Apache is probably not executing php files properly. Make sure the apache modules are enabled. Screenshots of error: Firefox (something probably cached from before)

Chrome fresh navigation to page Run:

a2enmod php7.3

List of modules that should be enabled

# apachectl -M

Loaded Modules:
 core_module (static)
 so_module (static)
 watchdog_module (static)
 http_module (static)
 log_config_module (static)
 logio_module (static)
 version_module (static)
 unixd_module (static)
 access_compat_module (shared)
 alias_module (shared)
 auth_basic_module (shared)
 authn_core_module (shared)
 authn_file_module (shared)
 authz_core_module (shared)
 authz_host_module (shared)
 authz_user_module (shared)
 autoindex_module (shared)
 cgi_module (shared)
 cgid_module (shared)
 deflate_module (shared)
 dir_module (shared)
 env_module (shared)
 filter_module (shared)
 mime_module (shared)
 mpm_prefork_module (shared)
 negotiation_module (shared)
 php7_module (shared)
 reqtimeout_module (shared)
 rewrite_module (shared)
 setenvif_module (shared)
 status_module (shared)

apt install libapache2-mod-php7.4

apt install php8.2-fpm libapache2-mod-php8.2 &&
a2enmod php8.2 proxy_fcgi setenvif &&
a2enconf php8.2-fpm

• Debian 10 (after upgrade from Debian 9)
• Debian 12 (after upgrade from Debian 10 → 11)

When accessing graphs in url nagiosdomain.com/nagiosgraph/cgi-bin/showhost.cgi?host=localhost you get an error

JavaScript is disabled.
nagiosgraph.js is not installed or wrong version.

The issue could be in rrd filename (/usr/local/nagiosgraph/var/rrd) encoding the '“' character as ”%22“ which confuses javascript. Example of filename

Check%20www.karl.com___%7C%20isAlreadySelected%28panel%2C%20selectedValue%29%3B%5Cn%20%20%7D%22%20on%20https%3A%2F%2Fwww.karl.com%2Fde%2Fmantel_cod59141927re.html%20%7CRAUVX.rrd

Simply remove the offending file and reload nagios.

• nagios 4.2.2
• nagiosgraph 1.5.2
• Debian GNU/Linux 11 (bullseye)

Make sure you are using an up to date nrpe plugins.

• Debian 10 (after upgrade from Debian 9)

Add to check_if_traffic -g2 flag:

check_if_traffic!22!localhost!public!eth0!0,50000!0,75000!-Y -B -k -g2 --label

For example if changing whether or not notifications will be sent, nagios by default remembers state even after restart, so for notifications you will need to change it manually in /usr/local/nagios/var/retention.dat.

You could also remove this file (but first stop nagios and make a backup!) which will force reread of all configs and schedule the check for all hosts and services, so make sure you want this.

Happened when trying to make install nrpe version 3.2.0 on Debian 12. Used version nrpe 4.1.0.and problem went away.

Check the pid in nrpe.cfg and the nrpe.service file, The paths should match there.

~# grep pid /usr/local/nagios/etc/nrpe.cfg
pid_file=/var/run/nrpe.pid

systemctl cat nrpe
# /lib/systemd/system/nrpe.service
[Unit]
Description=Nagios Remote Plugin Executor
Documentation=http://www.nagios.org/documentation
After=var-run.mount nss-lookup.target network.target local-fs.target time-sync.target
Before=getty@tty1.service plymouth-quit.service xdm.service
Conflicts=nrpe.socket

[Install]
WantedBy=multi-user.target

[Service]
Type=simple
Restart=on-abort
PIDFile=/usr/local/nagios/var/nrpe.pid <<--- THIS

Here the fix was to update the nrpe.cfg with the path from systemd file.

• Icinga troubleshooting

• https://www.techrepublic.com/article/how-to-fix-apache-2-not-executing-php-files/
• https://support.nagios.com/forum/viewtopic.php?f=7&t=40351&p=205190#p205190
• https://serverfault.com/questions/805616/cannot-get-interface-speed-with-standard-mib-use-highperf-mib-g
• https://nagios.force.com/support/s/article/Understanding-retention-dat-and-modified-attributes-b41690d9?