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, while JASIG releases CAS for the simpler Tomcat environment.

The J2EE standard requires each application server to provide a set of standard services. Implicitly, this means that there are certain JAR files that define the standard APIs (Servlet, JPA, JSF, ...) that the application server has to provide for the running WAR. So generally there are some JAR files that the server uses for itself, some JAR files it shares with the WAR, and then the WAR itself contains private JAR files in its WEB-INF/lib directory. Tomcat and JBoss 4 or 5 solves this problem by creating separate directories of JAR files that are internal for the server or shared with the application. Unfortunately, JBoss has a lot more shared JAR files and they conflict with the JAR files that JASIG packages in the WAR it builds to run under the less extensive Tomcat environment. This created a lot of work reconciling the Maven Dependencies in CAS to match the JBoss 5 shared libraries.

JBoss EAP 6 has a modular system of libraries similar to OSGi. Each JAR file used by the system and potentially shared with applications is stored in a separate "module" directory. An application can access any of these libraries, but unlike the previous versions of JBoss it does not get a whole mass of JAR files automatically. The WAR has to include a JBoss specific configuration file naming the modules that it wants to access. By default, a WAR file will mostly use the JAR files in its WEB-INF/lib configuration and if JBoss internally uses a different version of the same JAR file, there is no conflict.

However, the J2EE standard still requires JBoss or any other application server to automatically provide the Servlet API, JPA, and other API defining JAR files automatically to every WAR. This potentially poses a problem if there is a JAR file that JBoss has decided is part of the standard API but CAS needs to use its own local copy. We discover that the logging libraries (SLF4J and log4j) fall in this category. JBoss EAP 6 will share its logging libraries and will screw up CAS logging unless you include a WEB-INF/jboss-deployment-structure.xml file that tells JBoss to exclude the org.apache.log4j, org.slf4j, and org.slf4j.impl modules from the default shared module list. If you don't do this, then the WEB-INF/classes/log4j.xml file is read and it creates the log files, but you don't get any log messages in them (or anywhere else for that matter).

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.