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Chapter 11. Cookies, Authentication, and Advanced Requests

Not every document can be fetched with a simple GET or POST request. Many pages require authentication before you can access them, some use cookies to keep track of the different users, and still others want special values in the Referer or User-Agent headers. This chapter shows you how to set arbitrary headers, manage cookies, and even authenticate using LWP. You'll be able to make your LWP programs appear to be Netscape or Internet Explorer, log in to a protected site, and work with sites that use cookies.

For example, suppose you're automating a web-based purchasing system. The server requires you to log in, then issues you a cookie to prove you've been authenticated. You must then send this cookie back to the server with every request you make.

Or, more mundanely, suppose you're extracting information from one of the many web sites that check the User-Agent header in your requests. If your User-Agent doesn't identify yours as a recent version of Netscape or Internet Explorer, the server sends you back an "Upgrade your browser" page. You need to set the User-Agent header to make it appear that you are using Netscape or Internet Explorer.

11.1. Cookies

HTTP was originally designed as a stateless protocol, meaning that each request is totally independent of other requests. But web site designers felt the need for something to help them identify the user of a particular session. The mechanism that does this is called a cookie. This section gives some background on cookies so you know what LWP is doing for you.

An HTTP cookie is a string that an HTTP server can send to a client, which the client is supposed to put in the headers of any future requests that it makes to that server. Suppose a client makes a request to a given server, and the response headers consist of this:

Date: Thu, 28 Feb 2002 04:29:13 GMT
Server: Apache/1.3.23 (Win32)
Content-Type: text/html
Set-Cookie: foo=bar; expires=Thu, 20 May 2010 01:23:45 GMT; path=/

This means that the server wants all further requests from this client to anywhere on this site (i.e., under /) to be accompanied by this header line:

Cookie: foo=bar

That header should be present in all this browser's requests to this site, until May 20, 2010 (at 1:23:45 in the morning), after which time the client should never send that cookie again.

A Set-Cookie line can fail to specify an expiration time, in which case this cookie ends at the end of this "session," where "session" is generally seen as ending when the user closes all browser windows. Moreover, the path can be something more specific than /. It can be, for example, /dahut/, in which case a cookie will be sent only for URLs that begin http://thishost/dahut/. Finally, a cookie can specify that this site is not just on this one host, but also on all other hosts in this subdomain, so that if this host is search.mybazouki.com, cookies should be sent to any hostname under mybazouki.com, including images.mybazouki.com, ads.mybazouki.com, extra.stuff.mybazouki.com, and so on.

All those details are handled by LWP, and you need only make a few decisions for a given LWP::UserAgent object:

  • Should it implement cookies at all? If not, it will just ignore any Set-Cookie: headers from the server and will never send any Cookie: headers.

  • Should it load cookies when it starts up? If not, it will start out with no cookies.

  • Should it save cookies to some file when the browser object is destroyed? If not, whatever cookies it has accumulated will be lost.

  • What format should the cookies file be in? Currently the choices are either a format particular to LWP, or Netscape cookies files.

11.1.1. Enabling Cookies

By default, an LWP::UserAgent object doesn't implement cookies. To make an LWP::UserAgent object that implements cookies is as simple as this:

my $browser = LWP::UserAgent->new( );
$browser->cookie_jar( {} );

However, that browser object's cookie jar (as we call its HTTP cookie database) will start out empty, and its contents won't be saved anywhere when the object is destroyed. Incidentally, the above code is a convenient shortcut for what one previously had to do:

# Load LWP class for "cookie jar" objects
use HTTP::Cookies;
my $browser = LWP::UserAgent->new( );
my $cookie_jar = HTTP::Cookies->new( );
$browser->cookie_jar( $cookie_jar );

There's not much point to using the long form when you could use the short form instead, but the longer form becomes preferable when you're adding options to the cookie jar.

11.1.2. Loading Cookies from a File

To start the cookie jar by loading from a particular file, use the file option to the HTTP::Cookies new method:

use HTTP::Cookies;
my $cookie_jar = HTTP::Cookies->new(
   file     => "/some/where/cookies.lwp",
);
my $browser = LWP::UserAgent->new;
$browser->cookie_jar( $cookie_jar );

In that case, the file is read when the cookie jar is created, but it's never updated with any new cookies that the $browser object will have accumulated.

To read the cookies from a Netscape cookies file instead of from an LWP-format cookie file, use a different class, HTTP::Cookies::Netscape, which is just like HTTP::Cookies, except for the format that it reads and writes:

use HTTP::Cookies::Netscape;
my $cookie_jar = HTTP::Cookies::Netscape->new(
   file => "c:/program files/netscape/users/shazbot/cookies.txt",
);
my $browser = LWP::UserAgent->new;
$browser->cookie_jar( $cookie_jar );

11.1.3. Saving Cookies to a File

To make LWP write out its potentially changed cookie jar to a file when the object is no longer in use, add an autosave => 1 parameter:

use HTTP::Cookies;
my $cookie_jar = HTTP::Cookies->new(
   file     => "/some/where/cookies.lwp",
   autosave => 1,
);
my $browser = LWP::UserAgent->new;
$browser->cookie_jar( $cookie_jar );

At time of this writing, using autosave => 1 with HTTP::Cookies::Netscape has not been sufficiently tested and is not recommended.

11.1.4. Cookies and the New York Times Site

Suppose that you have felt personally emboldened and empowered by all the previous chapters' examples of pulling data off of news sites, especially the examples of simplifying HTML in Chapter 10, "Modifying HTML with Trees". You decide that a great test of your skill would be to write LWP code that downloads the stories off various newspapers' web sites and saves them all in a format (either plain text, highly simplified HTML, or even WML, if you have an html2wml tool around) that your ancient but trusty 2001-era PDA can read. Thus, you can spend your commute time on the train (or bus, tube, el, metro, jitney, T, etc.) merrily flipping through the day's news stories from papers all over the world.

Suppose also that you have the basic HTML-simplifying code in place (so we shall not discuss it further), and the LWP code that downloads stories from all the newspapers is working fine—except for the New York Times site. And you can't imagine why it's not working! You have a simple HTML::TokeParser program that gets the main page, finds all the URLs to stories in it, and downloads them one at a time. You verify that those routines are working fine. But when you look at the files that it claims to be successfully fetching and saving ($response->is_success returns true and everything!), all you see for each one is a page that says "Welcome to the New York Times on the Web! Already a member? Log in!" When you look at the exact same URL in Netscape, you don't see that page at all, but instead you see the news story that you want your LWP program to be accessing.

Then it hits you: years ago, the first time you accessed the New York Times site, it wanted you to register with an email address and a password. But you haven't seen that screen again, because of... HTTP cookies! You riffle through your Netscape HTTP cookies file, and lo, there you find:

.nytimes.com TRUE / FALSE 1343279235 RMID 809ac0ad1cff9a6b

Whatever this means to the New York Times site, it's apparently what differentiates your copy of Netscape when it's accessing a story URL, from your LWP program when it's accessing that URL.

Now, you could simply hardwire that cookie into the headers of the $browser->get( ) request's headers, but that involves recalling exactly how lines in Netscape cookie databases translate into headers in HTTP request. The optimally lazy solution is to simply enable cookie support in this LWP::UserAgent object and have it read your Netscape cookie database. So just after where you started off the program with this:

use LWP;
my $browser = LWP::UserAgent->new( );

Add this:

use HTTP::Cookies::Netscape;
my $cookie_jar = HTTP::Cookies::Netscape->new(
 'file' => 'c:/program files/netscape/users/me/cookies.txt'
);
$browser->cookie_jar($cookie_jar);

With those five lines of code added, your LWP program's requests to the New York Times's server will carry the cookie that says that you're a registered user. So instead of giving your LWP program the "Log in!" page ad infinitum, the New York Times's server now merrily serves your program the news stories. Success!



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