Ubuntu Manpage: LWP - The World-Wide Web library for Perl

NAME

       LWP - The World-Wide Web library for Perl

SYNOPSIS

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

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 communication. 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.

• Supports parsing of HTML forms.

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

• Supports HTTP cookies.

• Some simple command line clients, for instance "lwp-request" and "lwp-download".

HTTP STYLE COMMUNICATION

       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 <URL:http://www.w3.org/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 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:

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

       •  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.

       •  headers contains additional information about the request and can also used to describe  the  content.
          The headers are a set of keyword/value pairs.

       •  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:

       •  code is a numerical value that indicates the overall outcome of the request.

       •  message is a short, human readable string that corresponds to the code.

       •  headers contains additional information about the response and describe the content.

       •  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:

       is_success()
          The request 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 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 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 configure how it will interact with the network and
       with your application.

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

       •  agent specifies the name that your application uses when it presents itself on the network.

       •  from 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.

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

       •  proxy   and   no_proxy    specify    if    and    when    to    go    through    a    proxy    server.
          <URL:http://www.w3.org/History/1994/WWW/Proxies/>

       •  credentials provides 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-classing "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;
         my $ua = LWP::UserAgent->new;
         $ua->agent("MyApp/0.1 ");

         # Create a request
         my $req = HTTP::Request->new(POST => 'http://search.cpan.org/search');
         $req->content_type('application/x-www-form-urlencoded');
         $req->content('query=libwww-perl&mode=dist');

         # 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 $res->status_line, "\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 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 a GET request you might want to add a "If-Modified-Since" or "If-None-Match" header to make the
request conditional.

For a 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 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 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 internally 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 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.

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

CPAN Requests
URLs with scheme "cpan:" are redirected to the a suitable CPAN mirror. If you have your own local mirror
of CPAN you might tell LWP to use it for "cpan:" URLs by an assignment like this:

$LWP::Protocol::cpan::CPAN = "file:/local/CPAN/";

Suitable CPAN mirrors are also picked up from the configuration for the CPAN.pm, so if you have used that
module a suitable mirror should be picked automatically. If neither of these apply, then a redirect to
the generic CPAN http location is issued.

Example request to download the newest perl:

$req = HTTP::Request->new(GET => "cpan:src/latest.tar.gz");

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::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 definitions.

        LWP                -- This file.  Library version number and documentation.
        LWP::MediaTypes    -- MIME types configuration (text/html etc.)
        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

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 communication through a proxy server.  See the
           description of the "env_proxy" method in LWP::UserAgent.

       PERL_LWP_ENV_PROXY
           If set to  a  TRUE  value,  then  the  "LWP::UserAgent"  will  by  default  call  "env_proxy"  during
           initialization.  This makes LWP honor the proxy variables described above.

       PERL_LWP_SSL_VERIFY_HOSTNAME
           The  default  "verify_hostname" setting for "LWP::UserAgent".  If not set the default will be 1.  Set
           it as 0 to disable hostname verification (the default prior to libwww-perl 5.840.

       PERL_LWP_SSL_CA_FILE
       PERL_LWP_SSL_CA_PATH
           The file and/or directory where the trusted  Certificate  Authority  certificates  is  located.   See
           LWP::UserAgent for details.

       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 compatibility with old times.

AUTHORS

       LWP was made possible by contributions from Adam Newby, Albert  Dvornik,  Alexandre  Duret-Lutz,  Andreas
       Gustafsson,  Andreas  Koenig, Andrew Pimlott, Andy Lester, Ben Coleman, Benjamin Low, Ben Low, Ben Tilly,
       Blair Zajac, Bob Dalgleish, BooK, Brad Hughes, Brian J. Murrell, Brian McCauley, Charles C.  Fu,  Charles
       Lane,  Chris  Nandor,  Christian  Gilmore,  Chris  W.  Unger, Craig Macdonald, Dale Couch, Dan Kubb, Dave
       Dunkin, Dave W. Smith, David Coppit, David Dick, David D. Kilzer, Doug  MacEachern,  Edward  Avis,  erik,
       Gary  Shea,  Gisle Aas, Graham Barr, Gurusamy Sarathy, Hans de Graaff, Harald Joerg, Harry Bochner, Hugo,
       Ilya Zakharevich, INOUE Yoshinari, Ivan Panchenko, Jack Shirazi, James Tillman, Jan Dubois, Jared  Rhine,
       Jim  Stern,  Joao Lopes, John Klar, Johnny Lee, Josh Kronengold, Josh Rai, Joshua Chamas, Joshua Hoblitt,
       Kartik Subbarao, Keiichiro Nagano, Ken Williams, KONISHI Katsuhiro,  Lee  T  Lindley,  Liam  Quinn,  Marc
       Hedlund,  Marc  Langheinrich,  Mark  D.  Anderson, Marko Asplund, Mark Stosberg, Markus B Krueger, Markus
       Laker, Martijn Koster, Martin Thurn, Matthew  Eldridge,  Matthew.van.Eerde,  Matt  Sergeant,  Michael  A.
       Chase,  Michael  Quaranta,  Michael  Thompson,  Mike  Schilli, Moshe Kaminsky, Nathan Torkington, Nicolai
       Langfeldt, Norton Allen, Olly Betts, Paul J. Schinder, peterm, Philip  GuentherDaniel  Buenzli,  Pon  Hwa
       Lin, Radoslaw Zielinski, Radu Greab, Randal L. Schwartz, Richard Chen, Robin Barker, Roy Fielding, Sander
       van Zoest, Sean M. Burke, shildreth, Slaven Rezic, Steve A Fink, Steve Hay, Steven Butler, Steve_Kilbane,
       Takanori Ugai, Thomas Lotterer, Tim Bunce, Tom Hughes, Tony Finch, Ville Skyttae, Ward Vandewege, William
       York, Yale Huang, and Yitzchak Scott-Thoennes.

       LWP  owes  a  lot  in motivation, design, and code, to the libwww-perl library for Perl4 by Roy Fielding,
       which included 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.

COPYRIGHT

         Copyright 1995-2009, 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 CPAN as well as:

         http://github.com/libwww-perl/libwww-perl

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

perl v5.20.2                                       2015-12-05                                           LWP(3pm)

NAME

SYNOPSIS

DESCRIPTION

HTTP STYLE COMMUNICATION

NETWORK SUPPORT

OVERVIEW OF CLASSES AND PACKAGES

MORE DOCUMENTATION

ENVIRONMENT

AUTHORS

COPYRIGHT

AVAILABILITY