Perform a Forward and Reverse DNS Lookup in VBScript – DNSLookup – ReverseDNSLookup – IsIP

Necessity is the mother of invention as they say. Today I had to write a couple of scripts to translate DNS names into IP addresses and vice versa. Since there is no easy way to do this in VBScript, I whipped up two (well three) quick little functions that use the nslookup command to preform the lookup using the computers default DNS name servers.

They are pretty straight forward to use, for the DNSLookup function you can pass any IP address and it will try to resolve. If it is successful it will return the fully qualified domain name. (Aliases are not returned). If it fails it will return “Failed.” It also checks the string that you pass with another new function called IsIP. This function checks any string you pass to see if it qualifies as a legit IP. If it is an IP the IsIP function returns “True”. If not then it returns “False”.

The ReverseDNSLookup will translate a fully qualified domain name into an IP address. Or if you have DNS suffix appending turned on then it will resolve computer names that are not fully qualified. If it fails to resolve to an IP the function returns “Failed.”

Function ReverseDNSLookup(sIPAddress)
  'This script is provided under the Creative Commons license located
  'at http://creativecommons.org/licenses/by-nc/2.5/ . It may not
  'be used for commercial purposes with out the expressed written consent
  'of NateRice.com

  If Not IsIP(sIPAddress) Then
    ReverseDNSLookup = "Failed: Invalid IP."
    Exit Function
  End If

  Const OpenAsDefault = -2
  Const FailIfNotExist = 0
  Const ForReading = 1

  Set oShell = CreateObject("WScript.Shell")
  Set oFSO = CreateObject("Scripting.FileSystemObject")
  sTemp = oShell.ExpandEnvironmentStrings("%TEMP%")
  sTempFile = sTemp & "\" & oFSO.GetTempName

  oShell.Run "%comspec% /c nslookup " & sIPAddress & ">" & sTempFile, 0, True

  Set fFile = oFSO.OpenTextFile(sTempFile, ForReading, FailIfNotExist, OpenAsDefault)
  sResults = fFile.ReadAll
  fFile.Close
  oFSO.DeleteFile (sTempFile)

  If InStr(sResults, "Name:") Then
    aNameTemp = Split(sResults, "Name:")
    aName = Split(Trim(aNameTemp(1)), Chr(13))
  
    ReverseDNSLookup = aName(0)
  Else
    ReverseDNSLookup = "Failed."
  End If
  
  Set oShell = Nothing
  Set oFSO = Nothing
End Function

Function DNSLookup(sAlias)
  'This script is provided under the Creative Commons license located
  'at http://creativecommons.org/licenses/by-nc/2.5/ . It may not
  'be used for commercial purposes with out the expressed written consent
  'of NateRice.com

  If len(sAlias) = 0 Then
    DNSLookup = "Failed."
    Exit Function
  End If

  Const OpenAsDefault = -2
  Const FailIfNotExist = 0
  Const ForReading = 1

  Set oShell = CreateObject("WScript.Shell")
  Set oFSO = CreateObject("Scripting.FileSystemObject")
  sTemp = oShell.ExpandEnvironmentStrings("%TEMP%")
  sTempFile = sTemp & "\" & oFSO.GetTempName

  oShell.Run "%comspec% /c nslookup " & sAlias & ">" & sTempFile, 0, True

  Set fFile = oFSO.OpenTextFile(sTempFile, ForReading, FailIfNotExist, OpenAsDefault)
  sResults = fFile.ReadAll
  fFile.Close
  oFSO.DeleteFile (sTempFile)

  aIP = Split(sResults, "Address:")

  If UBound(aIP) < 2 Then
    DNSLookup = "Failed."
  Else
    aIPTemp = Split(aIP(2), Chr(13))
    DNSLookup = trim(aIPTemp(0))
  End If
  
  Set oShell = Nothing
  Set oFSO = Nothing
End Function

Function IsIP(sIPAddress)
  'This script is provided under the Creative Commons license located
  'at http://creativecommons.org/licenses/by-nc/2.5/ . It may not
  'be used for commercial purposes with out the expressed written consent
  'of NateRice.com

  aOctets = Split(sIPAddress,".")
  
  If IsArray(aOctets) Then
    If UBound(aOctets) = 3 Then
      For Each sOctet In aOctets
        On Error Resume Next
        sOctet = Trim(sOctet)
        sOctet = sOctet + 0
        On Error Goto 0
        
        If IsNumeric(sOctet) Then
          If sOctet < 0 Or sOctet > 256 Then
            IsIP = False
            Exit Function
          End If
        Else
          IsIP = False
          Exit Function
        End If
      Next
      IsIP = True
    Else
      IsIP = False
    End If
  Else
    IsIP = False
  End If
End Function