The "nagios.conf" file supplied with the "nagios-www" packagte version
3.5.1-2.3 is using the Apache 2.2 syntax.
This leads to the Apache 2.4 web server not starting:
systemd[1]: Starting The Apache Webserver...
start_apache2[2053]: AH00526: Syntax error on line 15 of
/etc/apache2/conf.d/nagios.conf:
start_apache2[2053]: Invalid command 'Order', perhaps misspelled or defined by
a module not included in the server configuration
systemd[1]: apache2.service: Main process exited, code=exited, status=1/FAILURE
start_apache2[2314]: AH00526: Syntax error on line 15 of
/etc/apache2/conf.d/nagios.conf:
start_apache2[2314]: Invalid command 'Order', perhaps misspelled or defined by
a module not included in the server configuration
systemd[1]: apache2.service: Control process exited, code=exited status=1
systemd[1]: Failed to start The Apache Webserver.
systemd[1]: apache2.service: Unit entered failed state.
systemd[1]: apache2.service: Failed with result 'exit-code'.
------------------------------------------------------------------
The correct Apache 2.4 syntax is explained here:
<https://httpd.apache.org/docs/2.4/upgrading.html>
------------------------------------------------------------------
The required changes to nagios.conf are:
*** /etc/apache2/conf.d/nagios.conf 2016-06-28 05:00:12.000000000 +0200
--- ./nagios.conf 2017-07-05 14:13:23.295772497 +0200
***************
*** 12,19 ****
# SSLRequireSSL
Options ExecCGI
AllowOverride None
! Order allow,deny
! Allow from all
# Order deny,allow
# Deny from all
# Allow from 127.0.0.1
--- 12,18 ----
# SSLRequireSSL
Options ExecCGI
AllowOverride None
! Require all granted
# Order deny,allow
# Deny from all
# Allow from 127.0.0.1
***************
*** 29,36 ****
# SSLRequireSSL
Options None
AllowOverride None
! Order allow,deny
! Allow from all
# Order deny,allow
# Deny from all
# Allow from 127.0.0.1
--- 28,34 ----
# SSLRequireSSL
Options None
AllowOverride None
! Require all granted
# Order deny,allow
# Deny from all
# Allow from 127.0.0.1