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14 ” HTTP Server: Apache X-Powered-By: PHP/5.2.5 Cache-Control: private, s-maxage=0, max-age=0, must-revalidate Content-Language: en Last-Modified: Mon, 21 Jul 2008 02:06:27 GMT Content-Length: 53631 Content-Type: text/html; charset=utf-8 Connection: close [body...] Aside from headers, the main difference in content between requests and responses is in the contents of the request line versus the status line. • 1.0 is the minimum HTTP protocol version under which the response can be correctly interpreted by the client. • 200 is a response status code and OK is its corresponding human-readable description. It indicates the result of the server attempting to process the request, in this case that the request was successful. Status codes are divided into five classes distinguished by the first digit of the code. Belo w is a brief summary of each class. See section 10 of RFC 2616 for further descriptions of circumstances under which specific status codes may be received. • 1xx Informational: Request received, continuing process. • 2xx Success: Request received, understood, and accepted. • 3xx Redirection: Client must take additional action to complete the request. • 4xx Client Error: Request could not be fulfilled because of a client issue. • 5xx Server Error: Request was valid but the server failed to process it. M o ving right along, let us examine headers in more depth.
HTTP ” 15 H eaders An all-purpose method of communicating a variety of information related to requests and responses, headers are used by the client and server to accomplish a number of things including retention of state using cookies and identity verification using HTTP authentication. This section will deal with those that are particularly applicable to web scraping applications. F or more information, see section 14 of RFC 2616. Cookies HTTP is designed to be a stateless protocol. That is, once a server returns the response for a request, it effectively “ forgets ” about the request. It may log information about the request and the response it delivered, but it does not retain any sense of state for the same client between requests. Cookies are a method of circumventing this using headers. H ere is ho w they work. • The client issues a request. • In its response, the server includes a Set-Cookie header. The header value is comprised of name-value pairs each with optional associated attribute-value pairs. • In subsequent requests, the client will include a Cookie header that contains the data it received in the Set-Cookie response header. Cookies are frequently used to restrict access to certain content, most often by requiring some form of identity authentication before the target application will indicate that a cookie should be set. M ost client libraries have the capability to handle parsing and resending cookie data as appropriate, though some require explicit instruction before they will do so . F or more information on cookies, see RFC 2109 or its later (though less widely adopted) rendition RFC 2965. One of the aforementioned attributes, “expires,” is used to indicate when the client should dispose of the cookie and not persist its data in subsequent requests. This attribute is optional and its presence or lack thereof is the defining factor in whether or not the cookie is what’s called a session cookie. If a cookie has no expiration value
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