This tutorial will show you how to perform a DNS query using the .NET System.Net class, ASP.NET 2.0 and VB.NET
The .NET Framework offers a number of types that makes accessing networked resources easy.
To perform a DNS query, we will need to first import the System.Net namespace. The System.Net namespace contains the Dns.GetHostByName method, IPHostEntry container class and IPAddress class we will use to perform our DNS query.
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We'll put our code in a subroutine called DNSLookup() and call it from the btnSubmit_Click() event.
When the btnSubmit_Click() event fires it calls our DNSLookup() function. This function creates a new instance of the IPHostEntry container class and assigns it a host entry returned from the Dns.GetHostByName method. It then create an array of IPAddress objects using the AddressList of our host entry object. After that, it iterates through each ip and adds it to the text box.
Protected Sub DNSLookup(ByVal domain As String)
Try
'performs the DNS lookup
Dim he As IPHostEntry = Dns.GetHostEntry(domain)
Dim ip_addrs As IPAddress() = he.AddressList
txtIPs.Text = ""
For Each ip As IPAddress In ip_addrs
txtIPs.Text = ip.ToString()
Next ip Catch ex As System.Exception
lblStatus.Text = ex.ToString() End Try |
We have two textboxes, a submit button, and a label on the front end for user interaction. The front end .aspx page looks something like this:
<table>
<tr>
<td align="right" bgcolor="#eeeeee" class="header1">Domain Name:</td> <td bgcolor="#FFFFFF"><asp:textbox ID="txtDomain" runat="server"></asp:textbox><br /> <asp:button ID="btnSubmit" runat="server" OnClick="btnSubmit_Click" Text="Submit" /></td> </tr> <tr>
<td width="100" align="right" bgcolor="#eeeeee" class="header1">IP Address(es) Assigned to Domain Name:</td> <td bgcolor="#FFFFFF"> <br /> <asp:textbox ID="txtIPs" runat="server"></asp:textbox><br /> <br /> <asp:label ID="lblStatus" runat="server"></asp:label></td> </tr> |
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The flow for the code behind page is as follows:
Imports System.Net
Partial Public Class _Default : Inherits System.Web.UI.Page
Protected Sub btnSubmit_Click(ByVal sender As Object, ByVal e As EventArgs)
DNSLookup(txtDomain.Text) End Sub Protected Sub DNSLookup(ByVal domain As String)
Try
'performs the DNS lookup
Dim he As IPHostEntry = Dns.GetHostEntry(domain)
Dim ip_addrs As IPAddress() = he.AddressList
txtIPs.Text = ""
For Each ip As IPAddress In ip_addrs
txtIPs.Text = ip.ToString()
Next ip Catch ex As System.Exception
lblStatus.Text = ex.ToString() End Try End Sub |
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