Using CAS for single sign-on#

CAS is an open-source Single Sign On (SSO) provider that allows multiple applications and services to authenticate against it and brokers those authentication requests to a back-end authentication provider. This module allows Guacamole to redirect to CAS for authentication and user services. This module must be layered on top of other authentication extensions that provide connection information, as it only provides user authentication.

Warning

You will need to restart the Guacamole web application in order to complete configuration. Doing this will disconnect all active users, so please:

Do this only at a time that you can tolerate service unavailability, such as a scheduled maintenance window.
Keep in mind that configuration errors may prevent Guacamole from starting back up.

Installing/Enabling the CAS authentication extension#

Guacamole is configured differently depending on whether Guacamole was installed natively or using the provided Docker images. The documentation here covers both methods.

Native Webapp (Tomcat)

Native installations of Guacamole under Apache Tomcat or similar are configured by modifying the contents of GUACAMOLE_HOME (Guacamole’s configuration directory), which is located at /etc/guacamole by default and may need to be created first:

Download guacamole-auth-sso-1.6.0.tar.gz from the release page for Apache Guacamole 1.6.0 and extract it.
Create the GUACAMOLE_HOME/extensions directory, if it does not already exist.
Copy the cas/guacamole-auth-sso-cas-1.6.0.jar file from the contents of the archive to GUACAMOLE_HOME/extensions/.
Proceed with the configuring Guacamole for the newly installed extension as described below. The extension will be loaded after Guacamole has been restarted.

Note

Download and documentation links for all officially supported extensions for a particular version of Guacamole are always provided in the release notes for that version. The copy of the documentation you are reading now is from Apache Guacamole 1.6.0.

If you are using a different version of Guacamole, please locate that version within the release archives and consult the documentation for that release instead.

Container (Docker)

Docker installations of Guacamole include a bundled copy of Apache Tomcat and are configured using environment variables. The startup process of the Docker image automatically populates GUACAMOLE_HOME (Guacamole’s configuration directory) based on the values of these variables.

If deploying Guacamole using Docker Compose:

You will need to add at least one relevant environment variable to the environment section of your guacamole/guacamole container, such as the CAS_ENABLED environment variable:

CAS_ENABLED: "true"

If instead deploying Guacamole by running docker run manually:

The same environment variable(s) will need to be provided using the -e option. For example:

$ docker run --name some-guacamole \
    -e CAS_ENABLED="true" \
    -d -p 8080:8080 guacamole/guacamole

If CAS_ENABLED is set to false, the extension will NOT be installed, even if other related environment variables have been set. This can be used to temporarily disable usage of an extension without needing to remove all other related configuration.

You don’t strictly need to set CAS_ENABLED if other related environment variables are provided, but the extension will be installed only if at least one related environment variable is set.

Required configuration#

Native Webapp (Tomcat)

Guacamole’s CAS support requires specifying two properties that describe the CAS authentication server and the Guacamole deployment. These properties are absolutely required in all cases, as they dictate how Guacamole should connect to CAS and how CAS should redirect users back to Guacamole once their identity has been confirmed.

If deploying Guacamole natively, you will need to add a section to your guacamole.properties that looks like the following:

cas-authorization-endpoint: https://cas.example.net
cas-redirect-uri: https://guac.example.net

The properties that must be set in all cases for any Guacamole installation using this extension are:

cas-authorization-endpoint: The URL of the CAS authentication server. This should be the full path to the base of the CAS installation.
cas-redirect-uri: The URI to redirect back to upon successful authentication. Normally this will be the full URL of your Guacamole installation.

Container (Docker)

Guacamole’s CAS support requires specifying two environment variables that describe the CAS authentication server and the Guacamole deployment. These environment variables are absolutely required in all cases, as they dictate how Guacamole should connect to CAS and how CAS should redirect users back to Guacamole once their identity has been confirmed.

If deploying Guacamole using Docker Compose, you will need to add a set of environment variables to the environment section of your guacamole/guacamole container that looks like the following:

CAS_AUTHORIZATION_ENDPOINT: 'https://cas.example.net'
CAS_REDIRECT_URI: 'https://guac.example.net'

If instead deploying Guacamole by running docker run manually, these same environment variables will need to be provided using the -e option. For example:

$ docker run --name some-guacamole \
    -e CAS_AUTHORIZATION_ENDPOINT="https://cas.example.net" \
    -e CAS_REDIRECT_URI="https://guac.example.net" \
    -d -p 8080:8080 guacamole/guacamole

The environment variables that must be set in all cases for any Docker-based Guacamole installation using this extension are:

CAS_AUTHORIZATION_ENDPOINT: The URL of the CAS authentication server. This should be the full path to the base of the CAS installation.
CAS_REDIRECT_URI: The URI to redirect back to upon successful authentication. Normally this will be the full URL of your Guacamole installation.

Additional configuration (optional)#

Native Webapp (Tomcat)

Additional optional properties are available to control how CAS-related data is processed, including whether CAS ClearPass should be used and how user group memberships should be derived:

cas-clearpass-key

If using CAS ClearPass to pass the SSO password to Guacamole, this parameter specifies the private key file to use to decrypt the password. See the section on ClearPass below.

cas-group-attribute

The CAS attribute that determines group membership, typically “memberOf”. This parameter is only required if using CAS to define user group memberships. If omitted, groups aren’t retrieved from CAS, and all other group-related properties for CAS are ignored.

cas-group-format

The format that CAS will use for its group names. Possible values are plain, for groups that are simple text names, or ldap, for groups that are represented as LDAP DNs. If set to ldap, group names are always determined from the last (leftmost) attribute of the DN. If omitted, plain is used by default.

This property has no effect if cas-group-attribute is not set.

cas-group-ldap-base-dn

The base DN to require for LDAP-formatted CAS groups. If specified, only CAS groups beneath this DN will be included, and all other CAS groups will be ignored.

This property has no effect if cas-group-format is not ldap.

cas-group-ldap-attribute

The LDAP attribute to require for LDAP-formatted CAS groups. If specified, only CAS groups that use this attribute for the name of the group will be included. Note that LDAP group names are always determined from the last (leftmost) attribute of the DN. Specifying this property will only have the effect of ignoring any groups that do not use the specified attribute to represent the group name.

This property has no effect if cas-group-format is not ldap.

Container (Docker)

Additional optional environment variables are available to control how CAS-related data is processed, including whether CAS ClearPass should be used and how user group memberships should be derived:

CAS_CLEARPASS_KEY

If using CAS ClearPass to pass the SSO password to Guacamole, this parameter specifies the private key file to use to decrypt the password. See the section on ClearPass below.

CAS_GROUP_ATTRIBUTE

CAS_GROUP_FORMAT

This property has no effect if cas-group-attribute is not set.

CAS_GROUP_LDAP_BASE_DN

The base DN to require for LDAP-formatted CAS groups. If specified, only CAS groups beneath this DN will be included, and all other CAS groups will be ignored.

This property has no effect if cas-group-format is not ldap.

CAS_GROUP_LDAP_ATTRIBUTE

This property has no effect if cas-group-format is not ldap.

Controlling login behavior#

Guacamole loads authentication extensions in order of priority, and evaluates authentication attempts in this same order. This has implications for how the Guacamole login process behaves when an SSO extension is present:

If the SSO extension has priority:: Users that are not yet authenticated will be immediately redirected to the configured identity provider. They will not see a Guacamole login screen.
If a non-SSO extension has priority:: Users that are not yet authenticated will be presented with a Guacamole login screen. Additionally, links to the configured identity provider(s) will be available for users that wish to log in using SSO.

The default priority of extensions is dictated by their filenames, with extensions that sort earlier alphabetically having higher priority than others. This can be overridden by explicitly setting the extension priority.

Automatically redirecting all unauthenticated users#

To ensure users are redirected to the CAS identity provider immediately (without a Guacamole login screen), ensure the CAS extension has priority over all others:

extension-priority: cas

Completing installation#

Native Webapp (Tomcat)

Guacamole will only reread its configuration and load newly-installed extensions during startup, so Tomcat will need to be restarted before these changes can take effect. Restart Tomcat and give the new functionality a try.

You do not need to restart guacd.

Hint

If Guacamole does not come back online after restarting Tomcat, check the logs. Configuration problems may prevent Guacamole from starting up, and any such errors will be recorded in Tomcat’s logs.

Container (Docker)

The environment variables that configure the behavior of Docker can only be set at the time the Docker container is created. To apply these configuration changes, you will need to recreate the container.

If your Guacamole container was deployed using Docker Compose:: Simply making the desired changes to your docker-compose.yml and running docker compose up is sufficient. Docker Compose will automatically recognize that the environment variables of the container have changed and recreate it.
If your Guacamole container was deployed manually (using docker run):: You wll need to manually use docker rm to remove the old container and then manually recreate it with docker run and the new environment variables.

Hint

If Guacamole does not come back online after recreating the container, check the Docker logs. Configuration problems may prevent Guacamole from starting up, and any such errors will be recorded in the Docker logs for the Guacamole container.

Using CAS ClearPass#

CAS has a function called ClearPass that can be used to cache the password used for SSO authentication and make that available to services at a later time. Configuring the CAS server for ClearPass is beyond the scope of this article - more information can be found on the Apereo CAS wiki at the following URL: https://apereo.github.io/cas.

Once you have CAS configured for credential caching, you need to configure the service with a keypair for passing the credential securely. The public key gets installed on the CAS server, while the private key gets configured with the cas-clearpass-key property. The private key file needs to be in RSA PKCS8 format.

Using CAS for single sign-on

Contents

Using CAS for single sign-on#

Installing/Enabling the CAS authentication extension#

Required configuration#

Additional configuration (optional)#

Controlling login behavior#

Automatically redirecting all unauthenticated users#

Presenting unauthenticated users with a login screen#

Completing installation#

Using CAS ClearPass#