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By default, on logout the CAS server will display the CAS logout page.
To make the CAS server redirect to the portal page after logout, modify $CAS_TOMCAT_HOME/webapps/cas/WEB-INF/cas-servlet.xml
to include the followServiceRedirects="true" parameter:
<bean id="logoutController" class="org.jasig.cas.web.LogoutController"
p:centralAuthenticationService-ref="centralAuthenticationService"
p:logoutView="casLogoutView"
p:warnCookieGenerator-ref="warnCookieGenerator"
p:ticketGrantingTicketCookieGenerator-ref="ticketGrantingTicketCookieGenerator"
p:followServiceRedirects="true"/>
The portal page will be then immediately redirected to the CAS login page. Thus, you will see the CAS login page instead of the portal page.