e.g., national, which has been languishing...
## We have to provide NTP service. The guests sync to our clock.
ntpclient inbound $ntp_servers
+## Provide NTP service to untrusted clients.
+iptables -A inbound -p udp -j ACCEPT \
+ --source-port 123 --destination-port 123 \
+ -s 172.29.198.0/23
+ip6tables -A inbound -p udp -j ACCEPT \
+ --source-port 123 --destination-port 123 \
+ -s 2001:470:9740::/48
+
## Guaranteed black hole. Put this at the very front of the chain.
run iptables -I INPUT -d 212.13.198.78 -j DROP
run ip6tables -I INPUT -d 2001:ba8:0:1d9::ffff -j DROP