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Internet Domain Name System

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The Dns class is a static class that retrieves information about a specific host from the Internet Domain Name System (DNS).

The host information from a GetHostByName query is returned in an instance of the IPHostEntry class. If the specified host has more than one entry in the DNS database, IPHostEntry contains multiple Internet Protocol (IP) addresses and aliases.

The following example queries the DNS database for information about a host that is called www.contoso.com:

IPHostEntry hostInfo = Dns.GetHostByName("www.contoso.com");

The Resolve method queries a DNS server for the IP address that is associated with a host name, such as www.contoso.com, or for an IP address in dotted-quad notation, such as 192.168.1.2.

When the host name is a DNS-style host name that is associated with multiple IP addresses, only the first IP address that resolves to that host name is returned, as in the following example:

IPHostEntry hostInfo = Dns.Resolve("www.contoso.com");

TCP and UDP

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Applications can use TCP and UDP services with the TcpClient, TcpListener, and UdpClient classes. These classes, which are built on top of the Socket class, represent the data that is sent and received from the network as streams. These classes also take care of the details of creating a connection.

Using the TcpClient Class

You use the TcpClient class to request data from an Internet resource that uses TCP. The methods and properties of TcpClient abstract the details for creating a Socket instance that requests and receives data through TCP. The connection to the Internet is represented as a stream; therefore data can be read and written in a standard manner.

The following code demonstrates how to set up a TcpClient object to connect to a server on TCP port 14:

TcpClient tcpc = new TcpClient(serverURI, 14); Stream s = tcpc.GetStream();

Byte[] read = new Byte[32];

int bytes = s.Read(read, 0, read.Length);

String strInData = Encoding.ASCII.GetString(read);

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The System.Net.Sockets namespace contains an implementation of the

Windows Sockets interface. All other network access classes in the

System.Net namespace are built on top of this implementation of sockets.

The .NET Framework’s Socket class is a managed code version of the socket services that are provided by the WinSock32 API. If you are familiar with the Winsock API, you should be comfortable using the Socket class to develop applications. In most cases, the Socket class methods simply marshal data into their native Microsoft Win32® counterparts and handle necessary security checks.

while (bytes > 0)

{

bytes = mySocket.Receive(RecvBytes, RecvBytes.Length, 0);

strRetPage = strRetPage + ASCII.GetString(RecvBytes, 0, bytes);

}

}

catch (Exception e)

{

Console.WriteLine("Exception {0}",e.ToString());

}

return strRetPage;

}

}

Handling Errors

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The WebRequest and WebResponse classes throw system exceptions, such as InvalidArgumentException, and Web-specific exceptions, which are instances of WebException and are thrown by the GetResponse method.

Each WebException includes a Status property that contains a value from the WebExceptionStatus class. You can examine the Status property to determine the particular error that has occurred and take the proper steps to resolve the error.

The following table describes some of the possible values for the Status property. For a complete list of values, see the .NET Framework SDK documentation.

When the Status property is WebExceptionStatus.ProtocolError, a

WebResponse that contains the response from the server is available. You can examine this response to determine the actual source of the protocol error.

The following example shows how to catch a WebException; the invalid URL argument in the WebRequest.Create call will throw an exception:

using System; using System.Net; using System.IO; using System.Text;

class App

{

public static void Main(string[] args)

{

try

{

//Create a request instance

//Note invalid URL will throw exception WebRequest myRequest =

WebRequest.Create("http://localhost_bad_URL");

//Get the response.

WebResponse myResponse = myRequest.GetResponse();

//Get a readable stream from the server. StreamReader sr = new StreamReader(

myResponse.GetResponseStream(), Encoding.ASCII);

int length = 1024;

char [] Buffer = new char[1024]; int bytesread = 0;

//Read from the stream and write data to Console. bytesread = sr.Read( Buffer, 0, length);

while( bytesread > 0 )

{

Console.Write( Buffer, 0, bytesread); bytesread = sr.Read( Buffer, 0, length);

}

sr.Close();

myResponse.Close();

}

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Your application can provide security for sending and receiving data over the

Internet by using a Web proxy, Secure Sockets Layer (SSL) encryption,

Internet authentication, and the NET Framework’s code access permissions.