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

You can configure {security} to use Public Key Infrastructure (PKI) certificates
to authenticate users in {es}. This requires clients to present X.509
certificates.

NOTE: You cannot use PKI certificates to authenticate users in {kib}.

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 Names
(DNs) from the user certificates to {security} roles in the 
<<security-api-role-mapping,role-mapping API>> or 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 and enable client authentication to use PKI.

For more information, see {xpack-ref}/pki-realm.html[PKI User Authentication].

. 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 SSL/TLS layer is accepted
for authentication. The username is the common name (CN) extracted from the DN
of the certificate.

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 DN as the username, you
can specify a regex to extract the desired username. For example, the regex in
the following configuration extracts the email address from the DN:

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

. Restart {es}.

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

. Enable client authentication on the desired network layers (transport or http).
+
--
//TBD: This step might need to be split into a separate topic with additional details
//about setting up client authentication. 
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,`xpack.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 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. For example:

[source, yaml]
------------------------------------------------------------
xpack:
  security:
    authc:
      realms:
        pki:
          pki1:
            truststore:
              path: "/path/to/pki_truststore.jks"
              password: "x-pack-test-password"
------------------------------------------------------------

The `certificate_authorities` option can be used as an alternative to the
`truststore.path` setting.
--

. 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. 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,js]
--------------------------------------------------
PUT _xpack/security/role_mapping/users
{
  "roles" : [ "user" ],
  "rules" : { "field" : {
    "dn" : "cn=John Doe,ou=example,o=com" <1>
  } },
  "enabled": true
}
--------------------------------------------------
// CONSOLE
<1> The distinguished name (DN) of a PKI user.

Or, alternatively, configured in a role-mapping file:
[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 disinguished 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 
{xpack-ref}/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.

--
