I used Dual WAN mode. WAN and WAN1 (via lan1) used PPPoE Connect to ISP.
Just test again, and compared the file /tmp/filter_rules, the difference is as follows(MARK RED COLOR):
Option NO
*filter
:INPUT ACCEPT [0:0]
:FORWARD DROP [0:0]
:OUTPUT ACCEPT [0:0]
:FUPNP - [0:0]
:SECURITY - [0:0]...
ASUS N66U Merlin 380.66_4
Advanced Settings>Firewall, after i check "Respond ICMP Echo (ping) Request from WAN" YES Option, I can not access the Internet, Check NO, I can access the Internet. I think it is a bug, need to fix. thanks.