Getting the hostname from an IP address (or vice versa) is no great magic, and can be done easily with PowerShell, by simply leveraging .Net to do the work.
We’ve all done something like this to get resolve an IP address:
[System.Net.Dns]::GetHostEntry("69.69.95.133").HostName
Or this to get an IP address from a hostname:
[System.Net.DNS]::GetHostAddresses("www.fortypoundhead.com").IPAddressToString
But did you ever notice that sea of red when you specify an invalid IP address or hostname? When you do that, you’ll get a big,red error message similar to the following:
[System.Net.Dns]::gethostentry("128.254.95.254").HostName
Exception calling "GetHostEntry" with "1" argument(s): "No such host is known"
At line:1 char:1
+ [System.Net.Dns]::gethostentry("128.254.95.254").HostName
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : NotSpecified: (:) [], MethodInvocationException
+ FullyQualifiedErrorId : SocketException
I recently got asked how that nonsense could be avoided. Well, the answer is to simply encapsulate the process in a function that wraps up the .Net method to get the data back. It’s pretty simple, and here is the code:
[CmdletBinding()]
Param(
[Parameter(Mandatory=$True,Position=1)]
[string]$IPAddress
)
# noise if we fail
$ErrorActionPreference = "silentlycontinue"
$Result = $null
# Pass the IP to .Net for name resolution.
$result = [System.Net.Dns]::gethostentry($IPAddress)
# process the results
If ($Result)
{
$MyResult = [string]$Result.HostName
}
Else
{
$MyResult = "unresolved"
}
# Send it to the output
$MyResult
Alternatively, you could also plug this directly into a standalone script. For example, lets say that you have a list of IP addresses in a file that you want to resolve to host names. No problem:# Get list from file, initialize empty array
$ListOfIPs=Get-content ".\hosts.txt"
$ResultList = @()
# roll through the list, resolving as we
# go with the .Net DNS resolver
foreach ($IP in $ListOfIPs)
{
# We don't want to see any errors as we go, right?
$ErrorActionPreference = "silentlycontinue"
$Result = $null
# status to user, just so they know that something
# is still happening, then pass the current IP
# to .Net for name resolution.
write-host "resolving $IP"
$result = [System.Net.Dns]::gethostentry($IP)
# Enter into the array, with the returned results.
If ($Result)
{
$ResultList += "$IP," + [string]$Result.HostName
}
Else
{
$ResultList += "$IP,unresolved"
}
}
# send it out to a file, and inform the user we are done
$ResultList | Out-File .\resolved.txt
write-host "name resolution complete"
Again, pretty straightforward.
First, it grabs the list of IP addresses from a file called hosts.txt. Next, the script runs through the list, resolving the IP addresses (if it can), storing the results in $ResultList. Finally, the output is sent to another file called resolved.txt
I hope this is useful to someone, and drop a note in the comments if you have questions.