CentOS Stream 8
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Nagios 4 : Notification Setting2021/06/03
 
This is the Email Notification Settings section.
It is enabled on some items by default but if you change it, configure like follows.
[1]
Install and start SMTP Server, refer to here.
[2] Set notification recipients. And aldo Nagios sends notifications with [mail] command, so install it.
[root@dlp ~]#
dnf -y install mailx
[root@dlp ~]#
vi /etc/nagios/objects/contacts.cfg
# line 32 : set recipient email address

email
root@localhost
[root@dlp ~]#
systemctl restart nagios

[3] It's possible to change notification setting on Nagios admin site.
Login to it and click [Services] link, then services list is displayed. The icons which are displayed near a service name are just the one which notification is disabled.
(HTTP and SSH corresponds to the example below)
To enable notification on a service, click service name first.
[4] Click [Enable notifications for this service].
[5] Click [Commit] button.
[6] That's OK all.
[7] If notification is enabled and a service has some troubles, the notification like follows are sent to the recipients you set. 
From nagios@dlp.srv.world  Wed Jun  2 22:51:29 2021
Return-Path: <nagios@dlp.srv.world>
X-Original-To: root@localhost
Delivered-To: root@localhost
Date: Wed, 02 Jun 2021 22:51:29 -0500
To: root@localhost
Subject: ** PROBLEM Service Alert: localhost/SSH is CRITICAL **
User-Agent: Heirloom mailx 12.5 7/5/10
Content-Type: text/plain; charset=us-ascii
From: nagios@dlp.srv.world
Status: R

***** Nagios *****

Notification Type: PROBLEM

Service: SSH
Host: localhost
Address: 127.0.0.1
State: CRITICAL

Date/Time: Wed Jun 2 22:51:29 CDT 2021

Additional Info:

connect to address 127.0.0.1 and port 22: Connection refused
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