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depot/third_party/nixpkgs/nixos/doc/manual/development/etc-overlay.section.md

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/etc via overlay filesystem

::: {.note} This is experimental and requires a kernel version >= 6.6 because it uses new overlay features and relies on the new mount API. :::

Instead of using a custom perl script to activate /etc, you activate it via an overlay filesystem:

{
  system.etc.overlay.enable = true;
}

Using an overlay has two benefits:

  1. it removes a dependency on perl
  2. it makes activation faster (up to a few seconds)

By default, the /etc overlay is mounted writable (i.e. there is a writable upper layer). However, you can also mount /etc immutably (i.e. read-only) by setting:

{
  system.etc.overlay.mutable = false;
}

The overlay is atomically replaced during system switch. However, files that have been modified will NOT be overwritten. This is the biggest change compared to the perl-based system.

If you manually make changes to /etc on your system and then switch to a new configuration where system.etc.overlay.mutable = false;, you will not be able to see the previously made changes in /etc anymore. However the changes are not completely gone, they are still in the upperdir of the previous overlay in /.rw-etc/upper.