My Corporate DNS Server was lying to me, often.

I was trying to copy some files to a group to a computers. What I’d do is check if the computer is alive first, by doing a network ping. If the computer answered the Ping, then the computer was alive, so I would then copy files to it.

It mostly worked, except when it didn’t. Here is the clue which lead me to the answer:

C:\>ping alfred

Pinging ALFRED [192.168.1.5] with 32 bytes of data: Reply from 192.168.1.5: bytes=32 time<1ms TTL=128 … C:\>ping powergirl

Pinging POWERGIRL [192.168.1.5] with 32 bytes of data: Reply from 192.168.1.5: bytes=32 time<1ms TTL=128

C:\>

The DNS server was returning the same IP address for two different computers. As POWERGIRL was powered on, it answered the Ping request for ALFRED. When I tried to copy to ALFRED, it failed as the file copy was trying to verify that ALFRED was at 192.168.1.5, and it wasn’t.

The cause? The DNS server was retaining IP addresses for 24 hours. Sometimes a computer’s IP will change more often than that. So the DNS server ended up with 2 or more DNS computer records with the same IP.
The fix? Tell the DNS server to only retain DNS records for 2 hours. (for the more technically minded, we adjusted the Time To Live value).

How I originally detected the DNS server was lying to me?
I pinged the computer (ALFRED) to get it’s IP address (192.168.1.5).
I then asked IP address 192.168.1.5 what it’s computer name was. In the example above, it was actually POWERGIRL.

The code to do that is in the Reachable function, below:

Function Reachable(strComputer) Dim wmiQuery, objWMIService, objPing, objStatus,sIPAddress,sComputerName wmiQuery = "Select * From Win32_PingStatus Where Address = '" & strComputer & "'" Set objWMIService = GetObject("winmgmts:\\.\root\cimv2") Set objPing = objWMIService.ExecQuery(wmiQuery) For Each objStatus In objPing If IsNull(objStatus.StatusCode) Or objStatus.Statuscode<>0 Then Reachable = False 'if computer is unreacable, return false Else Reachable = True 'if computer is reachable, return true 'wscript.echo strComputer & " : " & objStatus.ProtocolAddress sComputerName = strComputer sIPAddress = objStatus.ProtocolAddress If objStatus.ProtocolAddressResolved <> "" then wscript.echo strComputer & " : " & objStatus.ProtocolAddressResolved End If On Error Resume Next ' ask the computer at IP address sIPAddress, what it's name is. Set objWMIService2 = GetObject("winmgmts:" & "{impersonationLevel=impersonate}!\\" & sIPAddress & "\root\cimv2") Set colComputers2 = objWMIService2.ExecQuery ("Select * from Win32_ComputerSystem")

        For Each objComputer2 in colComputers2
            If strComputer <> objComputer2.Name Then
                ‘ dump out that the computer we PING is not the computer which actually answered.
                WScript.Echo strComputer + " : " +objComputer2.Name
            Else
                ‘ the computer which answered is the computer we pinged.
                WScript.Echo objComputer2.Name
            End if
        Next
        On Error GoTo 0
      End If
Next

End Function

My Corporate DNS Server was lying to me, often.

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