What is LDAP?


LDAP, Lightweight Directory Access Protocol, is an Internet protocol that email and other programs use to look up information from a server.

Every email program has a personal address book, but how do you look up an address for someone who’s never sent you email? How can an organization keep one centralized up-to-date phone book that everybody has access to?

That question led software companies such as Microsoft, IBM, Lotus, and Netscape to support a standard called LDAP. “LDAP-aware” client programs can ask LDAP servers to look up entries in a wide variety of ways. LDAP servers index all the data in their entries, and “filters” may be used to select just the person or group you want, and return just the information you want. For example, here’s an LDAP search translated into plain English: “Search for all people located in Chicago whose name contains “Fred” that have an email address. Please return their full name, email, title, and description.”

LDAP is not limited to contact information, or even information about people. LDAP is used to look up encryption certificates, pointers to printers and other services on a network, and provide “single signon” where one password for a user is shared between many services. LDAP is appropriate for any kind of directory-like information, where fast lookups and less-frequent updates are the norm.

As a protocol, LDAP does not define how programs work on either the client or server side. It defines the “language” used for client programs to talk to servers (and servers to servers, too). On the client side, a client may be an email program, a printer browser, or an address book. The server may speak only LDAP, or have other methods of sending and receiving data—LDAP may just be an add-on method.

If you have an email program (as opposed to web-based email), it probably supports LDAP. Most LDAP clients can only read from a server. Search abilities of clients (as seen in email programs) vary widely. A few can write or update information, but LDAP does not include security or encryption, so updates usually requre additional protection such as an encrypted SSL connection to the LDAP server.

LDAP also defines: Permissions, set by the administrator to allow only certain people to access the LDAP database, and optionally keep certain data private. Schema: a way to describe the format and attributes of data in the server. For example: a schema entered in an LDAP server might define a “groovyPerson” entry type, which has attributes of “instantMessageAddress”, and “coffeeRoastPreference”. The normal attributes of name, email address, etc., would be inherited from one of the standard schemas, which are rooted in X.500 (see below).

LDAP was designed at the University of Michigan to adapt a complex enterprise directory system (called X.500) to the modern Internet. X.500 is too complex to support on desktops and over the Internet, so LDAP was created to provide this service “for the rest of us.”

LDAP servers exist at three levels: There are big public servers, large organizational servers at universities and corporations, and smaller LDAP servers for workgroups. Most public servers from around year 2000 have disappeared, although directory.verisign.com exists for looking up X.509 certificates. The idea of publicly listing your email address for the world to see, of course, has been crushed by spam.

While LDAP didn’t bring us the worldwide email address book, it continues to be a popular standard for communicating record-based, directory-like data between programs.

dn: cn=John Doe,dc=example,dc=com
 cn: John Doe
 givenName: John
 sn: Doe
 telephoneNumber: +1 888 555 6789
 telephoneNumber: +1 888 555 1232
 mail: john@example.com
 manager: cn=Barbara Doe,dc=example,dc=com
 objectClass: inetOrgPerson
 objectClass: organizationalPerson
 objectClass: person
 objectClass: top

dn is the name of the entry; it’s not an attribute nor part of the entry. “cn=John Doe” is the entry’s RDN, and “dc=example,dc=com” is the DN of the parent entry, where dc denotes Domain Component. The other lines show the attributes in the entry. Attribute names are typically mnemonic strings, like “cn” for common name, “dc” for domain component, “mail” for e-mail address and “sn” for surname.

A server holds a subtree starting from a specific entry, e.g. “dc=example,dc=com” and its children. Servers may also hold references to other servers, so an attempt to access “ou=department,dc=example,dc=com” could return a referral or continuation reference to a server which holds that part of the directory tree. The client can then contact the other server. Some servers also support chaining, which means the server contacts the other server and returns the results to the client.

LDAP rarely defines any ordering: The server may return the values of an attribute, the attributes in an entry, and the entries found by a search operation in any order. This follows from the formal definitions – an entry is defined as a set of attributes, and an attribute is a set of values, and sets need not be ordered.

The client may request the following operations:

  • Start TLS — use the LDAPv3 Transport Layer Security (TLS) extension for a secure connection
  • Bind — authenticate and specify LDAP protocol version
  • Search — search for and/or retrieve directory entries
  • Compare — test if a named entry contains a given attribute value
  • Add a new entry
  • Delete an entry
  • Modify an entry
  • Modify Distinguished Name (DN) — move or rename an entry
  • Abandon — abort a previous request
  • Extended Operation — generic operation used to define other operations
  • Unbind — close the connection (not the inverse of Bind)
< ?php
$ldap = ldap_connect('ldap.lettucecode.com');

if(!$ldap) {
  die('Could not connect to LDAP server.');
}

if(!ldap_bind($ldap, $ldap_user, $password)) {
  die('Could not bind to LDAP server.');
}

$base_dn = 'o=lettucecode.com,o=com';
$search = ldap_search($ldap, $base_dn, 'uid='.$username);
if(ldap_count_entries($ldap, $search) < 1) {
  die('Username not found');
}

$info = ldap_get_entries($ldap, $search);

if(isset($info[0]['name'])) {
  echo 'Username '.$username.' has name '.$info[0]['name'];
} else {
  echo 'Entry -name- not found in username '.$username;
}

ldap_close($ldap);
?>

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