Been googling all over the net for a solution and every time I end up with the most elaborate detailed guides as to how to setup internet sharing, setup nat etc, I'm after something simple.

I have 2 networks.

10.1.1.0/24
10.1.5.0/24

My linux box has got both interfaces configured as 10.1.1.5 and 10.1.5.1
10.1.1.1 is the default gw to get out to the net, these are internal networks.

network 10.1.5.0 is only connected on this box, all machines can get anywhere apart from 10.1.5.x addresses.
On the box itself I can ping 10.1.5.x addresses but from the outside I cannot so it doesnt' seem that my linux box is doing any routing for the outside.

Basically I need this machine to route between 10.1.1.0 and 10.1.5.0 networks and I can't get it working.
What do I need to configure/enable?

Thanks

should just be an "echo 1 > /sys/proc/net/ipv4/ip_forward" (probably not the *exact* path..) to enable the routing between the interfaces. you won't need any source natting or anything as long as you have a route back from elsewhere to these sources.

Actually it's
which will enable packet forwarding (i.e. allows packets to pass from one network to another).

Also, if you want "Packet Forwarding" to be enbled by default when you start this machine you should edit /etc/sysctl.conf to contain:
Happy hunting