What is LDAP and how does LDAP authentication work



LDAP stands for “Lightweight Directory Access Protocol”. It is a simplification of the X.500 Directory Access Protocol (DAP) used to access directory information. A directory is essentially a special-purpose database optimized to handle identity-related information.

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The LDAP standard also defines a data model based on the X.500 data model. It is a hierarchical data model, with objects arranged in a hierarchical structure, and each object containing a collection of attributes. The overall structure of any particular directory is defined by its schema, much like a database schema defines the tables and columns.

LDAP defines a “Bind” operation that authenticates the LDAP connection and establishes a security context for subsequent operations on that connection. There are two authentication methods defined in RFC 4513, simple and SASL. The simple authentication method has the LDAP client send the username (as a LDAP distinguished name) and password (in clear text) to the LDAP server.

The LDAP server looks up the object with that username in the directory, compares the password provided to the password(s) stored with the object, and authenticates the connection if they match. Because the password is provided in clear text, LDAP simple Binds should only be done over a secure TLS connection. SASL Bind is more involved, and allows the client and server to negotiate a particular authentication mechanism, which they then use to authenticate the LDAP connection.

Lightweight Directory Access Protocol its working: To access the LDAP service, the LDAP client first must authenticate itself to the service. That is, it must tell the LDAP server who is going to be accessing the data so that the server can decide what the client is allowed to see and do. If the client authenticates successfully to the LDAP server, then when the server subsequently receives a request from the client, it will check whether the client is allowed to perform the request.

This process is called access control. In LDAP, authentication is supplied in the "bind" operation. Ldapv3 supports three types of authentication: anonymous, simple and SASL authentication. A client that sends a LDAP request without doing a "bind" is treated as an anonymous client. Simple authentication consists of sending the LDAP server the fully qualified DN of the client (user) and the client's clear-text password. This mechanism has security problems because the password can be read from the network. To avoid exposing the password in this way, you can use the simple authentication mechanism within an encrypted channel (such as SSL), provided that this is supported by the LDAP server. Finally, SASL is the Simple Authentication and Security Layer (RFC 2222).

It specifies a challenge-response protocol in which data is exchanged between the client and the server for the purposes of authentication and establishment of a security layer on which to carry out subsequent communication. By using SASL, LDAP can support any type of authentication agreed upon by the LDAP client and server. The Cyrus-SASL package is available at the following URL: http://asg.web.cmu.edu/sasl/sasl....

Further on authenticating users to access information from your Directory Tree, your LDAP server can authenticate users from other services too (Sendmail, Login, Ftp, etc.). This is accomplished migrating specific user information to your LDAP server and using a mechanism called PAM (Pluggable Authentication Module). The authentication module for LDAP is available as a tar ball on the following address:http://www.padl.com/OSS/pam_ldap...

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