Updated

EonaCat.DnsTester/MainForm.Designer.cs

@@ -47,6 +47,8 @@
             this.CustomDns2 = new System.Windows.Forms.TextBox();
             this.lslStatus = new System.Windows.Forms.Label();
             this.StatusBox = new System.Windows.Forms.ListBox();
+            this.chkDns1 = new System.Windows.Forms.CheckBox();
+            this.chkDns2 = new System.Windows.Forms.CheckBox();
             this.SuspendLayout();
             // 
             // ResultView
@@ -224,13 +226,39 @@
             this.StatusBox.Size = new System.Drawing.Size(2209, 376);
             this.StatusBox.TabIndex = 23;
             // 
-            // TesterUI
+            // chkDns1
+            // 
+            this.chkDns1.AutoSize = true;
+            this.chkDns1.Checked = true;
+            this.chkDns1.CheckState = System.Windows.Forms.CheckState.Checked;
+            this.chkDns1.Location = new System.Drawing.Point(277, 205);
+            this.chkDns1.Name = "chkDns1";
+            this.chkDns1.Size = new System.Drawing.Size(245, 36);
+            this.chkDns1.TabIndex = 24;
+            this.chkDns1.Text = "Test dns server";
+            this.chkDns1.UseVisualStyleBackColor = true;
+            // 
+            // chkDns2
+            // 
+            this.chkDns2.AutoSize = true;
+            this.chkDns2.Checked = true;
+            this.chkDns2.CheckState = System.Windows.Forms.CheckState.Checked;
+            this.chkDns2.Location = new System.Drawing.Point(1229, 205);
+            this.chkDns2.Name = "chkDns2";
+            this.chkDns2.Size = new System.Drawing.Size(245, 36);
+            this.chkDns2.TabIndex = 25;
+            this.chkDns2.Text = "Test dns server";
+            this.chkDns2.UseVisualStyleBackColor = true;
+            // 
+            // MainForm
             // 
             this.AutoScaleDimensions = new System.Drawing.SizeF(240F, 240F);
             this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Dpi;
             this.AutoSize = true;
             this.AutoSizeMode = System.Windows.Forms.AutoSizeMode.GrowAndShrink;
             this.ClientSize = new System.Drawing.Size(2517, 1693);
+            this.Controls.Add(this.chkDns2);
+            this.Controls.Add(this.chkDns1);
             this.Controls.Add(this.StatusBox);
             this.Controls.Add(this.lslStatus);
             this.Controls.Add(this.CustomDns2);
@@ -247,7 +275,7 @@
             this.Icon = ((System.Drawing.Icon)(resources.GetObject("$this.Icon")));
             this.Margin = new System.Windows.Forms.Padding(8, 7, 8, 7);
             this.MaximizeBox = false;
-            this.Name = "TesterUI";
+            this.Name = "MainForm";
             this.StartPosition = System.Windows.Forms.FormStartPosition.CenterScreen;
             this.Text = "EonaCat.DnsTester";
             this.Load += new System.EventHandler(this.TesterUI_Load);
@@ -276,6 +304,8 @@
         private System.Windows.Forms.TextBox CustomDns2;
         private System.Windows.Forms.Label lslStatus;
         private System.Windows.Forms.ListBox StatusBox;
+        private System.Windows.Forms.CheckBox chkDns1;
+        private System.Windows.Forms.CheckBox chkDns2;
     }
 }
 

EonaCat.DnsTester/MainForm.cs

@@ -6,6 +6,7 @@ using System.Net.Sockets;
 using System.Security.Cryptography;
 using System.Text;
 using System.Text.RegularExpressions;
+using System.Threading.Tasks;
 using System.Windows.Forms;
 
 namespace EonaCat.DnsTester
@@ -15,6 +16,8 @@ namespace EonaCat.DnsTester
         private bool UseCustomDnsServers = false;
         public string DNS1, DNS2;
 
+        public bool IsRunning { get; private set; }
+
         public MainForm()
         {
             InitializeComponent();
@@ -27,14 +30,28 @@ namespace EonaCat.DnsTester
             int CleanNamesCount = 0;
 
             if (this.UseCustomDnsServers)
+            {
+				if (chkDns1.Checked)
 				{
 					DNS1 = CustomDns1.Text;
+				}
+
+				if (chkDns2.Checked)
+				{
 					DNS2 = CustomDns2.Text;
 				}
+            }
             else
+            {
+				if (chkDns1.Checked)
 				{
 					DNS1 = DnsList1.SelectedValue.ToString();
+				}
+
+				if (chkDns2.Checked)
+				{
 					DNS2 = DnsList2.SelectedValue.ToString();
+				}
 
             }
 
@@ -62,7 +79,17 @@ namespace EonaCat.DnsTester
 			ResultView.Update();
 			Application.DoEvents();
 
+			if (IsRunning)
+			{
+				return;
+			}
+
+			Task.Run(() => 
+			{
+                IsRunning = true;
                 CheckDNS(CleanNames, CleanNamesCount, DNS1, DNS2);
+				IsRunning = false;
+			});
 
 		}
 
@@ -233,13 +260,13 @@ namespace EonaCat.DnsTester
 			DNSsocket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReceiveTimeout, DNSReceiveTimeout);
 
 			IPEndPoint dnsEP1 = null;
-            if (!string.IsNullOrWhiteSpace(DNSAddress1))
+            if (chkDns1.Checked && !string.IsNullOrWhiteSpace(DNSAddress1))
 			{
 				dnsEP1 = new IPEndPoint(IPAddress.Parse(DNSAddress1), IPPort);
 			}
 			
 			IPEndPoint dnsEP2 = null;
-            if (!string.IsNullOrWhiteSpace(DNSAddress2))
+            if (chkDns2.Checked && !string.IsNullOrWhiteSpace(DNSAddress2))
 			{
 				dnsEP2 = new IPEndPoint(IPAddress.Parse(DNSAddress2), IPPort);
 			}
@@ -247,20 +274,23 @@ namespace EonaCat.DnsTester
 			// Start the clock
 			StartTime = DateTime.Now.Ticks;
 
-			// i=0; // for testing - do one
 			for (i = 0; i < URLNamescount; i++)
 			{
 				URLNameStart = URLNames[i].Substring(0, URLNames[i].IndexOf("."));
 				DomainName = URLNames[i].Substring(URLNames[i].IndexOf(".") + 1, URLNames[i].Length - URLNames[i].IndexOf(".") - 1);
-				// Build the query 
+
+				if (chkDns1.Checked)
+				{
 					QueryString = TransactionID1 + TypeString + (char)URLNameStart.Length + URLNameStart + (char)DomainName.Length + DomainName + TrailerString;
 					Sendbytes = Encoding.ASCII.GetBytes(QueryString);
 					if (dnsEP1 != null)
 					{
 						DNSsocket.SendTo(Sendbytes, Sendbytes.Length, SocketFlags.None, dnsEP1);
 					}
+				}
 
-				// send the same message to both DNS servers except for the transaction ID
+				if (chkDns2.Checked)
+				{
 					QueryString = TransactionID2 + TypeString + (char)URLNameStart.Length + URLNameStart + (char)DomainName.Length + DomainName + TrailerString;
 					Sendbytes = Encoding.ASCII.GetBytes(QueryString);
 					if (dnsEP2 != null)
@@ -268,6 +298,7 @@ namespace EonaCat.DnsTester
 						DNSsocket.SendTo(Sendbytes, Sendbytes.Length, SocketFlags.None, dnsEP2);
 					}
 				}
+			}
 
 			byte[] Receivebytes = new byte[512];
 
@@ -275,8 +306,6 @@ namespace EonaCat.DnsTester
 			{
 			// wait for a response up to timeout
 			more: DNSsocket.Receive(Receivebytes);
-
-
 				// make sure the message returned is ours
 				if (Receivebytes[0] == Sendbytes[0] && (Receivebytes[1] == 0x31) || (Receivebytes[1] == 0x32))
 				{