DnsServer = 'your.dns.server'

# This uses DnsShell to get the zones, but you can edit this to read
# zones from a file, or whatever, to avoid DnsShell.
# It uses dnscmd.exe below and parses its output, and then uses the
# (apparently obsolete) [Net.Dns] class' GetHostByAddress() method
# to look up the IP
####
#### Zones must be in the form 58.10.in-addr.arpa (for 10.58.0.0/16).
####

Import-Module .\DnsShell # this has to exist ( http://dnsshell.codeplex.com/ )
$Zones = Get-DnsZone -Server $DnsServer
$ReverseZones = $Zones | Where { $_.ZoneName -like '*.arpa' } | Select -ExpandProperty ZoneName # @('58.10.in-addr.arpa') # = 

foreach ($z in $ReverseZones) {
    Write-Host -Fore Green "Processing zone: $z" -NoNewline
    $Records = dnscmd.exe $DnsServer /ZonePrint $z # Get all records and parse them looking for duplicates below.
    #Write-Host -Fore Yellow " -- Num lines:" $Records.Count
    $Records | ForEach-Object -Begin { $IpHash = @{}; $Ctr = 0 } `
        -Process {
            if ($_ -match '^([\d.]+)\s+.*PTR\s+(\S+)\.$') {
                $IpEndReversed = $matches[1]
                #Write-Verbose $IpEndReversed
                $Ctr++
                Start-Sleep -Milliseconds 25
                $TempArray = $z -replace '\.in-addr\.arpa$' -split '\.'
                $IpStart = ($TempArray[-1..-($TempArray.Count)]) -join '.'
                $TempArray = $IpEndReversed -split '\.'
                $IpEnd = ($TempArray[-1..-($TempArray.Count)]) -join '.'
                #Write-Verbose $IpEnd
                $Ip = "$IpStart.$IpEnd"
                
                #Write-Verbose "Looking up $Ip..."
                $ErrorActionPreference = 'Stop'
                try {
                    $Dns = [Net.Dns]::GetHostByAddress($Ip)
                    # This means we found a duplicate. Send an object down the pipeline.
                    if ($Dns.Aliases -match '\S') {
                        New-Object psobject -Property @{
                            IP = $Ip
                            HostName = $Dns.HostName -join ', '
                            Aliases = $Dns.Aliases -join ', '
                            Error = $null
                        }
                    }
                }
                catch {
                    New-Object psobject -Property @{
                        IP = $Ip
                        HostName = $null
                        Aliases = $null
                        Error = $_
                    }
                }
                $ErrorActionPreference = 'Continue'
            }
        }
    Write-Host -Fore Yellow " -- Num records:" $Ctr
    #Write-Verbose "Processed $Ctr PTR records for $z"
    #Start-Sleep -Seconds 3
}

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