NAT external IP to look like internal

[thabangk]

Hi All

I have a linux firewall with 2 NICS
eth0 [192.168.0.20]
eth1 [41.31.167.23]

and i have an internal router which is on [192.168.0.10]

and it forwards the internal traffic to [10.10.0.0\24] range and it only allow this range only 192.168.0.0/24 range to the destination [10.10.0.0\24]

So i want to do port forwarding to [10.10.0.21] on the firewall but the external ip traffic should be natted as if it comes from my internal range [10.10.0.0\24]

Can someone please help me,the answer is needed tommorrow and im a beginner on this iptables thing