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Unit 9: Web Techniques
The most common status message people get to see is 404 Not Found, which simply means Notes
that document you requested does not exists. This is either because it really does not exists
or because you entered the URL wrong. When a 404 is returned it is usually displayed on the
browser screen in whatever default format is used by that browser. The server may also transmit
a detailed error report page along with an error message if the resource call was unsuccessful.
9.1.2 The HTTP Header
People who know just enough HTML to be dangerous encounter the term HTTP header and
may think that is corresponds in some way to the document header in an HTML document. This
is not the case. The HTML document header is something you have coded into the document
between <head> tags, and, as far as the server is concerned, is part of the document content
being sent. The HTML header is information the author has provided for the client application
about the document. The HTTP header, on the other hand, is information the client and server
provide each other about the transmission process for the document.
If you need a concrete example, think of the HTML header as the date and address written at
the top of a business letter, while the HTTP header is the address written on the outside of the
envelope. They may both be addresses, but they are physically different things in physically
different locations.
The HTTP header contains details about the transaction between the client and server, with
slight variations depending on whether it is a request or a response. The header information
can be grouped into three different categories. These are:
General
General information fields contain information about either the client or the server. General
information can be as general as nothing but the current date and time.
Entity
Entity information fields contain information about the data being transmitted. Common entity
information is the date on which the document or resource was lastly modified or the address
of the document requesting this one.
Request/Response
The request/response fields contain information about the client and server configuration,
including what sort of documents the client can accept and what sort of requests the server
can accept. This information includes the server name and version for the server, and the client
application name and version for the client. It also includes the platform being used by the
client or the server. This information is often used by client or server applications to customize
the request or response for the needs of the application on the other end of the connection. It
can also be used to specify what sort of documents the client can receive, so, for example, the
server knows not to try to send images or audio files to a text-only interface.
Each header field is delimited by a line break at the end. In other words, each data field is
written on its own line. The end of the header section is delimited by one or more blank lines.
In computer terms, a blank line is nothing but some form of newline character. So the end of
the header section is actually delimited by a sequence of line break characters with nothing
between them.
A sample request header might look as follows:
GET /utils/servervars.php HTTP/1.1
Accept: text/html, image/png, image/jpeg, image/gif, */*
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