First enable IPv4 packet forwarding by editing /etc/sysctl.conf, uncomment the following line:

net.ipv4.ip_forward=1

Then add the following lines to /etc/rc.local (so the rule is enabled on reboot):

iptables -t nat -A POSTROUTING -s 192.168.0.0/16 -o eth0 -j MASQUERADE
iptables -A FORWARD -s 192.168.1.0/16 -o eth0 -j ACCEPT
iptables -A FORWARD -d 192.168.1.0/16 -m state --state ESTABLISHED,RELATED -i eth3 -j ACCEPT

# forward ssh to workstation with the internal ip 192.168.1.2
iptables -t nat -I PREROUTING -p tcp -i eth0 --dport 22 -j DNAT --to-destination 192.168.1.2:22
iptables -A FORWARD -i eth0 -o eth3 -p tcp --dport 22 -j ACCEPT

• https://help.ubuntu.com/10.04/serverguide/C/firewall.html