NixOS's etc activation removes files that were in a previous generation's
environment.etc but absent from the current one -- even real copies, not
only symlinks. LXC tarballs bake the host key into environment.etc (via
NIXOS_HOST_KEYS_DIR), but every subsequent nixos-rebuild switch lacks that
env var, so the key is removed as "obsolete". sops-nix derives its age
decryption key from /etc/ssh/ssh_host_ed25519_key, so deletion cascades
into "Error getting data key: 0 successful groups required, got 0" for
every sops secret on the host.
Fix: two activation scripts bracket the etc step.
preserveSshHostKey (no deps, runs before etc): copies the live key to
/run (tmpfs) before etc can delete it.
restoreSshHostKey (deps=[etc], runs after etc): reinstalls via `install`
if etc removed the key. The resulting file is not tracked in either
generation's environment.etc, so subsequent rebuilds leave it alone.
scripts/recover-hosts.sh: restore both private and public key files (not
just the private key), use install(1) for atomic mode setting, and add a
post-rebuild sops-nix verification step to confirm success.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014zT1L6hmsq6i1evAEH7dmi
