Blog GL2i

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====== DNS ======

Here I want to show how huge the difference is between a string and an object.

===== Initial request =====

Let's suppose you have to perform a lot of DNS modifications because a service provider of yours has changed IP server. Since you are wise, you are going to follow the rule: Trust but verify.

  - Check input: maybe some mistakes in the request
  - Provide a preCheck report to the requester

This is the example we are going to deal with:

<csv>
A_Record,Provider
google.ca,p1
google.com,p1
google.fr,p2
google.toto,p3
</csv>

===== Trap to avoid =====

Sysadmins are used to execute nslookup command to query DNS servers. Let's put it into a powershell script.

<file powershell dns_v1.ps1 >
Import-Csv -Path input_dns.csv | foreach {
  nslookup $_.A_Record 8.8.8.8
}
</file>

Output is a nightmare.

<code>
nslookup : Non-authoritative answer:
At line:1 char:1
+ nslookup google.ca 8.8.8.8
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (Non-authoritative answer::String) [], RemoteException
    + FullyQualifiedErrorId : NativeCommandError
 
Server:  dns.google
Address:  8.8.8.8

Name:    google.ca
Addresses:  2607:f8b0:4020:805::2003
	  172.217.13.131
</code>

Here is an issues list:

  - Quite difficult to parse results
  - nslookup is not a built-in Powershell command, as a result we don't get answer as kind of DNS object
  - Really hard to create preCheck output file

===== the right tool for the right problem =====

Recent versions of PowerShell includes the perfect command for this request: Resolve-Dns.

<file powershell dns_v2.ps1 >
Import-Csv -Path input_dns.csv | foreach {
  Resolve-DnsName $_.A_Record -Type A -Server 8.8.8.8
}
</file>

Now script outputs something we can easily manage and export.

<code>
Name                                           Type   TTL   Section    IPAddress                                
----                                           ----   ---   -------    ---------                                
google.ca                                      A      49    Answer     172.217.13.131                           
google.com                                     A      299   Answer     172.217.13.110                           
google.fr                                      A      299   Answer     172.217.13.163                           
Resolve-DnsName : google.toto : DNS name does not exist
At line:2 char:7
+       Resolve-DnsName $_.A_Record -Type A -Server 8.8.8.8
+       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : ResourceUnavailable: (google.toto:String) [Resolve-DnsName], Win32Exception
    + FullyQualifiedErrorId : DNS_ERROR_RCODE_NAME_ERROR,Microsoft.DnsClient.Commands.ResolveDnsName
</code>

We still raise an exception from google.toto record. We are going to use a try-catch block to handle this.

<file powershell dns_v3.ps1>
Import-Csv -Path input_dns.csv | foreach {
    $currentLine = $_
    $record = $currentLine.A_Record
    try {
        $result = Resolve-DnsName $record -Type A -Server 8.8.8.8 2> $null
        Write-Host $result.getType().fullname
    } catch {
        Write-Host ("Error with record: {0}" -f $record)
    }   
}
</file>

When the DNS A record exists, we get an object from Microsoft.DnsClient.Commands.DnsRecord_A class. Otherwise we catch the exception properly.


<code>
Microsoft.DnsClient.Commands.DnsRecord_A
Microsoft.DnsClient.Commands.DnsRecord_A
Microsoft.DnsClient.Commands.DnsRecord_A
Error with record: google.toto

Name                                           Type   TTL   Section    IPAddress                                
----                                           ----   ---   -------    ---------                                
google.ca                                      A      299   Answer     172.217.13.131                           
google.com                                     A      299   Answer     172.217.13.110                           
google.fr                                      A      299   Answer     172.217.13.163 
</code>

===== Final script =====

Reminder: we want to verify if all DNS A records provided by the requester are correct.

<file powershell dns_final.ps1>
$results = @()

Import-Csv -Path input_dns.csv | foreach {
    $currentLine = $_
    $record = $currentLine.A_Record
    try {
        $result = Resolve-DnsName $record -Type A -Server 8.8.8.8 2> $null
        Write-Host $result.getType().fullname
        $exportLine = $result | Select-Object -Property Name,Type,IPAddress
        $exportLine | Add-Member -MemberType NoteProperty -Name "Status" -Value "ok"
    } catch {
        Write-Host ("Error with record: {0}" -f $record)
        $exportLine = New-Object psobject -Property @{
            'Name' = $record;
            'Type' = 'A';
            'IPAddress' = '';
            'Status' = 'ko';
        }
    }   
    $results += $exportLine
}

$results | Export-Csv -Path dnsPreCheck.csv -Delimiter "," -Encoding UTF8 -NoTypeInformation
</file>

From the result we can easily identify names without DNS A records. Mission accomplished :-)

<csv>
"Name","Type","IPAddress","Status"
"google.ca","A","172.217.13.131","ok"
"google.com","A","172.217.13.110","ok"
"google.fr","A","172.217.13.163","ok"
"google.toto","A","","ko"
</csv>