The previous problem was due to incorrectly added iptable rules
But now I Just found out that my configuration wasn't working perfectly. After a vpn connection, my wan interface on the router is screwed. DNS no longer works. I have to reboot the router to get it back to normal through ssh remotely. Further investigation found that a default route to the ppp0 interface was added somehow when the vpn connection was made. The resolv.conf was also cleared. It is probably done by the default pppd configuration used for wan pppoe connection. Anyone has clue how to correct this problem?