LDAP Authentication

Introduction

Zend\Authentication\Adapter\Ldap supports web application authentication with LDAP services. Its features include username and domain name canonicalization, multi-domain authentication, and failover capabilities. It has been tested to work with Microsoft Active Directory and OpenLDAP, but it should also work with other LDAP service providers.

This documentation includes a guide on using Zend\Authentication\Adapter\Ldap, an exploration of its API, an outline of the various available options, diagnostic information for troubleshooting authentication problems, and example options for both Active Directory and OpenLDAP servers.

Usage

To incorporate Zend\Authentication\Adapter\Ldap authentication into your application quickly, even if you’re not using Zend\Mvc, the meat of your code should look something like the following:

use Zend\Authentication\AuthenticationService;
use Zend\Authentication\Adapter\Ldap as AuthAdapter;
use Zend\Config\Reader\Ini as ConfigReader;
use Zend\Config\Config;
use Zend\Log\Logger;
use Zend\Log\Writer\Stream as LogWriter;
use Zend\Log\Filter\Priority as LogFilter;

$username = $this->getRequest()->getPost('username');
$password = $this->getRequest()->getPost('password');


$auth = new AuthenticationService();

$configReader = new ConfigReader();
$configData = $configReader->fromFile('./ldap-config.ini');
$config = new Config($configData, true);

$log_path = $config->production->ldap->log_path;
$options = $config->production->ldap->toArray();
unset($options['log_path']);

$adapter = new AuthAdapter($options,
                           $username,
                           $password);

$result = $auth->authenticate($adapter);

if ($log_path) {
    $messages = $result->getMessages();

    $logger = new Logger;
    $writer = new LogWriter($log_path);

    $logger->addWriter($writer);

    $filter = new LogFilter(Logger::DEBUG);
    $writer->addFilter($filter);

    foreach ($messages as $i => $message) {
        if ($i-- > 1) { // $messages[2] and up are log messages
            $message = str_replace("\n", "\n  ", $message);
            $logger->debug("Ldap: $i: $message");
        }
    }
}

Of course, the logging code is optional, but it is highly recommended that you use a logger. Zend\Authentication\Adapter\Ldap will record just about every bit of information anyone could want in $messages (more below), which is a nice feature in itself for something that has a history of being notoriously difficult to debug.

The Zend\Config\Reader\Ini code is used above to load the adapter options. It is also optional. A regular array would work equally well. The following is an example ldap-config.ini file that has options for two separate servers. With multiple sets of server options the adapter will try each, in order, until the credentials are successfully authenticated. The names of the servers (e.g., ‘server1’ and ‘server2’) are largely arbitrary. For details regarding the options array, see the Server Options section below. Note that Zend\Config\Reader\Ini requires that any values with “equals” characters (=) will need to be quoted (like the DNs shown below).

[production]

ldap.log_path = /tmp/ldap.log

; Typical options for OpenLDAP
ldap.server1.host = s0.foo.net
ldap.server1.accountDomainName = foo.net
ldap.server1.accountDomainNameShort = FOO
ldap.server1.accountCanonicalForm = 3
ldap.server1.username = "CN=user1,DC=foo,DC=net"
ldap.server1.password = pass1
ldap.server1.baseDn = "OU=Sales,DC=foo,DC=net"
ldap.server1.bindRequiresDn = true

; Typical options for Active Directory
ldap.server2.host = dc1.w.net
ldap.server2.useStartTls = true
ldap.server2.accountDomainName = w.net
ldap.server2.accountDomainNameShort = W
ldap.server2.accountCanonicalForm = 3
ldap.server2.baseDn = "CN=Users,DC=w,DC=net"

The above configuration will instruct Zend\Authentication\Adapter\Ldap to attempt to authenticate users with the OpenLDAP server s0.foo.net first. If the authentication fails for any reason, the AD server dc1.w.net will be tried.

With servers in different domains, this configuration illustrates multi-domain authentication. You can also have multiple servers in the same domain to provide redundancy.

Note that in this case, even though OpenLDAP has no need for the short NetBIOS style domain name used by Windows, we provide it here for name canonicalization purposes (described in the Username Canonicalization section below).

The API

The Zend\Authentication\Adapter\Ldap constructor accepts three parameters.

The $options parameter is required and must be an array containing one or more sets of options. Note that it is an array of arrays of Zend\Ldap\Ldap options. Even if you will be using only one LDAP server, the options must still be within another array.

Below is print_r() output of an example options parameter containing two sets of server options for LDAP servers s0.foo.net and dc1.w.net (the same options as the above INI representation):

Array
(
    [server2] => Array
        (
            [host] => dc1.w.net
            [useStartTls] => 1
            [accountDomainName] => w.net
            [accountDomainNameShort] => W
            [accountCanonicalForm] => 3
            [baseDn] => CN=Users,DC=w,DC=net
        )

    [server1] => Array
        (
            [host] => s0.foo.net
            [accountDomainName] => foo.net
            [accountDomainNameShort] => FOO
            [accountCanonicalForm] => 3
            [username] => CN=user1,DC=foo,DC=net
            [password] => pass1
            [baseDn] => OU=Sales,DC=foo,DC=net
            [bindRequiresDn] => 1
        )

)

The information provided in each set of options above is different mainly because AD does not require a username be in DN form when binding (see the bindRequiresDn option in the Server Options section below), which means we can omit a number of options associated with retrieving the DN for a username being authenticated.

Note

What is a Distinguished Name?

A DN or “distinguished name” is a string that represents the path to an object within the LDAP directory. Each comma-separated component is an attribute and value representing a node. The components are evaluated in reverse. For example, the user account CN=Bob Carter,CN=Users,DC=w,DC=net is located directly within the CN=Users,DC=w,DC=net container. This structure is best explored with an LDAP browser like the ADSI Edit MMC snap-in for Active Directory or phpLDAPadmin.

The names of servers (e.g. ‘server1’ and ‘server2’ shown above) are largely arbitrary, but for the sake of using Zend\Config\Reader\Ini, the identifiers should be present (as opposed to being numeric indexes) and should not contain any special characters used by the associated file formats (e.g. the ‘.‘INI property separator, ‘&‘ for XML entity references, etc).

With multiple sets of server options, the adapter can authenticate users in multiple domains and provide failover so that if one server is not available, another will be queried.

Note

The Gory Details: What Happens in the Authenticate Method?

When the authenticate() method is called, the adapter iterates over each set of server options, sets them on the internal Zend\Ldap\Ldap instance, and calls the Zend\Ldap\Ldap::bind() method with the username and password being authenticated. The Zend\Ldap\Ldap class checks to see if the username is qualified with a domain (e.g., has a domain component like alice@foo.net or FOO\alice). If a domain is present, but does not match either of the server’s domain names (foo.net or FOO), a special exception is thrown and caught by Zend\Authentication\Adapter\Ldap that causes that server to be ignored and the next set of server options is selected. If a domain does match, or if the user did not supply a qualified username, Zend\Ldap\Ldap proceeds to try to bind with the supplied credentials. if the bind is not successful, Zend\Ldap\Ldap throws a Zend\Ldap\Exception\LdapException which is caught by Zend\Authentication\Adapter\Ldap and the next set of server options is tried. If the bind is successful, the iteration stops, and the adapter’s authenticate() method returns a successful result. If all server options have been tried without success, the authentication fails, and authenticate() returns a failure result with error messages from the last iteration.

The username and password parameters of the Zend\Authentication\Adapter\Ldap constructor represent the credentials being authenticated (i.e., the credentials supplied by the user through your HTML login form). Alternatively, they may also be set with the setUsername() and setPassword() methods.

Server Options

Each set of server options in the context of Zend\Authentication\Adapter\Ldap consists of the following options, which are passed, largely unmodified, to Zend\Ldap\Ldap::setOptions():

Server Options

Name	Description
host	The hostname of LDAP server that these options represent. This option is required.
port	The port on which the LDAP server is listening. If useSsl is TRUE, the default port value is 636. If useSsl is FALSE, the default port value is 389.
useStartTls	Whether or not the LDAP client should use TLS (aka SSLv2) encrypted transport. A value of TRUE is strongly favored in production environments to prevent passwords from be transmitted in clear text. The default value is FALSE, as servers frequently require that a certificate be installed separately after installation. The useSsl and useStartTls options are mutually exclusive. The useStartTls option should be favored over useSsl but not all servers support this newer mechanism.
useSsl	Whether or not the LDAP client should use SSL encrypted transport. The useSsl and useStartTls options are mutually exclusive, but useStartTls should be favored if the server and LDAP client library support it. This value also changes the default port value (see port description above).
username	The DN of the account used to perform account DN lookups. LDAP servers that require the username to be in DN form when performing the “bind” require this option. Meaning, if bindRequiresDn is TRUE, this option is required. This account does not need to be a privileged account; an account with read-only access to objects under the baseDn is all that is necessary (and preferred based on the Principle of Least Privilege).
password	The password of the account used to perform account DN lookups. If this option is not supplied, the LDAP client will attempt an “anonymous bind” when performing account DN lookups.
bindRequiresDn	Some LDAP servers require that the username used to bind be in DN form like CN=Alice Baker,OU=Sales,DC=foo,DC=net (basically all servers except AD). If this option is TRUE, this instructs Zend\Ldap\Ldap to automatically retrieve the DN corresponding to the username being authenticated, if it is not already in DN form, and then re-bind with the proper DN. The default value is FALSE. Currently only Microsoft Active Directory Server (ADS) is known not to require usernames to be in DN form when binding, and therefore this option may be FALSE with AD (and it should be, as retrieving the DN requires an extra round trip to the server). Otherwise, this option must be set to TRUE (e.g. for OpenLDAP). This option also controls the default accountFilterFormat used when searching for accounts. See the accountFilterFormat option.
baseDn	The DN under which all accounts being authenticated are located. This option is required. if you are uncertain about the correct baseDn value, it should be sufficient to derive it from the user’s DNS domain using DC= components. For example, if the user’s principal name is alice@foo.net, a baseDn of DC=foo,DC=net should work. A more precise location (e.g., OU=Sales,DC=foo,DC=net) will be more efficient, however.
accountCanonicalForm	A value of 2, 3 or 4 indicating the form to which account names should be canonicalized after successful authentication. Values are as follows: 2 for traditional username style names (e.g., alice), 3 for backslash-style names (e.g., FOO\alice) or 4 for principal style usernames (e.g., alice@foo.net). The default value is 4 (e.g., alice@foo.net). For example, with a value of 3, the identity returned by Zend\Authentication\Result::getIdentity() (and Zend\Authentication\AuthenticationService::getIdentity(), if Zend\Authentication\AuthenticationService was used) will always be FOO\alice, regardless of what form Alice supplied, whether it be alice, alice@foo.net, FOO\alice, FoO\aLicE, foo.net\alice, etc. See the Account Name Canonicalization section in the Zend\Ldap\Ldap documentation for details. Note that when using multiple sets of server options it is recommended, but not required, that the same accountCanonicalForm be used with all server options so that the resulting usernames are always canonicalized to the same form (e.g., if you canonicalize to EXAMPLE\username with an AD server but to username@example.com with an OpenLDAP server, that may be awkward for the application’s high-level logic).
accountDomainName	The FQDN domain name for which the target LDAP server is an authority (e.g., example.com). This option is used to canonicalize names so that the username supplied by the user can be converted as necessary for binding. It is also used to determine if the server is an authority for the supplied username (e.g., if accountDomainName is foo.net and the user supplies bob@bar.net, the server will not be queried, and a failure will result). This option is not required, but if it is not supplied, usernames in principal name form (e.g., alice@foo.net) are not supported. It is strongly recommended that you supply this option, as there are many use-cases that require generating the principal name form.
accountDomainNameShort	The ‘short’ domain for which the target LDAP server is an authority (e.g., FOO). Note that there is a 1:1 mapping between the accountDomainName and accountDomainNameShort. This option should be used to specify the NetBIOS domain name for Windows networks, but may also be used by non-AD servers (e.g., for consistency when multiple sets of server options with the backslash style accountCanonicalForm). This option is not required but if it is not supplied, usernames in backslash form (e.g., FOO\alice) are not supported.
accountFilterFormat	The LDAP search filter used to search for accounts. This string is a printf()-style expression that must contain one ‘%s’ to accommodate the username. The default value is ‘(&(objectClass=user)(sAMAccountName=%s))’, unless bindRequiresDn is set to TRUE, in which case the default is ‘(&(objectClass=posixAccount)(uid=%s))’. For example, if for some reason you wanted to use bindRequiresDn = true with AD you would need to set accountFilterFormat = ‘(&(objectClass=user)(sAMAccountName=%s))’.
optReferrals	If set to TRUE, this option indicates to the LDAP client that referrals should be followed. The default value is FALSE.

Note

If you enable useStartTls = TRUE or useSsl = TRUE you may find that the LDAP client generates an error claiming that it cannot validate the server’s certificate. Assuming the PHP LDAP extension is ultimately linked to the OpenLDAP client libraries, to resolve this issue you can set “TLS_REQCERT never” in the OpenLDAP client ldap.conf (and restart the web server) to indicate to the OpenLDAP client library that you trust the server. Alternatively, if you are concerned that the server could be spoofed, you can export the LDAP server’s root certificate and put it on the web server so that the OpenLDAP client can validate the server’s identity.

Collecting Debugging Messages

Zend\Authentication\Adapter\Ldap collects debugging information within its authenticate() method. This information is stored in the Zend\Authentication\Result object as messages. The array returned by Zend\Authentication\Result::getMessages() is described as follows

Debugging Messages

Messages Array Index	Description
Index 0	A generic, user-friendly message that is suitable for displaying to users (e.g., “Invalid credentials”). If the authentication is successful, this string is empty.
Index 1	A more detailed error message that is not suitable to be displayed to users but should be logged for the benefit of server operators. If the authentication is successful, this string is empty.
Indexes 2 and higher	All log messages in order starting at index 2.

In practice, index 0 should be displayed to the user (e.g., using the FlashMessenger helper), index 1 should be logged and, if debugging information is being collected, indexes 2 and higher could be logged as well (although the final message always includes the string from index 1).

Common Options for Specific Servers

Options for Active Directory

For ADS, the following options are noteworthy:

Options for Active Directory

Name	Additional Notes
host	As with all servers, this option is required.
useStartTls	For the sake of security, this should be TRUE if the server has the necessary certificate installed.
useSsl	Possibly used as an alternative to useStartTls (see above).
baseDn	As with all servers, this option is required. By default AD places all user accounts under the Users container (e.g., CN=Users,DC=foo,DC=net), but the default is not common in larger organizations. Ask your AD administrator what the best DN for accounts for your application would be.
accountCanonicalForm	You almost certainly want this to be 3 for backslash style names (e.g., FOO\alice), which are most familiar to Windows users. You should not use the unqualified form 2 (e.g., alice), as this may grant access to your application to users with the same username in other trusted domains (e.g., BAR\alice and FOO\alice will be treated as the same user). (See also note below.)
accountDomainName	This is required with AD unless accountCanonicalForm 2 is used, which, again, is discouraged.
accountDomainNameShort	The NetBIOS name of the domain that users are in and for which the AD server is an authority. This is required if the backslash style accountCanonicalForm is used.

Note

Technically there should be no danger of accidental cross-domain authentication with the current Zend\Authentication\Adapter\Ldap implementation, since server domains are explicitly checked, but this may not be true of a future implementation that discovers the domain at runtime, or if an alternative adapter is used (e.g., Kerberos). In general, account name ambiguity is known to be the source of security issues, so always try to use qualified account names.

Options for OpenLDAP

For OpenLDAP or a generic LDAP server using a typical posixAccount style schema, the following options are noteworthy:

Options for OpenLDAP

Name	Additional Notes
host	As with all servers, this option is required.
useStartTls	For the sake of security, this should be TRUE if the server has the necessary certificate installed.
useSsl	Possibly used as an alternative to useStartTls (see above).
username	Required and must be a DN, as OpenLDAP requires that usernames be in DN form when performing a bind. Try to use an unprivileged account.
password	The password corresponding to the username above, but this may be omitted if the LDAP server permits an anonymous binding to query user accounts.
bindRequiresDn	Required and must be TRUE, as OpenLDAP requires that usernames be in DN form when performing a bind.
baseDn	As with all servers, this option is required and indicates the DN under which all accounts being authenticated are located.
accountCanonicalForm	Optional, but the default value is 4 (principal style names like alice@foo.net), which may not be ideal if your users are used to backslash style names (e.g., FOO\alice). For backslash style names use value 3.
accountDomainName	Required unless you’re using accountCanonicalForm 2, which is not recommended.
accountDomainNameShort	If AD is not also being used, this value is not required. Otherwise, if accountCanonicalForm 3 is used, this option is required and should be a short name that corresponds adequately to the accountDomainName (e.g., if your accountDomainName is foo.net, a good accountDomainNameShort value might be FOO).