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- [Channels](command-ref/files/channels.md) - [Channels](command-ref/files/channels.md)
- [Default Nix expression](command-ref/files/default-nix-expression.md) - [Default Nix expression](command-ref/files/default-nix-expression.md)
- [Architecture](architecture/architecture.md) - [Architecture](architecture/architecture.md)
- [Protocols](protocols/protocols.md)
- [Serving Tarball Flakes](protocols/tarball-fetcher.md)
- [Glossary](glossary.md) - [Glossary](glossary.md)
- [Contributing](contributing/contributing.md) - [Contributing](contributing/contributing.md)
- [Hacking](contributing/hacking.md) - [Hacking](contributing/hacking.md)

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# Protocols
This chapter documents various developer-facing interfaces provided by
Nix.

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# Serving Tarball Flakes
Tarball flakes are served as regular tarballs via HTTP or the file
system (for `file://` URLs).
An HTTP server can return an "immutable" flakeref appropriate for lock
files. This allows users to specify a tarball flake input in
`flake.nix` that requests the latest version of a flake
(e.g. `https://example.org/hello/latest.tar.gz`), while `flake.lock`
will record a URL whose contents will not change
(e.g. `https://example.org/hello/<revision>.tar.gz`). To do so, the
server must return a `Link` header with the `rel` attribute set to
`immutable`, as follows:
```
Link: <flakeref>; rel="immutable"
```
(Note the required `<` and `>` characters around *flakeref*.)
*flakeref* must be a tarball flakeref. It can contain flake attributes
such as `narHash`, `rev` and `revCount`. If `narHash` is included, its
value must be the NAR hash of the unpacked tarball (as computed via
`nix hash path`). Nix checks the contents of the returned tarball
against the `narHash` attribute. The `rev` and `revCount` attributes
are useful when the tarball flake is a mirror of a fetcher type that
has those attributes, such as Git or GitHub. They are not checked by
Nix.
```
Link: <https://example.org/hello/442793d9ec0584f6a6e82fa253850c8085bb150a.tar.gz
?rev=442793d9ec0584f6a6e82fa253850c8085bb150a
&revCount=835
&narHash=sha256-GUm8Uh/U74zFCwkvt9Mri4DSM%2BmHj3tYhXUkYpiv31M%3D>; rel="immutable"
```
(The linebreaks in this example are for clarity and must not be included in the actual response.)
For tarball flakes, the value of the `lastModified` flake attribute is
defined as the timestamp of the newest file inside the tarball.