Hello. I'm not sure if this will be picked up by everyone interested, but as I had trouble with this issue I thought I would post my solution. If somebody wants to add it to a wiki fine.
My issue was that I had external (in Apache) authentication but needed a JAAS sign-on and authorisation. AJP13 was setting the RemoteUser via a cookie. I used a version of the DatabaseServerLoginModule for authorisation based on the RemoteUser.

To trigger the Signon - I needed first to code the following simple Servlet:

/**
*
* Simple Servlet to trigger JAAS Login - passing the RemoteUser as username.
*
* @author Jim Brady
*/
public class LoginServlet extends HttpServlet {

public void init() throws ServletException {
}

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doPost(request,response);
}

public void doPost(HttpServletRequest request, HttpServletResponse response) {
// retrieve form parameter values from request
Log log = LogFactory.getLog(this.getClass());

try {
log.info("User = " + request.getRemoteUser());
response.sendRedirect("j_security_check?j_username="
+ request.getRemoteUser() + "&j_password=");
} catch (Exception e) {
log.warn("Whoah", e);
}
}
}

This was integrated via the following items in web.xml.

<servlet-mapping>
<servlet-name>LoginServlet</servlet-name>
<url-pattern>*.login</url-pattern>
</servlet-mapping>


<servlet-name>LoginServlet</servlet-name>
<servlet-class>appl.web.login.LoginServlet</servlet-class>


<login-config>
<auth-method>FORM</auth-method>
<realm-name>APACHE</realm-name>
<form-login-config>
<form-login-page>/login/apache.login</form-login-page>
<form-error-page>/login/loginError.htm</form-error-page>
</form-login-config>
</login-config>

It took a long while to work out how to do this, so I hope it helps some other people out there!

The above only works for Jetty - not for Tomcat. For Tomcat see
http://www.jboss.com/index.html?module=bb&op=viewtopic&t=23094