OpenSearch/x-pack/docs/en/security/authentication/configuring-pki-realm.asciidoc

[role="xpack"]
[[configuring-pki-realm]]
=== Configuring a PKI realm

You can configure {es} to use Public Key Infrastructure (PKI) certificates to
authenticate users. This requires clients connecting directly to {es} to
present X.509 certificates. The certificates must first be accepted for
authentication on the SSL/TLS layer on {es}. Only then they are optionally
further validated by a PKI realm.

Users may also use PKI certificates to authenticate to {kib}, however this
requires some <<pki-realm-for-proxied-clients,additional configuration>>. On
{es}, this configuration enables {kib} to act as a proxy for SSL/TLS
authentication and to submit the client certificates to {es} for further
validation by a PKI realm.

For more general information, see {stack-ov}/pki-realm.html[PKI user authentication].

[float]
[role="xpack"]
[[pki-realm-for-direct-clients]]
==== PKI authentication for clients connecting directly to {es}

To use PKI in {es}, you configure a PKI realm, enable client authentication on
the desired network layers (transport or http), and map the Distinguished Name
(DN) from the Subject field in the user certificate to roles by using the
<<security-api-role-mapping,role-mapping API>> or the role-mapping file.

You can also use a combination of PKI and username/password authentication. For
example, you can enable SSL/TLS on the transport layer and define a PKI realm to
require transport clients to authenticate with X.509 certificates, while still
authenticating HTTP traffic using username and password credentials. You can
also set `xpack.security.transport.ssl.client_authentication` to `optional` to
allow clients without certificates to authenticate with other credentials.

IMPORTANT: You must enable SSL/TLS with client authentication to use PKI when
clients connect directly to {es}.

. Add a realm configuration for a `pki` realm to `elasticsearch.yml` under the
`xpack.security.authc.realms.pki` namespace.
If you are configuring multiple realms, you should 
explicitly set the `order` attribute. See <<ref-pki-settings>> for all of the 
options you can set for a `pki` realm.
+
--
For example, the following snippet shows the most basic `pki` realm configuration:

[source, yaml]
------------------------------------------------------------
xpack:
  security:
    authc:
      realms:
        pki:
          pki1:
            order: 1
------------------------------------------------------------

With this configuration, any certificate trusted by the {es} SSL/TLS layer is
accepted for authentication. The username is the common name (CN) extracted
from the DN in the Subject field of the end-entity certificate. This
configuration does not permit PKI authentication to {kib}.

IMPORTANT: When you configure realms in `elasticsearch.yml`, only the
realms you specify are used for authentication. If you also want to use the
`native` or `file` realms, you must include them in the realm chain.

If you want to use something other than the CN of the Subject DN as the
username, you can specify a regex to extract the desired username. The regex is
applied on the Subject DN. For example, the regex in the following
configuration extracts the email address from the Subject DN:

[source, yaml]
------------------------------------------------------------
xpack:
  security:
    authc:
      realms:
        pki:
          pki1:
            username_pattern: "EMAILADDRESS=(.*?)(?:,|$)"
------------------------------------------------------------

NOTE: If the regex is too restrictive and does not match the Subject DN of the
client's certificate, then the realm does not authenticate the certificate.

--

. Restart {es} because realm configuration is not reloaded automatically. If
you're following through with the next steps, you might wish to hold the
restart for last.

. <<configuring-tls,Enable SSL/TLS>>.

. Enable client authentication on the desired network layers (transport or http).
+
--

When clients connect directly to {es} and are not proxy-authenticated, the PKI
realm relies on the TLS settings of the node's network interface. The realm can
be configured to be more restrictive than the underlying network connection.
That is, it is possible to configure the node such that some connections
are accepted by the network interface but then fail to be authenticated by the
PKI realm. However, the reverse is not possible. The PKI realm cannot
authenticate a connection that has been refused by the network interface.

In particular this means:

* The transport or http interface must request client certificates by setting
  `client_authentication` to `optional` or `required`.
* The interface must _trust_ the certificate that is presented by the client
  by configuring either the `truststore` or `certificate_authorities` paths,
  or by setting `verification_mode` to `none`. See 
  <<ssl-tls-settings,`ssl.verification_mode`>> for an explanation of this
  setting.
* The _protocols_ supported by the interface must be compatible with those
  used by the client.

The relevant network interface (transport or http) must be configured to trust
any certificate that is to be used within the PKI realm. However, it is possible to
configure the PKI realm to trust only a _subset_ of the certificates accepted
by the network interface. This is useful when the SSL/TLS layer trusts clients 
with certificates that are signed by a different CA than the one that signs your 
users' certificates.

To configure the PKI realm with its own truststore, specify the
`truststore.path` option. The path must be located within the Elasticsearch
configuration directory (ES_PATH_CONF). For example:

[source, yaml]
------------------------------------------------------------
xpack:
  security:
    authc:
      realms:
        pki:
          pki1:
            truststore:
              path: "pki1_truststore.jks"
------------------------------------------------------------

If the truststore is password protected, the password should be configured by
adding the appropriate `secure_password` setting to the {es} keystore.  For
example, the following command adds the password for the example realm above:

[source, shell]
------------------------------------------------------------
bin/elasticsearch-keystore add \
xpack.security.authc.realms.pki.pki1.truststore.secure_password
------------------------------------------------------------

The `certificate_authorities` option can be used as an alternative to the
`truststore.path` setting, when the certificate files are PEM formatted
. The setting accepts a list. The two options are exclusive, they cannot be both used
simultaneously.
--

. Map roles for PKI users.
+
--
You map roles for PKI users through the <<security-role-mapping-apis,role
mapping APIs>> or by using a file stored on each node. Both configuration
options are merged together. When a user authenticates against a PKI realm, the
privileges for that user are the union of all privileges defined by the roles
to which the user is mapped.

You identify a user by the distinguished name in their certificate.
For example, the following mapping configuration maps `John Doe` to the
`user` role:

Using the role-mapping API:

[source,console]
--------------------------------------------------
PUT /_security/role_mapping/users
{
  "roles" : [ "user" ],
  "rules" : { "field" : {
    "dn" : "cn=John Doe,ou=example,o=com" <1>
  } },
  "enabled": true
}
--------------------------------------------------

<1> The distinguished name (DN) of a PKI user.

Or, alternatively, configured inside a role-mapping file. The file's path
defaults to `ES_PATH_CONF/role_mapping.yml`. You can specify a different path (which must be within
ES_PATH_CONF) by using the `files.role_mapping` realm setting (e.g.
`xpack.security.authc.realms.pki.pki1.files.role_mapping`):

[source, yaml]
------------------------------------------------------------
user: <1>
  - "cn=John Doe,ou=example,o=com" <2>
------------------------------------------------------------
<1> The name of a role.
<2> The distinguished name (DN) of a PKI user.

The distinguished name for a PKI user follows X.500 naming conventions which
place the most specific fields (like `cn` or `uid`) at the beginning of the
name, and the most general fields (like `o` or `dc`) at the end of the name.
Some tools, such as _openssl_, may print out the subject name in a different
format.

One way that you can determine the correct DN for a certificate is to use the
<<security-api-authenticate,authenticate API>> (use the relevant PKI
certificate as the means of authentication) and inspect the metadata field in
the result. The user's distinguished name will be populated under the `pki_dn`
key. You can also use the authenticate API to validate your role mapping.

For more information, see 
{stack-ov}/mapping-roles.html[Mapping Users and Groups to Roles].

NOTE: The PKI realm supports
{stack-ov}/realm-chains.html#authorization_realms[authorization realms] as an
alternative to role mapping.

--

[float]
[role="xpack"]
[[pki-realm-for-proxied-clients]]
==== PKI authentication for clients connecting to {kib}

By default, the PKI realm relies on the node's network interface to perform the
SSL/TLS handshake and extract the client certificate. This behaviour requires
that that clients connect directly to {es} so that their SSL connection is
terminated by the {es} node.  If SSL/TLS authenticatication is to be performed
by {kib}, the PKI realm must be configured to permit delegation.

Specifically, when clients presenting X.509 certificates connect to {kib},
{kib} performs the SSL/TLS authentication. {kib} then forwards the client's
certificate chain, by calling an {es} API, to have them further validated by
the PKI realms that have been configured for delegation.

To permit authentication delegation for a specific {es} PKI realm, start by
configuring the realm for the usual case, as detailed in the
<<pki-realm-for-direct-clients>>
section. Note that you must explicitly configure a `truststore` (or,
equivalently `certificate_authorities`) even though it is the same trust
configuration that you have configured on the network layer.  Afterwards,
simply toggle the `delegation.enabled` realm setting to `true`.  This realm is
now allowed to validate delegated PKI authentication (after restarting {es}).

NOTE: PKI authentication delegation requires that the
`xpack.security.authc.token.enabled` setting be `true` and that SSL/TLS be
configured (without SSL/TLS client authentication).

NOTE: {kib} also needs to be
{kibana-ref}/kibana-authentication.html#pki-authentication[configured to allow
PKI certificate authentication].

A PKI realm with `delegation.enabled` still works unchanged for clients
connecting directly to {es}. Directly authenticated users, and users that are PKI
authenticated by delegation to {kib} both follow the same
{stack-ov}/mapping-roles.html[role mapping rules] or
{stack-ov}/realm-chains.html#authorization_realms[authorization realms
configurations].

However, if you use the <<security-role-mapping-apis,role mapping APIs>>,
you can distinguish between users that are authenticated by delegation and
users that are authenticated directly. The former have the
extra fields `pki_delegated_by_user` and `pki_delegated_by_realm` in the user's
metadata. In the common setup, where authentication is delegated to {kib}, the
values of these fields are `kibana` and `reserved`, respectively. For example,
the following role mapping rule will assign the `role_for_pki1_direct` role to
all users that have been authenticated directly by the `pki1` realm, by
connecting to {es} instead of going through {kib}:

[source,console]
--------------------------------------------------
PUT /_security/role_mapping/direct_pki_only
{
  "roles" : [ "role_for_pki1_direct" ],
  "rules" : {
    "all": [
      {
        "field": {"realm.name": "pki1"}
      },
      {
        "field": {
          "metadata.pki_delegated_by_user": null <1>
        }
      }
    ]
  },
  "enabled": true
}
--------------------------------------------------

<1> only when this metadata field is set (it is *not* `null`) the user has been
authenticated in the delegation scenario.