The CAS Server is a Spring Web Application, but it is designed according to standard layers or functional components that can be plugged into a standard J2EE diagram. There are three main components, each of which can be subdivided into subcomponents.
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. It tries to use every possible feature of Spring, so even people who know Spring discover there are extra manuals to read if they want to understand CAS. This page is designed to quickstart a new CAS administrator or developer with high level pointers to where things are and how they fit together. Maybe you don't need the details.
Spring is a Java framework that loads Java classes and creates Java objects based on XML configuration files. A reference to the object is then "injected" to another Java object by calling the method that sets a property value. For example, when tickets are generated they have to be assigned an ID. The class that creates tickets has a property that holds a reference to an object that generates the ID string. JASIG provides an ID generation class that should satisfy most requirements, but if you find it inadequate you can write your own ID generation class with special local properties. Then you change the XML file that creates the ID generation bean to use your classname instead of the JASIG classname. Spring now creates an object of your class instead of the JASIG standard class and sets the property in the ticket generation object to point to it. Ticket ID strings are now generated by your class.
The good news is that CAS has a well defined set of layers represented by interfaces, and these layers can be described using the standard J2EE terminology. The next step is then to identify the key configuration components for each layer or interface and how they fit into the overall CAS process.
- The front end is a "Presentation Layer" that handles arriving HTTP requests. It is divided into two parts:
- The end user login from the browser is handled by a Spring Webflow configured in XML. If the user is already logged in (the CAS Cookie is present) then the request may be handled automatically as a Single SignOn. If not, the flow can optionally look for non-interactive forms of authentication like a User X.509 certificate and private key installed in the Browser. Otherwise, the flow presents the CAS Login page and gets the userid and password. Finally it generates the Cookie and returns the Service Ticket ID string.
- However, CAS also gets administrative requests and Service Ticket validation requests (/serviceValidate). These are handled by the Spring MVC mechanism, which is basically the Spring version of a Servlet.
- The Business Layer (where J2EE would have EJBs) validates the login (by verifying the userid/password against a backend system such as Kerberos or LDAP), and it creates Tickets including the Login TGT and Service Tickets.
- Tickets are stored in a Ticket Cache. Normally this is just an in-memory collection, although it can be replicated (for clustered failover) or stored in a shared database. Since Tickets can be persisted (even though they frequently aren't) they become Entities in a logical Persistence Layer.
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CAS 3 uses two different Spring APIs to configure and code the Presentation Layer.
Protocol (Validate) Requests
The calls to validate Service Tickets and issue Proxy Tickets are processed by Spring MVC configured in the WEB-INF/cas-servlet.xml file.Spring "Action" Beans are configured that expose a method that receives the Request and Response object of an incoming HTTP operation.
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<bean id="serviceValidateController" class="org.jasig.cas.web.ServiceValidateController"
p:validationSpecificationClass="org.jasig.cas.validation.Cas20WithoutProxyingValidationSpecification"
p:centralAuthenticationService-ref="centralAuthenticationService"
p:proxyHandler-ref="proxy20Handler"
p:argumentExtractor-ref="casArgumentExtractor" />
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Each Action Bean is called when the end of a request URL contains a string mapped to the Bean (again in cas-servlet.xml):
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<prop
key="/serviceValidate">
serviceValidateController
</prop>
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Login Webflow
The "/login" function is mapped to an entirely separate API called Spring WebFlow. WebFlow is a common computer design pattern called a "State Machine" coded as a sequence of XML elements in the WEB-INF/login-webflow.xml file. In some sense, a WebFlow demonstrates how to replace a dozen lines of Java with a hundred lines of considerably more complex XML (but at least you are not coding).
Each state is represented by an XML element that either performs a simple EL test or else calls a Bean method. The result of the test or return value of the method is "success" or "error" and generates a transition (a GOTO) to another state.
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<action-state id="startAuthenticate">
<action bean="x509Check" />
<transition on="success" to="sendTicketGrantingTicket" />
<transition on="error" to="viewLoginForm" />
</action-state>
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The most important secondary internal service, and the most important Spring configuration element, is the AuthenticationManager. An AuthenticationManager handles supports login requests, validating the user Credentials and generating (after a sucessful login) the TGT.
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Credentials are created by the Presentation Layer from something in the HTTP request. In the simple case, the Userid and Password are extracted from the form and combined to create a Credential object. If the container is configured to accept User Certificates, an X.509 Certificate can be extracted from the Request object and presented as a Credential. If SPNEGO is enabled, then the Browser of a client logged into a Kerberos 5 realm (typically an Active Directory Domain) can present a Kerberos 5 Service Ticket as proof of identity.
Each AuthenticationHandler configured to the AuthenticationManager is called in turn. If it supports the type of Credential object, it can claim and validate the Credential. In the case of a Userid/Password from a Form, this means sending these field on to a backend system (a Kerberos 5 KDC, an LDAP directory, a JDBC database) that maintains a list of userids and passwords and can verify that the pair of fields are valid.
However, the container has often already done a lot of the validation. a Kerberos 5 Service Ticket as proof of identity. Of course in the real world, most credentials consist of a userid and password string.
In the Spring configuration file, the AuthenticationManager has a property that gets a List of AuthenticationHandlers. Each Handler is called in turn and is given a chance to accept or pass on a particular type of credential. An X.509 Handler will only look at Certificate credentials and will pass on userid/password credentials. There can be more than one userid/password Handler if you want to authenticate users to multiple backend systems. Spring configuration allows you to plug in a Handler that does LDAP if you want to authenticate passwords using LDAP, or JAAS if you want to authenticate passwords using Kerberos.
When X.509 User Certificates are enabled to Tomcat or JBoss, then the container has already verified that the Certificate was digitally signed by a Certificate Authority in its list of configured "trusted" CAs. With SPNEGO, all the Kerberos 5 keys and tickets protocol has already been done down in the presentation layer using the Java interface to the system GSSAPI service. So some AuthenticationHandlers really validate the Credentials, while others simply verify that the type of object corresponds to something the container or Presentation Layer has already validated and then returns "true" because such Credentials are implicitly trusted.
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- The user types something into a form. Since he is an end user, it should be as simple as possible. Suppose he types "gilbert".
- The AuthenticationHandler validates the Credential object containing "gilbert". However, in order to authenticate to a particular back end service, it may be necessary to transform what was typed in to something specific to that particular back end service. At Yale, if you want to use Kerberos 5 protocol to the MIT Kerberos servers you would use Principal name "gilbert@NET.YALE.EDU", while using the same protocol against Active Directory would use the same JaasAuthenticationHandler configuration and simply transfom transform the typed in userid to "gilbert@YU.YALE.EDU".
- However, at Yale "gilbert" is a Netid that is maintained across a number of back end system. Any system protected against unauthorized modification might be used to authenticate this person, and while it may be of administrative interest how the authentication was done, when some CAS Service validates a Service Ticket it expects to get the naked Netid "gilbert". So even though a fully qualified Principal name may be used with the back end service, the unqualified name would be stored by CAS in the Ticket space.
- That works fine until Yale joins one or more federations of universities and allows cross-institutional login. Now to support legacy systems, the unqualified "gilbert" would be returned to all Yale services that expect that CAS only authenticates local Yale users. New services that are ready to accept fully qualified cross-institutional Principal names might get back "gilbert@harvard.edu" and would realize that this is almost certainly some other person named "gilbert" at a different institution and, therefore, someone completely different from the local Netid "gilbert" at Yale.
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JAAS (Kerberos) Password Handler
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<bean class="org.jasig.cas.authentication.handler.support.JaasAuthenticationHandler"
p:realm="CASJAASconfig" />
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JAAS is a rather complicated standard feature of Java that has its own more comprehensive description page.
The configuration example given above tells CAS to authenticate the userid and password through JAAS using the bundle of parameters in the JAAS "login configuration" file named "CASJAASconfig". To determine exactly what JAAS is going to do, you have to search the environment, find the login configuration file, and lookup the name.
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LDAP (AD) Password Handler
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<bean class="org.jasig.cas.adaptors.ldap.BindLdapAuthenticationHandler">
<property name="filter" value="sAMAccountName=%u" />
<property name="searchBase" value="dc=yu,dc=yale,dc=edu" />
<property name="contextSource" ref="ldapContextSource" />
<property name="ignorePartialResultException" value="yes" />
</bean>
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X.509 Certificate Handler
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<bean class="org.jasig.cas.adaptors.x509.authentication.handler.support.X509CredentialsAuthenticationHandler"
p:trustedIssuerDnPattern="CN=Yale University ITS Issuing Certifying Authority.+"
p:maxPathLength="3"
p:maxPathLengthAllowUnspecified="true"
/>
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X.509 Certificate Resolver
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<bean class="org.jasig.cas.adaptors.x509.authentication.principal.X509CertificateCredentialsToIdentifierPrincipalResolver"
p:identifier="$CN" />
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