CAS Authentication

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.

Downloading the CAS authentication extension

Guacamole’s SSO extensions are available separately from the main guacamole.war. The link for this and all other officially-supported and compatible extensions for a particular version of Guacamole are provided on the release notes for that version. You can find the release notes for current versions of Guacamole here: http://guacamole.apache.org/releases/.

The SSO extensions are packaged together in a .tar.gz file containing one extension for each supported SSO method:

SSO Method	Extension
CAS	cas/guacamole-auth-sso-cas-1.5.5.jar
OpenID Connect	openid/guacamole-auth-sso-openid-1.5.5.jar
SAML	saml/guacamole-auth-sso-saml-1.5.5.jar

The extension for the desired SSO method, in this case guacamole-auth-sso-cas-1.5.5.jar from within the cas/ subdirectory, must ultimately be placed in GUACAMOLE_HOME/extensions.

Installing CAS authentication

Guacamole extensions are self-contained .jar files which are located within the GUACAMOLE_HOME/extensions directory. If you are unsure where GUACAMOLE_HOME is located on your system, please consult Configuring Guacamole before proceeding.

To install the CAS authentication extension, you must:

1. Create the GUACAMOLE_HOME/extensions directory, if it does not already exist.

2. Copy guacamole-auth-sso-cas-1.5.5.jar within GUACAMOLE_HOME/extensions.

3. Configure Guacamole to use CAS authentication, as described below.

Configuring Guacamole for CAS Authentication

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 the CAS and how CAS should redirect users back to Guacamole once their identity has been confirmed:

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 optional properties are available to control how CAS tokens are 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.

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 setting the extension-priority property within guacamole.properties.

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

Presenting unauthenticated users with a login screen

To ensure users are given a normal Guacamole login screen and have the option to log in with traditional credentials or with CAS, ensure the CAS extension does not have priority:

extension-priority: *, cas

Completing the installation

Guacamole will only reread guacamole.properties and load newly-installed extensions during startup, so your servlet container will need to be restarted before CAS authentication can be used. Doing this will disconnect all active users, so be sure that it is safe to do so prior to attempting installation. When ready, restart your servlet container and give the new authentication a try.

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.