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

Building on the answer by Martin Liversage, I've added some comments that explain what's going on, and adjusted to return multiple records if they exist.

My example also concatenates multiple strings in a TXT record rather than separating by line breaks.

I don't know if the if (dnsRecord.wType == DNS_TYPE_TEXT) line is really necessary considering that constraint is in the arguments to the DnsQuery function, but I preserved it from Martin's answer anyway.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Runtime.InteropServices;
using System.Text;

namespace Util
{
    /// <summary>
    /// Based on https://stackoverflow.com/a/11884174 (Martin Liversage)
    /// </summary>
    class DnsInterop
    {
        private const short DNS_TYPE_TEXT = 0x0010;
        private const int DNS_QUERY_STANDARD = 0x00000000;
        private const int DNS_ERROR_RCODE_NAME_ERROR = 9003;
        private const int DNS_INFO_NO_RECORDS = 9501;


        public static IEnumerable<string> GetTxtRecords(string domain)
        {
            var results = new List<string>();
            var queryResultsSet = IntPtr.Zero;
            DnsRecordTxt dnsRecord;

            try
            {
                // get all text records
                // pointer to results is returned in queryResultsSet
                var dnsStatus = DnsQuery(
                  domain,
                  DNS_TYPE_TEXT,
                  DNS_QUERY_STANDARD,
                  IntPtr.Zero,
                  ref queryResultsSet,
                  IntPtr.Zero
                );

                // return null if no records or DNS lookup failed
                if (dnsStatus == DNS_ERROR_RCODE_NAME_ERROR
                    || dnsStatus == DNS_INFO_NO_RECORDS)
                {
                    return null;
                }

                // throw an exception if other non success code
                if (dnsStatus != 0)
                    throw new Win32Exception(dnsStatus);

                // step through each result
                for (
                    var pointer = queryResultsSet; 
                    pointer != IntPtr.Zero; 
                    pointer = dnsRecord.pNext)
                {
                    dnsRecord = (DnsRecordTxt)
                        Marshal.PtrToStructure(pointer, typeof(DnsRecordTxt));

                    if (dnsRecord.wType == DNS_TYPE_TEXT)
                    {
                        var builder = new StringBuilder();

                        // pointer to array of pointers
                        // to each string that makes up the record
                        var stringArrayPointer = pointer + Marshal.OffsetOf(
                            typeof(DnsRecordTxt), "pStringArray").ToInt32();

                        // concatenate multiple strings in the case of long records
                        for (var i = 0; i < dnsRecord.dwStringCount; ++i)
                        {
                            var stringPointer = (IntPtr)Marshal.PtrToStructure(
                                stringArrayPointer, typeof(IntPtr));

                            builder.Append(Marshal.PtrToStringUni(stringPointer));
                            stringArrayPointer += IntPtr.Size;
                        }

                        results.Add(builder.ToString());
                    }
                }
            }
            finally
            {
                if (queryResultsSet != IntPtr.Zero)
                {
                    DnsRecordListFree(queryResultsSet, 
                        (int)DNS_FREE_TYPE.DnsFreeRecordList);
                }
            }

            return results;
        }


        [DllImport("Dnsapi.dll", EntryPoint = "DnsQuery_W", 
            ExactSpelling = true, CharSet = CharSet.Unicode, 
            SetLastError = true)]
        static extern int DnsQuery(string lpstrName, short wType, int options, 
            IntPtr pExtra, ref IntPtr ppQueryResultsSet, IntPtr pReserved);


        [DllImport("Dnsapi.dll")]
        static extern void DnsRecordListFree(IntPtr pRecordList, int freeType);


        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
        struct DnsRecordTxt
        {
            public IntPtr pNext;
            public string pName;
            public short wType;
            public short wDataLength;
            public int flags;
            public int dwTtl;
            public int dwReserved;
            public int dwStringCount;
            public string pStringArray;
        }


        enum DNS_FREE_TYPE
        {
            DnsFreeFlat = 0,
            DnsFreeRecordList = 1,
            DnsFreeParsedMessageFields = 2
        }
    }
}

Answer 2

Funny how all the websites has this wrong

SPF is not TXT

you can have a txt record with no SPF and SPF with no TXT, so a TXT lookup will not show the SPF

Answer 3

Even though .NET has a lot of support for networking including doing host name to address mapping it lacks a general way to query DNS.

However, you can use P/Invoke to call the DnsQuery function directly. The API is somewhat cumbersome but it is not impossible to create the correct P/Invoke signature for your requirement.

A SPF record is stored as a TXT record in DNS. The corresponding structure you will have to work with is the DNS_TXT_DATA structure. If you can find an example of querying a MX record you can reuse the code and use DNS_TYPE_TEXT for the query type and unmarshal the data to a DNS_TXT_DATA structure.

Or you could just use this code:

using System.ComponentModel;
using System.Runtime.InteropServices;

public String DnsGetTxtRecord(String name) {
  const Int16 DNS_TYPE_TEXT = 0x0010;
  const Int32 DNS_QUERY_STANDARD = 0x00000000;
  const Int32 DNS_ERROR_RCODE_NAME_ERROR = 9003;
  const Int32 DNS_INFO_NO_RECORDS = 9501;
  var queryResultsSet = IntPtr.Zero;
  try {
    var dnsStatus = DnsQuery(
      name,
      DNS_TYPE_TEXT,
      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);
    DnsRecordTxt dnsRecord;
    for (var pointer = queryResultsSet; pointer != IntPtr.Zero; pointer = dnsRecord.pNext) {
      dnsRecord = (DnsRecordTxt) Marshal.PtrToStructure(pointer, typeof(DnsRecordTxt));
      if (dnsRecord.wType == DNS_TYPE_TEXT) {
        var lines = new List<String>();
        var stringArrayPointer = pointer
          + Marshal.OffsetOf(typeof(DnsRecordTxt), "pStringArray").ToInt32();
        for (var i = 0; i < dnsRecord.dwStringCount; ++i) {
          var stringPointer = (IntPtr) Marshal.PtrToStructure(stringArrayPointer, typeof(IntPtr));
          lines.Add(Marshal.PtrToStringUni(stringPointer));
          stringArrayPointer += IntPtr.Size;
        }
        return String.Join(Environment.NewLine, lines);
      }
    }
    return null;
  }
  finally {
    const Int32 DnsFreeRecordList = 1;
    if (queryResultsSet != IntPtr.Zero)
      DnsRecordListFree(queryResultsSet, DnsFreeRecordList);
  }
}

[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);

[DllImport("Dnsapi.dll")]
static extern void DnsRecordListFree(IntPtr pRecordList, Int32 freeType);

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
struct DnsRecordTxt {
  public IntPtr pNext;
  public String pName;
  public Int16 wType;
  public Int16 wDataLength;
  public Int32 flags;
  public Int32 dwTtl;
  public Int32 dwReserved;
  public Int32 dwStringCount;
  public String pStringArray;
}

Answer 4

You basically need to do a DNS request asking for the MX/SPF record of the domain. There's a few examples of doing this in C# around. There's a library at http://mailsystem.codeplex.com/ that has a Validator class with GetMxRecords to do this that you might find useful

Answer 5

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;
}

Answer 6

I have the same problem, and managed to find two three solutions:

The nslookup solution

You can get the SPF by typing the following command in the command line:

nslookup -type=TXT <hostname>

You can automate this in C# using System.Diagonstics.Process, as described in this blog post.

The DNS.NET Resolver project

I found this CodeProject article about DNS resolution. It comes with demo project. I ran the project and got the following result for stackexchange.com:

Note: Make sure that the QType field is set to TXT before you press the Send button

The section highlighted in yellow represents the SPF record. I haven't yet dug into the code to see how it's done, but this seems like a good alternative to the nslookup solution above.

[Update] The ARSoft.Tools.Net project

If all you need to do is check whether a domain supports a mail server, you can use the ARSoft.Tools.Net library (also available as a NuGet Package).

After installing the package, I managed to perform the SPF check with this code:

var spfValidator = new ARSoft.Tools.Net.Spf.SpfValidator();

var mailIpAddress = IPAddress.Parse("X.X.X.X");
var domain = "example.com";
var senderAddress = "sender@example.com";

ARSoft.Tools.Net.Spf.SpfQualifier result = 
    spfValidator.CheckHost(mailIpAddress, domain, senderAddress);