Nagios XI - Apply Configuration Fails - Backend login to the Core Configuration failed

Problem Description

Apply Configuration fails with the following error:

Backend login to the Core Config Manager failed.

An error occurred while attempting to apply your configuration to Nagios Core.
Monitoring engine configuration files have been rolled back to their last known good checkpoint.

This problem can be caused by the Nagios XI server being unable to resolve "localhost".

Diagnosing The Problem

Open an SSH session to your Nagios XI server and execute the following commands:

su nagios
cd /usr/local/nagiosxi/scripts
./reconfigure_nagios.sh

You receive the following output:

URL: http://localhost/nagiosxi/includes/components/ccm/
CMDLINE
/usr/bin/wget --save-cookies nagiosql.cookies --keep-session-cookies http://localhost/nagiosxi/includes/components/ccm/ --no-check-certificate --post-data 'submit=Login&hidelog=true&loginSubmitted=true&username=nagiosxi&password=n@gweb' -O nagiosql.login--2016-08-03 10:10:46--  http://localhost/nagiosxi/includes/components/ccm/
Resolving localhost... failed: Name or service not known.
wget: unable to resolve host address “localhost”
NAGIOSQL LOGIN FAILED!

You can see in the last three lines that the wget command is complaining that it cannot resolve localhost.

If you execute the following command you can see the contents of the /etc/hosts file:

cat /etc/hosts

Here is the output:

127.0.0.1    localhost.localdomain    localhost.localdomain    localhost4    localhost4.localdomain4        xi-c6x-x64
::1    localhost.localdomain    localhost.localdomain    localhost6    localhost6.localdomain6        xi-c6x-x64

If you look at the output you can see that there are no "localhost" entries, there are similar entries but not one for "localhost".

Resolving The Problem

Edit your /etc/hosts file and make sure there are localhost entries. For example:

127.0.0.1    localhost.localdomain    localhost.localdomain    localhost4    localhost4.localdomain4    localhost    xi-c6x-x64
::1    localhost.localdomain    localhost.localdomain    localhost6    localhost6.localdomain6    localhost    xi-c6x-x64

After making these changes, "Apply Configuration" from Core Configuration Manager and your problem should be resolved.

NOTE:

Newer versions of Nagios XI no longer use wget, so if you still are receiving the error message after the above change, run the following commands in a root SSH session on the Nagios server to diagnose the problem:

chage -l nagios
 su - nagios
 cd /usr/local/nagiosxi/scripts
 time ./reconfigure_nagios.sh

This should show what is causing the Backend login error. See the Strict_Trans_Tables KB article for possible resolution.

Final Thoughts

For any support related questions please visit the Nagios Support Forums at:

http://support.nagios.com/forum/