Wednesday, March 26, 2008

Testing DNS records for scavenging

The commands in this post provide an easy method to export zone records from a Microsoft DNS server, and check whether the aging interval indicates this record would be scavenged given a hypthetical No refresh+Refresh interval.


  1. Export the records using dnscmd, which includes the aging timestamp for each record
  2. Run the following VBscript to convert the hours-since-epoch to a human readable datetime and compare against the DNS No-Refresh+Refresh interval.

For example:

  1. dnscmd %dnsserver% /enumrecords %zone% @ /continue > DNS_Records.txt
  2. cscript DNSScavengeTest.vbs DNS_Records.txt

'--- DNSScavengeTest.vbs ---

Const ForReading = 1
Const ForWriting = 2
Const ForAppending = 8
Const AGING_TOKEN = "[Aging:"

Const DDNS_NO_REFRESH = 7        ' The dynamic DNS no refresh period, where an update classified as a refresh will not be accepted for the record
Const DDNS_REFRESH = 7            ' The dynamic DNS refresh period, during which an update will be accepted for the record
Const GMT_OFFSET = +10            ' Offset in hours to adjust the resultant times based on the current GMT timezone

Set objFSO = CreateObject("Scripting.FileSystemObject")

If WScript.Arguments.Count = 1 Then 
    strFileName = WScript.Arguments(0) 
    wscript.echo "Specify a filename containing the output of dnscmd. eg DNSScavengeTest.vbs DNS_Records.txt"
End If

If Not objFSO.FileExists(strFileName) Then
    WScript.Echo "Error: " & strFileName & " file not found."
End If

Set objTextStream = objFSO.OpenTextFile(strFileName, ForReading)
strZoneRecords = objTextStream.ReadAll

For Each strLine in Split(strZoneRecords, vbCRLF)
    intStart = InStr(1, strLine, AGING_TOKEN, 1) 
    If intStart <> 0 Then                                ' Does this line contain an aging value?
        intStart = intStart + Len(AGING_TOKEN)
        intEnd = InStr(intStart, strLine, "]")
        If intEnd <> 0 Then intLength = intEnd - intStart
        strHost = Left(strLine, InStr(strLine, " ")-1)                ' Yes, extract the host

        intAging = Mid(strLine, intStart, intLength)                ' Extract the aging value, expressed in the decimal number of hours since 01/01/1601
        dtmDate = DateAdd("h", intAging, "01/01/1601 00:00:00 AM")        ' Convert to a date timestamp
        dtmDate = DateAdd("h", GMT_OFFSET, dtmDate)                ' Add the current GMT offset

        intDiff = DateDiff("h", dtmDate, Now)                    ' The difference between now and the timestampe
        intHourDiff = intDiff - ((DDNS_NO_REFRESH * 24) + (DDNS_REFRESH * 24))    ' Based on the dynamic DNS no-refresh and refresh periods combined
        If intHourDiff > 0 Then                            ' Is this a positive number, indicating the record will be scavenged
            intDay = CInt(intHourDiff / 24)                    ' Yes, convert to a number of days for output
            WScript.Echo strHost & ", " & dtmDate & ", " & intDay + DDNS_NO_REFRESH + DDNS_REFRESH & " days ago"    'This record would be scavenged
            WScript.Echo "*" & strHost & ", " & dtmDate            ' This record won't be scavenged
            intDay = 0
        End If
    End If

Wayne's World of IT (WWoIT), Copyright 2008 Wayne Martin.

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About Me

I’ve worked in IT for over 20 years, and I know just about enough to realise that I don’t know very much.