iptables -A INPUT -i lo -p udp --destination-port 123 -j ACCEPT
iptables -A INPUT -p udp --source-port 123:123 -m state --state ESTABLISHED -j ACCEPT

iptables -A OUTPUT -o lo -p udp --source-port 123 -j ACCEPT
iptables -A OUTPUT -p udp --destination-port 123:123 -m state --state NEW,ESTABLISHED -j ACCEPT