How the proxy server works ?

 How the proxy server works
Once you have configured your web browser as described, your browser will load a CGI script each time it is restarted. This script redefines the function that determines how to fetch the URL that is being requested. If the URL matches a pattern or a site that requires a proxy, it will connect to the proxy server rather than going directly to the site.

——————————————————————————–
(USR = User on IBM.Net or another ISP; PXY = BOL web proxy server; SRV = web server restricted to UCLA)

Step 1: User starts browser, connects to proxy server to get the Proxy Automatic Configuration (PAC) file.
  fetch request: proxy.ucla.edu/cgi/proxy

    +—–+                    +—–+
    |  U  |                    |  P  |
    |  S  | ==================>|  X  |
    |  R  |                    |  Y  |
    +—–+                    +—–+

Step 2: Proxy server returns “proxy”, which configures
  browser to do proxying.

    +—–+                    +—–+
    |  U  | “proxy” loaded     |  P  |
    |  S  |<================== |  X  |
    |  R  |                    |  Y  |
    +—–+                    +—–+

Step 3: User requests URL, URL compared against function retrieved
  from “proxy”, connection route is determined.

Step 4a: Route for URL returned from function is “DIRECT”
  (no proxy required).  Browser retrieves URL directly.  (*Note: At this
  point, every URL returns “DIRECT” unless it is one that is known to
  restrict access to ucla.edu*)

    +—–+                    +—–+
    |  U  | fetch URL          |  S  |
    |  S  |<==================>|  R  |
    |  R  |                    |  V  |
    +—–+                    +—–+

Step 4b1: Route for URL returned from function is “PROXY xxxxx”
  (proxy required).  Browser contacts proxy server and requests page.

    +—–+                    +—–+                    +—–+
    |  U  | request URL        |  P  |                    |  S  |
    |  S  | ==================>|  X  |                    |  R  |
    |  R  |                    |  Y  |                    |  V  |
    +—–+                    +—–+                    +—–+

Step 4b2: Proxy contacts server and requests page.

    +—–+                    +—–+                    +—–+
    |  U  |                    |  P  | request URL        |  S  |
    |  S  |                    |  X  | ==================>|  R  |
    |  R  |                    |  Y  |                    |  V  |
    +—–+                    +—–+                    +—–+

Step 4b3: Server sees request from ucla.edu domain (proxy address)
  and returns URL to proxy.

    +—–+                    +—–+                    +—–+
    |  U  |                    |  P  | return URL         |  S  |
    |  S  |                    |  X  |<================== |  R  |
    |  R  |                    |  Y  |                    |  V  |
    +—–+                    +—–+                    +—–+

Step 4b4: Proxy returns URL to User.

    +—–+                    +—–+                    +—–+
    |  U  | return URL         |  P  |                    |  S  |
    |  S  |<================== |  X  |                    |  R  |
    |  R  |                    |  Y  |                    |  V  |
    +—–+                    +—–+                    +—–+
 

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