LinuxQuestions.org - problems with iptables NAT

I'm trying to get an iptables script working for my linux box so that I can basically use it as a router. The setup is fairly weird for reasons beyond my control, so bear with me while I outline it (This is why I couldn't find any examples etc to help me through it).

I've got a ADSL router which is set to do NAT itself, but it's one I cannot admin and have to go get somebody to change it every time I want a new port forwarded, and bridging it is out of the question, so I've just gotten them to forward every port through to my linux box and I'm redistributing them from there. So essentially I'm routing data back out on the same interface it's comming in on (eth0, the only interface on the whole machine)

I can route ports to itself easily, for example I tested routing 3000 to 6667 and my IRC could find the IRCD. When I attempt to route to other ip's though I get errors. When I routed port 3100 on the linux server back to 2000 on my windows machine and listened with hyperterminal and then telnetted to port 3100 on the linux box I recieved a connection in (My windows firewall noticed it) and then hyperterminal stopped listening and the connection failed. I also tried routing it out to a friend's machine who was hosting an IRCD and the IRCD reported "Can't allocate fd for socks on [@IP.REMOVED.WEIRDNUMBER]" and failed to connect.

Below is the whole script I am using to create my iptables. A few comments to help everyone trying to read:
My windows machine: 192.168.1.99
My linux box: 192.168.1.100
My internet IP is non-static so I have used 0.0.0.0/0 to represent all IP's.

This is pretty much the first time I've used iptables, and the below was put together from reading a few tutorials. hopefully somebody can show me what I've done wrong.

Code:

#!/bin/sh



#EDITABLE CONSTANTS

VALID_ADMIN_IP=192.168.1.99

VNC_SERVER_MAX_COUNT=10

SELF=192.168.1.100

SELF_EXTERNAL=0.0.0.0/0

FIGJAM=192.168.1.99



#DO NOT EDIT THESE VARIABLES

VNC_START_PORT=5901

let "VNC_END_PORT=$VNC_START_PORT+$VNC_SERVER_MAX_COUNT-1"



#FUNCTIONS

route_port()

{

        source_port=$1

        dest_ip=$2

        dest_port=$3

        protocol=$4

        if [ -z "$protocol" ]; then

                protocol="tcp"

        fi



        iptables -A FORWARD -p $protocol --dport $dest_port -j ACCEPT

        iptables -t nat -A PREROUTING -p $protocol -d $SELF_EXTERNAL --dport $source_port -j DNAT --to $dest_ip:$dest_port

}



#Clear IP tables

iptables -F -t filter

iptables -F -t nat



#Set default security levels

iptables -P INPUT DROP

iptables -P OUTPUT ACCEPT

iptables -P FORWARD DROP



#Create exceptions to rules

# --------------- PUBLIC ACCESS ---------------

# Apache and FTP and IRCD

iptables -A INPUT -p TCP --destination-port http -j ACCEPT

iptables -A INPUT -p TCP --destination-port ftp-data:ftp -j ACCEPT

iptables -A INPUT -p TCP --destination-port 6667 -j ACCEPT

# Allow pinging

iptables -A INPUT -p ICMP --icmp-type echo-request -j ACCEPT

iptables -A INPUT -p ICMP --icmp-type echo-reply -j ACCEPT

# Allow DNS

iptables -A INPUT -p udp --sport 53 --dport 1024:65535 -j ACCEPT

#  --------------- ADMIN ONLY  ---------------

# SSH, VNC, SAMBA, SWAT

iptables -A INPUT -p TCP -s $VALID_ADMIN_IP --destination-port ssh -j ACCEPT

iptables -A INPUT -p TCP -s $VALID_ADMIN_IP --destination-port $VNC_START_PORT:$VNC_END_PORT -j ACCEPT

iptables -A INPUT -p TCP -s $VALID_ADMIN_IP --destination-port 135:139 -j ACCEPT

iptables -A INPUT -p TCP -s $VALID_ADMIN_IP --destination-port 901 -j ACCEPT





# ------------- ROUTE-O-LICIOUS --------------

# I think I need this, but I may just be stupid

iptables -A POSTROUTING -t nat -s 192.168.1.0/255.255.255.0 -d 0/0 -j MASQUERADE

echo 1 > /proc/sys/net/ipv4/ip_forward



# Azureus

route_port 6881 $FIGJAM 6881 tcp

# Total Annihilation

route_port 2300:2400 $FIGJAM 2300:2400 tcp

route_port 2300:2400 $FIGJAM 2300:2400 udp

route_port 47624 $FIGJAM 47624 tcp

route_port 3000 66.216.103.243 6667 tcp

route_port 3100 $FIGJAM 2000 tcp

If you got to here, thanks for your time even if you can't help :)