Configuration Directories and Precedence

The default configuration is set during compilation, so configuration is only needed when it is necessary to deviate from those defaults. The main configuration file is either in /usr/lib/systemd/ or /etc/systemd/ and contains commented out entries showing the defaults as a guide to the administrator. Local overrides can be created by creating drop-ins, as described below. The main configuration file can also be edited for this purpose (or a copy in /etc/ if it's shipped in /usr/) however using drop-ins for local configuration is recommended over modifications to the main configuration file.

In addition to the "main" configuration file, drop-in configuration snippets are read from /usr/lib/systemd/*.conf.d/, /usr/local/lib/systemd/*.conf.d/, and /etc/systemd/*.conf.d/. Those drop-ins have higher precedence and override the main configuration file. Files in the *.conf.d/ configuration subdirectories are sorted by their filename in lexicographic order, regardless of in which of the subdirectories they reside. When multiple files specify the same option, for options which accept just a single value, the entry in the file sorted last takes precedence, and for options which accept a list of values, entries are collected as they occur in the sorted files.

When packages need to customize the configuration, they can install drop-ins under /usr/. Files in /etc/ are reserved for the local administrator, who may use this logic to override the configuration files installed by vendor packages. Drop-ins have to be used to override package drop-ins, since the main configuration file has lower precedence. It is recommended to prefix all filenames in those subdirectories with a two-digit number and a dash, to simplify the ordering of the files. This also defined a concept of drop-in priority to allow distributions to ship drop-ins within a specific range lower than the range used by users. This should lower the risk of package drop-ins overriding accidentally drop-ins defined by users.

To disable a configuration file supplied by the vendor, the recommended way is to place a symlink to /dev/null in the configuration directory in /etc/, with the same filename as the vendor configuration file.