PHP : Internet Services - [17.9] Using LDAP for User Authentication

17.9.1 Problem

You want to restrict parts of your site to authenticated users. Instead of verifying people against a database or using HTTP Basic authorization, you want to use an LDAP server. Holding all user information in an LDAP server makes centralized user administration easier.

17.9.2 Solution

Use PEAR's Auth class, which supports LDAP authentication:

$options = array('host'     => 'ldap.example.com',
                 'port'     => '389',
                 'base'     => 'o=Example Inc., c=US',
                 'userattr' => 'uid');

$auth = new Auth('LDAP', $options);

// begin validation 
// print login screen for anonymous users
$auth->start();

if ($auth->getAuth()) {
    // content for validated users
} else {
    // content for anonymous users
}

// log users out
$auth->logout();

17.9.3 Discussion

LDAP servers are designed for address storage, lookup, and retrieval, and so are better to use than standard databases like MySQL or Oracle. LDAP servers are very fast, you can easily implement access control by granting different permissions to different groups of users, and many different programs can query the server. For example, most email clients can use an LDAP server as an address book, so if you address a message to "John Smith," the server replies with John's email address, jsmith@example.com.

PEAR's Auth class allows you to validate users against files, databases, and LDAP servers. The first parameter is the type of authentication to use, and the second is an array of information on how to validate users. For example:

$options = array('host'     => 'ldap.example.com',
                 'port'     => '389',
                 'base'     => 'o=Example Inc., c=US',
                 'userattr' => 'uid');

$auth = new Auth('LDAP', $options);

This creates a new Auth object that validates against an LDAP server located at ldap.example.com and communicates over port 389. The base directory name is o=Example Inc., c=US, and usernames are checked against the uid attribute. The uid field stands for user identifier. This is normally a username for a web site or a login name for a general account. If your server doesn't store uid attributes for each user, you can substitute the cn attribute. The common name field holds a user's full name, such as "John Q. Smith."

The Auth::auth( ) method also takes an optional third parameter — the name of a function that displays the sign-in form. This form can be formatted however you wish; the only requirement is that the form input fields must be called username and password. Also, the form must submit the data using POST.

$options = array('host'     => 'ldap.example.com',
                 'port'     => '389',
                 'base'     => 'o=Example Inc., c=US',
                 'userattr' => 'uid');

function pc_auth_ldap_signin() {
    print<<<_HTML_
<form method="post" action="$_SERVER[PHP_SELF]">
Name: <input name="username" type="text"><br />
Password: <input name="password" type="password"><br />
<input type="submit" value="Sign In">
</form>
_HTML_;
}

$auth = new Auth('LDAP', $options, 'pc_auth_ldap_signin');

Once the Auth object is instantiated, authenticate a user by calling Auth::start( ) :

$auth->start();

If the user is already signed in, nothing happens. If the user is anonymous, the sign-in form is printed. To validate a user, Auth::start( ) connects to the LDAP server, does an anonymous bind, and searches for an address in which the user attribute specified in the constructor matches the username passed in by the form:

$options['userattr'] =  = $_POST['username']

If Auth::start( ) finds exactly one person that fits this criteria, it retrieves the designated name for the user, and attempts to do an authenticated bind, using the designated name and password from the form as the login credentials. The LDAP server then compares the password to the userPassword attribute associated with the designated name. If it matches, the user is authenticated.

You can call Auth::getAuth( ) to return a boolean value describing a user's status:

if ($auth->getAuth( )) {
    print 'Welcome member! Nice to see you again.';
} else {
    print 'Welcome guest. First time visiting?';
}

The Auth class uses the built-in session module to track users, so once validated, a person remains authenticated until the session expires, or you explicitly log them out with:

$auth->logout( );