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Nov 15 2009

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system monitoring with munin

A while back a friend and colleague of mine introduced me to the server monitoring tool called munin which he had installed on one of the servers he maintains. It looked interesting enough for me to stick it on my “to do” list for my VPSs. Having a bunch of relevant stats presented in graphical form all in one place would be useful. So this weekend I decided to install it on both my mail and web VPS and my tor node.

Munin can be installed in a master/slave configuration where one server acts as the main monitoring station and periodically polls the others for updated stats. This is the setup I chose, and now this server (my web and mail host) acts as the master and my tor node is a slave. Each server in the cluster must be set to run the munin-node monitor (which listens by default on port 4949) to allow munin itself to connect and gather stats for display. The configuration file allows you to restrict connections to specific IP addresses. On the main node I limit this to local loopback whilst on the tor node I allow the master to connect in addition to local loopback. And just to be on the safe side, I reinforced this policy in my iptables rules.

The graphs are drawn using RRDtool, which can be a little heavy on CPU usage, certainly too heavy for the slugs which ruled out my installing the master locally rather than on one of the VPSs. But the impact on my bytemark host looks perfectly acceptable so far.

One of the neatest things about munin is its open architecture. Statistics are all collected via a series of plugins. These plugins can be written in practically any scripting language you care to name. In the plugins which came by default with the standard debian install of munin I found plugins mostly written as shell scripts with the occasional perl script. However, a couple of the additional scripts I installed were written in php and python. The standard set of plugins covers most of what you would expect to monitor on a linux server (cpu, memory i/o, process stats, mail traffic etc). but there were two omissions which were quite important to me. One was for lighttpd, the other for tor. I found suitable candidates on-line pretty quickly though. The tor monitor plugin can be found on the munin exchange site (a repository of third party plugins). I couldn’t find a lighttpd plugin there but eventually picked one up from here (thomas is clearly not a perl fan).

Most plugins (at least those supplied by default in the the debian package) “just work”, but some do need a little extra customisation. For example the “ip_ ” plugin (which monitors network traffic on specified IP addresses) gets its stats from iptables and assumes that you have an entry of the form:

-A INPUT -d 192.168.1.1
-A OUTPUT -s 192.168.1.1

at the top of your iptables config file. You also need to ensure that the “ip_” plugin is correctly named with the suffix formed of the IP address to be monitored (e.g. “ip_” becomes “ip_192.168.1.1″). The simplest way to do this (and certainly the best way if you wish to monitor multiple addresses) is to ensure that the symlink from “/etc/munin/plugins/ip_” to “/usr/share/munin/plugins/ip_” is named correctly. Thus (in directory /etc/munin/plugins):

ln -s /usr/share/munin/plugins/ip_ ip_192.168.1.1

The lighttpd plugin I found also needs a little bit of work before you can see any useful stats. The plugin connects to lighty’s “server status” URL to gather its information. So you need to ensure that you have loaded the mod_status module in your lighty config file and that you have specified the URL correctly (any name will do, it just has to be consistent in both the lighty config and the plugin). It is also worth restricting access to the URL to local loopback if you are not going to access the stats directly from a browser from elsewhere. This sort of entry in your config file should do:

server.modules += ( “mod_status” )

$HTTP["remoteip"] == “127.0.0.1″ {
status.status-url = “/server-status”
}

The tor plugin connects to the tor control port (9051 by default) but this port is normally not configured because it poses a security risk if configured incorrectly. Unless you also specify one of “HashedControlPassword” or “CookieAuthentication”, in the tor config file, then setting this option will cause tor to allow any process on the local host to control it. This is a “bad thing” (TM). If you choose to use the tor plugin, then you should ensure that access to the control port is locked down. The tor plugin assumes that you will use “CookieAuthentication”, but the path to the cookie is set incorrectly for the standard debian install (which sets the tor data directory to /var/lib/tor rather than the standard /etc/tor).

So far it all looks good, but I may add further plugins (or remove less useful ones) as I experiment with munin over the next few weeks.

Permanent link to this article: http://baldric.net/2009/11/15/system-monitoring-with-munin/