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 via ASP.NET and C#? Basically i want to verify/check SPF records on a domain if its supporting out own mail web server.

Answer 1

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 = "[email protected]";

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

Answer 2

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;
}