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LWP(3)			       User Contributed Perl Documentation			   LWP(3)

       LWP - The World-Wide Web library for Perl

	 use LWP;
	 print "This is libwww-perl-$LWP::VERSION\n";

       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:

       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, https, gopher, ftp, news, file, and mailto

       o  Provides a full object oriented interface or a very simple procedural interface.

       o  Supports the basic and digest authorization schemes.

       o  Supports transparent redirect handling.

       o  Supports access through proxy servers.

       o  Provides parser for robots.txt files and a framework for constructing robots.

       o  Implements HTTP content negotiation algorithm that can be used both in protocol modules
	  and in server scripts (like CGI scripts).

       o  Supports HTTP cookies.

       o  A simple command line client application called "lwp-request".

       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

       o  The HTTP protocol is based on a request/response paradigm. A client establishes a con-
	  nection 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, fol-
	  lowed 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".  The fact that the
       class name uses "HTTP::" as a prefix only implies that we use the HTTP model of communica-
       tion.  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:

       o  The method is a short string that tells what kind of request this is.  The most common
	  methods are GET, PUT, POST and HEAD.

       o  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.

       o  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.

       o  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:

       o  The code is a numerical value that indicates 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 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, 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:

	  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 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".  Every libwww-perl application that
       wants to communicate should create at least one object of this class. The main method pro-
       vided 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 configure how it will interact with
       the network and with your application.

       o  The timeout specifies how much time we give remote servers to respond before the
	  library disconnects and creates an internal timeout response.

       o  The agent specifies 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 run-
	  ning the application.  If this is set, then the address will be sent to the servers
	  with every request.

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

       o  The proxy and no_proxy attributes specify if and when to go through a proxy server.

       o  The credentials provide a way to set up user names and passwords needed to access cer-
	  tain services.

       Many applications want even more control over how they interact with the network and they
       get this by sub-classing "LWP::UserAgent".  The library includes a sub-class, "LWP::Robo-
       tUA", 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');

	 # 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 nor-
       mally created for each request sent.

       This section discusses the various protocol schemes and the HTTP style methods that head-
       ers 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 GET request you might want to add a "If-Modified-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 "applica-
       tion/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 forward all types of request through the HTTP protocol module.  See
       LWP::UserAgent for documentation of this.

       HTTPS Requests

       HTTPS requests are HTTP requests over an encrypted network connection using the SSL proto-
       col 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 con-
       ditional on the content of the server certificate.  If the certificate subject does not
       match, no request is sent to the server and an internally generated error response is
       returned.  The value of the "If-SSL-Cert-Subject" header is interpreted as a Perl regular

       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 authorization 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 transfer 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.


	 $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 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.


	 $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');
	 This is the content of the message that we are sending to
	 the world.

       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 let-
       ter) in the request URL path itself.


	 $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 "Con-
       tent-Type" and "Content-Encoding" in the response are guessed based on the file suffix.


	 $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.


	 $req = HTTP::Request->new(POST => 'mailto:libwww@perl.org');
	 $req->header(Subject => "subscribe");
	 $req->content("Please subscribe me to the libwww-perl mailing list!\n");

       This table should give you a quick overview of the classes provided by the library. Inden-
       tation 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::Authen::Basic -- Handle 401 and 407 responses

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

       All modules contain detailed information on the interfaces they provide.  The lwpcook man-
       page 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-mir-
       ror" are implemented.

       The following environment variables are used by LWP:

	   The "LWP::MediaTypes" functions will look for the .media.types and .mime.types files
	   relative to you home directory.

	   These environment variables can be set to enable communication through a proxy server.
	   See the description of the "env_proxy" method in LWP::UserAgent.

	   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 applications fails
	   after you installed LWP-5.60 or better.

	   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.

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

       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 Spaf-
       ford, 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 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.

       The latest version of this library is likely to be available from CPAN as well as:


       The best place to discuss this code is on the <libwww@perl.org> mailing list.

libwww-perl-5.65			    2002-05-31					   LWP(3)
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