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Realm chains

Realms live within a realm chain. It is essentially a prioritized list of configured realms (typically of various types). Realms are consulted in ascending order (that is to say, the realm with the lowest order value is consulted first). You must make sure each configured realm has a distinct order setting. In the event that two or more realms have the same order, the node will fail to start.

During the authentication process, Elastic Stack security features consult and try to authenticate the request one realm at a time. Once one of the realms successfully authenticates the request, the authentication is considered to be successful. The authenticated user is associated with the request, which then proceeds to the authorization phase. If a realm cannot authenticate the request, the next realm in the chain is consulted. If all realms in the chain cannot authenticate the request, the authentication is considered to be unsuccessful and an authentication error is returned (as HTTP status code 401).

Some systems (e.g. Active Directory) have a temporary lock-out period after several successive failed login attempts. If the same username exists in multiple realms, unintentional account lockouts are possible. For more information, see Users are frequently locked out of Active Directory.

The default realm chain contains the file and native realms. To explicitly configure a realm chain, you specify the chain in the elasticsearch.yml file. If your realm chain does not contain file or native realm or does not disable them explicitly, file and native realms will be added automatically to the beginning of the realm chain in that order. To opt-out from the automatic behaviour, you can explicitly configure the file and native realms with the order and enabled settings.

The following snippet configures a realm chain that enables the file realm as well as two LDAP realms and an Active Directory realm, but disables the native realm.

xpack.security.authc.realms:
  file.file1:
      order: 0

  ldap.ldap1:
      order: 1
      enabled: false
      url: 'url_to_ldap1'
      ...

  ldap.ldap2:
      order: 2
      url: 'url_to_ldap2'
      ...

  active_directory.ad1:
      order: 3
      url: 'url_to_ad'

  native.native1:
      enabled: false

As can be seen above, each realm has a unique name that identifies it. Each type of realm dictates its own set of required and optional settings. That said, there are settings that are common to all realms.

Delegating authorization to another realm

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Some realms have the ability to perform authentication internally, but delegate the lookup and assignment of roles (that is, authorization) to another realm.

For example, you may wish to use a PKI realm to authenticate your users with TLS client certificates, then lookup that user in an LDAP realm and use their LDAP group assignments to determine their roles in Elasticsearch.

Any realm that supports retrieving users (without needing their credentials) can be used as an authorization realm (that is, its name may appear as one of the values in the list of authorization_realms). See Looking up users without authentication for further explanation on which realms support this.

For realms that support this feature, it can be enabled by configuring the authorization_realms setting on the authenticating realm. Check the list of supported settings for each realm to see if they support the authorization_realms setting.

If delegated authorization is enabled for a realm, it authenticates the user in its standard manner (including relevant caching) then looks for that user in the configured list of authorization realms. It tries each realm in the order they are specified in the authorization_realms setting. The user is retrieved by principal - the user must have identical usernames in the authentication and authorization realms. If the user cannot be found in any of the authorization realms, authentication fails.

See Configuring authorization delegation for more details.

Delegated authorization requires that you have a subscription that includes custom authentication and authorization realms.

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