Monitor your traffic with MRTG

The Multi Router Traffic Grapher is a free tool that can help you monitor the traffic on your network. This article from InformIT discusses its function, where to find it and the complimentary software you will need to use it.

MRTG is the Multi Router Traffic Grapher, a piece of free software released under the GNU General Public License. In the middle of a crisis, or when you are debugging an immediate network problem, MRTG will allow you to...  view the traffic patterns of many networks at once and quickly determine if one or more is experiencing an abnormal traffic load. The fact that the graphs display the history of the network is key.

In practice, it can be difficult to tell from immediate bandwidth and packet-per-second counts alone whether a network is operating normally. If a 100 Mbps link is carrying 85 Mbps of traffic, is this heavy but normal use or is the network straining under an attack? By having the history of the network available, you can look for sudden changes that might account for an operational problem. A denial-of-service attack that attempts to exhaust the available bandwidth on a network nearly always presents as a sudden, sustained increase in traffic levels; the attackers do not have much to gain by slowly ramping up the attack over a period of time.

When you are not tending to an immediate problem, MRTG is useful for studying trends in traffic on your network. It will help you understand how traffic is distributed across your network, plan capacity needs for the future, and so on.

While MRTG is most often used to collect data from router interfaces, it can also collect traffic data from switches or servers. In this way, you can monitor the bandwidth use of a particular machine. In fact, MRTG can be configured to collect any statistical data that a device makes available via SNMP.

MRTG is available at http://www.mrtg.org/. It relies on a few pieces of software not included in the distribution. In particular, it requires:

• Perl 5.005 or greater
• The GD library
• The PNG library
• The zlib library

You will not need external SNMP software because MRTG comes with its own SNMP implementation. Install documentation is available from the doc/ directory in the distribution, but on a modern Linux system, MRTG will build without any special instructions.

Read more about MRTG and see graphical examples at InformIT.