# phpman > perldoc > LWP
## NAME
LWP - The World-Wide Web library for Perl
## SYNOPSIS
use LWP;
print "This is libwww-perl-$[LWP::VERSION](https://www.chedong.com/phpMan.php/perldoc/LWP%3A%3AVERSION/markdown)\n";
## DESCRIPTION
The libwww-perl collection is a set of Perl modules which provides a simple and consistent
application programming interface (API) to the World-Wide Web. The main focus of the library is
to provide classes and functions that allow you to write WWW clients. The library also contain
modules that are of more general use and even classes that help you implement simple HTTP
servers.
Most modules in this library provide an object oriented API. The user agent, requests sent and
responses received from the WWW server are all represented by objects. This makes a simple and
powerful interface to these services. The interface is easy to extend and customize for your own
needs.
The main features of the library are:
* Contains various reusable components (modules) that can be used separately or together.
* Provides an object oriented model of HTTP-style communication. Within this framework we
currently support access to "http", "https", "gopher", "ftp", "news", "file", and "mailto"
resources.
* Provides a full object oriented interface or a very simple procedural interface.
* Supports the basic and digest authorization schemes.
* Supports transparent redirect handling.
* Supports access through proxy servers.
* Provides parser for robots.txt files and a framework for constructing robots.
* Supports parsing of HTML forms.
* Implements HTTP content negotiation algorithm that can be used both in protocol modules and
in server scripts (like CGI scripts).
* Supports HTTP cookies.
* Some simple command line clients, for instance "lwp-request" and "lwp-download".
## HTTP STYLE COMMUNICATION
The libwww-perl library is based on HTTP style communication. This section tries to describe
what that means.
Let us start with this quote from the HTTP specification document
<>:
* The HTTP protocol is based on a request/response paradigm. A client establishes a connection
with a server and sends a request to the server in the form of a request method, URI, and
protocol version, followed by a MIME-like message containing request modifiers, client
information, and possible body content. The server responds with a status line, including the
message's protocol version and a success or error code, followed by a MIME-like message
containing server information, entity meta-information, and possible body content.
What this means to libwww-perl is that communication always take place through these steps:
First a *request* object is created and configured. This object is then passed to a server and
we get a *response* object in return that we can examine. A request is always independent of any
previous requests, i.e. the service is stateless. The same simple model is used for any kind of
service we want to access.
For example, if we want to fetch a document from a remote file server, then we send it a request
that contains a name for that document and the response will contain the document itself. If we
access a search engine, then the content of the request will contain the query parameters and
the response will contain the query result. If we want to send a mail message to somebody then
we send a request object which contains our message to the mail server and the response object
will contain an acknowledgment that tells us that the message has been accepted and will be
forwarded to the recipient(s).
It is as simple as that!
### The Request Object
The libwww-perl request object has the class name [HTTP::Request](https://www.chedong.com/phpMan.php/perldoc/HTTP%3A%3ARequest/markdown). The fact that the class name
uses "HTTP::" as a prefix only implies that we use the HTTP model of communication. It does not
limit the kind of services we can try to pass this *request* to. For instance, we will send
[HTTP::Requests](https://www.chedong.com/phpMan.php/perldoc/HTTP%3A%3ARequests/markdown) both to ftp and gopher servers, as well as to the local file system.
The main attributes of the request objects are:
* method is a short string that tells what kind of request this is. The most common methods are
GET, PUT, POST and HEAD.
* uri is a string denoting the protocol, server and the name of the "document" we want to
access. The uri might also encode various other parameters.
* headers contains additional information about the request and can also used to describe the
content. The headers are a set of keyword/value pairs.
* content is an arbitrary amount of data.
### The Response Object
The libwww-perl response object has the class name [HTTP::Response](https://www.chedong.com/phpMan.php/perldoc/HTTP%3A%3AResponse/markdown). The main attributes of
objects of this class are:
* code is a numerical value that indicates the overall outcome of the request.
* message is a short, human readable string that corresponds to the *code*.
* headers contains additional information about the response and describe the content.
* content is an arbitrary amount of data.
Since we don't want to handle all possible *code* values directly in our programs, a libwww-perl
response object has methods that can be used to query what kind of response this is. The most
commonly used response classification methods are:
### is_success
The request was successfully received, understood or accepted.
### is_error
The request failed. The server or the resource might not be available, access to the resource
might be denied or other things might have failed for some reason.
### The User Agent
Let us assume that we have created a *request* object. What do we actually do with it in order
to receive a *response*?
The answer is that you pass it to a *user agent* object and this object takes care of all the
things that need to be done (like low-level communication and error handling) and returns a
*response* object. The user agent represents your application on the network and provides you
with an interface that can accept *requests* and return *responses*.
The user agent is an interface layer between your application code and the network. Through this
interface you are able to access the various servers on the network.
The class name for the user agent is [LWP::UserAgent](https://www.chedong.com/phpMan.php/perldoc/LWP%3A%3AUserAgent/markdown). Every libwww-perl application that wants to
communicate should create at least one object of this class. The main method provided by this
object is request(). This method takes an [HTTP::Request](https://www.chedong.com/phpMan.php/perldoc/HTTP%3A%3ARequest/markdown) object as argument and (eventually)
returns a [HTTP::Response](https://www.chedong.com/phpMan.php/perldoc/HTTP%3A%3AResponse/markdown) object.
The user agent has many other attributes that let you configure how it will interact with the
network and with your application.
* timeout specifies how much time we give remote servers to respond before the library
disconnects and creates an internal *timeout* response.
* agent specifies the name that your application uses when it presents itself on the network.
* from can be set to the e-mail address of the person responsible for running the application.
If this is set, then the address will be sent to the servers with every request.
* parse_head specifies whether we should initialize response headers from the "" section
of HTML documents.
* proxy and no_proxy specify if and when to go through a proxy server.
<>
* credentials provides a way to set up user names and passwords needed to access certain
services.
Many applications want even more control over how they interact with the network and they get
this by sub-classing [LWP::UserAgent](https://www.chedong.com/phpMan.php/perldoc/LWP%3A%3AUserAgent/markdown). The library includes a sub-class, [LWP::RobotUA](https://www.chedong.com/phpMan.php/perldoc/LWP%3A%3ARobotUA/markdown), for robot
applications.
### An Example
This example shows how the user agent, a request and a response are represented in actual perl
code:
# Create a user agent object
use [LWP::UserAgent](https://www.chedong.com/phpMan.php/perldoc/LWP%3A%3AUserAgent/markdown);
my $ua = [LWP::UserAgent](https://www.chedong.com/phpMan.php/perldoc/LWP%3A%3AUserAgent/markdown)->new;
$ua->agent("MyApp/0.1 ");
# Create a request
my $req = [HTTP::Request](https://www.chedong.com/phpMan.php/perldoc/HTTP%3A%3ARequest/markdown)->new(POST => '');
$req->content_type('application/x-www-form-urlencoded');
$req->content('query=libwww-perl&mode=dist');
# Pass request to the user agent and get a response back
my $res = $ua->request($req);
# Check the outcome of the response
if ($res->is_success) {
print $res->content;
}
else {
print $res->status_line, "\n";
}
The $ua is created once when the application starts up. New request objects should normally
created for each request sent.
## NETWORK SUPPORT
This section discusses the various protocol schemes and the HTTP style methods that headers may
be used for each.
For all requests, a "User-Agent" header is added and initialized from the "$ua->agent" attribute
before the request is handed to the network layer. In the same way, a "From" header is
initialized from the $ua->from attribute.
For all responses, the library adds a header called "Client-Date". This header holds the time
when the response was received by your application. The format and semantics of the header are
the same as the server created "Date" header. You may also encounter other "Client-XXX" headers.
They are all generated by the library internally and are not received from the servers.
HTTP Requests
HTTP requests are just handed off to an HTTP server and it decides what happens. Few servers
implement methods beside the usual "GET", "HEAD", "POST" and "PUT", but CGI-scripts may
implement any method they like.
If the server is not available then the library will generate an internal error response.
The library automatically adds a "Host" and a "Content-Length" header to the HTTP request before
it is sent over the network.
For a GET request you might want to add an "If-Modified-Since" or "If-None-Match" header to make
the request conditional.
For a POST request you should add the "Content-Type" header. When you try to emulate HTML