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Trong tài liệu C# Network Programming (Trang 138-142)

For applications that require a connectionless socket, the UdpClient class provides a simple interface to UDP sockets. You may be wondering why there is not a listener version of the UDP helper class. The answer is simple:

you don t need one. Remember, UDP is a connectionless protocol, so there is no such thing as a client or server;

there are only UDP sockets either waiting for or sending data. You do not need to bind the UDP socket to a specific address and wait for incoming data.

The UdpClient constructors follow the same format as TcpClient they allow you to specify the amount of information necessary to create the UDP socket or reference a remote address and UDP port.

The Receive() method allows you to receive data on the UDP port. There is no connection session established, so when a UDP port receives data, you do not necessarily know where it came from (unless you specified it in the UdpClient constructor). To compensate for this, the Receive() method includes an IPEndPoint parameter that is filled with the IP information from the remote host. This is done as follows:

IPEndPoint remoteclient = new IPEndPoint(IPAddress.Any, 0);

byte[] recv = newconn.Receive(ref remoteclient);

string data = Encoding.ASCII.GetString(recv);

ConsoleWriteLine("From: {0}", remoteclient.ToString());

ConsoleWriteLine(" Data: "{0}", data);


The UdpClient object is instantiated using a port number on which you want the application to accept UDP packets.

Note that the remoteclient object is instantiated using the IPAddress.Any value. This is used as a dummy value that will be replaced with the information from the remote client in the Receive() method.

The Send() method has three formats. If the UdpClient object references a remote host address and port, the Send() method does not need to specify the destination of the data.

However, if the UdpClient object does not reference a remote host, the Send() method must do that. This can be accomplished with either an IPEndPoint object, or a string hostname or address value and an integer port number.

Sending data would look like this:

UdpClient newclient = new UdpClient(8001);

IPEndPoint remotehost = new

IPEndPoint(IPAddress.Parse(""), 8001);

byte[] bytes = Encoding.ASCII.GetBytes("test string");

newclient.Send(bytes, bytes.Length, remotehost);


The port number used on the UdpClient constructor does not have to be the same as for the remote host, but it often helps to keep them the same in order to track the ports that are being used by particular applications. The remote host address information is stored in an IPEndPoint object and used as a parameter in the Send() method.


This chapter covers a lot of ground in discussing the history of network programming and its relationships to C#

network programming. Network programming got its real push in the early days of Unix systems with the Berkeley socket library. Version 4.2 of the BSD Unix operating system included a common socket interface to assist programmers in communicating with other systems across a network.

The Windows socket interface, Winsock, builds on the existing Unix network socket programming model, adding many Winsock-specific functions to enhance the standard socket programming model. Many of the enhancements center around adding the capability for asynchronous socket I/O. The WSAAsyncSelect() and WSAEventSelect() functions enable Winsock programs to use standard Windows messages and events to signal socket events.

The .NET Framework offers a socket interface to access the Winsock APIs. The System.Net_.Sockets namespace provides the Socket class that uses managed code to provide Winsock functionality to C# programs. In addition to the normal Socket class, C# also offers the TcpClient, TcpListener, and UdpClient classes. These classes take care of much of the programming overhead associated with creating sockets, and they provide an easy interface for novice network programmers to work from.

As you can see, network addresses are crucial to socket programming. Unfortunately, these days there are lots of options for how to reference an address on the Internet (or even a local network). The next chapter describes how to use the Domain Name Service (DNS) classes in C# to resolve hostnames into IP addresses for your network


Chapter 4: DNS and C#


IP addresses and network programs go hand in hand. When writing socket programs, you must indicate the IP address of either the local host for server applications, or a remote host for client applications. Sometimes this is not an easy thing to do equating IP addresses and hostnames can be complicated. If all we ever needed were IP addresses, life would be a lot simpler for the network programmer.

In the real world, however, everywhere you look books, magazines, websites, television advertisements, and all the rest you see hostnames thrown at you. The Internet has made the hostname a common household item. Because of this, almost all network programs must allow customers to enter hostnames as well as IP addresses, and it is up to the network programmer to ensure that the program can find the IP address that is properly associated with the hostname. The primary control mechanism for accomplishing this is the Domain Name System (DNS), which is used to control hostnames and provide information to hosts wanting to find the IP address of another hostname.

This chapter discusses the basics of DNS and how you can use DNS to resolve hostnames in your C# network programs. First, you ll look at an overview of DNS processes, and then you ll see how DNS is used on standard Windows workstations and servers. Finally, you ll get an in-depth look at how to use DNS in your C# network programs, along with sample programs showing the C# DNS classes in action.

The History of Computer Names

Back in the old days (the 80s) when the Internet was small, it wasn t too hard to locate other computers on the network. Each system on the Internet contained a database that matched hostnames with assigned IP addresses.

Internet hostnames could be anything the system administrator wanted to use Fred, Barney, acctg1, anything. A central clearinghouse kept track of all the hostnames and IP addresses used on the Internet and published a master database containing all the address information. Usually once a week or so, individual system administrators downloaded the master database of hostnames and addresses to their local systems.

This system worked fine for a while, but it wasn t long before it didn t scale very well. As more and more hosts were added to the Internet, the database grew. And of course, as it grew, the longer it took to download, and the more space it took up on the individual systems. Clearly a dramatic improvement was needed, and that improvement was the Domain Name System (DNS).

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