Install Nagios NRPE Agents on Debian 11/Debian 10

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This guide describes how to easily install Nagios NRPE agents on Debian 11/Debian 10. If you want to monitor your Debian hosts using Nagios server, then you need to have the NRPE agents installed on these hosts. Nagios Remote Plugin Executor (NRPE) allows you to remotely execute Nagios plugins on other Linux/Unix machines to query machine metrics such as disk usage, CPU load, etc.

Add Hosts to Nagios Server for Monitoring

Before you can now start to monitor hosts using Nagios server, you need to add them hosts to Nagios Server.

Follow the guide below to add hosts to Nagios server for monitoring.

Add Hosts to Nagios Server For Monitoring

Install Nagios NRPE Agents on Debian 11/Debian 10

Nagios NRPE agents:

Install Nagios NRPE Agent From Default Debian Repos

NRPE agents are available on the default Debian repositories.

On Debian 11;

apt-cache policy nagios-nrpe-server
nagios-nrpe-server:
  Installed: (none)
  Candidate: 4.0.3-1
  Version table:
     4.0.3-1 500
        500 http://deb.debian.org/debian bullseye/main amd64 Packages

NRPE 4.0.3, which is the current stable latest release as of this writing is available for installation.

apt install nagios-nrpe-server

On Debian 10

On Debian 10, the default repos do not have the latest version of NRPE.

apt-cache policy nagios-nrpe-server
nagios-nrpe-server:
  Installed: (none)
  Candidate: 3.2.1-2
  Version table:
     3.2.1-2 500
        500 http://deb.debian.org/debian buster/main amd64 Packages

To ensure you install the latest version of Debian 10, you can install Backport repos;

echo 'deb http://deb.debian.org/debian buster-backports main' > /etc/apt/sources.list.d/backports.list

Then run command below installs NRPE agent on Debian 10.

apt update

Note that Backports repos are disabled by default and hence, you need to explicitly specify that you want to install from Backports.

apt install -t buster-backports nagios-nrpe-server

Confirm installed version once the installation completes.

nrpe -V
NRPE - Nagios Remote Plugin Executor
Version: 4.0.3

Running NRPE on Debian 11/Debian 10

Run the commands below to start and enable nrpe to run on system boot.

systemctl enable --now nagios-nrpe-server

To check the status of NRPE agent;

systemctl status nagios-nrpe-server
● nagios-nrpe-server.service - Nagios Remote Plugin Executor
     Loaded: loaded (/lib/systemd/system/nagios-nrpe-server.service; enabled; vendor preset: enabled)
     Active: active (running) since Thu 2021-11-18 13:48:57 EAT; 9min ago
       Docs: http://www.nagios.org/documentation
   Main PID: 775 (nrpe)
      Tasks: 1 (limit: 4678)
     Memory: 1012.0K
        CPU: 7ms
     CGroup: /system.slice/nagios-nrpe-server.service
             └─775 /usr/sbin/nrpe -c /etc/nagios/nrpe.cfg -f

Nov 18 13:48:57 debian11 systemd[1]: Started Nagios Remote Plugin Executor.
Nov 18 13:48:57 debian11 nrpe[775]: Starting up daemon
Nov 18 13:48:57 debian11 nrpe[775]: Server listening on 0.0.0.0 port 5666.
Nov 18 13:48:57 debian11 nrpe[775]: Server listening on :: port 5666.
Nov 18 13:48:57 debian11 nrpe[775]: Listening for connections on port 5666
Nov 18 13:48:57 debian11 nrpe[775]: Allowing connections from: 127.0.0.1,::1

Open NRPE Port on Firewall

NRPE uses port TCP 5666 by default. If firewall is running, open this port to allow external checks from Nagios Monitoring server.

ufw allow from <NAGIOS_IP> to any port 5666 proto tcp comment "Allow Connection from Nagios Server for Metrics check"

If you are using your IP tables;

apt install iptables-persistent -y
iptables -I INPUT -p tcp -s <NAGIOS_IP> --dport 5666 -j ACCEPT
iptables-save > /etc/iptables/rules.v4

Install Nagios NRPE Agent from Source Code

You can as well build Nagios NRPE agent from the source code.

To install Nagios NRPE from source code:

Install Required Build Packages

apt update
apt install -y autoconf automake gcc libc6 libmcrypt-dev make libssl-dev wget

Download NRPE Source Archive

Next, download the current release version of NRPE source code from the releases page.

wget https://github.com/NagiosEnterprises/nrpe/releases/download/nrpe-4.0.3/nrpe-4.0.3.tar.gz

Install Nagios NRPE Agent from Source Code

Extract the source archive

tar xzf nrpe-4.0.3.tar.gz

Compile the agent;

cd nrpe-4.0.3/
./configure --enable-command-args

Sample summary;

...
*** Configuration summary for nrpe 4.0.3 2020-04-28 ***:

 General Options:
 -------------------------
 NRPE port:    5666
 NRPE user:    nagios
 NRPE group:   nagios
 Nagios user:  nagios
 Nagios group: nagios


Review the options above for accuracy.  If they look okay,
type 'make all' to compile the NRPE daemon and client
or type 'make' to get a list of make options.
make all

Create NRPE nagios user and group;

make install-groups-users

Install NRPE binary files;

make install

Install NRPE configuration files;

make install-config

The configs are placed under /usr/local/nagios/etc directory as nrpe.cfg.

Update services file to define NRPE service and respective port on which it listens on;

echo "nrpe		5666/tcp			# Nagios NRPE" >> /etc/services

Install NRPE service;

make install-init

This creates a systemd unit file, /lib/systemd/system/nrpe.service.

Hence you can start and enable NRPE to run on boot;

systemctl enable --now nrpe

You can then enable the service to be accessible on firewall as shown above.

Install Nagios Plugins From Source

Download plugins from downloads page.

wget https://nagios-plugins.org/download/nagios-plugins-2.3.3.tar.gz
tar xzf nagios-plugins-2.3.3.tar.gz
cd nagios-plugins-2.3.3
./configure
make
make install

The plugins are installed under /usr/local/nagios/libexec/ directory.

How to Configure NRPE Agent

When installed from the APT repos, the default Nagios NRPE configuration is /etc/nagios/nrpe.cfg.

When installed from the source, the default configuration file is /usr/local/nagios/etc/nrpe.cfg.

The configuration files are well commented. This is how the config looks like with no comments;

grep -vE "^.*#|^$" /usr/local/nagios/etc/nrpe.cfg
log_facility=daemon debug=0
pid_file=/usr/local/nagios/var/nrpe.pid
server_port=5666
nrpe_user=nagios
nrpe_group=nagios
allowed_hosts=127.0.0.1,::1
dont_blame_nrpe=0
allow_bash_command_substitution=0
command_timeout=60
connection_timeout=300
disable_syslog=0
command[check_users]=/usr/local/nagios/libexec/check_users -w 5 -c 10
command[check_load]=/usr/local/nagios/libexec/check_load -r -w .15,.10,.05 -c .30,.25,.20
command[check_hda1]=/usr/local/nagios/libexec/check_disk -w 20% -c 10% -p /dev/hda1
command[check_zombie_procs]=/usr/local/nagios/libexec/check_procs -w 5 -c 10 -s Z
command[check_total_procs]=/usr/local/nagios/libexec/check_procs -w 150 -c 200

In most cases, the configs above would suffice to monitor the host agent metrics.

However, there are a few parameters that you might want to update:

allowed_hosts=127.0.0.1,::1

Update this parameter to include the IP address of the monitoring server. For example, in my setup, the IP address of the Nagios server is 192.168.58.22. Hence, this parameter is updated such that it looks like;

allowed_hosts=127.0.0.1,192.168.58.22

The others include the commands to check various metrics. Below are my updated commands based on host agent.

command[check_users]=/usr/local/nagios/libexec/check_users -w 5 -c 10
command[check_load]=/usr/local/nagios/libexec/check_load -r -w 15,10,5 -c 30,25,20
command[check_disk]=/usr/local/nagios/libexec/check_disk -w 20% -c 10% -p /dev/sda1
command[check_zombie_procs]=/usr/local/nagios/libexec/check_procs -w 5 -c 10 -s Z
command[check_total_procs]=/usr/local/nagios/libexec/check_procs -w 150 -c 200

The NRPE installed from the APT, the plugins path is set to /usr/lib/nagios/plugins/. The commands above would look like;

command[check_users]=/usr/lib/nagios/plugins/check_users -w 5 -c 10
command[check_load]=/usr/lib/nagios/plugins/check_load -r -w 15,10,5 -c 30,25,20
command[check_disk]=/usr/lib/nagios/plugins/check_disk -w 20% -c 10% -p /dev/sda1
command[check_zombie_procs]=/usr/lib/nagios/plugins/check_procs -w 5 -c 10 -s Z
command[check_total_procs]=/usr/lib/nagios/plugins/check_procs -w 150 -c 200

In general, this is how my configuration is like;

log_facility=daemon
debug=0
pid_file=/run/nagios/nrpe.pid
server_port=5666
nrpe_user=nagios
nrpe_group=nagios
allowed_hosts=127.0.0.1,::1
dont_blame_nrpe=0
allow_bash_command_substitution=0
command_timeout=60
connection_timeout=300
disable_syslog=0
command[check_users]=/usr/lib/nagios/plugins/check_users -w 5 -c 10
command[check_load]=/usr/lib/nagios/plugins/check_load -r -w 15,10,5 -c 30,25,20
command[check_disk]=/usr/lib/nagios/plugins/check_disk -w 20% -c 10% -p /dev/sda1
command[check_zombie_procs]=/usr/lib/nagios/plugins/check_procs -w 5 -c 10 -s Z
command[check_total_procs]=/usr/lib/nagios/plugins/check_procs -w 150 -c 200
include=/etc/nagios/nrpe_local.cfg
include_dir=/etc/nagios/nrpe.d/

Restart NRPE when done configuring it;

systemctl restart nagios-nrpe-server.service

If installed from the source, then use nrpe.service.

systemctl restart nrpe.service

Check to see if port 5666/tcp is listening;

ss -altnp | grep 5666
LISTEN    0         5                  0.0.0.0:5666             0.0.0.0:*        users:(("nrpe",pid=10345,fd=4))                                                
LISTEN    0         5                     [::]:5666                [::]:*        users:(("nrpe",pid=10345,fd=5))

If you already added hosts to Nagios and defined the services, then such is the example of how they should look on Nagios dashboard.

Hosts;

Install Nagios NRPE Agents on Debian 11/Debian 10

Services;

Install Nagios NRPE Agents on Debian 11/Debian 10

That is just it on how to install Nagios NRPE agents on Debian 11/Debian 10.

Further Reading

Nagios Core Documentation

Other Tutorials

Install Nagios NRPE Agent on Rocky Linux 8

How to Install Nagios Plugins and NRPE agents on CentOS 7/RHEL 7/Fedora 29

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