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+# fakeNss {#sec-fakeNss}
+
+Provides `/etc/passwd` and `/etc/group` files that contain `root` and `nobody`, allowing user/group lookups to work in binaries that insist on doing those.
+This might be a better choice than a custom script running `useradd` and related utilities if you only need those files to exist with some entries.
+
+`fakeNss` also provides `/etc/nsswitch.conf`, configuring NSS host resolution to first check `/etc/hosts` before checking DNS, since the default in the absence of a config file (`dns [!UNAVAIL=return] files`) is quite unexpected.
+
+It also creates an empty directory at `/var/empty` because it uses that as the home directory for the `root` and `nobody` users.
+The `/var/empty` directory can also be used as a `chroot` target to prevent file access in processes that do not need to access files, if your container runs such processes.
+
+The user entries created by `fakeNss` use the `/bin/sh` shell, which is not provided by `fakeNss` because in most cases it won't be used.
+If you need that to be available, see [`dockerTools.binSh`](#sssec-pkgs-dockerTools-helpers-binSh) or provide your own.
+
+## Inputs {#sec-fakeNss-inputs}
+
+`fakeNss` is made available in Nixpkgs as a package rather than a function, but it has two attributes that can be overridden and might be useful in particular cases.
+For more details on how overriding works, see [](#ex-fakeNss-overriding) and [](#sec-pkg-override).
+
+`extraPasswdLines` (List of Strings; _optional_)
+
+: A list of lines that will be added to `/etc/passwd`.
+  Useful if extra users need to exist in the output of `fakeNss`.
+  If `extraPasswdLines` is specified, it will **not** override the `root` and `nobody` entries created by `fakeNss`.
+  Those entries will always exist.
+
+  Lines specified here must follow the format in {manpage}`passwd(5)`.
+
+  _Default value:_ `[]`.
+
+`extraGroupLines` (List of Strings; _optional_)
+
+: A list of lines that will be added to `/etc/group`.
+  Useful if extra groups need to exist in the output of `fakeNss`.
+  If `extraGroupLines` is specified, it will **not** override the `root` and `nobody` entries created by `fakeNss`.
+  Those entries will always exist.
+
+  Lines specified here must follow the format in {manpage}`group(5)`.
+
+  _Default value:_ `[]`.
+
+## Examples {#sec-fakeNss-examples}
+
+:::{.example #ex-fakeNss-dockerTools-buildImage}
+# Using `fakeNss` with `dockerTools.buildImage`
+
+This example shows how to use `fakeNss` as-is.
+It is useful with functions in `dockerTools` to allow building Docker images that have the `/etc/passwd` and `/etc/group` files.
+This example includes the `hello` binary in the image so it can do something besides just have the extra files.
+
+```nix
+{ dockerTools, fakeNss, hello }:
+dockerTools.buildImage {
+  name = "image-with-passwd";
+  tag = "latest";
+
+  copyToRoot = [ fakeNss hello ];
+
+  config = {
+    Cmd = [ "/bin/hello" ];
+  };
+}
+```
+:::
+
+:::{.example #ex-fakeNss-overriding}
+# Using `fakeNss` with an override to add extra lines
+
+The following code uses `override` to add extra lines to `/etc/passwd` and `/etc/group` to create another user and group entry.
+
+```nix
+{ fakeNss }:
+fakeNss.override {
+  extraPasswdLines = ["newuser:x:9001:9001:new user:/var/empty:/bin/sh"];
+  extraGroupLines = ["newuser:x:9001:"];
+}
+```
+:::