[role="xpack"]
[[realms]]
=== Realms

Authentication in the {stack} {security-features} is handled by one or more
authentication services called _realms_. A _realm_ is used to resolve and
authenticate users based on authentication tokens. The {security-features}
provide the following built-in realms:

_native_::
An internal realm where users are stored in a dedicated {es} index.
This realm supports an authentication token in the form of username and password,
and is available by default when no realms are explicitly configured. The users
are managed via the {ref}/security-api.html#security-user-apis[user management APIs]. 
See <<native-realm>>.

_ldap_::
A realm that uses an external LDAP server to authenticate the
users. This realm supports an authentication token in the form of username and
password, and requires explicit configuration in order to be used. See
<<ldap-realm>>.

_active_directory_::
A realm that uses an external Active Directory Server to authenticate the
users. With this realm, users are authenticated by usernames and passwords.
See <<active-directory-realm>>.

_pki_::
A realm that authenticates users using Public Key Infrastructure (PKI). This
realm works in conjunction with SSL/TLS and identifies the users through the
Distinguished Name (DN) of the client's X.509 certificates. See <<pki-realm>>.

_file_::
An internal realm where users are defined in files stored on each node in the
{es} cluster. This realm supports an authentication token in the form
of username and password and is always available. See <<file-realm>>.

_saml_::
A realm that facilitates authentication using the SAML 2.0 Web SSO protocol.
This realm is designed to support authentication through {kib} and is not
intended for use in the REST API.  See <<saml-realm>>.

_kerberos_::
A realm that authenticates a user using Kerberos authentication. Users are
authenticated on the basis of Kerberos tickets. See <<kerberos-realm>>.

The {stack} {security-features} also support custom realms. If you need to
integrate with another authentication system, you can build a custom realm
plugin. For more information, see
<<custom-realms>>.

==== Internal and external realms

Realm types can roughly be classified in two categories:

Internal::  Realms that are internal to Elasticsearch and don't require any
communication with external parties. They are fully managed by {stack}
{security-features}. There can only be a maximum of one configured realm per
internal realm type. The {security-features} provide two internal realm types:
`native` and `file`.

External::  Realms that require interaction with parties/components external to
{es}, typically, with enterprise grade identity management systems. Unlike
internal realms, there can be as many external realms as one would like - each
with its own unique name and configuration. The {stack} {security-features}
provide the following external realm types: `ldap`, `active_directory`, `saml`,
`kerberos`, and `pki`.