Local Configuration

You can customize the system’s configuration for managed components with config files that are located in /etc/local/*.

Every component that supports customizing its configuration creates a directory writable by service users, such as /etc/local/firewall. The specific format and allowed filenames depend on the specifics of each component and are documented separately.

Changes to the files in the local configuration directory are picked up automatically upon the next run of our configuration agent (generally every 10 minutes) but you can also explicitly trigger it by running:

$ sudo fc-manage --build

This will update the machine’s system configuration, which includes copying the local configuration files into the Nix store. Your custom config is thus versioned along the general system config (in case we have to revert to an older configuration version) and is atomically loaded and activated.

To inspect the result of this call, you can check the journal:

$ journalctl --since -1h --unit fc-manage


fc-manage is our utility that updates a system’s configuration and calls the underlying NixOS commands.

The basic call to apply changed configuration is:

$ sudo fc-manage --build
# Short form
$ sudo fc-manage -b

This will pick up locally changed configuration but will not perform general OS updates or fetch new data from our configuration management database (like adding new users or IPs).

The call to perform extensive updates including potential OS updates (the “channel”) and changes from the configuration management database (CMDB, directory, “ENC”) is:

$ sudo fc-manage --directory --channel
# Short form:
$ sudo fc-manage -ec

A mixed form (no OS updates but include changes from the CMDB) is:

$ sudo fc-manage --directory --build
# Short form:
$ sudo fc-manage -eb

Custom NixOS-native configuration

You can put custom NixOS configuration (called modules) in /etc/local/nixos. See custom.nix.example for the basic structure of a NixOS module. All options offered by NixOS and our platform code can be set there.


Care must be taken to avoid breaking the system. Overriding options already set by the platform can be dangerous.

Run sudo fc-manage -b to activate the changes (may restart services!).

For more information about writing NixOS modules, refer to the NixOS manual

Look up NixOS options here (Channel: 21.05 should be selected):