REMOTE_USER not being set by Apache2?
<Location />
    AuthBasicProvider ldap
    AuthLDAPURL ldap://mainframe/ou=People,dc=dtch,dc=com?uid?sub
    AuthzLDAPAuthoritative off
    AuthType Basic
    AuthName "LDAP"
    require valid-user

    Options Indexes FollowSymLinks MultiViews +Includes
</Location>

JKWorersFile  /path/to/workers.properties
JkShmFile     /path/to/mod_jk.shm
JkLogFile     /path/to/mod_jk.log
JkLogLevel    info
JkOptions     +ForwardKeySize +ForwardURICompat -ForwardDirectories

worker.list=worker1

worker.worker1.port=8009
worker.worker1.host=127.0.0.1
worker.worker1.type=ajp13

...

JkMount /*.jsp worker1
JkMount /sng/* worker1

<filter>
    <filter-name>RemoteUser</filter-name>
    <filter-class>path.to.RemoteUser</filter-class>
</filter>
<filter-mapping>
    <filter-name>RemoteUser</filter-name>
    <url-pattern>/*</url-pattern>
    <dispatcher>REQUEST</dispatcher>
</filter-mapping>

//...

public void doFilter( ServletRequest request, ServletResponse response, FilterChain chain )
    throws IOException, ServletException
{
    if( request instanceof HttpServletRequest )
    {
        HttpServletRequest httpServletRequest = ( HttpServletRequest )request;
        String username = httpServletRequest.getHeader( "REMOTE_USER" );

        if( username == null )
        {
            // enter here every time
        }

        // ...
    }

    //...
}

Basic XXXX:YYYY