I'm facing same problem with iPlanet. I want to add <listener> in web.xml file.
My web server has web-app_2_2.dtd file. But this file don't have entry for
<listener>, then I found that web-app_2_3.dtd has this entry. I have to use this
file. I'm using iPlanet 6.

So please let me know that how I do that ?

Can I add new DTD file to Web Server ? How ?
Can I install any patch to have web-app_2_3.dtd file on Web Server ?

This is how my web.xml file is

<?xml version="1.0" encoding="ISO-8859-1" ?>
<!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application
2.3//EN" "http://java.sun.com/dtd/web-app_2_3.dtd">
<web-app>
<listener>
<listener-class>CRSSessionListener</listener-class>
</listener>
</web-app>

Whenever I include web-app_2_3.dtd file in web.xml, I'm getting error that
'External entity 'web-app_2_3.dtd' not found. And if I'm not using
web-app_2_3.dtd file then web server could not parse web.xml file.