Passive Icinga Checks: icinga-pusher

I use Icinga to monitor the availability of my Debian/OpenWRT/etc machines. I have relied on server-side checks on the Icinga system that monitor the externally visible operations of the services that I care about. In theory, monitoring externally visible properties should be good enough. Recently I had one strange incident that was due to an out of disk space on one system. This prompted me to revisit my thinking, and to start monitor internal factors as well. This would allow me to detect problems before they happen, such as an out of disk space condition.

Another reason that I only had server-side checks was that I didn’t like the complexity of the Icinga agent nor wanted to open up for incoming SSH connections from the Icinga server on my other servers. Complexity and machine-based authorization tend to lead to security problems so I prefer to avoid them. The manual mentions agents that use the REST API which was that start of my journey into something better.

What I would prefer is for the hosts to push their self-test results to the central Icinga server. Fortunately, there is a Icinga REST API in modern versions of Icinga (including version 2.10 that I use). The process-check-result API can be used to submit passive check results. Getting this up and running required a bit more research and creativity than I would have hoped for, so I thought it was material enough for a blog post. My main requirement was to keep complexity down, hence I ended up with a simple shell script that is run from cron. None of the existing API clients mentioned in the manual appealed to me.

Prepare the Icinga server with some configuration changes to support the push clients (replace blahonga with a fresh long random password).

icinga# cat > /etc/icinga2/conf.d/api-users.conf
object ApiUser "pusher" {
  password = "blahonga"
  permissions = [ "actions/process-check-result" ]
}
^D
icinga# icinga2 feature enable api && systemctl reload icinga2

Then add some Service definitions and assign it to some hosts, to /etc/icinga2/conf.d/services.conf:

apply Service "passive-disk" {
  import "generic-service"
  check_command = "passive"
  check_interval = 2h
  assign where host.vars.os == "Debian"
}
apply Service "passive-apt" {
  import "generic-service"
  check_command = "passive"
  check_interval = 2h
  assign where host.vars.os == "Debian"
 }

I’m using a relaxed check interval of 2 hours because I will submit results from a cron job that is run every hour. The next step is to setup the machines to submit the results. Create a /etc/cron.d/icinga-pusher with the content below. Note that % characters needs to be escaped in crontab files. I’m running this as the munin user which is a non-privileged account that exists on all of my machines, but you may want to modify this. The check_disk command comes from the monitoring-plugins-basic Debian package, which includes other useful plugins like check_apt that I recommend.

30 * * * * munin /usr/local/bin/icinga-pusher `hostname -f` passive-apt /usr/lib/nagios/plugins/check_apt

40 * * * * munin /usr/local/bin/icinga-pusher `hostname -f` passive-disk "/usr/lib/nagios/plugins/check_disk -w 20\% -c 5\% -X tmpfs -X devtmpfs"

My icinga-pusher script requires a configuration file with some information about the Icinga setup. Put the following content in /etc/default/icinga-pusher (again replacing blahonga with your password):

ICINGA_PUSHER_CREDS="-u pusher:blahonga"
ICINGA_PUSHER_URL="https://icinga.yoursite.com:5665"
ICINGA_PUSHER_CA="-k"

The parameters above are used by the icinga-pusher script. The ICINGA_PUSHER_CREDS contain the api user credentials, either a simple "-u user:password" combination or it could be "--cert /etc/ssl/yourclient.crt --key /etc/ssl/yourclient.key". The ICINGA_PUSHER_URL is the base URL of your Icinga setup, for the API port which is usually 5665. The ICINGA_PUSHER_CA is "--cacert /etc/ssl/icingaca.crt" or "-k" to not use any CA verification (not recommended!).

Below is the script icinga-pusher itself. Some error handling has been removed for brevity — I have put the script in a separate “icinga-pusher” git repository which will be where I make any updates to this project in the future.

#!/bin/sh

# Copyright (C) 2019 Simon Josefsson.
# Released under the GPLv3+ license.

. /etc/default/icinga-pusher

HOST="$1"
SERVICE="$2"
CMD="$3"

OUT=$($CMD)
RC=$?

oIFS="$IFS"
IFS='|'
set -- $OUT
IFS="$oIFS"

OUTPUT="$1"
PERFORMANCE="$2"

data='{ "type": "Service", "filter": "host.name==\"'$HOST'\" && service.name==\"'$SERVICE'\"", "exit_status": '$RC', "plugin_output": "'$OUTPUT'", "performance_data": "'$PERFORMANCE'" }'

curl $ICINGA_PUSHER_CA $ICINGA_PUSHER_CREDS \
     -s -H 'Accept: application/json' -X POST \
     "$ICINGA_PUSHER_URL/v1/actions/process-check-result" \
     -d "$data"

exit 0

What do you think? Is there a simpler way of achieving what I want? Thanks for reading.

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