#7472 (Cut Through Forwarding breaks non-standard port ...
https://svn.dd-wrt.com/ticket/747221.09.2021 · Instead of getting forwarded to my NAT'd IPs/ports (ex. port 32400, FTP via SSL with port range forwarding rules) simply timeout. I tested this release with a clean reset (NVRAM reset and power restart) and as soon as I configure CTF +FA with a custom Port Forwarding (NAT/QoS tab-->Port Forwarding) rule (port 32400), traffic to my NAT'd device stops/times out.
Iptables command - DD-WRT Wiki
https://wiki.dd-wrt.com/wiki/index.php/IptablesFirewall Forwarded Ports. If you have enabled SPI firewall feature on DD-WRT, your router is pretty much protected. However one practical use of iptables is protecting certain ports forwarded to internal IP addresses. The simplest way to do this is: Create port forwarding to internal IP addresses using the DD-WRT "Port Forwarding" web interface