Question

I know how to look up a hostname from an IPv4 in VB using the GetHostByAddr Windows API call (this works great). However, that function does not allow one to specify the DNS server to use. Sometimes the default company DNS servers are fine, but other times I need to specify an external DNS server for lookups, and I don’t think doing a shell nslookup and parsing the output is the best method, here.

Note: this is actually going to be used as VBA code in an Excel workbook to help someone else do his job, and it’s not worth writing a big application when some simple functionality is all he needs.

I thought I had possibly found an answer in the API call getnameinfo but careful reading seems to indicate it does not offer a servername parameter.

After some intense searching, I found reference to the pExtra parameter to the DNSQuery function. But I don’t even know how to begin to use that in VB6.

Could anyone help me out in any way with doing a DNS lookup from VB6, specifying the servername to use?

A full working solution would of course be nice, but I’m willing to work: just point me in the right direction.

UPDATE: For some odd reason it didn’t click that DNSQuery was a Windows API call. It just didn’t sound like one. I certainly would have been able to make more headway on the problem if I’d gathered that one tiny detail.

Answer 1

Try this:

Option Explicit

Private Declare Function DnsQuery Lib "dnsapi" Alias "DnsQuery_A" (ByVal strname As String, ByVal wType As Integer, ByVal fOptions As Long, ByVal pServers As Long, ppQueryResultsSet As Long, ByVal pReserved As Long) As Long
Private Declare Function DnsRecordListFree Lib "dnsapi" (ByVal pDnsRecord As Long, ByVal FreeType As Long) As Long
Private Declare Function lstrlen Lib "kernel32" (ByVal straddress As Long) As Long
Private Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (Destination As Any, ByVal Source As Long, ByVal Length As Long)
Private Declare Function inet_ntoa Lib "ws2_32.dll" (ByVal pIP As Long) As Long
Private Declare Function inet_addr Lib "ws2_32.dll" (ByVal sAddr As String) As Long

Private Const DnsFreeRecordList         As Long = 1
Private Const DNS_TYPE_A                As Long = &H1
Private Const DNS_QUERY_BYPASS_CACHE    As Long = &H8

Private Type VBDnsRecord
    pNext           As Long
    pName           As Long
    wType           As Integer
    wDataLength     As Integer
    flags           As Long
    dwTel           As Long
    dwReserved      As Long
    prt             As Long
    others(35)      As Byte
End Type

Private Sub Command1_Click()
    MsgBox Resolve("google.com", "208.67.222.222")
End Sub

Private Function Resolve(sAddr As String, Optional sDnsServers As String) As String
    Dim pRecord     As Long
    Dim pNext       As Long
    Dim uRecord     As VBDnsRecord
    Dim lPtr        As Long
    Dim vSplit      As Variant
    Dim laServers() As Long
    Dim pServers    As Long
    Dim sName       As String

    If LenB(sDnsServers) <> 0 Then
        vSplit = Split(sDnsServers)
        ReDim laServers(0 To UBound(vSplit) + 1)
        laServers(0) = UBound(laServers)
        For lPtr = 0 To UBound(vSplit)
            laServers(lPtr + 1) = inet_addr(vSplit(lPtr))
        Next
        pServers = VarPtr(laServers(0))
    End If
    If DnsQuery(sAddr, DNS_TYPE_A, DNS_QUERY_BYPASS_CACHE, pServers, pRecord, 0) = 0 Then
        pNext = pRecord
        Do While pNext <> 0
            Call CopyMemory(uRecord, pNext, Len(uRecord))
            If uRecord.wType = DNS_TYPE_A Then
                lPtr = inet_ntoa(uRecord.prt)
                sName = String(lstrlen(lPtr), 0)
                Call CopyMemory(ByVal sName, lPtr, Len(sName))
                If LenB(Resolve) <> 0 Then
                    Resolve = Resolve & " "
                End If
                Resolve = Resolve & sName
            End If
            pNext = uRecord.pNext
        Loop
        Call DnsRecordListFree(pRecord, DnsFreeRecordList)
    End If
End Function