This script provides configurable via rc.conf initializing of ng_netflow
nodes and connects them to ng_ipfw node. Maybe the right place for this
would be /usr/share/examples/netgraph.

Fix: 

#!/bin/sh

# PROVIDE: ng_netflow_ipfw
# BEFORE: ipfw
# REQUIRE: FILESYSTEMS

# ng_netflow_ipfw_nodeN_enable (bool): used for temporary disabling one node
# ng_netflow_ipfw_nodeN_cookie (int): specify cookie number for ng_ipfw (default=N)
# ng_netflow_ipfw_nodeN_collector : "ipaddr:port" of corresponding collector
# ng_netflow_ipfw_nodeN_atimeout (int) : active timeout for ng_netflow node
# ng_netflow_ipfw_nodeN_itimeout (int) : inactive timeout for ng_netflow node
#
#  For more information about specific parameters please refer to ng_ipfw(4) and
#   ng_netflow(4) man pages.
#

. /etc/rc.subr

name="ng_netflow_ipfw"
rcvar="ng_netflow_ipfw_enable"
start_cmd="ng_netflow_ipfw_start"
stop_cmd="ng_netflow_ipfw_stop"
required_modules="netgraph ng_ipfw ng_netflow ng_ksocket"

ng_netflow_ipfw_start()
{
        local node ngctl
        ngctl="/usr/sbin/ngctl"
        node=0
        local singlenode=0
        if [ -n "$1" ]; then
                singlenode=1
                node=$1
        fi

        while true; do
                eval _enable=\$ng_netflow_ipfw_node${node}_enable
                if [ -z "${_enable}" ]; then # node not defined, finish
                        break
                fi
                if ! checkyesno _enable; then # node defined, but disabled, skip
                        continue
                fi
                eval _cookie=\${ng_netflow_ipfw_node${node}_cookie:-\"${node}\"}
                eval _collector=\${ng_netflow_ipfw_node${node}_collector:-\"127.0.0.1:9995\"}
                eval _atimeout=\${ng_netflow_ipfw_node${node}_atimeout:-\"1800\"}
                eval _itimeout=\${ng_netflow_ipfw_node${node}_itimeout:-\"15\"}

                echo "mkpeer ipfw: netflow ${_cookie} iface0
                name ipfw:${_cookie} netflow${node}
                msg netflow${node}: setdlt { iface=0 dlt=12 }
                msg netflow${node}: settimeouts { inactive=${_itimeout} active=${_atimeout} }
                mkpeer netflow${node}: ksocket export inet/dgram/udp
                msg netflow${node}:export connect inet/${_collector}" | ${ngctl} -f -

                if [ "${singlenode}" = 1 ]; then
                        break
                fi
                node=$(( ${node} + 1 ))
        done
}

ng_netflow_ipfw_stop()
{
        local node ngctl
        ngctl="/usr/sbin/ngctl"
        node=0
        local singlenode=0
        if [ -n "$1" ]; then
                singlenode=1
                node=$1
        fi

        while true; do
                eval _enable=\$ng_netflow_ipfw_node${node}_enable
                if [ -z "${_enable}" ]; then # node not defined, finish
                        break
                fi
                if ! checkyesno _enable; then # node defined, but disabled, skip
                        continue
                fi

                echo "rmhook netflow${node}: export
                rmhook netflow${node}: iface0" | ${ngctl} -f -

                if [ "${singlenode}" = 1 ]; then
                        break
                fi
                node=$(( ${node} + 1 ))
        done
}

load_rc_config $name
run_rc_command $@