Chapter 2
Host: http://www.debian.com
Connection: close
User-Agent: Python-urllib/3.4The first line is called the request line. It is comprised of the request method, which
is GET in this case, the path to the resource, which is / here, and the HTTP version,
1.1. The rest of the lines are request headers. Each line is comprised of a header
name followed by a colon and a header value. The request in the preceding output
only contains headers, it does not have a body.
Headers are used for several purposes. In a request they can be used for passing
extra data, such as cookies and authorization credentials, and for asking the server
for preferred formats of resources.
For example, an important header is the Host header. Many web server applications
provide the ability to host more than one website on the same server using the same
IP address. DNS aliases are set up for the various website domain names, so they all
point to the same IP address. Effectively, the web server is given multiple hostnames,
one for each website it hosts. IP and TCP (which HTTP runs on), can't be used to
tell the server which hostname the client wants to connect to because both of them
operate solely on IP addresses. The HTTP protocol allows the client to supply the
hostname in the HTTP request by including a Host header.
We'll look at some more request headers in the following section.
Here's an example of a response:
HTTP/1.1 200 OK
Date: Sun, 07 Sep 2014 19:58:48 GMT
Content-Type: text/html
Content-Length: 4729
Server: Apache
Content-Language: en<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">\n...The first line contains the protocol version, the status code, and the status message.
Subsequent lines contain the headers, a blank line, and then the body. In the
response, the server can use headers to inform the client about things such as the
length of the body, the type of content the response body contains, and the cookie
data that the client should store.