/etc/config/firewall. note: /etc/firewall.user is for manual iptables commands and is basically a shell script. I only recommend this if /etc/config/firewall doesn't do what you want (like multiport).
config redirect option src wan option src_dport 8080 option dest lan option dest_ip 192.168.1.3 option dest_port 80 option proto tcp
So options src and dest merely define the too and from interfaces (to be honest I'm not sure they actually do anything. as I'm not seeing any changes to what's in iptables). src_dport is what external port you want to listen on. dest_port is what you want it to go to. dest_ip is what ip you want it to go to and proto is the protocol it listens on (yes you have to specify separately for udp if what you're forwarding uses both tcp and udp).
After you've edited and saved the file to fit your case run
/etc/init.d/firewall restartand it should work
IMPORTANT: there's a bug in 7.x and pre 8.09.2 with 2.4-bcrm kernels and netfilter code... it 'causes port drift. if the bug is affecting you. (it did me) reboot the system. you will be able to see it with a -j LOG iptables rule. the output was thus for me
as you can see the destination port(DPT) is 82 instead of 80 like it's supposed to be. 8.09.2 should be out in a few weeks it's currently at rc2.
IN=eth0.1 OUT=br-lan SRC=220.127.116.11 DST=192.168.1.3 LEN=44 TOS=0x00 PREC=0x20 TTL=47 ID=63450 DF PROTO=TCP SPT=54402 DPT=82 WINDOW=5840 RES=0x00 SYN URGP=0