SAML offers Multi Tenancy, meaning that a single target application (WAR) can be secured with multiple Keycloak realms. The realms can be located on the same Keycloak instance or on different instances.
To do this, the application must have multiple keycloak-saml.xml adapter configuration files.
While you could have multiple instances of your WAR with different adapter configuration files deployed to different context-paths, this may be inconvenient and you may also want to select the realm based on something other than context-path.
Keycloak makes it possible to have a custom config resolver, so you can choose which adapter config is used for each request. In SAML, the configuration is only interesting in the login processing; once the user is logged in, the session is authenticated and it does not matter if the keycloak-saml.xml returned is different. For that reason, returning the same configuration for the same session is the correct way to go.
To achieve this, create an implementation of org.keycloak.adapters.saml.SamlConfigResolver. The following example uses the Host header to locate the proper configuration and load it and the associated elements from the applications' Java classpath:
package example;
import java.io.InputStream;
import org.keycloak.adapters.saml.SamlConfigResolver;
import org.keycloak.adapters.saml.SamlDeployment;
import org.keycloak.adapters.saml.config.parsers.DeploymentBuilder;
import org.keycloak.adapters.saml.config.parsers.ResourceLoader;
import org.keycloak.adapters.spi.HttpFacade;
import org.keycloak.saml.common.exceptions.ParsingException;
public class SamlMultiTenantResolver implements SamlConfigResolver {
@Override
public SamlDeployment resolve(HttpFacade.Request request) {
String host = request.getHeader("Host");
String realm = null;
if (host.contains("tenant1")) {
realm = "tenant1";
} else if (host.contains("tenant2")) {
realm = "tenant2";
} else {
throw new IllegalStateException("Not able to guess the keycloak-saml.xml to load");
}
InputStream is = getClass().getResourceAsStream("/" + realm + "-keycloak-saml.xml");
if (is == null) {
throw new IllegalStateException("Not able to find the file /" + realm + "-keycloak-saml.xml");
}
ResourceLoader loader = new ResourceLoader() {
@Override
public InputStream getResourceAsStream(String path) {
return getClass().getResourceAsStream(path);
}
};
try {
return new DeploymentBuilder().build(is, loader);
} catch (ParsingException e) {
throw new IllegalStateException("Cannot load SAML deployment", e);
}
}
}
You must also configure which SamlConfigResolver implementation to use with the keycloak.config.resolver context-param in your web.xml:
<web-app>
...
<context-param>
<param-name>keycloak.config.resolver</param-name>
<param-value>example.SamlMultiTenantResolver</param-value>
</context-param>
</web-app>