Managing netbootGuid

  • Article

The attribute on AD computer objects netbootGuid is very important to allowing some deployment options (RIS and WDS) to locate the computer object the hardware belongs to. Unfortunately certain activities, such as replacing the system board, swapping in new hardware but using a pre-existing computer name, or creating a new VM using an existing VHD can invalidate the accuracy of the netbootGuid attribute stored in AD.

I wrote the script below (more precisely cobbled together since my VBScript is very rusty) to automatically check to make sure the netbootGuid is correct and update it if it isn't. This script can be deployed as a startup script via Group Policies to ensure every time the computer boots, the netbootGuid will be updated if needed. However, in order for this to work, the ACLs on the computer objects must be changed to allow SELF to write to the netbootGuid property.

References (from whence I cobbled):

Note: I'm not entirely fond of having to write out to a temp file to turn the GUID into a byte array. But it works. I'm open to feedback on alternate methods as to how I can rewrite the function baConvertGuidToByteArray without writing out to the temporary file. Thanks in advance.

Note: Updated code 1/6/2014 based on comment below.

<VBScript>

 'https://support.microsoft.com/kb/302467
'The sample uses WMI to return the UUID on the system.
'If a UUID can not be found on the system it returns all F's.
'What RIS does in this case is it uses a zero'd out version of the MAC 
'address of the NIC the machine is booting off of. 
'This sample will return the value required to set the 
'netbootGUID attribute

Option Explicit

Call UpdateNetbootGuid(guidGetUUID, szGetDn)

Function guidGetUUID
    Dim SystemSet, SystemItem, NetworkAdapterSet, NetworkAdapter

    Set SystemSet = GetObject("winmgmts:").InstancesOf ("Win32_ComputerSystemProduct")
    For Each SystemItem In SystemSet
        If SystemItem.UUID = "FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF" Then
            Set NetworkAdapterSet = GetObject("winmgmts:").InstancesOf ("Win32_NetworkAdapter")
             For Each NetworkAdapter In NetworkAdapterSet
                If NetworkAdapter.AdapterType = "Ethernet 802.3" And NetworkAdapter.Description <> "Packet Scheduler Miniport" Then
                    guidGetUUID = "00000000-0000-0000-0000-" & Replace(NetworkAdapter.MACAddress, ":", "")
                End If
            Next
        Else
            guidGetUUID = SystemItem.UUID
        End If            
    Next
End Function

Function szGetDN
    Dim objSysInfo 
    Set objSysInfo = CreateObject("ADSystemInfo")
    'Set DN to upper Case
    szGetDN = UCase(objSysInfo.ComputerName)
End Function

Sub UpdateNetbootGuid(guidUUID, szComputerDn)
    Dim oComputer

    'Get parentcontainer
    Set oComputer = GetObject("LDAP://" & szComputerDn)

    If ByteArrayToGuid(oComputer.netbootGuid) <> guidUUID Then
        oComputer.Put "netbootGuid", baConvertGuidToByteArray(guidUUID)
        oComputer.SetInfo
    End If

    'Clean up
     Set oComputer = Nothing
End Sub

Function ByteArrayToGuid(arrbytOctet)
If Not IsEmpty(arrbytOctet) Then
    ByteArrayToGuid = _
        Right("0" & Hex(AscB(MidB(arrbytOctet, 4, 1))), 2) & _
        Right("0" & Hex(AscB(MidB(arrbytOctet, 3, 1))), 2) & _
        Right("0" & Hex(AscB(MidB(arrbytOctet, 2, 1))), 2) & _
        Right("0" & Hex(AscB(MidB(arrbytOctet, 1, 1))), 2) & _
        "-" & _
        Right("0" & Hex(AscB(MidB(arrbytOctet, 6, 1))), 2) & _
        Right("0" & Hex(AscB(MidB(arrbytOctet, 5, 1))), 2) & _
        "-" & _
        Right("0" & Hex(AscB(MidB(arrbytOctet, 8, 1))), 2) & _
        Right("0" & Hex(AscB(MidB(arrbytOctet, 7, 1))), 2) & _
        "-" & _
        Right("0" & Hex(AscB(MidB(arrbytOctet, 9, 1))), 2) & _
        Right("0" & Hex(AscB(MidB(arrbytOctet, 10, 1))), 2) & _
        "-"  & _
        Right("0" & Hex(AscB(MidB(arrbytOctet, 11, 1))), 2) & _
        Right("0" & Hex(AscB(MidB(arrbytOctet, 12, 1))), 2) & _
        Right("0" & Hex(AscB(MidB(arrbytOctet, 13, 1))), 2) & _
        Right("0" & Hex(AscB(MidB(arrbytOctet, 14, 1))), 2) & _
        Right("0" & Hex(AscB(MidB(arrbytOctet, 15, 1))), 2) & _
        Right("0" & Hex(AscB(MidB(arrbytOctet, 16, 1))), 2)
    End If
End Function

Function baConvertGuidToByteArray(ByVal strHexString)
    Dim fso, stream, temp, ts, n, szScrubbedString
    Set fso = CreateObject ("scripting.filesystemobject") 
    Set stream = CreateObject ("adodb.stream")
    Const TemporaryFolder = 2

    temp = fso.GetSpecialFolder(TemporaryFolder) & fso.gettempname () 

    Set ts = fso.createtextfile (temp) 

    szScrubbedString = Replace(strHexString, "-", "")

    ts.write Chr("&h" & Mid(szScrubbedString, 7, 2))
    ts.write Chr("&h" & Mid(szScrubbedString, 5, 2))
    ts.write Chr("&h" & Mid(szScrubbedString, 3, 2))
    ts.write Chr("&h" & Mid(szScrubbedString, 1, 2))
    ts.write Chr("&h" & Mid(szScrubbedString, 11, 2))
    ts.write Chr("&h" & Mid(szScrubbedString, 9, 2))
    ts.write Chr("&h" & Mid(szScrubbedString, 15, 2))
    ts.write Chr("&h" & Mid(szScrubbedString, 13, 2))
    ts.write Chr("&h" & Mid(szScrubbedString, 17, 2))
    ts.write Chr("&h" & Mid(szScrubbedString, 19, 2))
    ts.write Chr("&h" & Mid(szScrubbedString, 21, 2))
    ts.write Chr("&h" & Mid(szScrubbedString, 23, 2))
    ts.write Chr("&h" & Mid(szScrubbedString, 25, 2))
    ts.write Chr("&h" & Mid(szScrubbedString, 27, 2))
    ts.write Chr("&h" & Mid(szScrubbedString, 29, 2))
    ts.write Chr("&h" & Mid(szScrubbedString, 31, 2))

    ts.close 

    stream.type = 1 
    stream.open 
    stream.loadfromfile temp 

    baConvertGuidToByteArray = stream.read 

    stream.close 
    fso.deletefile temp 

    Set stream = Nothing 
    Set fso = Nothing 
End Function

</VBScript>