The IPTables plugin allows to collect some statistics on IPTables rules: incoming and outgoing traffic in bytes and number of packets.
In order to gather all the data on a chain:
<Plugin "iptables"> Chain "filter" "OUTPUT" </Plugin>
In order to gather the data on a precise rule:
root$ iptables -A CUSTOM -p tcp --dport 22 -m comment --comment "UniqueComment" -j ACCEPT
<Plugin "iptables"> Chain "filter" "CUSTOM" "UniqueComment" </Plugin>
With this configuration, it is thus possible to check the traffic on a precise port.