LWP
LWP(3) User Contributed Perl Documentation LWP(3)
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:
o Contains various reusable components (modules) that can be used sep-
arately 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 resources.
o Provides a full object oriented interface or a very simple procedu-
ral 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 construct-
ing robots.
o Supports parsing of HTML forms.
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 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 sec-
tion 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/>:
o 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 pre-
vious 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 parame-
ters 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:
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 key-
word/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:
is_success()
The request was was successfully received, understood or accepted.
is_error()
The request failed. The server or the resource might not be avail-
able, 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 actu-
ally 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 com-
munication 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.
o The timeout specifies how much time we give remote servers to
respond before the library disconnects and creates an internal time-
out 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 running 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 head-
ers from the <head> section of HTML documents.
o The proxy and no_proxy attributes specify if and when to 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
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;
$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 inter-
nal 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 reg-
ular 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 mecha-
nisms (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 proto-
col. 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 (sur-
prise!) 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::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
MORE DOCUMENTATION
All modules contain detailed information on the interfaces they pro-
vide. 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.
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_USE_HTTP_10
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.
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 K
perl v5.8.6 2007-08-05 LWP(3)
