The HyperText Transfer Protocol in Java

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400 Bad Request: A generic error code indicating that the request could not be understood by the server 404 Not Found: The requested document does not exist on this server 505 HTTP Version Not Supported: The request HTTP protocol version is not supported by the server
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How would you like to see a real HTTP response message This is very easy to do! First Telnet into your favorite WWW server Then type in a one-line request message for some object that is housed on the server For example, if you can logon to a Unix machine, type: telnet wwweurecomfr 80 GET /~ross/indexhtml HTTP/10 (Hit the carriage return twice after typing the second line) This opens a TCP connection to port 80 of the host wwweurecomfr and then sends the HTTP GET command You should see a response message that includes the base HTML file of Professor Ross's homepage If you'd rather just see the HTTP message lines and not receive the object itself, replace GET with HEAD Finally, replace /~ross/index html with /~ross/bananahtml and see what kind of response message you get In this section we discussed a number of header lines that can be used within HTTP request and response messages The HTTP specification (especially HTTP/11) defines many, many more header lines that can be inserted by browsers, Web servers and network cache servers We have only covered a small fraction of the totality of header lines We will cover a few more below and another small fraction when we discuss network Web caching at the end of this chapter A readable and comprehensive discussion of HTTP headers and status codes is given in [Luotonen 1998] An excellent introduction to the technical issues surrounding the Web is [Yeager 1996] How does a browser decide which header lines it includes in a request message How does a Web server decide which header lines it includes in a response messages A browser will generate header lines as a function of the browser type and version (eg, an HTTP/10 browser will not generate any 11 header lines), user configuration of browser (eg, preferred language) and whether the browser currently has a cached, but possibly out-of-date, version of the object Web servers behave similarly: there are different products, versions, and configurations, all of which influence which header lines are included in response messages
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224 User-Server Interaction: Authentication and Cookies
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We mentioned above that an HTTP server is stateless This simplifies server design, and has permitted engineers to develop very highperforming Web servers However, it is often desirable for a Web site to identify users, either because the server wishes to restrict user access or because it wants to serve content as a function of the user identity HTTP provides two mechanisms to help a server identify a user: authentication and cookies
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Many sites require users to provide a username and a password in order to access the documents housed on the server This requirement is referred to as authentication HTTP provides special status codes and headers to help sites perform authentication Let us walk through an example to get a feel for how these special status codes and headers work Suppose a client requests an object from a server, and the server requires user authorization 1 The client first sends an ordinary request message with no special header lines 2 The server then responds with empty entity body and with a 401 Authorization Required status code In this response message the server includes the WWW-Authenticate: header, which specifies the details about how to perform authentication (Typically, it indicates to the user needs to provide a username and a password) 3 The client receives the response message and prompts the user for a username and password The client resends the request message, but this time includes an Authorization: header line, which includes the username and password After obtaining the first object, the client continues to send the username and password in subsequent requests for objects on the server (This typically continues until the client closes his browser However, while the browser remains open, the username and password are cached, so the user is not prompted for a username and password for each object it requests!) In this manner, the site can identify the user for every request
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file:///D|/Downloads/Livros/computa o/Computer%20NetDown%20Approach%20Featuring%20the%20Internet/httphtm (7 of 14)20/11/2004 15:51:52
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