# nix-cache architecture

This repository configures `nix-cache` as a **binary cache server** and a **remote builder** for other hosts.

## Important design notes

- This is **not** a shared `/nix/store` setup.
- Every machine still keeps and uses its own local `/nix/store`.
- Clients prefer `http://nix-cache` for substitutes and keep `https://cache.nixos.org/` as fallback.
- Clients can offload builds to `nix-cache` through SSH (`nix.distributedBuilds`).
- Client hosts import `modules/nix-cache/client.nix` and, when remote building is enabled, `modules/nix-cache/remote-builder-client.nix`.
- The `nix-cache` host imports `modules/nix-cache/server.nix`.

## Binary cache signing key

`modules/nix-cache/client.nix` hardcodes every client's trust in one
specific public key (`cache.local-1:usoWYanY3Kpq2+kDIS2nhWoLZiRxanmdysdzqCFBHW4=`).
That means whichever host is currently playing the `nix-cache` role has to
use that *exact* keypair — not a freshly generated one — or no client will
accept substitutes from it (they'd just silently fall back to building
from source). So unlike most per-host secrets, this one can't be
self-generated on first boot; it's managed via sops-nix like every other
secret in this repo, sourced from `secrets/nix-cache.yaml`'s
`cache-priv-key` entry (`modules/nix-cache/server.nix`).

**Adding or rotating the value:**

```bash
nix-shell -p sops --run 'sops secrets/nix-cache.yaml'
```

Add (or replace) a `cache-priv-key` entry with the private key file's exact
contents. If you don't have it yet, generate a keypair once:

```bash
nix-store --generate-binary-cache-key nix-cache-1 cache-priv.pem cache-pub.pem
```

— paste `cache-priv.pem`'s contents into the `cache-priv-key` entry above,
delete both local files afterward, and update
`trusted-public-keys` in `modules/nix-cache/client.nix` (and every already-built
client) to match `cache-pub.pem` if this is a genuine rotation rather than
a first-time bootstrap. Any `nixos-configurations.*-nix-cache` host picks
the new key up automatically on next activation — no more manual
`/etc/nix/cache-priv.pem` install step.

## Remote builder SSH keys

Each client authenticates as `nixremote` using its **own default root SSH
identity** (`/root/.ssh/id_ed25519`) — not a separately-named or shared
keypair. If a client doesn't have one yet:

```bash
sudo ssh-keygen -t ed25519 -N '' -f /root/.ssh/id_ed25519
```

Then add its `.pub` contents as a new entry in `vars.remoteBuilderAuthorizedKeys`
(`variables.nix`) and rebuild `nix-cache` to pick it up (that list is
declarative — an imperative `ssh-copy-id nixremote@nix-cache` won't stick;
it gets overwritten on every rebuild). Verify with:

```bash
sudo ssh -i /root/.ssh/id_ed25519 nixremote@nix-cache nix-store --version
```

The committed `remoteBuilderAuthorizedKeys` entries are public SSH keys
only. Keep the matching private keys on client hosts and out of the
repository.

nix-cache's own SSH *host* key is trusted declaratively via
`programs.ssh.knownHosts` in `modules/nix-cache/remote-builder-client.nix`,
sourced from `vars.nixCacheHostKey` (`variables.nix`) — every client rebuild
picks it up automatically, so distributed builds don't fail with "Host key
verification failed" on a client that has never manually SSH'd to nix-cache
before. If nix-cache's host key is ever rotated or the host rebuilt from
scratch, update `vars.nixCacheHostKey` to match its new
`/etc/ssh/ssh_host_ed25519_key.pub`.

## Manual verification

After deployment:

```bash
curl http://nix-cache/nix-cache-info
nix store ping --store http://nix-cache
nix show-config | grep -E 'substituters|trusted-public-keys|builders-use-substitutes'
sudo ssh -i /root/.ssh/id_ed25519 nixremote@nix-cache nix-store --version
nix build nixpkgs#hello --builders 'ssh://nixremote@nix-cache x86_64-linux /root/.ssh/id_ed25519 4 2 big-parallel,kvm,nixos-test,benchmark' -L
nix path-info -r nixpkgs#hello
curl -I "http://nix-cache/$(basename "$(nix path-info nixpkgs#hello)").narinfo"
```