Question

In VB6 I’m making a call to the Windows API DnsQuery.

Private Declare Function DnsQuery Lib "dnsapi" Alias "DnsQuery_A" ( _
   ByVal lpstrName As String, _
   ByVal wType As Integer, _
   ByVal Options As Long, _
   ByVal pServers As Long, _
   ppQueryResultsSet As Long, _
   ByVal pReserved As Long) As Long

Private Type VBDnsRecord
   pNext           As Long
   pName           As Long
   'Name            As String ' Commented out, see question
   wType           As Integer
   wDataLength     As Integer
   Flags           As Long
   dwTtl           As Long
   dwReserved      As Long
   ptrData         As Long
   Others(35)      As Byte
End Type

My declaration of the struct came from here. I presume Others(35) is to provide a big enough container when the actual struct that is returned is larger than expected (there are some variable-length types UNIONed in there). See the DNS_RECORD structure for more details.

So, I have 2 questions:

1. Strings in VB are really, behind the scenes, double word pointers (4 bytes, aka a Long). For a bit I thought I could declare Name as a String since this would just place the pointer in there and work correctly (as when passing VB Strings into API calls). However, I’m guessing the application terminations I got are because it’s a C-style string and not a VB-style string, and VB looks in the memory location just before the start of the string for a length value, and gets some random garbage and blows up. Is that a reasonable guess?

2. My call to DnsQuery works when I use am returning a DNS RR type of DNS_PTR_DATA or DNS_A_DATA, but when I try DNS_TXT_DATA it’s blowing up. Can someone else spot what I’m doing wrong? Look at Case DNS_TYPE_TEXT, and see my comments below.

   RetVal = DnsQuery(DnsName,
   QueryType, DNS_QUERY_BYPASS_CACHE,
   pServers, pDnsRecord, 0)
       If RetVal = 0 Then
          pNext = pDnsRecord
          Do While pNext <> 0
             CopyMemory DnsRecord, pNext, Len(DnsRecord)
             Select Case DnsRecord.wType
                Case DNS_TYPE_A
                   Ptr = inet_ntoa(DnsRecord.ptrData)
                   TempString = String(lstrlen(Ptr), 0)
                   CopyMemory ByVal TempString, Ptr, Len(TempString)
                Case DNS_TYPE_PTR, DNS_TYPE_NS, DNS_TYPE_CNAME,
   DNS_TYPE_DNAME, DNS_TYPE_MB,
   DNS_TYPE_MD, DNS_TYPE_MF,
   DNS_TYPE_MG, DNS_TYPE_MR
                   Ptr = DnsRecord.ptrData
                   TempString = String$(lstrlen(Ptr), 0)
                   CopyMemory ByVal TempString, Ptr, Len(TempString)
                Case DNS_TYPE_TEXT, DNS_TYPE_HINFO, DNS_TYPE_ISDN,
   DNS_TYPE_TEXT, DNS_TYPE_X25
                   Dim TextData As Dns_Txt_Data
                   Ptr = DnsRecord.ptrData
                   CopyMemory VarPtr(TextData), Ptr, Len(TextData)
                   Stop
                Case Else
                   TempString = "unhandled resource record type"
            End Select
            If Not FullRecord Then
               DnsLookup = "   " & TempString
               Exit Do
            Else
               DnsLookup = DnsLookup & " " & vbCrLf &
   DnsTypeNameFromCode(DnsRecord.wType)
   & " " & TempString
            End If
            pNext = DnsRecord.pNext
         Loop
   

   Now, when I put a breakpoint on the CopyMemory line and inspect the value of Ptr, I expect something in the millions or higher, indicating that it is a pointer, only I’m getting the value 1 (which explains why everything blows up when I try to copy from that memory location). This seems to indicate to me that instead of a pointer to the expected DNS_TXT_DATA struct, I’m getting the count of strings. When I examine Other(0) through Other(3), they all have values which make me suspect the next four bytes are a pointer. So what gives? Why is this struct just coming in “inline” but the others come in as pointers to the start of the struct?

I appreciate any help!

Answer 1

1) C strings and VB strings different and you can’t just substitute one for another. You’ll need to explicitly convert the C string to a VB string. In other words, yes, your guess is reasonable.

2) I took a quick look at the Microsoft docs, which say that DNS_TXT_DATA is a count followed by a pointer to a string (which is what you’re seeing). DNS_PTR_DATA, on the other hand, is a point to the resulting record. So what you’re describing seems to match the documentation.