< C++, C# & VB .NET Asynchronous Name Resolution Examples | Main | Chap 8: Client Sockets Communication >
Chapter 7 Part 4:
IP Addressing and DNS
What do we have in this chapter 7 Part 4?
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Reverse Name Lookup
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C++ IP Resolution Program Example
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C# IP Resolution Program Example
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VB .NET IP Resolution Program Example
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Reverse Name Lookup
So far, we’ve seen how to resolve a name to an IP address, but what if you have an IP address and want to find what name is associated with the address? DNS provides reverse name lookup to perform this type of lookup. The .NET Framework Dns class performs DNS reverse name lookup using GetHostByAddress(IPAddress). This function returns any names associated with the IP address in an IPHostEntry object if an IP is associated with a name. If a name can’t be found, a SocketException will be thrown. The IPv6 protocol in Windows XP and Windows Server 2003 does not register its reverse lookup information with DNS, which means that a reverse query on an IPv6 address will fail. In DNS, more than one name can be associated with an IP address. As a result, the IPHostEntry object returned by GetHostByAddress() can return multiple names. The HostName field of IPHostEntry will contain the primary host name associated with the IP address. The Aliases field will contain any additional names that might also be associated with an IP. The following code fragment demonstrates how to perform a reverse name lookup:
C#
try { IPHostEntry IPHost = Dns.GetHostEntry("10.1.2.3"); Console.WriteLine("The Primary Host name is: " + IPHost.HostName.ToString());
// Print out any Aliases that are found if (IPHost.Aliases.Length > 0) { Console.WriteLine("Additional names are:"); foreach(string Alias in IPHost.Aliases) { Console.WriteLine(Alias); } } } catch (Exception e) { Console.WriteLine("GetHostEntry() failed with error: " + e.Message); } |
Visual Basic .NET
Try
Dim IPHost As IPHostEntry = Dns.GetHostEntry("10.1.2.3")
Console.WriteLine("The Primary Host name is: " + IPHost.HostName.ToString())
' Print out any Aliases that are found
If (IPHost.Aliases.Length > 0) Then
Console.WriteLine("Additional names found are: ")
Dim CurAlias As String
For Each CurAlias In IPHost.Aliases
Console.WriteLine(CurAlias)
Next
End If
Catch e As Exception
Console.WriteLine("GetHostEntry() failed with error: " + e.Message)
End Try
Performing DNS reverse name lookup using GetHostByAddress() can take some time, so it might cause your application to block on the call. Blocking on this call can make your application appear unresponsive, as described with forward name lookup. Therefore, two asynchronous counterpart methods, BeginHostByAddress() and EndHostByAddress(), are available for reverse name lookup that use the .NET Framework asynchronous pattern.
The following example is a sample that demonstrates how to synchronously perform reverse name lookup using the Dns class.
C++ IP Resolution Program Example
Create a new CLR console application and you can use ResolveIPCP as the project and solution names if you want.
Add the following code.
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// ResolveIPCP.cpp : main project file. /// <summary> /// This sample demonstrates how to perform reverse name lookup using /// the DNS service. Reverse name lookup is finding what names are /// associated with an IP address. The sample will print the primary /// host name found for an IP address followed by any additional names /// if they are found. To run this sample, simply run ResolveIP and /// supply an IP address as a command line parameter. /// </summary>
#include "stdafx.h"
using namespace System; using namespace System::Net;
[STAThread] int main(array<System::String ^> ^args) { if (args->Length < 1) { Console::WriteLine("Usage: {0} <IP address to query>", Environment::CommandLine); Console::WriteLine("Usage: {0} 1.2.1.4>", Environment::CommandLine); return 0; }
try { IPHostEntry^ IPHost = Dns::GetHostEntry(args[0]->ToString()); Console::WriteLine("GetHostEntry() is OK..."); Console::WriteLine("The Primary Host name is: " + IPHost->HostName->ToString());
// Print out any Aliases that are found if (IPHost->Aliases->Length > 0) { Console::WriteLine("\nAdditional names found are:"); for each(String^ Alias in IPHost->Aliases) { Console::WriteLine(Alias); } } Console::WriteLine("\nNo Aliases found..."); } catch (Exception^ e) { Console::WriteLine("GetHostEntry() failed with error: " + e->Message); }
return 0; } |
Build and run the project. The following is the output sample.
C# IP Resolution Program Example
Create a new console application project. You can use the project and solution name as shown in the following Figure if you want.
Add the following code.
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using System; using System.Net;
namespace ResolveIPChap6CS { /// <summary> /// Summary description for ResolveIP. /// </summary> class Program { /// <summary> /// This sample demonstrates how to perform reverse name lookup using /// the DNS service. Reverse name lookup is finding what names are /// associated with an IP address. The sample will print the primary /// host name found for an IP address followed by any additional names /// if they are found. To run this sample, simply run ResolveIP and /// supply an IP address as a command line parameter. /// </summary> [STAThread] static void Main(string[] args) { if (args.Length < 1) { Console.WriteLine("Usage: Executable_file_name <IP address to query>"); return; }
try { IPHostEntry IPHost = Dns.GetHostEntry(args[0].ToString()); Console.WriteLine("GetHostEntry() is OK..."); Console.WriteLine("The Primary Host name is: " + IPHost.HostName.ToString());
// Print out any Aliases that are found if (IPHost.Aliases.Length > 0) { Console.WriteLine("Additional names found are:"); foreach(string Alias in IPHost.Aliases) { Console.WriteLine(Alias); } } Console.WriteLine("No Aliases found..."); } catch (Exception e) { Console.WriteLine("GetHostEntry() failed with error: " + e.Message); } } } } |
The following are the sample outputs for ipv4 and ipv6.
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VB .NET IP Resolution Program Example
Create a new console application project. You can use the project and solution name as shown in the following Figure if you want.
Add the following code.
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Imports System Imports System.Net
Module Module1 ' This sample demonstrates how to perform reverse name lookup using ' the DNS service. Reverse name lookup is finding what names are ' associated with an IP address. The sample will print the primary ' host name found for an IP address followed by any additional names ' if they are found. To run this sample, simply run ResolveIP and ' supply an IP address as a command line parameter. Sub Main() ' Parse command line arguments if any Dim args As String() = Environment.GetCommandLineArgs()
If (args.Length < 2) Then Console.WriteLine("Usage: Executable_file_name <IP address to query>") Exit Sub End If
Try Dim IPHost As IPHostEntry = Dns.GetHostEntry(args(1).ToString())
Console.WriteLine("GetHostEntry() is OK...") Console.WriteLine("The Primary Host name is: " + IPHost.HostName.ToString())
' Print out any Aliases that are found If (IPHost.Aliases.Length > 0) Then Console.WriteLine("Additional names found are:") Dim CurAlias As String
For Each CurAlias In IPHost.Aliases Console.WriteLine(CurAlias) Next End If Console.WriteLine("No Aliases were found...") Catch e As Exception Console.WriteLine("GetHostEntry() failed with error: " + e.Message) End Try End Sub End Module |
The following is the sample output.
There are many more features that can be implemented other than the name resolution and services. You can find those services in the nslookup program (dig program in Linux/Unix) and screenshots are shown below.
