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What is a Web Proxy Server?

A Web proxy server is a specialized HTTP server. The primary use of a proxy server is to allow internal clients access to the Internet from behind a firewall. Anyone behind a firewall can now have full Web access past the firewall host with minimum effort and without compromising security.

The proxy server listens for requests from clients within the firewall and forwards these requests to remote internet servers outside the firewall. The proxy server reads responses from the external servers and then sends them to internal client clients.

In the usual case, all the clients within a given subnet use the same proxy server. This makes it possible for the proxy to cache documents efficiently that are requested by a number of clients.

People using a proxy server should feel as if they are getting responses directly from remote servers.

Clients without Domain Name Services (DNS) can still use the Web. The proxy IP address is the only information they need. Organizations using private network address spaces such as the class A net 10.*.*.* can still use the Internet as long as the proxy is visible to both the private internal net and the Internet.

Most proxy servers are implemented on a per-access method basis. Proxy servers can allow or deny internet requests according to the protocol of the requests. For instance a proxy server can allow calls to FTP servers while denying calls to HTTP servers.

When Web Proxy Servers are Useful
You can use a proxy server in a number of ways, including:

* Permitting and restricting client access to the Internet based on the client IP address
* Caching documents for internal documents
* Selectively controlling access to the Internet and subnets based on the submitted URL
* Providing Internet access for companies using private networks
* Converting data to HTML format so it is readable by a browser

An Ordinary Web Transaction Via a Server
Many clients have their own IP address and a direct connection to servers on the Internet. When a normal HTTP request is made by the browser, the HTTP server gets only the path and keyword portion of the requested URL. Other parts of the URL, such as the protocol specifier "http:" and the host name, are clear to the remote HTTP server. The remote server knows that it is an HTTP server, and it knows the host machine that on which it is running. The requested path specifies the document or a CGI program on the local file system of the server, or some other resource available from that server.

When a user enters:

http://mycompany.com/information/ProxyDetails.html

The browser converts it to:

GET /information/ProxyDetails.html

Communication Via a Proxy Server
The proxy server acts as both a server system and a client system. It is a server when accepting HTTP requests from browsers, and acts as a client system when its browser software connects to remote servers to retrieve documents.

The proxy server uses the header fields passed to it by the browser without modification when it connects to the remote server. This means the browser does not lose any functionality when going through a proxy.

A complete proxy server should be able to communicate all the Web protocols, the most important ones being HTTP, FTP, Gopher, and WAIS. Proxies that handle only a single Internet protocol, such as HTTP, are possible, but a Web browser would then require access to other proxy servers to handle the remaining protocols.

When a browser sends a request through a proxy server, the browser always uses HTTP for the transactions with the proxy server. This is true even when the user wants to access a remote server that uses another protocol; for example, FTP.

Instead of specifying only the pathname and search keywords to the proxy server, the browser specifies the full URL. This way the proxy server has all the information necessary to make the actual request to the remote server specified in the request URL, using the protocol specified in the URL.

The browser connects to the server running on mycompany.com and issues the command and waits for a response. In this example, the browser makes a request to the HTTP server and specifies the requested resource relative to that server; there is no protocol nor host name specifier in the URL.

Advantages and Disadvantages of Caching Documents
Caching documents means storing documents locally so users do not have to connect to a remote server to get files. When a local browser requests a file, the server checks its cache to see if it has the document. If the file exists in the cache, the server serves the local copy to the browser. If you cache documents you need to decide:

* Which documents are used frequently enough to justify keeping them locally
* How long you can keep the documents in cache before fetching more recent copies.

Proxy Server-to-Proxy Server Linking
Chaining proxy servers lets you run a proxy server as a local cache on behalf of a department within an organization. The individual departments have control over the server and cache. These departmental proxy servers can connect to a proxy server on a firewall between the Internet and the organization.

Any restrictions for access set for the organization proxy server take precedence over access restrictions set for the departmental proxy servers.

For example, departmental proxy server 1 might be set to allow all URL requests. The organizational proxy server, as corporate policy, might be set to deny all URL requests for certain online publications. A request for one of these publications coming into proxy server 1 would be forwarded to the organizational proxy server. The organizational proxy server would then deny the request.

Want to learn about Python? What is Python? Python is the web programming language that the Google search engine was made with! Python is used to create dynamic web pages or web applications. Python code is interpreted by the web server which then outputs a web page to the browsers that access it.

Learn about Python's capabilities
Python is available on multiple platforms. It can be used for managing email, connecting to other web sites, processing form information, storing information in a database, creating cookies in a web browser, and much more. It can connect to almost any type of database but it has a strong relationship with its free database buddy, MySQL. Python supports object oriented, imperative, and functional programming models.

Learn about Python's syntax
Syntax is the wording and way that the language has to be used for it to work. Python is very easy to understand so it is a great language for beginners to start learning how to program. Python is a very readable programming language. It often uses English keywords where other programming languages use punctuation. Here are some important things to learn about Python:
Sample Python Code

* Indentation. Rather than using brackets to specify what will be in a block of code (within an if statement, for example), Python uses indentation. This means white space is very important and if you don’t indent your code properly if could do something completely unexpected.
* Variables. Variables hold different types of data. They are loosely typed to allow specific types of data like strings, numbers, arrays, or lists.
* Comments. You can use the pound sign (#) to comment out a line or three quote marks (''' my comment ''') to comment out multiple lines.

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Want to learn about Ruby on Rails? What is Ruby on Rails? It is the hippest web programming language of the decade. It is a free, server-side, web application framework that allows you to create dynamic web pages and web applications very rapidly. Rudy on Rails code is interpreted by the web server which then outputs a web page to the browsers that access it. It is becoming more and more popular as people begin to recognize its power to quickly design and implement web applications. The fundamental Ruby on Rails principles include Convention over Configuration (CoC) and Don't repeat yourself (DRY). These philosophies have shaped Ruby on Rails so it very easy to quickly create new applications.

<%= "Hello World!"; %>

Hello World!

* How can I compile Perl

To compile Perl you need a C compiler on your machine. If your vendor did not include one with your system, then grab a binary version of gcc from the net,

check,
+ http://www.perl.com/pub/language/info/software.html#binary
+ http://www.perl.com/CPAN/ports/

If you are on a UNIX its probably best to compile Perl yourself from the source code distribution. If you are on Windows or MacOS, you will probably be best with a binary build.

* Perl scripting basics

If you are looking to understand a perl script, then these are common elements you may find,
+ # - comment and this line wont get executed.
+ $ - indicates that this is a variable
+ & - if a word starts with a & it is a call to a sub-routine
+ < - the file is being opened for a read operation + > - the file is being opened for a write operation
+ >> - the file is being opened for an append operation
+ (.) - this puts together two variables
+ @ - if a word start with this character it indicates a single-dimension array
+ % - if a word starting with this character indicates a two-dimensional array.
* Perl versions

Version 4
+ was the fourth major release (March 1991)
+ it's stable but,
+ is old, limited, and no longer maintained;
+ its last patch (4.036) was in 1992

Version 5
+ name for the fifth major release (October 1994)
+ rewrite of the original perl source code from releases 1 through 4
+ has been modularized, object-oriented, tweaked, trimmed, and optimized
+ interface is mostly the same

Version 6
+ rewrite of the current release of Perl by the Topaz project
+ 100% source-compatibility with previous releases
+ written in nominally portable C++

* Reporting Perl bugs

Want to learn about Perl? What is Perl? It is one of the oldest web programming language around. Practically every Linux based server comes with support for it by default. Perl is a server-side, scripting language that can output dynamic web pages or web applications. Perl code, like PHP, is run on the server and then gives HTML back to the web browser.

Learn about Perl's capabilities
Because Perl has been around since 1987, it is available on nearly every platform, Windows or Linux. Perl was originally developed for text manipulation but can now be used for managing email, connecting to other web sites or servers, processing form information, storing information in a database, reading and writing to text files, creating cookies in a web browser, and much more. Perl also support subroutines (blocks of code that can do specific things over and over).
Learn About Perl

Learn about Perl's syntax
Perl borrows a lot of syntax from the C programming language. Syntax is the wording and way that the language has to be used for it to work. Perl is designed to be practical so it can be easy to understand but it can also be complex so a higher-level language such as PHP would be a better choice for you if you are a beginning programmer. Perl uses typed variables that store a particular type of data (text, numbers, arrays, scalar, etc.). Here are some important things to learn about Perl:

* Filename. You can name your Perl files anything you like.
* Variables. Most variables are marked by a leading character which identifies the type of variable:
o $foo # a scalar
o @foo # an array
o %foo # a hash
o FOO # a file handle or constant
o &foo # a subroutine. (The & is optional)
* Comments. You can comment out code using the hash character (#).

You can code Perl in any text editor but it may help to use one that has syntax highlighting or other features. What does Perl code look like? Here is a sample:

$yesterday = parsedate('yesterday');
$printdate = strftime('%a, %b %d', localtime($yesterday));

open (FILE, ">$dir/index.html");
for (@index) {
if (/INSERT HERE/) {
print FILE "$_";
print FILE "$printdate\n";
if (($id % 5) == 0) {print FILE "\n"}
}
else {print FILE "$_"};
} # End for
close FILE;

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