Tag Archives: DNS

Parsing nslookup: Only Return the Resolved IP

It was late last Friday evening when a vendor and I worked though some maintenance. As a part of the work (which actually lasted well into the weekend), we needed to remove two DNS A records and replace them with CNAME records. I figured that before the change, I would return the current resolution results, so I would be able to ensure the DNS change took place, after the night’s weekend’s work.

You’re probably familiar with the standard output of nslookup.

PS> nslookup subdomain.mydomain.com
Server:  mydomain.com
Address:  10.10.10.1

Name:    subdomain.mydomain.com
Address:  10.20.30.40

These results indicate the name, subdomain.mydomain.com, resolves to an IP address of 10.20.30.40 when using the mydomain.com nameserver, at 10.10.10.1. I wanted to clean up this output, and since I had time to kill while fighting off sleep, I figured — why not? I started by piping the results of nslookup to the Select-String cmdlet looking for the string, ‘Address.’ As you can see in the example below, it returns both the IP of the server where the resolution took place, and the results. After that, there doesn’t seem to be a way to distinguish one line from the other. I needed another way to only return the results.

PS> nslookup subdomain.mydomain.com | Select-String Address

Address: 10.10.10.1
Address: 10.20.30.40

I decided to pipe the command above to the Get-Member cmdlet to see what kind of object the Select-String cmdlet returned. The property that jumped out at me right away was LineNumber — was it really going to be that easy?

   TypeName: Microsoft.PowerShell.Commands.MatchInfo

Name         MemberType Definition
----         ---------- ----------
Equals       Method     bool Equals(System.Object obj)
GetHashCode  Method     int GetHashCode()
GetType      Method     type GetType()
RelativePath Method     string RelativePath(string directory)
ToString     Method     string ToString(), string ToString(string directory)
Context      Property   Microsoft.PowerShell.Commands.MatchInfoContext Context {get;set;}
Filename     Property   string Filename {get;}
IgnoreCase   Property   bool IgnoreCase {get;set;}
Line         Property   string Line {get;set;}
LineNumber   Property   int LineNumber {get;set;}
Matches      Property   System.Text.RegularExpressions.Match[] Matches {get;set;}
Path         Property   string Path {get;set;}
Pattern      Property   string Pattern {get;set;}

Here’s the thing, the output of nslookup is always the same: this on a this line, and that on that line. This is likely why the LineNumber property looked so promising, so quickly. Now that we know we have this property, let’s try and use it. While my first line number guess worked (read, dumb luck), let’s do it wrong first. Consider again our most recent output below.

PS> nslookup subdomain.mydomain.com | Select-String Address

Address: 10.10.10.1
Address: 10.20.30.40

You might think you would want line 3 of our output, but that’s not correct. Even though Select-String has trimmed down the output, the line numbers are all still intact. Look at the very first example at the top of this post. Line 3 is blank — it’s line 5 that we want! Take a look.

PS> nslookup subdomain.mydomain.com | Select-String Address | Where-Object LineNumber -eq 5

Address: 10.20.30.40

If you want to store this as as a string, use the .ToString() method.

PS> (nslookup subdomain.mydomain.com | Select-String Address | Where-Object LineNumber -eq 5).ToString()
Address: 10.20.30.40

And, if you want to remove the word “address,” the colon, and the space, in order to only return the IP address, then you can parse the string by using the .Split() method. The example below splits the string at the space and return the last element — what’s on the right side of the space.

PS> (nslookup subdomain.mydomain.com | Select-String Address | Where-Object LineNumber -eq 5).ToString().Split(' ')[-1]
10.20.30.40

I hope this was helpful for someone. Until next time.

Find DNS Servers Being Used by DHCP Scopes

Download the Get-TMDhcpDNS function here: https://gallery.technet.microsoft.com/Find-DNS-Servers-Being-640978d1

There was a Microsoft TechNet forum post last week regarding trying to obtain the DNS servers being used by different DHCP Scopes (Here’s the link: https://social.technet.microsoft.com/Forums/en-US/58723d31-7586-40c3-acd2-183f20b49daf/how-to-dump-the-dns-options-for-each-dhcp-scope?forum=ITCG).

I originally thought the person wanted to get the DNS Servers listed in the Server Options (what the scopes use by default), until he (or she) better clued me in to wanting the DNS Servers listed in the Scope Options, when there was one. At times there won’t be any DNS Servers listed in the Scope Options, and it’ll use the Server Options instead.

Since that forum post, I wrapped up a few commands to create a full-service advanced function. Get-TMDhcpDNS function will collect the Scope Name, Scope ID, DNS Servers, and whether the DNS Servers assigned, are done so in the Scope Options or Server Options.

If you think this might be helpful for you or someone else, then please download it, test it, and rate it. Thanks, and here’s an example of the function in action:

PS C:\> Get-TMDhcpDNS -ComputerName 'dhcpsrv1.mydomain.com' | Format-Table -AutoSize

Name                  ScopeName  ScopeID     DNS                                    ScopeOrServerDNS
----                  ---------  -------     ---                                    ----------------
dhcpsrv1.mydomain.com Building01 10.10.10.0  10.10.10.10.20,10.10.10.21,10.10.10.22 Scope
dhcpsrv1.mydomain.com Building02 172.16.16.0 172.16.16.5,172.16.16.15               Server

Use the link above to download the function.

Update: There was an issue with the DNS Server (System.Object[]) when piping the function to Export-Csv. That’s been corrected in 1.0.2. Here’s a post I had to reference (again): http://learn-powershell.net/2014/01/24/avoiding-system-object-or-similar-output-when-using-export-csv/.