backup stuff

ansible/roles/backup_store/README.md

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+# `backup_store`
+
+Pulls already-encrypted backup artefacts from every source host onto
+`small-backups-box`, on a timer, and expires them per source.
+
+Generalises the hand-written `pull-backups.sh` that had one hardcoded source
+(`arbret`). That job's behaviour is preserved exactly: same source path, same
+90 days, same destination directory.
+
+## This host holds no key
+
+Everything pulled here is ciphertext produced by `backup_source` on the source
+host. The box cannot read any of it — the age identity lives only on lapy. That
+is deliberate: the machine holding every backup should not also be able to open
+them.
+
+## One failing source must not stop the others
+
+The script is `set -uo pipefail`, **not** `-e`. Each source runs in its own
+function, failures are counted, and the script exits non-zero at the end so
+systemd marks the unit failed. A dead host costs you that one source, not the
+whole run.
+
+This is the specific failure the whole plan exists to prevent: the laptop jobs
+aborted on first error and then silently produced empty directories for nine
+months.
+
+## Trust points one way
+
+The box authenticates with `~/.ssh/id_pull` to an unprivileged, dedicated
+account on each source (`backup-pull`, or `arbret` on prd-arbret), authorised
+with `restrict`. That account can read one directory and do nothing else — no
+sudo, no pty, no forwarding. A compromised backup box cannot reach into
+production.
+
+## Addressing: names, never IPs
+
+Sources are addressed by name. The job this replaced hardcoded spacey's IP; the
+droplet was later rebuilt, the address was recycled to a stranger, and the
+backup failed silently from 2025-12-01 while the directory listing still looked
+healthy.
+
+Two kinds of name are in play:
+
+- **Tailnet members** (vipy, memos-box, …) → MagicDNS names. These require a
+  headscale ACL grant from `tag:small-backups-box` to the source's `:22`; without
+  it the box cannot even resolve the peer, let alone reach it.
+- **spacey** is *not* a tailnet member — it is the headscale control server — so
+  its backup is pulled over the public internet via `headscale.contrapeso.xyz`,
+  which follows the host if the droplet is rebuilt.
+
+## Retention here is the long tail
+
+Sources keep a few days locally; this box keeps 90 (or whatever the source entry
+says). Losing the source's local copy is expected.