The certificate identity can be extracted from either Subject DN or Issuer DN using a regular expression as a filter. For example, the regular expression below will match the e-mail attribute:
```
emailAddress=(.*?)(?:,|$)
```
The regular expression filtering is applicable only if the `Identity Source` is set to either `Match SubjectDN using regular expression` or `Match IssuerDN using regular expression`.
The certificate identity mapping can be configured to map the extracted user identity to an existing user's username or e-mail or to a custom attribute which value matches the certificate identity. For example, setting the `Identity source` to _Subject's e-mail_ and `User mapping method` to _Username or email_ will have the X.509 client certificate authenticator use the e-mail attribute in the certificate's Subject DN as a search criteria to look up an existing user by username or by e-mail.
IMPORTANT: Please notice that if we disable `Login with email` at realm settings, the same rules will be applied to certificate authentication. In other words, users won't be able to log in using e-mail attribute.
IMPORTANT: Usage of `Certificate Serial Number and IssuerDN` as an identity source requires two custom attributes - one for serial number and the other for IssuerDN.
IMPORTANT: `SHA-256 Certificate thumbprint` is lowercase hexadecimal representation of SHA-256 certificate thumbprint.
IMPORTANT: Usage of `Full certificate in PEM format` as an identity source is limited to custom attributes mapped to external federation sources like LDAP. You must enable `Always Read Value From LDAP` in this case, because certificates cannot be stored in Keycloak database due to a length limitation.
The following sections describe how to configure {appserver_name}/Undertow and the {project_name} Server to enable X.509 client certificate authentication.
The `ssl` element contains the `keystore` element that defines how to load the server public key pair from a JKS keystore
`ssl/keystore/path`::
A path to a JKS keystore
`ssl/keystore/relative-to`::
Defines a path the keystore path is relative to
`ssl/keystore/keystore-password`::
The password to open the keystore
`ssl/keystore/alias` (optional)::
The alias of the entry in the keystore. Set it if the keystore contains multiple entries
`ssl/keystore/key-password` (optional)::
The private key password, if different from the keystore password.
`authentication/truststore`::
Defines how to load a trust store to verify the certificate presented by the remote side of the inbound/outgoing connection. Typically, the truststore contains a collection of trusted CA certificates.
`authentication/truststore/path`::
A path to a JKS keystore that contains the certificates of the trusted CAs (certificate authorities)
The value must match the name of the realm from the previous section
`https-listener/verify-client`::
If set to `REQUESTED`, the server will optionally ask for a client certificate. Setting the attribute to `REQUIRED` will have the server to refuse inbound connections if no client certificate has been provided.
* Using the up/down arrows, change the order of the "X509/Validate Username Form" by moving it above the "Browser Forms" execution, and set the requirement to "ALTERNATIVE"
Defines whether to use the canonical format to determine a distinguished name.
The format is described in detail in the official link:https://docs.oracle.com/javase/8/docs/api/javax/security/auth/x500/X500Principal.html#getName-java.lang.String-[Java API documentation] .
This option only affects the two User Identity Sources _Match SubjectDN using regular expression_ and _Match IssuerDN using regular expression_.
If you setup a new {project_name} instance it is recommended to enable this option. Leave this option disabled to remain beckward compatible with existing {project_name} instances.
`Enable Serial Number hexadecimal representation` (optional)::
An option to use hexadecimal representation of the Serial Number. See link:https://tools.ietf.org/html/rfc5280#section-4.1.2.2[RFC5280, Section-4.1.2.2]. Serial Number with sign bit set to 1 should be left padded with 00 octet. E.g. Serial number with decimal value _161_, or _a1_ in hexadecimal representation according to RFC5280 must be encoded as _00a1_. More details can be found: link:https://tools.ietf.org/html/rfc5280#appendix-B[RFC5280, appendix-B].
Defines a regular expression to use as a filter to extract the certificate identity. The regular expression must contain a single group.
`User Mapping Method`::
Defines how to match the certificate identity to an existing user. _Username or e-mail_ will search for an existing user by username or e-mail. _Custom Attribute Mapper_ will search for an existing user with a custom attribute which value matches the certificate identity. The name of the custom attribute is configurable.
A custom attribute which value will be matched against the certificate identity. Multiple custom attributes are relevant when attribute mapping is related to multiple values, e.g. 'Certificate Serial Number and IssuerDN'.
WARNING: It is highly recommended that attribute used here is read-only for the users. By default, only the `usercertificate` attribute is read-only by {project_name}.
If you use a different attribute name, you may need to add it to the list of read-only attributes. See the details in the link:#_read_only_user_attributes[Threat model mitigation chapter].
Verifies whether the certificate's KeyUsage extension bits are set. For example, "digitalSignature,KeyEncipherment" will verify if bits 0 and 2 in the KeyUsage extension are asserted. Leave the parameter empty to disable the Key Usage validation. See link:https://tools.ietf.org/html/rfc5280#section-4.2.1.3[RFC5280, Section-4.2.1.3]. The server will raise an error only when flagged as critical by the issuing CA and there is a key usage extension mismatch.
Verifies one or more purposes as defined in the Extended Key Usage extension. See link:https://tools.ietf.org/html/rfc5280#section-4.2.1.12[RFC5280, Section-4.2.1.12]. Leave the parameter empty to disable the Extended Key Usage validation. The server will raise an error only when flagged as critical by the issuing CA and there is a key usage extension mismatch.
If set, X.509 client certificate authentication will not prompt the user to confirm the certificate identity and will automatically sign in the user upon successful authentication.
* Select the "Bindings" tab, find the drop down for "Direct Grant Flow". Select the newly created X509 direct grant flow from the drop down and click on "Save".
When an HTTP request is sent directly to {project_name} server, the {appserver_name} undertow subsystem will establish an SSL handshake and extract the client certificate. The client certificate will be then saved to the attribute `javax.servlet.request.X509Certificate` of the HTTP request, as specified in the servlet specification. The {project_name} X509 authenticator will be then able to lookup the certificate from this attribute.
However, when the {project_name} server listens to HTTP requests behind a load balancer or reverse proxy, it may be the proxy server which extracts the client certificate and establishes the mutual SSL connection. A reverse proxy usually puts the authenticated client certificate in the HTTP header of the underlying request and forwards it to the back end {project_name} server. In this case, {project_name} must be able to look up the X.509 certificate chain from the HTTP headers instead of from the attribute of HTTP request, as is done for Undertow.
If {project_name} is behind a reverse proxy, you usually need to configure alternative provider of the `x509cert-lookup` SPI in {project_dirref}/standalone/configuration/standalone.xml. Along with the `default` provider, which looks up the certificate from the HTTP header, we also have two additional built-in providers: `haproxy` and `apache`, which are described next.
===== HAProxy certificate lookup provider
You can use this provider when your {project_name} server is behind an HAProxy reverse proxy. Configure the server like this:
In this example configuration, the client certificate will be looked up from the HTTP header, `SSL_CLIENT_CERT`, and the other certificates from its chain will be looked up from HTTP headers like `CERT_CHAIN_0` , `CERT_CHAIN_1`, ..., `CERT_CHAIN_9` . The attribute `certificateChainLength` is the maximum length of the chain, so the last one tried attribute would be `CERT_CHAIN_9` .
Consult the HAProxy documentation for the details of how the HTTP Headers for the client certificate and client certificate chain can be configured and their proper names.
The configuration is same as for the `haproxy` provider. Consult the Apache documentation on link:https://httpd.apache.org/docs/current/mod/mod_ssl.html[mod_ssl] and link:https://httpd.apache.org/docs/current/mod/mod_headers.html[mod_headers] for the details of how the HTTP Headers for the client certificate and client certificate chain can be configured and their proper names.
NOTE: NGINX link:http://nginx.org/en/docs/http/ngx_http_ssl_module.html#variables[SSL/TLS module] does not expose the client certificate chain, so Keycloak NGINX certificate lookup provider is rebuilding it using the link:{installguide_truststore_link}[{installguide_truststore_name}]. Please populate Keycloak truststore using keytool CLI with all root and intermediate CA's needed for rebuilding client certificate chain.
We do not have built-in support for other reverse proxy implementations. However, it is possible that other reverse proxies can be made to behave in a similar way to `apache` or `haproxy` and that some of those providers can be used. If none of those works, you may need to create your own implementation of the `org.keycloak.services.x509.X509ClientCertificateLookupFactory` and `org.keycloak.services.x509.X509ClientCertificateLookup` provider. See the link:{developerguide_link}[{developerguide_name}] for the details on how to add your own provider.
The host and the port number of a remote {project_name} server that has been configured to allow users authenticate with x.509 client certificates using the Direct Grant Flow.
