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nixos/doc/manual/development/etc-overlay.section.md
833f386ab3fdef51ed3c04b8bd7bdf4f06c937134ccf408900000003000004b9
# `/etc` via overlay filesystem {#sec-etc-overlay}

::: {.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:

```nix
{ 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:

```nix
{ 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`.
Chunks
0f3cdfe6 (1st chunk of `nixos/doc/manual/development/etc-overlay.section.md`)
Title: /etc via Overlay Filesystem
Summary
This experimental feature, requiring Linux kernel 6.6+, allows activating the `/etc` directory using an overlay filesystem instead of a custom Perl script. This approach eliminates a Perl dependency and speeds up activation. By default, the `/etc` overlay is writable, but it can be configured as immutable (read-only) by setting `system.etc.overlay.mutable = false`. During system configuration switches, the overlay is atomically replaced, but any files manually modified within `/etc` will not be overwritten. If `system.etc.overlay.mutable` is set to `false` after manual changes, those changes will no longer be visible in `/etc` but remain preserved in the `/.rw-etc/upper` directory of the previous overlay.