lwp(3)

NAME

LWP - The World-Wide Web library for Perl

SYNOPSIS

use LWP;
print "This is libwww-perl-$LWP::VERSION0;

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 commu: nication. 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.
� Implements HTTP content negotiation algorithm that can: be used both in protocol modules and in server scripts
(like CGI scripts).
� Supports HTTP cookies.
� A simple command line client application called: "lwp-request".

HTTP STYLE COMMUNICATION

The libwww-perl library is based on HTTP style communica
tion. This section tries to describe what that means.

Let us start with this quote from the HTTP specification
document <URL:http://www.w3.org/pub/WWW/Protocols/>:

� 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 mes
sage'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 acknowledg ment 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". 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::Request"s both to ftp and gopher servers, as well as to the local file system.
The main attributes of the request objects are:
� The method is a short string that tells what kind of: request this is. The most common methods are GET, PUT, POST and HEAD.
� The 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.
� The headers contain additional information about the: request and can also used to describe the content. The
headers are a set of keyword/value pairs.
� The content is an arbitrary amount of data.
The Response Object
The libwww-perl response object has the class name "HTTP::Response". The main attributes of objects of this class are:
� The code is a numerical value that indicates the over: all outcome of the request.
� The message is a short, human readable string that cor: responds to the code.
� The headers contain additional information about the: response and describe the content.
� The 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 classi fication methods are:
is_success(): The request was 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 rea
son.
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 inter face that can accept requests and return responses.
The user agent is an interface layer between your applica tion 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". 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" object as argument and (eventu ally) returns a "HTTP::Response" object.
The user agent has many other attributes that let you con figure how it will interact with the network and with your application.
� The timeout specifies how much time we give remote: servers to respond before the library disconnects and
creates an internal timeout response.
� The agent specifies the name that your application: should use when it presents itself on the network.
� The from attribute 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.
� The parse_head specifies whether we should initialize: response headers from the <head> section of HTML docu
ments.
� The proxy and no_proxy attributes specify if and when: to go through a proxy server.
<URL:http://www.w3.org/pub/WWW/Proxies/>
� The credentials provide 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-class ing "LWP::UserAgent". The library includes a sub-class, "LWP::RobotUA", 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;
$ua = LWP::UserAgent->new;
$ua->agent("MyApp/0.1 ");; # Create a request
my $req = HTTP::Request->new(POST =>; 'http://www.perl.com/cgi-bin/BugGlimpse');
$req->content_type('application/x-www-form-urlencoded');
$req->content('match=www&errors=0');; # 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 "Bad luck this time0;; }
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 ini
tialized 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 CGIscripts may implement any method they like.

If the server is not available then the library will gen
erate an internal error response.

The library automatically adds a "Host" and a "Con
tent-Length" header to the HTTP request before it is sent
over the network.

For GET request you might want to add a "If-Modi
fied-Since" or "If-None-Match" header to make the request
conditional.

For POST request you should add the "Content-Type" header.
When you try to emulate HTML <FORM> handling you should
usually let the value of the "Content-Type" header be
"application/x-www-form-urlencoded". See lwpcook for
examples of this.

The libwww-perl HTTP implementation currently support the
HTTP/1.1 and HTTP/1.0 protocol.

The library allows you to access proxy server through
HTTP. This means that you can set up the library to for
ward all types of request through the HTTP protocol mod
ule. See LWP::UserAgent for documentation of this.

HTTPS Requests

HTTPS requests are HTTP requests over an encrypted network
connection using the SSL protocol developed by Netscape.
Everything about HTTP requests above also apply to HTTPS
requests. In addition the library will add the headers
"Client-SSL-Cipher", "Client-SSL-Cert-Subject" and
"Client-SSL-Cert-Issuer" to the response. These headers
denote the encryption method used and the name of the
server owner.

The request can contain the header "If-SSL-Cert-Subject"
in order to make the request conditional on the content of
the server certificate. If the certificate subject does
not match, no request is sent to the server and an inter
nally generated error response is returned. The value of
the "If-SSL-Cert-Subject" header is interpreted as a Perl
regular expression.

FTP Requests

The library currently supports GET, HEAD and PUT requests.
GET retrieves a file or a directory listing from an FTP
server. PUT stores a file on a ftp server.

You can specify a ftp account for servers that want this
in addition to user name and password. This is specified
by including an "Account" header in the request.

User name/password can be specified using basic authoriza
tion or be encoded in the URL. Failed logins return an
UNAUTHORIZED response with "WWW-Authenticate: Basic" and
can be treated like basic authorization for HTTP.

The library supports ftp ASCII transfer mode by specifying
the "type=a" parameter in the URL. It also supports trans
fer of ranges for FTP transfers using the "Range" header.

Directory listings are by default returned unprocessed (as
returned from the ftp server) with the content media type
reported to be "text/ftp-dir-listing". The "File::Listing"
module provides methods for parsing of these directory
listing.

The ftp module is also able to convert directory listings
to HTML and this can be requested via the standard HTTP
content negotiation mechanisms (add an "Accept: text/html"
header in the request if you want this).

For normal file retrievals, the "Content-Type" is guessed
based on the file name suffix. See LWP::MediaTypes.

The "If-Modified-Since" request header works for servers
that implement the MDTM command. It will probably not
work for directory listings though.

Example:: $req = HTTP::Request->new(GET => 'ftp://me:pass; wd@ftp.some.where.com/');
$req->header(Accept => "text/html, */*;q=0.1");
News Requests
Access to the USENET News system is implemented through the NNTP protocol. The name of the news server is obtained from the NNTP_SERVER environment variable and defaults to "news". It is not possible to specify the hostname of the NNTP server in news: URLs.
The library supports GET and HEAD to retrieve news arti cles through the NNTP protocol. You can also post arti cles to newsgroups by using (surprise!) the POST method.
GET on newsgroups is not implemented yet.
Examples:: $req = HTTP::Request->new(GET =>; 'news:abc1234@a.sn.no');; $req = HTTP::Request->new(POST =>; 'news:comp.lang.perl.test');
$req->header(Subject => 'This is a test',
From => 'me@some.where.org');; $req->content(<<EOT);
This is the content of the message that we are sending; to
the world.
EOT
Gopher Request
The library supports the GET and HEAD methods for gopher requests. All request header values are ignored. HEAD cheats and returns a response without even talking to server.
Gopher menus are always converted to HTML.
The response "Content-Type" is generated from the document type encoded (as the first letter) in the request URL path itself.
Example:: $req = HTTP::Request->new(GET => 'gopher://go; pher.sn.no/');
File Request
The library supports GET and HEAD methods for file requests. The "If-Modified-Since" header is supported. All other headers are ignored. The host component of the file URL must be empty or set to "localhost". Any other host value will be treated as an error.
Directories are always converted to an HTML document. For normal files, the "Content-Type" and "Content-Encoding" in the response are guessed based on the file suffix.
Example:: $req = HTTP::Request->new(GET => 'file:/etc/passwd');
Mailto Request
You can send (aka "POST") mail messages using the library. All headers specified for the request are passed on to the mail system. The "To" header is initialized from the mail address in the URL.
Example:: $req = HTTP::Request->new(POST => 'mailto:lib; www@perl.org');
$req->header(Subject => "subscribe");
$req->content("Please subscribe me to the libwww-perl; mailing list!0);

OVERVIEW OF CLASSES AND PACKAGES

This table should give you a quick overview of the classes provided by the library. Indentation shows class inheri tance.: LWP::MemberMixin -- Access to member variables of Perl5; classes
LWP::UserAgent -- WWW user agent class
LWP::RobotUA -- When developing a robot applica

tions

LWP::Protocol -- Interface to various protocol

schemes

LWP::Protocol::http -- http:// access
LWP::Protocol::file -- file:// access
LWP::Protocol::ftp -- ftp:// access
...

LWP::Authen::Basic -- Handle 401 and 407 responses
LWP::Authen::Digest

HTTP::Headers -- MIME/RFC822 style header (used by

HTTP::Message)
HTTP::Message -- HTTP style message

HTTP::Request -- HTTP request
HTTP::Response -- HTTP response

HTTP::Daemon -- A HTTP server class

WWW::RobotRules -- Parse robots.txt files

WWW::RobotRules::AnyDBM_File -- Persistent RobotRules

Net::HTTP -- Low level HTTP client; The following modules provide various functions and defi
nitions.

LWP -- This file. Library version number

and documentation.
LWP::MediaTypes -- MIME types configuration (text/html

etc.)
LWP::Debug -- Debug logging module
LWP::Simple -- Simplified procedural interface for

common functions
HTTP::Status -- HTTP status code (200 OK etc)
HTTP::Date -- Date parsing module for HTTP date

formats
HTTP::Negotiate -- HTTP content negotiation calcula

tion
File::Listing -- Parse directory listings
HTML::Form -- Processing for <form>s in HTML doc

uments

ENVIRONMENT

The following environment variables are used by LWP:

HOME: The "LWP::MediaTypes" functions will look for the
.media.types and .mime.types files relative to you home directory.
http_proxy ftp_proxy xxx_proxy no_proxy: These environment variables can be set to enable com
munication through a proxy server. See the descrip
tion of the "env_proxy" method in LWP::UserAgent.
PERL_LWP_USE_HTTP_10: Enable the old HTTP/1.0 protocol driver instead of the
new HTTP/1.1 driver. You might want to set this to a
TRUE value if you discover that your old LWP applica
tions fails after you installed LWP-5.60 or better.
PERL_HTTP_URI_CLASS: Used to decide what URI objects to instantiate. The
default is "URI". You might want to set it to
"URI::URL" for compatiblity with old times.

BUGS

The library can not handle multiple simultaneous requests
yet. Also, check out what's left in the TODO file.

ACKNOWLEDGEMENTS

This package owes a lot in motivation, design, and code,
to the libwww-perl library for Perl 4, maintained by Roy
Fielding <fielding@ics.uci.edu>.

That package used work from Alberto Accomazzi, James
Casey, Brooks Cutter, Martijn Koster, Oscar Nierstrasz,
Mel Melchner, Gertjan van Oosten, Jared Rhine, Jack Shi
razi, Gene Spafford, Marc VanHeyningen, Steven E. Brenner,
Marion Hakanson, Waldemar Kebsch, Tony Sanders, and Larry
Wall; see the libwww-perl-0.40 library for details.

The primary architect for this Perl 5 library is Martijn
Koster and Gisle Aas, with lots of help from Graham Barr,
Tim Bunce, Andreas Koenig, Jared Rhine, and Jack Shirazi.

COPYRIGHT

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

AVAILABILITY

The latest version of this library is likely to be avail able from CPAN as well as:: http://www.linpro.no/lwp/
The best place to discuss this code is on the <lib www@perl.org> mailing list.

docs.sk

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