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2.2. HTTP

Now that we have a clearer understanding of URLs, let's return to the main focus of this chapter: HTTP, the protocol that clients and servers use to communicate on the Web.

The Secure Sockets Layer

HTTP is not a secure protocol, and many networking protocols (like ethernet) allow the conversation between two computers to be overheard by other computers on the same area of the network. The result is that it is very possible for a third party to eavesdrop on HTTP transactions and record authentication information, credit card numbers, and other important data.

Thus, Netscape developed the SSL (Secure Sockets Layer) protocol, which provides a secure communications channel that HTTP can operate across, while also providing security against eavesdropping and other privacy attacks. SSL has developed into an IETF standard and is now formally referred to as the TLS (Transport Layer Security) protocol (TLS 1.0 is essentially SSL 3.1). Not all browsers support TLS yet.

When your browser requests a URL that begins with https, it creates an SSL/TLS connection to the remote server and performs its HTTP transaction across this secure connection. Fortunately, you don't need to understand the details of how this works to write scripts, because the web server transparently manages it for you. Standard CGI scripts will work the same in a secure environment as in a standard one. When your CGI script receives a secure SSL/TLS connection, however, you are given additional information about the client and the connection, as we will see in the next chapter.

2.2.1. The Request and Response Cycle

When a web browser requests a web page, it sends a request message to a web server. The message always includes a header, and sometimes it also includes a body. The web server in turn replies with a reply message. This message also always includes a header and it usually contains a body.

There are two features that are important in understanding HTTP:

Figure 2-2 shows an example of an HTTP transaction. Say you told your browser you wanted a document at http://localhost/index.html. The browser would connect to the machine at localhost on port 80 and send it the following message:

GET /index.html HTTP/1.1
Host: localhost
Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, image/xbm, */*
Accept-Language: en
Connection: Keep-Alive
User-Agent: Mozilla/4.0 (compatible; MSIE 4.5; Mac_PowerPC)
Figure 2-2

Figure 2-2. The HTTP request/response cycle

Assuming that a web server is running and the path maps to a valid document, the server would reply with the following message:

HTTP/1.1 200 OK
Date: Sat, 18 Mar 2000 20:35:35 GMT
Server: Apache/1.3.9 (Unix)
Last-Modified: Wed, 20 May 1998 14:59:42 GMT
ETag: "74916-656-3562efde"
Content-Length: 141
Content-Type: text/html

<HEAD><TITLE>Sample Document</TITLE></HEAD>
  <H1>Sample Document</H1>
  <P>This is a sample HTML document!</P>

In this example, the request includes a header but no content. The response includes both a header and HTML content, separated by a blank line (see Figure 2-3).

Figure 2-3

Figure 2-3. The HTTP header/body message structure

2.2.2. HTTP Headers

If you are familiar with the format of Internet email, this header and body syntax may look familiar to you. Historically, the format of HTTP messages is based upon many of the conventions used by Internet email, as established by MIME (Multipurpose Internet Mail Extensions). Do not be tricked into thinking that HTTP and MIME headers are the same, however. The similarity extends only to certain fields, and many early similarities have changed in later versions of HTTP.

Here are the important things to know about header syntax:

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