Get SPF records from a Domain

Question

What are the ways to Check SPF records on a domain?

There is a website where i can do it manually using - http://www.mxtoolbox.com/SuperTool.aspx

How can i do it

Answer 1

We tried to use @martin-liversage's answer, but after some time running on hundrets of domains it failed on some memory problem. (Maybe there was some invalid/another type DNS record?) So i studied this exact WINAPI functions and structures used in this case and edited the solution acordingly.

Links to WINAPI documentation are included in code.

So here's our improved code, that's working 100% even in our case:

public String GetSpfRecord(String domain)
{
    // Definition of DNS params
    const Int16 DNS_TYPE_TXT = 0x0010;
    const Int32 DNS_QUERY_STANDARD = 0x00000001;
    const Int32 DNS_ERROR_RCODE_NAME_ERROR = 9003;
    const Int32 DNS_INFO_NO_RECORDS = 9501;

    DnsRecordA dnsRecord;
    var queryResultsSet = IntPtr.Zero;

    try
    {
        var dnsStatus = DnsQuery(
          domain,
          DNS_TYPE_TXT,
          DNS_QUERY_STANDARD,
          IntPtr.Zero,
          ref queryResultsSet,
          IntPtr.Zero
        );

        if (dnsStatus == DNS_ERROR_RCODE_NAME_ERROR || dnsStatus == DNS_INFO_NO_RECORDS)
            return null;

        if (dnsStatus != 0)
            throw new Win32Exception(dnsStatus);

        for (IntPtr pointer = queryResultsSet; pointer != IntPtr.Zero; pointer = dnsRecord.pNext)
        {
            // Copies data from memory (size of DnsRecordA) from adress pointer to new alocated memory and creates instance of pointer to this place.
            dnsRecord = (DnsRecordA)Marshal.PtrToStructure(pointer, typeof(DnsRecordA));

            // pokud se jedná o typ TXT
            if (dnsRecord.wType == DNS_TYPE_TXT)
            {
                // get pointer to informations in "Data" property (https://docs.microsoft.com/en-us/windows/win32/api/windns/ns-windns-dns_recorda)
                var dataPointer = pointer + Marshal.SizeOf(typeof(DnsRecordA));

                // Get the txtData
                var txtData = (DNS_TXT_DATAA)Marshal.PtrToStructure(dataPointer, typeof(DNS_TXT_DATAA));
                if (txtData.dwStringCount >= 1)
                {
                    string line = Marshal.PtrToStringUni(txtData.pStringArray[0]);

                    // only if record starts with "v=spf" (Getting only SPF records)
                    // Getting only first (here is always maximum of 1 record) and returning whole line
                    if (line.StartsWith("v=spf") && string.IsNullOrEmpty(result))
                    {
                        return line;
                    }
                }
            }
        }

        // no SPF record - returning null
        return null;
    }
    finally
    {
        const Int32 DnsFreeRecordList = 1;

        // always release the memory alocated for list of dns records
        if (queryResultsSet != IntPtr.Zero)
            DnsRecordListFree(queryResultsSet, DnsFreeRecordList);
    }
}

// https://docs.microsoft.com/en-us/windows/win32/api/windns/nf-windns-dnsquery_a
[DllImport("Dnsapi.dll", EntryPoint = "DnsQuery_W", ExactSpelling = true, CharSet = CharSet.Unicode, SetLastError = true)]
static extern Int32 DnsQuery(String lpstrName, Int16 wType, Int32 options, IntPtr pExtra, ref IntPtr ppQueryResultsSet, IntPtr pReserved);

// https://docs.microsoft.com/en-us/windows/win32/api/windns/nf-windns-dnsrecordlistfree
[DllImport("Dnsapi.dll")]
static extern void DnsRecordListFree(IntPtr pRecordList, Int32 freeType);

// https://docs.microsoft.com/en-us/windows/win32/api/windns/ns-windns-dns_recorda
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
struct DnsRecordA
{
    public IntPtr pNext;
    public String pName;
    public Int16 wType;
    public Int16 wDataLength;
    public Int32 flags;
    public Int32 dwTtl;
    public Int32 dwReserved;

    // Commented, because i'm getting this value dynamicaly (it can also be another structure type which might cause some problems)
    //public DNS_TXT_DATA Data;
}

// https://docs.microsoft.com/en-us/windows/win32/api/windns/ns-windns-dns_txt_dataa
[StructLayout(LayoutKind.Sequential)]
struct DNS_TXT_DATAA
{
    /// DWORD->unsigned int
    public uint dwStringCount;

    /// PSTR[1]
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 1, ArraySubType = UnmanagedType.SysUInt)]
    internal IntPtr[] pStringArray;
}