IPCOP in between Router and Private LAN
 

I am from Bhutan using ipcop as my firewall for the last 8 years, recently there has been change in my network system as follows.
Router is connected to ISP with IPVPN
IPCOP 1.4.21 RED connected to router (172.30.112.1 G0/0)-with 172.30.112.100

GREEN is connected network switch with 172.30.112.101
DHCP SERVER with 172.30.112.2
The problem is the users from the LAN cannot get access to internet. Anyone have the same problem can help me with this senario.

Your adressing is wrong, but we need more info about the netmask.
All interfaces on a router must be on different networks, as I read it you have 172.30.112.# on all interfaces. Even if you subnetted it the adressing is still wrong as you have 172.30.112.1 on RED and 172.30.112.2 on GREEN.
Easiest is to use different /24 networks on all interfaces, like this:
RED 172.30.112.0/24
GREEN 172.30.113.0/24
ORANGE (if exists) 172.30.114.0/24