Since HTTP is a stateless protocol, the mechanisms for adding "state" (such as "logged in") have to use one or more of the following methods:

• Mangled URLs (including a session key somewhere in the URL)
• Basic Authentication (login box, usually managed in the .htaccess file)
• Hidden fields (if your application goes from one form to another)
• Cookies (but be sure to provide a way to logout easily, or time it out)
• Firing off a separate web server for each session (useful in low-volume applications)

Each has advantages and disadvantages. Be sure to steer clear of anyone that advises you to use any of the following:

• IP address, even if attached to browser type (it's not unique for proxies, and can even change within a single session)
• The Referer (accessed through $ENV{HTTP_REFERER}) which can be trivially faked and is stripped automatically at some corporate firewalls
• Any kind of client-side session management using Java or Javascript (some firewalls actually strip this stuff, making your site useless to them, and all security-conscious people have this stuff turned off anyway)

That should get you started. For more information on some of the session topics, see my WebTechniques Perl column archives.

-- Randal L. Schwartz, Perl hacker

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