Blocking source ports after REDIRECT

[Trevor Turton <trevor@turton.co.za>]

I am running an untrusted web/app server and mail server on a Linux 
server, and choose to run then as non-root.  The apps must therefore use 
ports other than the well-known privileged ones.  I have set up iptables 
to redirect client requests addressed to the well-known ports to the 
ports that these apps open, e.g.
 25 -> 8025
 80 -> 8080
110 -> 8110
This works fine.  Out of obsessive neatness rather than for any good 
technical reason I would like to block direct access to the apps through 
their non-privileged (8xxx) ports.  When I insert a filter command to do 
this, it blocks the redirected traffic as well.  Does anyone have a 
suggestion for me?  The commands are listed below, with the one that 
breaks connectivity commented out with ##

Regards
Trevor Turton

#!/bin/bash
###############################################################################
# This script redirects various well-known services to local 
unprivileged ports
# and block the other privileged ports.
# 2004-04-13 Trevor Turton
###############################################################################
#
# turn on IP forwarding:
#
echo 1 > /proc/sys/net/ipv4/ip_forward
#
# redirect well-known services' ports to local unprivileged ports
#
/sbin/iptables -t nat -A PREROUTING -p tcp -i eth0 --dport 25  -j 
REDIRECT --to-ports 8025
/sbin/iptables -t nat -A PREROUTING -p tcp -i eth0 --dport 80  -j 
REDIRECT --to-ports 8080
/sbin/iptables -t nat -A PREROUTING -p tcp -i eth0 --dport 110 -j 
REDIRECT --to-ports 8110
/sbin/iptables -t nat -A PREROUTING -p tcp -i eth0 --dport 443 -j 
REDIRECT --to-ports 8443
#
# accept traffic addressed to the serviced tcp ports
#
/sbin/iptables -A INPUT -p tcp --dport 22  -j ACCEPT
/sbin/iptables -A INPUT -p tcp --dport 25  -j ACCEPT
/sbin/iptables -A INPUT -p tcp --dport 80  -j ACCEPT
/sbin/iptables -A INPUT -p tcp --dport 110 -j ACCEPT
/sbin/iptables -A INPUT -p tcp --dport 443 -j ACCEPT
#
# filter out traffic to remaining privileged tcp ports
#
/sbin/iptables -A INPUT -p tcp -i eth0 --dport 0:1023 -j DROP
## the following command blocks -eth0 --dport 25,80,110,443 as well!!
## /sbin/iptables -A INPUT -p tcp -i eth0 --dport 8000:8443 -j DROP
#
# accept traffic to serviced udp port (dns)
#
/sbin/iptables -A INPUT -p udp --dport 53  -j ACCEPT
#
# filter out traffic to remaining privileged udp ports
#
/sbin/iptables -A INPUT -p udp -i eth0 --dport 0:1023 -j DROP