IPv4 (v6 has been disabled)
IP Address: 192.168.0.1
Netmask:    255.255.255.128
Gateway:    192.168.0.250
METRIC:     1

IP            GW              Metric
192.168.0.0   192.168.0.250   1      # mine
192.168.0.0                   0      # system default
# plus some other routes that I don't think are important

# clear existing iptables rules
iptables -F
iptables -X
iptables --table nat -F
iptables --table nat -X
iptables --table mangle -F
iptables --table mangle -X

# the rest of these rules were adapted from a blog post and I don't fully understand how they work
iptables -t nat -A PREROUTING -i eth0 -s 192.168.0.0/24 -d 192.168.0.0/24 -p tcp --dport 80 -j ACCEPT
iptables -t nat -A PREROUTING -i eth0 -s 192.168.0.0/24 -p tcp --dport 80 -j DNAT --to 192.168.0.250:3128
iptables -t nat -I POSTROUTING -o eth0 -s 192.168.0.0/24 -d 192.168.0.250 -p tcp -j SNAT --to 192.168.0.1
iptables -I FORWARD -i eth0 -o eth0 -s 192.168.0.0/24 -d 192.168.0.250 -p tcp --dport 3128 -j ACCEPT