nix-config-new-host

---
name: nix-config-new-host
description:
  Use when adding a NixOS, macOS, or MicroVM host in this repo, including its outputs, networking,
  secrets, and eval-test wiring.
---

# Adding a host

Copy the closest existing host and change what differs. The current inventory and naming scheme are
in [hosts/README.md](../../../hosts/README.md).

## Core rules

1. **Three names, decided up front.** The directory (`hosts/idols-ai/`), the hostname
   (`hostName = "ai"`), and the configuration name (`ai-niri`) differ. Servers use the hostname as
   the configuration name; Niri desktops append `-niri`, because `just niri` deploys
   `$(hostname)-niri`. The `hostname` eval test encodes this.
2. **Secrets come from another repository.** The new host decrypts nothing until its host key is a
   recipient in `nix-secrets`; see the `nix-config-secrets` skill.
3. **The shared policy modules are not optional.** Several eval tests assert a policy for every
   configuration, so an under-wired host fails `just test` instead of failing in production.
4. **Build before you install.** `just test`, `just eval-host <name>`, and `just build-host <name>`
   pass before anything is partitioned or flashed. `disko` destroys the target disk, and
   partitioning and installing are user-run actions on a device the user has confirmed.

## 1. Pick the template

| New host                 | Copy from                                                                                         |
| ------------------------ | ------------------------------------------------------------------------------------------------- |
| Desktop workstation      | `hosts/idols-ai/` + `outputs/x86_64-linux/src/idols-ai.nix`                                       |
| Homelab server / VM host | `hosts/12kingdoms-shoryu/` + `outputs/x86_64-linux/src/12kingdoms-shoryu.nix`                     |
| Apple Silicon Linux      | `hosts/12kingdoms-shoukei/` + `outputs/aarch64-linux/src/12kingdoms-shoukei.nix`                  |
| macOS                    | `hosts/darwin-fern/` + `outputs/aarch64-darwin/src/fern.nix` (`darwinConfigurations`, no Colmena) |
| MicroVM guest            | `hosts/k8s/k3s-test-1-worker-1/` + `outputs/x86_64-linux/src/k3s-test-1-worker-1.nix`             |

A MicroVM guest is also registered in its VM host's `microvm.nix`, and is deployed with
`just microvm-deploy`. On a VM host with the `br0` bridge it also needs a `systemd.network.networks`
unit that attaches the guest's tap to `br0`; the tap name comes from the guest IP (`192.168.5.116`
to `vm116`). Some guest outputs also expose a Colmena node for evaluation or other workflows; do not
assume the physical-host deployment is done through Colmena.

## 2. Files to create or edit

1. `hosts/<dir>/default.nix` - sets `hostName` and imports the host's modules. Some hosts (`shoryu`,
   `shushou`, `youko`, `akane`) import `mylib.scanPaths ./.`, which pulls in **every other `.nix`
   file in the directory**; keep scratch files out of those.
2. `hosts/<dir>/hardware-configuration.nix` - generated on the target machine
   (`nixos-generate-config --show-hardware-config`), never copied from another host. Where the
   layout is declarative, add `disko-fs.nix` and record the install command in the host's
   `README.md`, as `hosts/idols-ai/README.md` does.
3. `home/hosts/linux/<name>.nix` or `home/hosts/darwin/<name>.nix` - only for a host with Home
   Manager; otherwise leave `home-modules` out.
4. `outputs/<system>/src/<name>.nix` - add the output types appropriate to the host:
   `nixosConfigurations.<name>` for NixOS, `darwinConfigurations.<name>` for macOS, and a
   `packages.<name>` installer image only where the platform provides one. Add `colmena.<name>` only
   for a host deployed through Colmena; it then needs `tags`, `ssh-user`, and usually `targetHost`.
   MicroVM guests also need the VM-host `microvm.nix` registration. Keep the leading comment about
   unused `args`: haumea passes them lazily and they are still required.
5. `vars/networking.nix` - `hostsAddr.<name> = { iface; ipv4; }` for a LAN host. That entry drives
   the static address, the SSH `Host` alias used for remote builds, and `known_hosts`, so a wrong
   `iface` takes the host offline at activation. Skip it for a DHCP or mobile host. If the host
   enables `modules.networking.mihomo` and runs systemd-resolved, it needs a DNS takeover tied to
   mihomo's lifecycle (`resolvectl dns`/`revert` in `ExecStartPost`/`ExecStopPost`, see
   `hosts/idols-ai/default.nix`); a static link DNS would kill DNS when mihomo dies.
6. `hosts/README.md` - add the host to the inventory.

Pin service user and group ids (`service-user-ids.nix`, as on `shoryu`) before the host has state on
disk. A dynamically allocated id that moves on a later rebuild orphans the files it owned
(`9187e4d9 fix(youko): pin dynamically-allocated service uid/gid (#320)`).

## 3. Tests that fail until the host is wired

[outputs/README.md](../../../outputs/README.md#which-tests-cover-a-new-host) lists which eval tests
check every configuration and which list hosts by name:

- `hostname`: a new `-niri` configuration needs a `specialExpected` entry, in the test for its
  platform (`ai-niri` in x86_64-linux, `shoukei-niri` in aarch64-linux).
- `security-*`, `kernel`, `nix-system-features`: apply to every configuration automatically.
- `home-manager`, `btrbk`, and the other host-listing tests: add the host if it should be covered.

`just test` fails (non-zero) unless the suite returns `true`.

## 4. Install and deploy

- First install: boot the ISO, partition with disko, install, then deploy normally. Partitioning,
  formatting, and installing destroy the target disk, so they are user-run actions on a device the
  user confirmed: check `lsblk`/`findmnt` first, name the exact device, and get authorization for it
  before any `destroy,format,mount`. Follow
  [nixos-installer/README.md](../../../nixos-installer/README.md) and the host's own README.
- Remote hosts: `just col <tag>` or the host's own recipe, once its key is a secrets recipient.
  Deploying is a separate impactful action; use the `nix-config-update` skill's staged deployment
  (confirm the target, preview the closure, and get authorization).
- The machine you are on: `just local` or `just niri`, which prompt for `sudo`, so the user runs
  them.

## 5. Verify

- `ssh <name> true`, then `systemctl --failed` and `journalctl -b -p err` on the host.
- Secrets decrypted, checked by mode and owner only.
- The role works: the service, VM, or desktop the host exists for.

More agent context in ryan4yin/nix-config

9 other files this repository gives its agents.