nix-config-debug · diff

git:20261010.3b63028 to git:20261010.9d69832

33 added, 21 removed. Audit A to A.

---

name: nix-config-debug

description: >-

Use when something here is broken or stops working: an eval or build error, a failed activation, a

dead or restarting unit, a mihomo or DNS outage, an unreachable host or MicroVM guest, or a

failing CI action.

---

# Debugging this repository

- Follow the global `systematic-debugging` skill for the process (root cause before any fix). This

- skill is only the map: which layer failed, and where to look. Rolling back, isolating a bad input

- bump, and the commands that destroy rollback points are in the `nix-config-update` skill.

+ Follow the global `diagnosing-bugs` skill for the process (a reproducible signal and root cause

+ before any fix). This skill is only the map: which layer failed, and where to look. Rolling back,

+ isolating a bad input bump, and the commands that destroy rollback points are in the

+ `nix-config-update` skill.

## Core rules

1. **Restore service first.** A down host or a dead session gets the previous generation before any

investigation. Rolling back the machine you are on needs `sudo`, so the user does it.

2. **Name the layer before touching code.** Eval, build, activation, and runtime failures have

different causes; a green `just test` says nothing about activation.

- 3. **Never get to green by weakening a check**: no disabled test, dropped assertion, or `mkForce`

- over the failing value.

- 4. **Preserve existing user changes.** Do not discard or stash them without authorization. If a

- clean baseline is needed, record the current diff and ask before isolating it.

## 1. Localize by layer

- | Layer | Symptom | Look with |

- | ------------- | ---------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |

- | eval | `just test` fails, an eval error | `just eval-host <host>` (already passes `--show-trace`) |

- | build | build error, hash mismatch, "marked as broken" | `just build-host <host>`, then `nix log <drv>` |

- | activation | the deploy fails after building | the deploy output; `journalctl -u home-manager-$USER -b` for Home Manager |

- | runtime | a unit is failed or restarting | `just list-failed`, `systemctl status <unit>`, `journalctl -u <unit> -b` |

- | proxy/DNS | proxied sites time out, or DNS still resolves after mihomo stops | `dig +short <site>` must return a fake-ip (`198.18.x.x`); a real IP means the resolver bypasses `dns-hijack`; on `ai` the `resolvectl dns`/`revert` hook in `hosts/idols-ai/default.nix` ties link DNS to mihomo's lifecycle and other mihomo hosts have no such hook, so read `resolvectl status` before concluding; a statically set link DNS would outlive a dead mihomo. Config and gotchas: [mihomo README](../../../modules/nixos/desktop/networking/mihomo/README.md); regenerate with `just mihomo-gen`, then restart `mihomo.service` |

- | boot | errors at boot, wrong kernel | `journalctl -b -p err`; `just history` for what is booted |

- | session | desktop or app misbehaves | `journalctl --user -b -p err`; the `nix-config-desktop` skill |

- | remote host | anything on a Colmena host | `ssh root@<host> journalctl -b -p err`, same commands over SSH |

- | MicroVM guest | guest down or unreachable | on the VM host: `systemctl status microvm@<guest>` and `microvm-tap-interfaces@<guest>`; then `br0` |

- | secrets | missing or unreadable `/run/agenix/<name>` | the `nix-config-secrets` skill |

- | shell | nushell exits 1 with `File not found` | whether `~/.secrets/` exists on the host; the `nix-config-secrets` skill |

- | CI | a GitHub Action keeps failing | `gh run list --workflow <wf>`, `gh run view <run> --log-failed` |

+ Each layer: symptom, then where to look.

+ - **eval** (`just test` fails, an eval error): `just eval-host <host>` (already passes

+ `--show-trace`)

+ - **build** (build error, hash mismatch, "marked as broken"): `just build-host <host>`, then

+ `nix log <drv>`

+ - **activation** (the deploy fails after building): the deploy output;

+ `journalctl -u home-manager-$USER -b` for Home Manager

+ - **runtime** (a unit is failed or restarting): `just list-failed`, `systemctl status <unit>`,

+ `journalctl -u <unit> -b`

+ - **proxy/DNS** (proxied sites time out, or DNS still resolves after mihomo stops):

+ `dig +short <site>` must return a fake-ip (`198.18.x.x`); a real IP means the resolver bypasses

+ `dns-hijack`; on `ai` the `resolvectl dns`/`revert` hook in `hosts/idols-ai/default.nix` ties link

+ DNS to mihomo's lifecycle and other mihomo hosts have no such hook, so read `resolvectl status`

+ before concluding; a statically set link DNS would outlive a dead mihomo. Config and gotchas:

+ [mihomo README](../../../modules/nixos/desktop/networking/mihomo/README.md); regenerate with

+ `just mihomo-gen`, then restart `mihomo.service`

+ - **boot** (errors at boot, wrong kernel): `journalctl -b -p err`; `just history` for what is booted

+ - **session** (desktop or app misbehaves): `journalctl --user -b -p err`; the `nix-config-desktop`

+ skill

+ - **remote host** (anything on a Colmena host): `ssh root@<host> journalctl -b -p err`, same

+ commands over SSH

+ - **MicroVM guest** (guest down or unreachable): on the VM host: `systemctl status microvm@<guest>`

+ and `microvm-tap-interfaces@<guest>`; then `br0`

+ - **secrets** (missing or unreadable `/run/agenix/<name>`): the `nix-config-secrets` skill

+ - **shell** (nushell exits 1 with `File not found`): whether `~/.secrets/` exists on the host; the

+ `nix-config-secrets` skill

+ - **CI** (a GitHub Action keeps failing): `gh run list --workflow <wf>`,

+ `gh run view <run> --log-failed`

## 2. Read the evaluated value

When `just test` fails, re-run the suite directly to get the trace (`nix eval` exits 0 even when the

suite returns `false`, so read its output):

```bash

nix eval .#evalTests --show-trace

```

The trace names the failing test under `outputs/<system>/tests/<name>/`. Read its `expr.nix` and

`expected.nix`, and evaluate the expression to see what actually came out before changing anything.

Most eval-layer questions are "what did this option actually end up as?". Ask the configuration

directly instead of reading modules:

```bash

nix eval .#nixosConfigurations.<host>.config.<option.path>

nix eval .#nixosConfigurations.<host>.config.systemd.services \

--apply 's: builtins.filter (n: builtins.match "microvm.*" n != null) (builtins.attrNames s)'

nix repl # then `:lf .` and inspect nixosConfigurations.<host>.config

```

Renamed or removed options are the most common eval break after an update; the error or warning

names the replacement. Treat deprecation warnings as failures waiting to happen.

## 3. Your change or the update?

`git status`, `git log --oneline -5 -- flake.lock`, and `git diff flake.lock` tell you which. If the

lock moved, isolate the input as described in the `nix-config-update` skill before reading code.

Preserve unrelated working-tree changes; do not use `git checkout`, `git stash`, or cleanup commands

to manufacture a clean tree without authorization.

## 4. Prove the fix

Re-run the exact command that failed, then `just test`, then `just build-host <host>` for every

affected host before anyone deploys. For a running service, confirm the live config is the one just

generated (service activation time against the file's mtime) and restart if they differ. Report the

evidence ("the unit is active, `just test` is `true`"), not the intent.