Collector Flow-Tools

[Ref: OpenBSD 4.8 amd64, 4.9 amd64 & i386]

Table of Contents
  1. Install
  2. Data Path
  3. Capturing netflow data
    • Validating Trafffic Flow
    • Start Capturing on Host Boot
  4. Storage Requirements

[Ref: flow-tools, Network Flow Analysis.]

Netflow collector(s) capture data exported from netflow sensors and archive them to disk.

This guide will look at installing a collector, configuring the collector (setting paths etc) and testing the capture process. For those wondering about the storage requirements, a sample storage use is shown below.

We capture netflow data using flow-tools.

a library and a collection of programs used to collect, send, process, and generate reports from NetFlow data. The tools can be used together on a single server or distributed to multiple servers for large deployments.

The above diagram shows the flow of data from 3 sensors, to a collector (which we are using OpenBSD for, and the collector archiving that data onto disk. The sensors can be appliances or full computers acting as routers or anything that manages network flow that is of interest to our collector.

The data we want to collect are "netflow" packets with traffic information exported from the "sensors".

Our sample collector configuration will use the following sample network layout.

Sensor IP-Address The IP-Address from where **sensor will broadcast** netflow packets. The IP-Address will also be used by the **collector to identify** traffic origination (which sensor broadcast which netflow packet.)
Collector IP-Address The Collector IP-Address where the netflow packets will be captured. This IP-Address is used by the **sensor to direct** the UDP packet.
UDP Port 12345 The UDP port number on which netflow packets are sent. By using different UDP ports for different sensors, a collector can alternatively differentiate multiple sensors.

Time Configuration

Network analysis is a time sensitive venture, make sure that the clocks on your sensors and collectors are synchronised or you will confuse everyone with the timelines of your logs, analysis, reports.

Sensing the Sensor

It is also useful to determine that the sensor data is reaching your collector's network interface before installing a collector. A simple tcpdump invocation should be sufficient to let you see whether traffic is coming from your sensor's ip address, to the collector's ip address at the specified port.

# tcpdump -nettti bnx0 udp and port 12345

Where bnx0 is the network interface configured with IP Address

1. Install

Install flow-tools from the OpenBSD Packages.

$ sudo pkg_add flow-tools

Dependencies the installation may install as well:

  • python
  • bzip2
  • sqlite3

2. Data Paths

Our installation will use the following paths.

Path Description
/etc/cfg Configuration files for netflow-tools copied from samples at /usr/local/share/examples/flow-tools/cfg including:
  • filter.cfg
  • stat.cfg - flow report definitions
# cp -R /usr/local/share/examples/flow-tools/cfg /etc
A lot confused about the filter/stat features, and configuration? Michael W. Lucas' book ["Network Flow Analysis"]( provides a number of example uses, and modifications to the above configuration files to help you gain a better view of your network traffic.
/var/netflow BASE Storage Location, all subsequent paths are rooted in this path. In production you might prefer /var/netflow as a separate partition with lots of space to ensure it doesn't mangle other work on the collector box. Refer to the sample storage utilisation below for a guesstimate of what you may need? But, run your collector to get a better understanding of your own requirements.
sensorXY /var/netflow/sensorXY is the Netflow capture directory, where *sensorXY* is the label and name for your sensor, which we reflect as the directory for netflow data exported from the sensor. Sample sensor names: wangateway, router1, finance_rtr

Michael's book is invaluable in your learning toolkit for an exhaustive set of tests, invaluable for validating software configuration is fully operational.

For our install paths, we need to create some directories.

# mkdir -p /var/netflow/sensorXY
# cp -R /usr/local/share/examples/flow-tools/cfg /etc

3. Capturing netflow data

[Ref: flow-capture

Item Details
Sensor IP-Address
Collector IP-Address
UDP Port 12345

Sensor data is collected using flow-capture which listens on the network interface (IP-Address) sensor specified 'destination address' and UDP port. The command-line capture process would be like the following,

flow-capture options ${collectorIP}/${sensorIP}/${port}
-p pidfile Configure the process ID file

-n rotations the number of times a new file per day. 

-S stat_interval log a timestamped message every stat_interval minutes indicating 
counters such as the number of flows received, packets processed, 
and lost flows. 

-w workdir set the workdir to /var/netflow. 

Let's start capturing some of that netflow data to local storage.

$ sudo flow-capture -p /var/run/ -n 287 -w /var/netflow/sensorXY \
    -S 5

More details on using flow-capture are also available at the manpage,

  • /12345 - our port selection matches what we've selected from our sensor, and is quite arbitrary for testing purposes, and you can use something more sane to your configuration.
  • -n 287 is about every 5 minutes.
  • -S 5 is set for 5 minutes, per Michael W. Lucas' book "Network Flow Analysis." notes the expected standard by supplementary analysis tools.

If the program installed correctly, and your sensor is sending your collector the netflow packet, you should be able (after executing flow-capture) to watch the work directory for data files, such as with:

$ ls /var/netflow/sensorXY/2012/2012-08/2012-08-12/

If you see a similar directory structure (YEAR/YEAR-MONTH/YEAR-MONTH-DAY) with file(s) then the capture process is working.

Validating Traffic Flow

A few quick ways of verifying, monitoring behaviour of the flow-capture process is by reviewing the:

  • Log files generated by flow-capture
  • Log messages generated by flow-capture
  • Traffic flow (tcpdump)
Log Files

When flow-capture is working correctly, data files will be stored in the specified directory, with data split into date folders, such as:


The file naming convention for the incremental files are:


Where ft signifies the log file is complete, and tmp signifies the log file is temporary/incomplete.

Log messages

flow-capture logs its messages, errors to syslog's /var/log/messages which can be monitored.

Start Capturing on Host Boot

To ensure flow-capture is started at each host start-up, we need to start the server using the rc(8) command script for system startup. The following is a sample script for inclusion in /etc/rc.local

File extract: /etc/rc.local


if [ -x ${CAPTURE} ]; then
    printf ' flow-capture'; ${CAPTURE} -p ${FLOWPID} -n 287 -w ${DATA}/${SENSOR} \
        -S 5 ${ipa_collector}/${ipa_sensor}/${port} && \
        echo "\t\t [OK]" || echo "\t\t [Failed]";

In our example, we are overly verbose hopefully to clarify the command-line options for flow-capture, but also so we can easily replicate the script should we use multiple hosts, or have multiple sensors we wish to capture.

4. Storage Requirements

How much disk space is required ? How long is a piece of string ?

2 months of data collection on 2 devices reveals the below, but all it really says is that storage

DeviceStorage Requirements
WAN Gateway751Mb
Internet Gateway1002Mb

Storage requirements are not onerous for today's standard hard drives clearing 1Tb.