Red Hat DIRECTORY SERVER 2.0 - GATEWAY Manual de usuario descargar pdf (Pagina 16)

Chapter 2. The site’s design

Multiple addresses:

A single system need not have a single IP address. It can have many and each can

have a different web site attached to it. This leads to two different server names ap-

pearing in standard URLs (i.e. there’s no :port element in the URL) but the two

server names correspond to the two different IP addresses of the system and corre-

spond to different web sites. The receiving web server uses the destination IP address

on the incoming query to distinguish web sites.

Multiple aliases

Also known as name-based virtual hosting, this is the most common form of virtual

hosting. The server only has a single IP address, but two different names in the DNS

correspond to that address. So chalk.dept.cam.ac.uk and cheese.dept.cam.ac.uk both

map on to the same IP address and therefore the same server. This raises the question

of how the web server can distinguish requests to the two different web sites.

Structures of HTTP queries and responses

To answer this question, we need to consider, brieﬂy, the nature of a web request.

Exactly what gets sent to a server when a URL is requested? (And for that matter,

what gets sent back?)

GET /index.html HTTP/1.1

Host: noether.csi.cam.ac.uk

User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.0.1) Gecko/20021003

Accept: text/html,text/plain;q=0.8,image/png,image/jpeg,image/gif;q=0.2,*/*;q=0.1

Accept-Language: en, es;q=0.50

Accept-Encoding: gzip, deflate, compress;q=0.9

Accept-Charset: ISO-8859-1, utf-8;q=0.66, *;q=0.33

Connection: keep-alive

Keep-Alive: 300

Figure 2-1. Incoming HTTP query for http://noether.csi.cam.ac.uk/index.html

To understand name-based virtual hosting consider just the ﬁrst two lines. The GET

request only refers to the local element of the URL. The second line speciﬁes the

hostname that is being asked for it.

HTTP Query Structure

GET

The ﬁrst line declares that this is a request from a client that wishes to read in-

formation from the server. GET is the most common HTTP method.

/index.html

The second term in the ﬁrst line is the local element of the URL requested. Note

that the leading part of the URL containing the server name has been stripped

out.

HTTP/1.1

The ﬁnal element declares that the query is couched in the language of version

1.1 of the HTTP standard.

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Red Hat DIRECTORY SERVER 2.0 - GATEWAY Manual de usuario Pagina 16