Ubuntu: Flushing IpTables Causes Loss of Internet Connection

If you are trying to disable the firewall, but lose Internet connection in the process, use this script:

iptables -F
iptables -X
iptables -t nat -F
iptables -t nat -X
iptables -t mangle -F
iptables -t mangle -X
iptables -P INPUT ACCEPT
iptables -P FORWARD ACCEPT
iptables -P OUTPUT ACCEPT

Source: https://help.ubuntu.com/community/IptablesHowTo#Disabling%20the%20firewall