but the traffic from 192.168.1.23 that i need to go to the internet i would like to have addressed as 161.31.31.23
You need SNAT (source NAT), not DNAT (destination NAT). I'll assume eth0 has address 161.31.31.23:
iptables -t nat -A POSTROUTING -o eth0 -j SNAT --to-source 161.31.31.23
or if eth0 has dinamically allocated IP address:
iptables -t nat -A POSTROUTING -o eth0 -j MASQUARADE
I'm not sure why you are assigning local IP address as an alias to your external interface (probably something you haven't mentioned), so I wan't comment on whether it is needed or not. Also, in case when you have multiple IP addresses (aliases) assigned to an interface, I'm not sure exactly how MASQUERADE target behaves. man iptables for more details and differences between SNAT and MASQUERADE targets.
-- Aleksandar Milivojevic <amilivojevic@xxxxxx> Pollard Banknote Limited Systems Administrator 1499 Buffalo Place Tel: (204) 474-2323 ext 276 Winnipeg, MB R3T 1L7
