Archived
beatzaplentyandClaude Sonnet 5
53b9a64826
nix-serve's secretKeyFile was a manual, undocumented-outside-a-comment `nix-store --generate-binary-cache-key` step per host -- easy to miss on a fresh nix-cache instance (as lxc-nix-cache testing just found: systemd fails the unit with EXIT_CREDENTIALS when LoadCredential can't find the source file, which nginx then reports as a 502 from clients). It also can't be regenerated per-host safely: modules/nix-cache/client.nix hardcodes every client's trust in one specific public key, so every nix-cache instance has to share the exact same keypair. Sourced from secrets/nix-cache.yaml's new cache-priv-key entry instead, via the same sops-nix pattern every other secret in this repo already uses. Verified the added value derives to the exact public key modules/nix-cache/client.nix already trusts before committing. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01La55Nsss8jZ7ZuzUV9mfot
75 lines
3.1 KiB
Markdown
75 lines
3.1 KiB
Markdown
# nix-cache architecture
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This repository configures `nix-cache` as a **binary cache server** and a **remote builder** for other hosts.
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## Important design notes
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- This is **not** a shared `/nix/store` setup.
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- Every machine still keeps and uses its own local `/nix/store`.
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- Clients prefer `http://nix-cache` for substitutes and keep `https://cache.nixos.org/` as fallback.
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- Clients can offload builds to `nix-cache` through SSH (`nix.distributedBuilds`).
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- Client hosts import `modules/nix-cache/client.nix` and, when remote building is enabled, `modules/nix-cache/remote-builder-client.nix`.
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- The `nix-cache` host imports `modules/nix-cache/server.nix`.
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## Binary cache signing key
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`modules/nix-cache/client.nix` hardcodes every client's trust in one
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specific public key (`cache.local-1:usoWYanY3Kpq2+kDIS2nhWoLZiRxanmdysdzqCFBHW4=`).
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That means whichever host is currently playing the `nix-cache` role has to
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use that *exact* keypair — not a freshly generated one — or no client will
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accept substitutes from it (they'd just silently fall back to building
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from source). So unlike most per-host secrets, this one can't be
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self-generated on first boot; it's managed via sops-nix like every other
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secret in this repo, sourced from `secrets/nix-cache.yaml`'s
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`cache-priv-key` entry (`modules/nix-cache/server.nix`).
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**Adding or rotating the value:**
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```bash
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nix-shell -p sops --run 'sops secrets/nix-cache.yaml'
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```
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Add (or replace) a `cache-priv-key` entry with the private key file's exact
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contents. If you don't have it yet, generate a keypair once:
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```bash
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nix-store --generate-binary-cache-key nix-cache-1 cache-priv.pem cache-pub.pem
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```
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— paste `cache-priv.pem`'s contents into the `cache-priv-key` entry above,
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delete both local files afterward, and update
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`trusted-public-keys` in `modules/nix-cache/client.nix` (and every already-built
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client) to match `cache-pub.pem` if this is a genuine rotation rather than
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a first-time bootstrap. Any `nixos-configurations.*-nix-cache` host picks
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the new key up automatically on next activation — no more manual
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`/etc/nix/cache-priv.pem` install step.
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## Remote builder SSH keys
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On each client, install the private key used to authenticate as `nixremote`:
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```bash
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sudo install -d -m 0700 /root/.ssh
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sudo install -m 0600 ./nixremote /root/.ssh/nixremote
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sudo ssh -i /root/.ssh/nixremote nixremote@nix-cache nix-store --version
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```
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On `nix-cache`, install the matching public key used by `nixremote` authorized keys.
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The committed `nixremote` authorized keys are public SSH keys only. Keep the
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matching private keys on client hosts and out of the repository.
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## Manual verification
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After deployment:
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```bash
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curl http://nix-cache/nix-cache-info
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nix store ping --store http://nix-cache
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nix show-config | grep -E 'substituters|trusted-public-keys|builders-use-substitutes'
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sudo ssh -i /root/.ssh/nixremote nixremote@nix-cache nix-store --version
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nix build nixpkgs#hello --builders 'ssh://nixremote@nix-cache x86_64-linux /root/.ssh/nixremote 4 2 big-parallel,kvm,nixos-test,benchmark' -L
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nix path-info -r nixpkgs#hello
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curl -I "http://nix-cache/$(basename "$(nix path-info nixpkgs#hello)").narinfo"
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```
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