Yale CAS Customizations

Generally speaking, one should try not to make modifications to off the shelf software. However, the advantage of Open Source software is that you have access to the source code if you need it. Sometime a single line change to the source accomplishes quickly something that could otherwise only be done with weeks or months of trial and error through the poorly documented Spring Configuration. Sometimes Yale has special needs for which the JASIG code has no solution.

The good news is that CAS has a logical modular design based on well defined, if sometimes poorly documented, interfaces. If you understand the architecture of Authentication Managers, Authentication Handlers, Credential to Principal Resolvers, and Ticket Caches, then you can create your own implementation of any specific interface and generate custom code that will easily migrate to new releases.

There are several problems that trigger a need for customization:

JBoss (not Tomcat)

Yale Production Services runs Java applications under JBoss. CAS was designed to run under the simpler Tomcat. This can produce a conflict in the Maven POM configuration file between the Version number of a dependency JAR library that was used to develop a particular release of CAS and a different Version number of the same library that was embedded in the version of JBoss you are deploying CAS under. So the standard JASIG POM has to be tweaked to change the version number and to mark certain libraries as "provided by the container" rather than as a library to be included with the application.

Yale has a Netid System (not just AD)

CAS added a password expiration mechanism that works well with Active Directory. While Yale uses AD to authenticate passwords, the Netid system that creates, activates, and changes passwords for Netids is driven by databases external to AD. In the past Yale has had its own custom version of Password Expiration logic, although in the future we may attempt to change AD enough to allow us to use the standard JASIG CAS module.

Occasional Loss of "Flow" Variables during Login

CAS uses Spring Webflow to manage the browser interface during login. Webflow is one of several technologies (like JBoss Weld and the new Java standard CDI) that organize information across a sequence of pages that perform a single transaction (like the checkout pages when you buy something over the Web). CAS has only the one userid and password page, but before and after that page there are a sequence of non-interactive processing steps to look for a user certificate presented by the browser, or generate the cookie and service ticket ID string. Using a Webflow configured in XML allows CAS customers to insert additional optional steps in the process though configuration rather than coding.

Like Weld and CDI, Spring Webflow maintains the values of previously set variables through the duration of a "Flow". They are discarded when the Flow is done. This is similar to the HTTP Session variables, but it operates on a subset of the Session activity. The browser maintains one Session with the Web server, but if you are doing two different things at the same time to the same Web server (say in two different tabs of the same browser) you can run two open Flows at the same time. So something has to keep them separate and keep their variables separate.

Unfortunately, this fails sometimes at Yale. It is not clear if anyone else experiences this problem with CAS, and it may be due to some local network problem or the way we configure the network. However, frequently enough the Flow seems to get broken when the user hits Enter on the page where he submits the userid and password. The JASIG CAS code stores one important piece of data in a Webflow variable (the service= string) that has to be available after this form is submitted, and it has no logic that can recover if the Flow is broken by some network problem. It is relatively trivial to program around this, because the service string could just as easily be stored in a hidden field of the form as in a Flow variable. However, this is a modification of the JASIG code that makes CAS more robust against failure of the Webflow mechanism for whatever reason.

Preferences

CAS has a large number of optional features, but they are all configured by the central administrator. There is no provision for the User to select optional CAS behavior. It seemed that this feature would be required in the future, so a "proof of concept" module has been created called CAS preferences. When the user goes to the /cas/preferences URL he is presented with a Form where he can select various check boxes and radio buttons. These choices are then written to a Cookie scoped to the CAS server and are available at the beginning of the login Webflow to influence the path through subsequent login sequences. This could be used to enable additional forms of authentication, special processing, or even additional logging to debug a problem someone else is having.

Statistics

There is a standard CAS page with basic server information, but Yale Production Services has a standard format expected for Opsview. So we used the requirement to add Yale specific monitoring data.