Force Transmission Bittorrent Client to use OpenVPN with IPTables

#!/bin/bash
export INTERFACE="tun0"
export VPNUSER="debian-transmission"
export LANIP="192.168.1.0/24"
export NETIF="eth0"
iptables -F -t nat
iptables -F -t mangle
iptables -F -t filter
# mark packets from $VPNUSER
iptables -t mangle -A OUTPUT ! --dest $LANIP  -m owner --uid-owner $VPNUSER -j MARK --set-mark 0x1
iptables -t mangle -A OUTPUT --dest $LANIP -p udp --dport 53 -m owner --uid-owner $VPNUSER -j MARK --set-mark 0x1
iptables -t mangle -A OUTPUT --dest $LANIP -p tcp --dport 53 -m owner --uid-owner $VPNUSER -j MARK --set-mark 0x1
iptables -t mangle -A OUTPUT ! --src $LANIP -j MARK --set-mark 0x1
# allow responses
iptables -A INPUT -i $INTERFACE -m conntrack --ctstate ESTABLISHED -j ACCEPT
# allow bittorrent
iptables -A INPUT -i $INTERFACE -p tcp --dport 59560 -j ACCEPT
iptables -A INPUT -i $INTERFACE -p tcp --dport 6443 -j ACCEPT
iptables -A INPUT -i $INTERFACE -p udp --dport 8881 -j ACCEPT
iptables -A INPUT -i $INTERFACE -p udp --dport 7881 -j ACCEPT
# block everything incoming on $INTERFACE
iptables -A INPUT -i $INTERFACE -j REJECT
# send DNS to google for $VPNUSER
iptables -t nat -A OUTPUT --dest $LANIP -p udp --dport 53  -m owner --uid-owner $VPNUSER  -j DNAT --to-destination 8.8.4.4
iptables -t nat -A OUTPUT --dest $LANIP -p tcp --dport 53  -m owner --uid-owner $VPNUSER  -j DNAT --to-destination 8.8.8.8
# let $VPNUSER access lo and $INTERFACE
iptables -A OUTPUT -o lo -m owner --uid-owner $VPNUSER -j ACCEPT
iptables -A OUTPUT -o $INTERFACE -m owner --uid-owner $VPNUSER -j ACCEPT
# all packets on $INTERFACE needs to be masqueraded
iptables -t nat -A POSTROUTING -o $INTERFACE -j MASQUERADE
# reject connections from predator ip going over $NETIF
iptables -A OUTPUT ! --src $LANIP -o $NETIF -j REJECT
export GATEWAYIP=`ifconfig $VPNIF | egrep -o '([0-9]{1,3}\.){3}[0-9]{1,3}' | egrep -v '255|(127\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3})' | tail -n1`
if [[ `ip rule list | grep -c 0x1` == 0 ]]; then
ip rule add from all fwmark 0x1 lookup $VPNUSER
fi
ip route replace default via $GATEWAYIP table $VPNUSER
ip route append default via 127.0.0.1 dev lo table $VPNUSER
ip route flush cache