LWP - Library for WWW access in Perl
SYNOPSIS
use LWP;
print "This is libwww-perl-$LWP::VERSION\n";
DESCRIPTION
Libwww-perl is a collection of Perl modules which provides
a simple and consistent 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, thus libwww-perl said to be a WWW client library.
The library also contain modules that are of more general
use.
The main architecture of the library is object oriented.
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 should be easy to extend and customize for your
needs.
The main features of the library are:
o Contains various reusable components (modules) that can
be used separately or together.
o Provides an object oriented model of HTTP-style
communication. Within this framework we currently
support access to http, gopher, ftp, news, file, and
mailto resources.
o The library be used through the full object oriented
interface or through a very simple procedural
interface.
o Support the basic and digest authorization schemes.
o Transparent redirect handling.
o Supports access through proxy servers.
o URL handling (both absolute and relative URLs are
supported).
o A parser for robots.txt files and a framework for
constructing robots.
o An experimental HTML parser and formatters (for
PostScript and plain text).
GUI browser called 'tkweb' is distributed with the Tk
extension for perl.
o An implementation of the HTTP content negotiation
algorithm that can be used both in protocol modules and
in server scripts (like CGI scripts).
o A simple command line client application called lwp-
request.
HTTP STYLE COMMUNICATION
The libwww-perl library is based on HTTP style
communication. This section try 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
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 has the class name HTTP::Request in
libwww-perl. The fact that the class name use HTTP:: as a
name 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 both to ftp and gopher servers, as
well as to the local file system.
The main attributes of the request objects are:
o The method is a short string that tells what kind of
request this is. The most used methods are GET, PUT,
POST and HEAD.
o The url is a string denoting the protocol, server and
the name of the "document" we want to access. The url
might also encode various other parameters.
o The headers contain additional information about the
request and can also used to describe the content. The
headers is a set of keyword/value pairs.
o The content is an arbitrary amount of data.
The Response Object
The request object has the class name HTTP::Response in
libwww-perl. The main attributes of objects of this class
are:
o The code is a numerical value that encode the overall
outcome of the request.
o The message is a short (human readable) string that
corresponds to the code.
o The headers contain additional information about the
response and they also describe the content.
o The content is an arbitrary amount of data.
Since we don't want to handle all possible code values
directly in our programs, the libwww-perl response object
have 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 was successfully received, understood
or accepted.
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 on to a user agent object
and this object will take care of all the things that need
to be done (low-level communication and error handling).
The user agent will give you back a response object. The
user agent represents your application on the network and
it provides you with an interface that can accept requests
and will return responses.
You should think about the user agent as 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 libwww-perl 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
kind. The main method provided by this object is
request(). This method takes an HTTP::Request object as
argument and will (eventually) return a HTTP::Response
object.
The user agent has many other attributes that lets you
configure how it will interact with the network and with
your application code.
o The timeout specify how much time we give remote
servers in creating responses before the library
disconnect and creates an internal timeout response.
o The agent specify the name that your application should
use when it presents itself on the network.
o 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.
o The use_alarm specify if it is OK for the user agent to
use the alarm(2) system to implement timeouts.
o The use_eval specify if the agent should raise an
exception (die in perl) if an error condition occur.
response headers from the <head> section of HTML
documents.
o The proxy and no_proxy specify if and when
communication should go through a proxy server.
<URL:http://www.w3.org/pub/WWW/Proxies/>
o The credentials provide a way to set up user names and
passwords that is needed to access certain services.
Many applications would want even more control over how
they interact with the network and they get this by
specializing the LWP::UserAgent by sub-classing. The
library provide a specialization called LWP::RobotUA that
is used by 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 = new LWP::UserAgent;
$ua->agent("AgentName/0.1 " . $ua->agent);
# Create a request
my $req = new HTTP::Request 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 time\n";
}
The $ua is created once when the application starts up.
New request objects are normally created for each request
sent.
NETWORK SUPPORT
This section goes through the various protocol schemes and
describe the HTTP style methods that are supported and the
headers that might have any effect.
For all requests, a "User-Agent" header is added and
initialized from the $ua->agent value before the request
For all responses, the library will add a header called
"Client-Date". This header will encode the time when the
response was received by your application. This format
and semantics of the header is just like the server
created "Date" header.
HTTP Requests
HTTP request are really just handed off to an HTTP server
and it will decide what happens. Few servers implement
methods beside the usual "GET", "HEAD", "POST" and "PUT"
but CGI-scripts can really 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 GET request you might want to add the "If-Modified-
Since" 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 the lwpcook
manpage for examples of this.
The libwww-perl HTTP implementation currently support the
HTTP/1.0 protocol. HTTP/0.9 servers are also handled
correctly.
The library allows you to access proxy server through
HTTP. This means that you can set up the library to
forward all types of request through the HTTP protocol
module. See the LWP::UserAgent manpage for documentation
of this.
FTP Requests
The library currently support GET, HEAD and PUT requests.
GET will retrieve a file or a directory listing from an
FTP server. PUT will store a file on a ftp server.
You can specify a ftp account for servers that want this
in addition user name and password. This is specified by
passing an "Account" header in the request.
User name/password can be specified using basic
authorization or be encoded in the URL. Bad logins return
The library support ftp ASCII transfer mode by specifying
the "type=a" parameter in the URL.
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 provide functionality 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).
The normal file retrievals, the "Content-Type" is guessed
based on the file name suffix. See the LWP::MediaTypes
manpage.
The "If-Modified-Since" header is not honored yet.
Example:
$req = HTTP::Request->new(GET => 'ftp://me:passwd@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 the news:-URLs.
The library support GET and HEAD to retrieve news articles
through the NNTP protocol. You can also post articles 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
The library supports the GET and HEAD method for gopher
request. All request header values are ignored. HEAD
cheats and will return 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://gopher.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:libwww-perl-request@ics.uci.edu');
$req->header("Subject", "subscribe");
$req->content("Please subscribe me to the libwww-perl mailing list!\n");
OVERVIEW OF CLASSES AND PACKAGES
This table should give you a quick overview of the classes
provided by the library. Indentation shows class
inheritance.
LWP::UserAgent -- WWW user agent class
LWP::RobotUA -- When developing a robot applications
LWP::Protocol -- Interface to various protocol schemes
LWP::Protocol::http -- http:// access
LWP::Protocol::file -- file:// access
LWP::Protocol::ftp -- ftp:// access
...
LWP::Socket -- Socket creation and IO
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
URI::URL -- Uniform Resource Locators
WWW::RobotRules -- Parse robots.txt files
WWW::RobotRules::AnyDBM_File -- Persistent RobotRules
HTML::Parser -- Parse HTML documents
HTML::TreeBuilder-- Build a HTML syntax tree
HTML::HeadParser -- Parse the <HEAD> section of a HTML document
HTML::LinkExtor -- Extract links from a HTML document
HTML::Element -- Building block for the HTML::TreeBuilder
HTML::Formatter -- Convert HTML syntax trees to readable formats
HTML::FormatText -- Output is plain text
HTML::FormatPS -- Output is PostScript
The following modules provide various functions and
definitions.
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 calculation
HTML::Entities -- Expand or unexpand entities in HTML text
File::Listing -- Parse directory listings
HTTP use the Base64 encoding at some places. The
QuotedPrint module is just included to make the MIME::
collection more complete.
MIME::Base64 -- Base64 encoding/decoding routines
MIME::QuotedPrint -- Quoted Printable encoding/decoding routines
and did not bother to make separate distributions for
them. Regard them as bonus, provided free for your
pleasure.
Font::AFM -- Parse Adobe Font Metric files
File::CounterFile -- Persistent counter class
MORE DOCUMENTATION
All modules contain detailed information on the interfaces
they provide. The the lwpcook manpage is the libwww-perl
cookbook that contain examples of typical usage of the
library. You might want to take a look at how the scripts
lwp-request, lwp-rget and lwp-mirror are implemented.
BUGS
The library can not handle multiple simultaneous requests
yet. The HTML:: modules are still experimental. 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
Shirazi, 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
Copyright 1995-1997, Gisle Aas
Copyright 1995, Martijn Koster
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
available from:
http://www.sn.no/libwww-perl/
The best place to discuss this code is on the <libwww-
perl@ics.uci.edu> mailing list.