LWP

LIB::LWP(1)           User Contributed Perl Documentation          LIB::LWP(1)



NAME
       LWP - Library for WWW access in Perl

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:

       ·  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, gopher,
          ftp, news, file, and mailto resources.

       ·  The library be used through the full object oriented interface or
          through a very simple procedural interface.

       ·  Support the basic and digest authorization schemes.

       ·  Transparent redirect handling.

       ·  Supports access through proxy servers.

       ·  URL handling (both absolute and relative URLs are supported).

       ·  A parser for robots.txt files and a framework for constructing
          robots.

       ·  An experimental HTML parser and formatters (for PostScript and plain
          text).

       ·  The library can cooperate with Tk.  A simple Tk-based GUI browser
          called 'tkweb' is distributed with the Tk extension for perl.

       ·  An implementation of the 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 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

       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:

       ·  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 is a set of
          keyword/value pairs.

       ·  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:

       ·  The code is a numerical value that encode the overall outcome of the
          request.

       ·  The message is a short (human readable) string that corresponds to
          the code.

       ·  The headers contain additional information about the response and
          they also 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, 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.

       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 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.

       ·  The timeout specify how much time we give remote servers in creating
          responses before the library disconnect and creates an internal
          timeout response.

       ·  The agent specify 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 use_alarm specify if it is OK for the user agent to use the
          alarm(2) system to implement timeouts.

       ·  The use_eval specify if the agent should raise an exception (die in
          perl) if an error condition occur.

       ·  The parse_head specify whether we should initialize response headers
          from the <head> section of HTML documents.

       ·  The proxy and no_proxy specify if and when communication should 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
          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 is handed to the network layer.
       In the same way, a "From" header is initialized from the $ua->from
       value.

       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 an UNAUTHORIZED response with
       "WWW-Authenticate: Basic" and can be treated as basic authorization for
       HTTP.

       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


       Gopher Request

       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::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::Socket        -- Socket creation and IO

        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

       The following modules does not have much to do with the World Wide Web,
       but are included just because I am lazy 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.




3rd Berkeley Distribution    perl 5.003, patch 07                  LIB::LWP(1)