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Nagios: Addition and Removal of Machines Without the Playbooks

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Will Parker edited this page Feb 1, 2021 · 2 revisions

Manual Setup for Nagios Clients

If, for whatever reason, a server needs to be monitored, but can't be setup via the Nagios_Ansible_Config_Tool, the following steps can be done manually.

Client Configuration:

The process to configure the client machines is fairly simple. However, the implementation will vary slightly by OS and architecture.

Key requirements:

  • Local user named ‘nagios’
  • SSH public key authentication (authorized_keys)
  • Nagios plugins

Setup Nagios User:

This will vary depending on the OS, but the following rules need to be satisfied:

  • The user must be called 'nagios'
  • The user must have permissions to access /usr/local/nagios/libexec/
  • The user must have a ~/.ssh/authorised_keys file defined, containing the Nagios Server's Nagios user's id_rsa.pub

The ssh key can be copied to the Nagios Server via: ssh-copy-id -i ~/.ssh/id_rsa.pub [email protected]

Install Nagios-Plugins:

Most distributions are able to install the nagios-plugins from their package manager.

i.e. On Ubuntu 20.04 / Linux Mint 20

sudo apt install nagios-plugins

If this isn't possible on your distribution, the common Nagios plugins can be built by following this guide.

Once this has been done, ensure that the plugins can be accessed at /usr/local/nagios/libexec - if this is not the case, symlink the folder to that location:

ln -s /usr/lib/nagios/plugins /usr/local/nagios/libexec

Server Definition:

With the client setup, the Nagios server needs to be told to monitor the client, and what services to monitor. The template for what we typically monitor a client for can be found here, with additional service definitions found in here. An overview of the checks we use can be found in the overview page. Copy and paste the templates / service checks required into a file at /usr/local/nagios/etc/servers/*HOSTNAME*.cfg, on the Nagios Server.

Different distributions will require different checks, depending on their package manager, and the method in which they sync time. For distributions that use systemd.timesyncd, use the check_timesync.cfg template. For distributions using NTP, use the check_ntp_timesync.cfg template.

There are several items that will need to be changed in these template files, to make the server definition specific to the machine. Here's a table of the values that need to be changed:

Template String Changed to Example
ReplaceHostName The machines name as it appears in the inventory.yml build-spearhead-freebsd12-x64-1
ReplaceAliasDescription A description of the machine Add by Ansible
ReplaceIPAddress IP Address of the Machine 185.131.222.224

Note: The Add by Ansible message is set when the machine has automatically been added via Ansible. When manually setting up machines, the description may be more important. i.e: ci.adoptopenjdk.net's message is AdoptOpenJDK - Jenkins server.

When adding a new machine to be monitored, it's important to add it to the correct hostgroup in the /usr/local/nagios/etc/objects/hostgroups.cfg file. If the machine is part of an already defined hostgroup (i.e. spearhead , ibmcloud, marist, etc), then all that's required is to add the hostname to the group's members field. If the machine is a new hostgroup / from a new provider, the hostgroup needs to be created, with the following template:

define hostgroup{
        hostgroup_name  <Provider_Name>
        alias           <Provider_Name>
        members         ,<Newly_Added_Machine_Hostname>
        }

After the server definition has been made and host groups updated, syntax and the rest of the nagios config files can be checked by running /usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg. If no errors have occurred, the service can be restarted by running sudo service nagios restart and the machine should be viewable at nagios.adoptopenjdk.net

Note: More information about additional services such as check_http, the package manager services, and passwd expiry can be found in the Nagios: Monitoring Additional Services page.

Clients with ICMP Disabled:

If a client has ICMP disabled and they are 'unpingable', the ping service will have to be removed from the server definition.

Additional Steps for Firewalled or NAT'd clients:

There are additional steps that are required to complete the client setup for systems where the Nagios server doesn't have direct access to them. (NAT'ed, Firewalled, etc) The following steps will configure a Reverse SSH Tunnel from the client system to the Nagios server allowing it to monitor the client across the tunnel. These steps assume the Client Configuration section has been completed.

All of these commands should be done as the Nagios user, on the respective machines

  • Copy the Nagios_RemoteTunnel.sh script to the client machine at ~/Nagios_RemoteTunnel.sh.

  • Ensure the script is always running

crontab -e
* * * * * ~/Nagios_RemoteTunnel.sh

Nagios Server Side:

  • Test the Reverse SSH Tunnel connection
# Change *PORTNUMBER* to the remote port number set in the script
/usr/local/nagios/libexec/check_by_ssh -H 127.0.0.1 -p *PORTNUMBER* -n lh -s c1:c2:c3 -C uptime -C uptime -C uptime

  • Manually update the server definition file at /usr/local/nagios/etc/servers/*HOSTNAME*.cfg , so the address field instead has 127.0.0.1 -p *PORTNUMBER*

  • Check the Nagios config and restart if no errors occur

/usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg
# If all is good restart nagios
service nagios restart

If all went well, the machine should be viewable at nagios.adoptopenjdk.net

Removing Machines from Nagios:

If machines that are being monitored by Nagios are being decommissioned, they will have to be manually removed from Nagios.

Note: Machines shouldn't be removed from Nagios, until they have been removed from inventory.yml / have a PR to remove them from inventory.yml.

On the Nagios Server, as the Nagios user:

# Remove the server definition
rm /usr/local/nagios/etc/servers/*HOSTNAME*.cfg
rm: remove write-protected regular file <*HOSTNAME*.cfg>? yes

When removing a machine, the /usr/local/nagios/etc/objects/hostgroups.cfg file needs to be updated, to remove the machine. This is a case of finding the hostgroup of the machine (i.e. spearhead,ibmcloud,marist) and removing the hostname from the members field.

Once this is done, the Nagios configuration can be checked, and restarted if all is well:

# Alternatively `check_nagios` has been aliased to this command
/usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg
# If all is good restart nagios
sudo /etc/init.d/nagios restart

If you don't have access to the Nagios server to do these steps, please raise an issue with Nagios: as a prefix, in the title.

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