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*.d configuration folders

Most system services and applications rely on configuration files for their settings and many packages come with pre-populated files (usually *.conf) with all defaults that can be edited and tailored based on your needs.

Editing the main .conf file can introduce challenges including:

  • The main configuration file could be overwritten during updates/upgrades
  • The main configuration file can change significantly from the default, making it difficult to read and interpret

*.d folders come to the rescue for both scenarios.

Many applications include specific folders, with ".d" in their name (i.e. conf.d, conf.modules.d) that contain:

  • Configuration snippets -> Chunks of the main config file with only specific overrides
  • Modular configurations -> Dedicated configuration files for specific functionalities
  • How to use them -> Syntax, flags to be enabled/disabled in the application to load files in the directory, etc.

It is a good practice to use them to immediately benefit from:

  • Better readability
  • Modular approach
  • Persistence across application or OS updates/upgrades

Example - httpd

Let's take httpd as an example.

By quickly inspecting its main folder, /etc/httpd/conf/, we can see that it has three configuration folders:

ls /etc/httpd/conf*
/etc/httpd/conf:
httpd.conf  magic

/etc/httpd/conf.d:
autoindex.conf  mod_dnssd.conf  README  userdir.conf  welcome.conf

/etc/httpd/conf.modules.d:
00-base.conf    00-dav.conf  00-mpm.conf       00-proxy.conf    01-cgi.conf  10-mod_dnssd.conf  README
00-brotli.conf  00-lua.conf  00-optional.conf  00-systemd.conf  10-h2.conf   10-proxy_h2.conf

Its main configuration file, httpd.conf stored in /etc/httpd/conf/ folder contains all the default configuration settings for a standard webserver to start and be operative.

Extract from httpd.conf file 
#
# This is the main Apache HTTP server configuration file.  It contains the
# configuration directives that give the server its instructions.
# See <URL:http://httpd.apache.org/docs/2.4/> for detailed information.
# In particular, see
# <URL:http://httpd.apache.org/docs/2.4/mod/directives.html>
# for a discussion of each configuration directive.
#
# See the httpd.conf(5) man page for more information on this configuration,
# and httpd.service(8) on using and configuring the httpd service.
#
# Do NOT simply read the instructions in here without understanding
# what they do.  They're here only as hints or reminders.  If you are unsure
# consult the online docs. You have been warned.
#
# Configuration and logfile names: If the filenames you specify for many
# of the server's control files begin with "/" (or "drive:/" for Win32), the
# server will use that explicit path.  If the filenames do *not* begin
# with "/", the value of ServerRoot is prepended -- so 'log/access_log'
# with ServerRoot set to '/www' will be interpreted by the
# server as '/www/log/access_log', where as '/log/access_log' will be
# interpreted as '/log/access_log'.

#
# ServerRoot: The top of the directory tree under which the server's
# configuration, error, and log files are kept.
#
# Do not add a slash at the end of the directory path.  If you point
# ServerRoot at a non-local disk, be sure to specify a local disk on the
# Mutex directive, if file-based mutexes are used.  If you wish to share the
# same ServerRoot for multiple httpd daemons, you will need to change at
# least PidFile.
#
ServerRoot "/etc/httpd"

#
# Listen: Allows you to bind Apache to specific IP addresses and/or
# ports, instead of the default. See also the <VirtualHost>
# directive.
#
# Change this to Listen on a specific IP address, but note that if
# httpd.service is enabled to run at boot time, the address may not be
# available when the service starts.  See the httpd.service(8) man
# page for more information.
#
#Listen 12.34.56.78:80
Listen 80
[...]
# Dynamic Shared Object (DSO) Support
#
# To be able to use the functionality of a module which was built as a DSO you
# have to place corresponding `LoadModule' lines at this location so the
# directives contained in it are actually available _before_ they are used.
# Statically compiled modules (those listed by `httpd -l') do not need
# to be loaded here.
#
# Example:
# LoadModule foo_module modules/mod_foo.so
#
Include conf.modules.d/*.conf
[...]
# Supplemental configuration
#
# Load config files in the "/etc/httpd/conf.d" directory, if any.
IncludeOptional conf.d/*.conf

By carefully reviewing the httpd.conf file, we can see that there are two specific directives, "Include conf.modules.d/*.conf" and "IncludeOptional conf.d/*.conf" that are looking for .conf files in the .d folders.

Each of these files contain specific configuration, either for the application or modules (i.e. SSL/TLS), and it is an accurate example of how configuration files can be well organized using .d folders.

Adding a custom configuration

An admin could edit the webserver configuration, to create a virtual host for example.net. To do so, it is enough to create a new file, in the conf.d folder, named example-net.conf with the following content:

Example custom configuration - example-net.conf 
# This is example config file for virtual host "example.net"

# Redirect only vhost. Use permanent url for same resource.
#<VirtualHost *:80>
#   ServerAlias example.net
#   RedirectPermanent / http://www.example.net
#</VirtualHost>

<VirtualHost *:80>
    ServerName www.example.net
    ServerAdmin webmaster@example.net
    DocumentRoot /home/services/httpd/vhosts/example.net
    ErrorLog logs/example.net-error_log
    TransferLog logs/example.net-access_log
</VirtualHost>

#<Directory "/home/services/httpd/vhosts/example.net">
#  AllowOverride None
#  Options None
#  <IfModule mod_authz_host.c>
#    Require all granted
#  </IfModule>
#</Directory>

Any change made to the conf.d and conf.modules.d can be made effective by reloading the httpd service, so to apply the new virtual host:

systemctl restart httpd