C# ПІДРУЧНИКИ / c# / MS Press - Msdn Training Programming Net Framework With C#

Web Proxy

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If your Web site uses a proxy to provide access to the Internet, you must configure a proxy instance to enable your application to communicate with the Web proxy.

Creating a Global Proxy Instance

The following example shows how to create a global proxy instance that enables any WebRequest to use a proxy to communicate with the Internet. The example assumes that the proxy server is named webproxy and that it communicates on port 80, the standard HTTP port.

WebProxy proxyObject = new WebProxy("http://webproxy:80/");

GlobalProxySelection.Select = proxyObject;

Overriding the Global Proxy Selection

You can override the global proxy selection by assigning an instance that implements the IWebProxy interface to the Proxy property of the WebRequest. The following code sends a WebRequest to http://www.contoso.com. The WebRequest overrides the global proxy selection with a proxy server that is named alternateproxy on port 80.

WebRequest req = WebRequest.Create("http://www.contoso.com/"); req.Proxy = new WebProxy("http://alternateproxy:80/");

Secure Sockets Layer

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The WebRequest and WebResponse classes use SSL automatically. The WebRequest object decides to use SSL on the basis of the URI that it is assigned. If the URI begins with https:, SSL is used. If the URI begins with http:, SSL is not used.

The following example illustrates the use of SSL:

String MyURI = "https://www.contoso.com/";

WebRequest wReq = WebRequest.Create(MyURI);

Internet Authentication

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The System.Net classes support a variety of client authentication mechanisms, including the standard Internet authentication methods: basic, digest, negotiate, NTLM, and Kerberos authentication, and custom methods that you can create.

Classes and Interfaces Used for Authentication

Authentication credentials are stored in the NetworkCredential and

CredentialCache classes, which implement the ICredentialLookup interface.

When one of these classes is queried for credentials, it returns an instance of the

NetworkCredential class.

The AuthenticationManager class manages the authentication process, while an authentication module class that implements the IAuthenticationModule interface performs the actual authentication process. You must register a custom authentication module with the AuthenticationManager before it can be used. Modules for the basic, digest, negotiate, NTLM, and Kerberos authentication methods are registered by default.

The NetworkCredential Class

The NetworkCredential class stores a set of credentials, which is associated with a single Internet resource and that is identified by a URI, and returns them in response to any call to the GetCredential method. The NetworkCredential class is typically used by applications that access a limited number of Internet resources or by applications that use the same set of credentials in all cases.

The CredentialCache Class

The CredentialCache class stores a collection of credentials for various Internet resources. When the GetCredential method is called, CredentialCache returns the proper set of credentials, as determined by the URI of the Internet resource and the requested authentication scheme. Because the CredentialCache class stores all of the credentials and provides them as requested, applications that use a variety of Internet resources with different authentication schemes benefit from using the CredentialCache class.

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The WebPermissions and SocketPermissions classes provide Internet security for applications that use System.Net. The WebPermissions class controls an application’s right to request data from a URI or to serve a URI to the Internet. The SocketPermissions class controls an application’s right to accept data on a local port or to contact applications through a transport protocol at another address that is based on the host, port number, and transport protocol.

You should choose the permission class on the basis of your application type. Applications that use WebRequest and its descendents should use the WebPermissions class to manage permissions. Applications that use socket-level access should use the SocketPermissions class to manage permissions.

WebPermissions and SocketPermissions define two permissions: accept and connect. Accept grants the application the right to answer an incoming connection from another party. Connect grants the application the right to initiate a connection to another party.

For WebPermissions, accept means that an application can export a particular URI anywhere on the Internet. Connect means that an application can access that URI, whether it is remote or local.

For SocketPermissions, accept means that an application can accept incoming connections on a local transport address. Connect means that an application can connect to a remote, or potentially local, transport address.

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The following recommendations will help you use the classes that are contained in System.Net more effectively:

!Whenever possible, use WebRequest and WebResponse, instead of typecasting to descendent classes.

Applications that use WebRequest and WebResponse can take advantage of new Internet protocols without extensive code changes.

!When writing ASP.NET applications that run on a server that uses the System.Net classes, it is often better, from a performance standpoint, to use the asynchronous methods for GetResponse and GetResponseStream.

!Set the ConnectionLimit property in the ServicePoint instance for your application.

The number of connections opened to an Internet resource can have a significant effect on network performance and throughput. By default, System.Net uses two connections per application for each host. Setting the ConnectionLimit property in the ServicePoint instance for your application can increase this number.

!When writing socket-level protocols, try to use the TcpClient or UdpClient classes, instead of writing directly to a socket.

The TcpClient and UdpClient classes encapsulate the creation of TCP and UDP sockets without requiring you to handle the details of the connection.

!When accessing sites that require credentials, use the CredentialCache class to create a cache of credentials, rather than supplying them with every request.

The CredentialCache class will search the cache to find the appropriate credential to present with a request, thus relieving you of the responsibility of creating and presenting credentials based on the URI.

Lab 11: Creating a DateTime Client/Server Application

Topic Objective

To introduce the lab.

Lead-in

In this lab, you will create a client application that uses the System.Net.Sockets

.TcpClient class to connect to and obtain date and time information from a server.

You will also create a server application that uses the

System.Net.Sockets

.TcpListener class to accept requests from and provide date and time information to clients.

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Objectives

After completing this lab, you will be able to:

!Create a client application that uses the System.Net.Sockets.TcpClient class to connect to and obtain date and time information from a server.

!Create a server application that uses the System.Net.Sockets.TcpListener class to accept requests from and provide date and time information to clients.

Lab Setup

Only solution files are associated with this lab. The solution files for this lab are in the folder <install folder>\Labs\Lab11\Solution.

Scenario

In this lab, you will create two Microsoft Visual Studio® .NET console applications: DateTimeClient and DateTimeServer. The DateTimeClient application will make a TCP connection to the DateTimeServer application and obtain a stream that contains date and time information. The DateTimeClient will read the stream and convert the stream’s ASCII data into a string that is then output to the console.

Estimated time to complete this lab: 45 minutes

Exercise 2

Creating the DateTime Client

In this exercise, you will create a client application. The client application takes as its runtime argument the name of the computer on which the server is running. The client connects to the server to obtain date and time information through TCP. The client then displays the date and time information on the console.

!Create the client application

1.In Visual Studio .NET, create a new C# console application project named

DateTimeClient in <install folder>\Labs\Lab11.

2.Rename the starting C# source file Datetimeclient.cs.

3.Add the following using statements:

using System.Net;

using System.Net.Sockets; using System.IO;

using System.Text;

4.Rename the wizard generated Class1 to Client.

5.Make the static Main method capable of handling runtime-supplied arguments.

6.In the Main method, add code to:

a.Create a TcpClient object.

b.Create a byte array object of size 32 bytes.

c.Check the number of runtime arguments. If the number of arguments is not one, print out an error message to the console, and exit. The message should state:

Please specify a server name in the command line

d.In a try/catch block, verify that the named server computer exists by calling the GetHostByName method of the Dns class. Catch any SocketException exceptions and write out to a console window the message “Cannot find server: ”, followed by the name of the server and the exception’s data. Then exit the application.

e.Connect to the named server using port 14.

f.Declare a variable of type Stream.

g.In a try/catch block, get the stream and assign it to the variable declared in the preceding step. Catch any InvalidOperationException exceptions and write out to a console window the message “Cannot connect to server ”, followed by the server’s name, and then exit the application.

h.Read all of the bytes in the stream into the byte array that you created in step 6b, and store the number of bytes that are read in an integer.

i.Convert the ASCII-encoded byte array into a string.

j.Write out to a console window a message that states the number of bytes received and the current date and time string retrieved from the server.