Securing Your Web Pages with Apache Page 2

But what's all this noise about 'discretionary' and 'mandatory,' you ask? Put simply, discretionary control (DAC) mechanisms check the validity of the credentials given them at the discretion of the user, and mandatory access controls (MAC) validate aspects that the user cannot control. For instance, anyone can tell you its username and password and you can then log in with them; which username and password you supply is at your discretion, and the system can't tell you apart from the real owner. Your DNA is something you can't change, though, and a control system that only allowed access to your pattern would never work for anyone else -- and you couldn't pretend to be someone else, either. This makes such a system a mandatory (also called non-discretionary) access control system.

In Web terms, and Apache terms in particular, discretionary controls are based on usernames and passwords, and mandatory controls are based on things like the IP address of the requesting client.

Another way to keep discretionary versus non-discretionary controls straight is to think about the way failures are handled: if you fail a discretionary check (such as if you misspell your password), you get another chance -- but if a mandatory check fails, you get a 'forbidden' error rather than 'not authorised,' and there's no way to say "give me another chance" without starting from scratch and requesting the page again as though for the first time. And unless something's changed on the server, even retrying isn't going to make a difference; you'll still be locked out.

Authentication versus Authorisation

Authentication is the process of verifying that credentials are correct -- that is, that the username is in the database and the password is correct for the username. Authorisation is the process of checking to see if a validated client is permitted to access a particular resource. For instance, Bob may have correctly supplied his username and password, but still not be able to access Jane's file because she hasn't included him in the authorisation list for it.

In Apache, almost all of the security-related modules (see a later section for a list) actually do both. The main feature that distinguishes them from each other is their authentication aspect; mostly, they let you store the valid credential information in one format or another. mod_auth, for instance, looks in normal text files for the username and password info, and mod_auth_dbm looks in a DBM database for it. They handle the authorisation side of their task in essentially identical ways, however.

The security modules are passed the information about what authentication databases to use via directives, such as AuthUserFile or AuthDBMGroupFile. The resource being protected is determined from the placement of the directives in the configuration files; in this example:

    <Directory /home/johnson/public_html>
        <Files foo.bar>
            AuthName "Foo for Thought"
            AuthType Basic
            AuthUserFile /home/johnson/foo.htpasswd
            Require valid-user

This article was originally published on Jun 29, 2000

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