Friday, January 9, 2009

Finding duplicate DNS records

Internal DNS in many organisations is usually a little messy and if tasked with cleaning it, my first step would be to identify duplicate records in DNS - either an IP address that is recorded against multiple host names or a host name with multiple host (A) records. Assuming you have a file containing one record per line, this is fairly simple to report on.

The following examples use dnscmd.exe, which assumes Windows 2000/2003 DNS, but you could use any input, as long as it contains a name and an IP.

The two commands:

  1. dnscmd %dnsServer% /enumrecords test.local @ /additional /continue | find /i /v "Aging:" | find /i "192.168" > DNSRecords.txt
  2. echo. > DuplicateIPs.txt & (for /f "tokens=1,4" %i in (DNSRecords.txt) do @if "%j" NEQ "" @find /i "%j" DuplicateIPs.txt >nul & if errorlevel 1 for /f "tokens=1" %m in ('"findstr /i "%j$" DNSRecords.txt | find /i /c "%j""') do @if %m GTR 1 (@echo %j,%m: & findstr /i "%j$" DNSRecords.txt & echo.) >> DuplicateIPs.txt) & type DuplicateIPs.txt

The first command uses dnscmd to enumerate the records from the root of the test.local zone and exclude aging (dynamic DNS) records. The last find command further filters the output by IP, which can be useful when targeting specific subnets/sites. You might also want to check aging dynamic records instead of static to see how well scavenging is working – if you do, just change the tokens to 1,5 instead of 1,4 (as the aging data is another token separating the name and IP).

The second command:
  1. Creates a new file called DuplicateIPs.txt in the current working directory
  2. Iterates through each line in the DNS record dump, extracting the first and fourth token (Name and IP)
  3. The commands in the first FOR loop check there is a value, then check the value hasn't already been processed in the duplicate list (otherwise you'd have duplicates of each duplicate) and then counts the duplicates and appends them to the file.
  4. The final command types the duplicate IP file created by the for loop iteration.

For example, supposing your DNS export contained the following records:

printer1 3600 A
printer2 3600 A
printer3 3600 A
printer4 3600 A
printer5 3600 A
printer6 3600 A
printer7 3600 A

After running the second command above, a file called DuplicateIPs.txt would be created and then typed to the prompt:,3:
printer1 3600 A
printer3 3600 A
printer6 3600 A,2:
printer4 3600 A
printer7 3600 A

The command below is modified to report duplicate names instead of IP addresses. This was done by using the first token (%i) instead of the second (%j), and modifying the findstr command to use a literal string search ending with a space rather than the regular expression EOL:

echo. > DuplicateIPs.txt & (for /f "tokens=1,4" %i in (DNSRecords.txt) do @if "%i" NEQ "" @find /i "%i" DuplicateIPs.txt >nul & if errorlevel 1 for /f "tokens=1" %m in ('"findstr /i /c:"%i " DNSRecords.txt | find /i /c "%i""') do @if %m GTR 1 (@echo %i,%m: & findstr /i /c:"%i " DNSRecords.txt & echo.) >> DuplicateIPs.txt) & type DuplicateIPs.txt

If you wanted a summary rather than the detail of each duplicate, you could also run the following command:

echo. > DuplicateIPSummary.txt & (for /f "tokens=1,4" %i in (DNSRecords.txt) do @if "%j" NEQ "" @find /i "%j" DuplicateIPSummary.txt >nul & if errorlevel 1 for /f "tokens=1" %m in ('"findstr /i "%j$" DNSRecords.txt | find /i /c "%j""') do @if %m GTR 1 (@echo %j,%m) >> DuplicateIPSummary.txt) & type DuplicateIPSummary.txt

In the example above, this would produce the following report:,3,2

I use this sort of command to generate reports on duplicates, in this case from DNS, but it could also be useful in DHCP, WINS, or any number of Active Directory objects/attributes. People (myself included) are often wary of automated processes that make changes, but this is an excellent example of how powerful read-only automated commands can be – you can take thousands of objects and produce a report in seconds to quickly identify inconsistencies in an environment.

Dnscmd Overview


Anonymous said...

Trying to use your DOS script to find duplicate names in the MS DNS export txt file... After removing the "data" column I was able to get the script to work. sortof..

It seems it's including any hostname that contains a matching ending... eg..


any suggestions?

Anonymous said...

findstr /i /c:"%i " DNSRecords.txt
doesn't work correctly for me.
With that expression both strings "otherpc " and "anotherpc " are considered to be equal.
I have to use :
findstr /i /l /c:"%i " DNSRecords.txt
to recognize these words.

Anonymous said...

Parameter /l in my previous post is wrong.

correct is /b :
findstr /i /b /c:"%i " DNSRecords.txt

All Posts

printQueue AD objects for 2003 ClusterVirtualCenter Physical to VirtualVirtual 2003 MSCS Cluster in ESX VI3
Finding duplicate DNS recordsCommand-line automation – Echo and macrosCommand-line automation – set
Command-line automation - errorlevels and ifCommand-line automation - find and findstrBuilding blocks of command-line automation - FOR
Useful PowerShell command-line operationsMSCS 2003 Cluster Virtual Server ComponentsServer-side process for simple file access
OpsMgr 2007 performance script - VMware datastores...Enumerating URLs in Internet ExplorerNTLM Trusts between 2003 and NT4
2003 Servers with Hibernation enabledReading Shortcuts with PowerShell and VBSModifying DLL Resources
Automatically mapping printersSimple string encryption with PowerShellUseful NTFS and security command-line operations
Useful Windows Printer command-line operationsUseful Windows MSCS Cluster command-line operation...Useful VMware ESX and VC command-line operations
Useful general command-line operationsUseful DNS, DHCP and WINS command-line operationsUseful Active Directory command-line operations
Useful command-linesCreating secedit templates with PowerShellFixing Permissions with NTFS intra-volume moves
Converting filetime with vbs and PowerShellDifference between bat and cmdReplica Domain for Authentication
Troubleshooting Windows PrintingRenaming a user account in ADOpsMgr 2007 Reports - Sorting, Filtering, Charting...
WMIC XSL CSV output formattingEnumerating File Server ResourcesWMIC Custom Alias and Format
AD site discoveryPassing Parameters between OpsMgr and SSRSAnalyzing Windows Kernel Dumps
Process list with command-line argumentsOpsMgr 2007 Customized Reporting - SQL QueriesPreventing accidental NTFS data moves
FSRM and NTFS Quotas in 2003 R2PowerShell Deleting NTFS Alternate Data StreamsNTFS links - reparse, symbolic, hard, junction
IE Warnings when files are executedPowerShell Low-level keyboard hookCross-forest authentication and GP processing
Deleting Invalid SMS 2003 Distribution PointsCross-forest authentication and site synchronizati...Determining AD attribute replication
AD Security vs Distribution GroupsTroubleshooting cross-forest trust secure channels...RIS cross-domain access
Large SMS Web Reports return Error 500Troubleshooting SMS 2003 MP and SLPRemotely determine physical memory
VMware SDK with PowershellSpinning Excel Pie ChartPoke-Info PowerShell script
Reading web content with PowerShellAutomated Cluster File Security and PurgingManaging printers at the command-line
File System Filters and minifiltersOpsMgr 2007 SSRS Reports using SQL 2005 XMLAccess Based Enumeration in 2003 and MSCS
Find VM snapshots in ESX/VCComparing MSCS/VMware/DFS File & PrintModifying Exchange mailbox permissions
Nested 'for /f' catch-allPowerShell FindFirstFileW bypassing MAX_PATHRunning PowerSell Scripts from ASP.Net
Binary <-> Hex String files with PowershellOpsMgr 2007 Current Performance InstancesImpersonating a user without passwords
Running a process in the secure winlogon desktopShadow an XP Terminal Services sessionFind where a user is logged on from
Active Directory _msdcs DNS zonesUnlocking XP/2003 without passwords2003 Cluster-enabled scheduled tasks
Purging aged files from the filesystemFinding customised ADM templates in ADDomain local security groups for cross-forest secu...
Account Management eventlog auditingVMware cluster/Virtual Center StatisticsRunning scheduled tasks as a non-administrator
Audit Windows 2003 print server usageActive Directory DiagnosticsViewing NTFS information with nfi and diskedit
Performance Tuning for 2003 File ServersChecking ESX/VC VMs for snapshotsShowing non-persistent devices in device manager
Implementing an MSCS 2003 server clusterFinding users on a subnetWMI filter for subnet filtered Group Policy
Testing DNS records for scavengingRefreshing Computer Account AD Group MembershipTesting Network Ports from Windows
Using Recovery Console with RISPAE Boot.ini Switch for DEP or 4GB+ memoryUsing 32-bit COM objects on x64 platforms
Active Directory Organizational Unit (OU) DesignTroubleshooting computer accounts in an Active Dir...260+ character MAX_PATH limitations in filenames
Create or modify a security template for NTFS perm...Find where a user is connecting from through WMISDDL syntax in secedit security templates

About Me

I’ve worked in IT for over 20 years, and I know just about enough to realise that I don’t know very much.