DNS name from IP address - powershell

Question

I need to create a list of IP addresses and DNS names. I
am trying to get DNS names from IP addresses. I have tried two ways:
try/catch but it ends afterwards.
Without and it just outputs DNS names that I can't relate to the IP addresses.
Here's what I have so far:
#try {
Get-Content C:\Users\pintose\Documents\IP_Address.txt | ForEach-Object
{([system.net.dns]::GetHostByAddress($_)).hostname >> C:\Users\user\Documents\hostname.txt}
# }
# catch {
if ($_.Exception.Message -like "*The requested name is valid*") {
Write-Output "UNREACHABLE" | Out-File C:\Users\user\Documents\hostname.txt }
# }

Answer 1

Try this solution:
$outFile = "C:\Users\user\Documents\hostname.txt"
Get-Content C:\Users\pintose\Documents\IP_Address.txt | ForEach-Object {
$hash = #{ IPAddress = $_
hostname = "n/a"
}
$hash.hostname = ([system.net.dns]::GetHostByAddress($_)).hostname
$object = New-Object psobject -Property $hash
Export-CSV -InputObject $object -Path $outFile -Append -NoTypeInformation
}
We create a objects, that have the IPaddress in it and a hostname n/a if it cannot be resolved. Then, the object gets exported into the file. You'll get something like:
192.0.0.1; Server1

Answer 2

This uses a workflow so it can do parallel foreach
Workflow Get-DNSNames([string[]]$IPAddresses){
foreach -parallel ($IP in $IPAddresses){
try{
#{$IP = $(([system.net.dns]::GetHostByAddress($IP)).hostname)}
}catch{
#{$IP = "N/A"}
}
}
}
$List = Get-DNSNames -IPAddresses $(Get-Content "C:\IPAddresses.txt").Split("[\r\n]")
$List | Out-File "C:\IPAddresses_Complete.txt"

Answer 3

You might want to try the other solutions offered here, but here are some things you might want to think about.
First, I'd recommend not putting the try{}catch{} around the whole of the first command. If you are looping through data and just one of them causes an exception, you risk not completing the task. Put the try{}catch{} around just the "risky" line of code:
Get-Content C:\Users\pintose\Documents\IP_Address.txt | Foreach-Object {
try {
([system.net.dns]::GetHostByAddress($_)).hostname >> C:\Users\user\Documents\hostname.txt
}
catch {
if ($_.Exception.Message -like "*The requested name is valid*") {
Write-Output "UNREACHABLE" | Out-File C:\Users\user\Documents\hostname.txt
}
}
}
When you catch the exception, you only write to the text file in the case that "the requested name is valid" (do you mean invalid?). You never write anything to the file otherwise. Thus, going back to your original code:
IF there is an exception caused by ANY of the IP addresses
AND the exception is NOT "the requested name is valid" (which I think might be a typo?)
THEN no error gets written to the file and the script ends without necessarily completing all the IP addresses.
Other things:
You use two methods to write to the file: >> and Out-File. Probably better to use the PowerShell cmdlet but with the -Append switch to ensure you append to the end of the file:
([system.net.dns]::GetHostByAddress($_)).hostname | Out-File C:\Users\user\Documents\hostname.txt -Append
Write-Output "UNREACHABLE" | Out-File C:\Users\user\Documents\hostname.txt -Append
#restless1987 has suggested a way to ensure you write both the IP address and the hostname (if determined) to the output file. I'd have a look at that to work out what is going on.
My final tip would be to be wary of reading in from .txt files with Get-Content. They often have trailing (blank) lines and you might want to try to ignore such blanks. Probably not a big issue in this case as it will just mean a failed DNS attempt, but I have seen such things wreak havoc on every mailbox in a (very) large company when used with other commands.

Answer 4

Another way...
$ip_list = Get-Content ./IP_Address.txt
foreach ($ip in $ip_list) {
try {
([system.net.dns]::GetHostByAddress($ip)).hostname |
Out-File -FilePath ./hostname.txt -Append
}
catch {
if ($_.Exception.Message -like "*The requested name is valid*") {
Write-Output "UNREACHABLE" | Out-File -FilePath './hostname.txt' -Append }
}
}

Answer 5

There are many tools that can accomplish this, but if you need a quick and dirty solution that you can run just about anywhere this will get the job done.
Using the eternally useful ps tools such as psloggedon /accepteula \\computername or ip address you can get who is currently logged in to check if this is the correct machine. For example:
c:\pstools>psloggedon /accepteula \\10.0.0.10
loggedon v1.33 - See who's logged on
Copyright ⌐ 2000-2006 Mark Russinovich
Sysinternals - www.sysinternals.com
Users logged on locally:
Error: could not retrieve logon time
NT AUTHORITY\LOCAL SERVICE
Error: could not retrieve logon time
NT AUTHORITY\NETWORK SERVICE
1/12/2015 8:06:51 AM DOMAIN\user
Error: could not retrieve logon time
NT AUTHORITY\SYSTEM
Users logged on via resource shares:
1/17/2015 2:26:43 PM DOMAIN\user
Now that you have confirmed that this IP address is the correct one for this user. We just need to lookup the IP address using nslookup.
C:\>nslookup 10.0.0.10
Server: server.domain.com
Address: 10.10.0.1
Name: Workstation07.server.domain.com
Address: 10.0.0.10
Now we know that the computer name for that computer is Workstation07.