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 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, thus libwww-perl is a WWW client
library. The library also contain modules that are of more
general use.
Most modules in this library are 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:
· 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.
· Cooperates with Tk. A simple Tk-based GUI browser
called 'tkweb' is distributed with the Tk extension for
perl.
· Implements HTTP content negotiation algorithm that can
be used both in protocol modules and in server scripts
(like CGI scripts).
· 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
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 used methods are GET, PUT,
POST and HEAD.
· 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.
· 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.
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 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 class. The main method provided by this object is
request(). This method takes an `HTTP::Request' object as
argument and (eventually) 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,
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 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 CGI-
scripts may implement any method they like.
erate 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 man
page for examples of this.
The libwww-perl HTTP implementation currently support the
HTTP/1.0 protocol. HTTP/0.9 servers are also handled cor
rectly.
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 the LWP::UserAgent manpage 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.
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.
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 the LWP::MediaTypes
manpage.
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: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 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->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://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->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 inheri
tance.
LWP::MemberMixin -- Access to member variables of Perl5 classes
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::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
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 calculation
File::Listing -- Parse directory listings
MORE DOCUMENTATION
All modules contain detailed information on the interfaces
they provide. 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 imple
mented.
BUGS
The library can not handle multiple simultaneous requests
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
Copyright 1995-2001, 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 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.
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