DDWRT advance setting: Block all traffic except HTTP HTTPS and FTP

add the following commands in DDWRT setting.
(Administration-->Commands-->paste commands-->Save Firewall.

iptables -I FORWARD 1 -p tcp -m multiport --dports 21,80,143,443,465,587,993,995,7001,7002 -j ACCEPT

DD-WRT:

1. Download DD-WRT K2.6 firmware (dd-wrt.v24-30880_NEWD-2_K2.6_mini-e4200.bin, dd-wrt.v24-30880_NEWD-2_K2.6_mega-nv60k.bin) from below link:

 http://ddwrt.stevejenkins.com/builds/30880/

detail refer to: https://www.stevejenkins.com/blog/2013/01/my-cisco-linksys-e4200-dd-wrt-settings-for-max-speed/