nixos/modules/platforms/lxc.nix

{ config, lib, modulesPath, flakeTarget, ... }:

let
  # Bakes this exact flake target's pre-generated SSH host key straight
  # into /etc/ssh/ -- mirrors modules/installer/host-keys.nix's
  # builtins.getEnv pattern (impure and empty under normal `nix
  # build`/`nix eval`, so this is a no-op unless explicitly opted into
  # with NIXOS_HOST_KEYS_DIR=... --impure), but places the key directly
  # rather than staging it under /etc/host-keys/ for a later manual copy
  # -- this is the whole system for a `lxc-*` host, built straight to a
  # pct-restorable tarball with no install step, so there's no later copy
  # step to stage for.
  #
  # Without this, config.system.build.tarball's built-in system just
  # generates a fresh host key at first boot like any other host would --
  # but sops-nix derives its decryption key from *this* file, and
  # .sops.yaml only trusts whatever key scripts/secrets/sync-host-keys.sh already
  # registered for this exact target name. A freshly-generated key can
  # never match that, so every secret (including this host's own login)
  # permanently fails to decrypt. Confirmed live: sops-install-secrets
  # errored with "Error getting data key: 0 successful groups required,
  # got 0" -- the container's actual host key's age fingerprint didn't
  # match the one registered in .sops.yaml at all.
  hostKeysDirStr = builtins.getEnv "NIXOS_HOST_KEYS_DIR";
  hasHostKeysDir = hostKeysDirStr != "" && builtins.pathExists hostKeysDirStr;
  hostKeysDir = /. + hostKeysDirStr;

  # flakeTarget ("${platform}-${buildType}") comes in via specialArgs from
  # flake.nix's mkTarget -- exactly the name scripts/secrets/sync-host-keys.sh
  # registers keys under. Deliberately not read back from
  # config.environment.etc."flake-target" (which is set to the same value)
  # -- this module also *contributes* to environment.etc below, and a
  # module reading the merged value of an option it's still defining is a
  # circular dependency (confirmed: "infinite recursion encountered").
  privKeyFile = hostKeysDir + "/${flakeTarget}_ssh_host_ed25519_key";
  pubKeyFile = hostKeysDir + "/${flakeTarget}_ssh_host_ed25519_key.pub";
  hasKeyForThisTarget =
    hasHostKeysDir
    && builtins.pathExists privKeyFile
    && builtins.pathExists pubKeyFile;
in
{
  # LXC containers share the host kernel — Proxmox starts them by exec'ing
  # /sbin/init directly, no bootloader/initrd involved — and Proxmox has its
  # own container hostname/network provisioning outside Nix. nixpkgs' own
  # virtualisation/proxmox-lxc.nix module already handles all of this
  # correctly (boot.isContainer, loader.initScript, systemd-networkd) and,
  # critically, provides config.system.build.tarball — a directly
  # `pct restore`-able container image, no nixos-install/bind-mount needed
  # (nixos-install refuses to touch the filesystem it's currently running
  # on, which is exactly what bind-mounting / onto /mnt for an installer
  # LXC container does).
  imports = [
    (modulesPath + "/virtualisation/proxmox-lxc.nix")
  ];

  proxmoxLXC = {
    # host.nix declares each host's real hostname (networking.hostName);
    # keep that instead of letting Proxmox's ambient container config win.
    manageHostName = true;
    # Unprivileged by default -- matches how these containers are actually
    # created (scripts/proxmox/create-proxmox-resource.sh reads this value
    # back to decide `pct create`'s --unprivileged flag, so the two stay
    # in sync).
    #
    # lxc-docker is the one exception: the kernel's NFS client doesn't set
    # FS_USERNS_MOUNT, so mounting NFS from inside *any* non-init user
    # namespace -- which is exactly what an unprivileged container's
    # UID-mapped root runs in -- is rejected at the VFS layer with EPERM,
    # no matter what Proxmox's own `mount=nfs;nfs4` container feature
    # allows at the AppArmor layer (confirmed live: TCP to the NFS server
    # succeeds, the server's export table matches the container's IP, and
    # `mount.nfs: Operation not permitted` still fires immediately with no
    # corresponding denial anywhere in the server's logs -- a kernel-level
    # rejection, not a network or export-permission one). Keying off
    # hostName rather than something docker-build-type-specific because
    # modules/build-types/docker.nix is also composed for linode-docker/
    # proxmox-docker, which don't import proxmox-lxc.nix at all --setting
    # this option there would break their eval with "option does not
    # exist" regardless of any mkIf guard, since mkIf only makes a value
    # conditional, not whether the option needs to exist somewhere.
    privileged = config.networking.hostName == "docker";
  };

  boot.loader = {
    grub.enable = false;
    systemd-boot.enable = false;
  };

  # NetworkManager depends on a running udevd to enumerate/classify devices,
  # which boot.isContainer disables (see nixpkgs' container-config.nix) —
  # that's what broke DHCP-hostname registration in Pi-hole. The imported
  # proxmox-lxc.nix module already switches networking to systemd-networkd
  # for the same reason; it just doesn't disable NetworkManager itself,
  # which modules/common/configuration.nix enables for every host.
  networking.networkmanager.enable = lib.mkForce false;

  environment.etc = lib.mkIf hasKeyForThisTarget {
    "ssh/ssh_host_ed25519_key" = {
      source = privKeyFile;
      mode = "0600";
    };
    "ssh/ssh_host_ed25519_key.pub" = {
      source = pubKeyFile;
      mode = "0644";
    };
  };

  # NixOS's etc activation removes any /etc file that was in the previous
  # generation's environment.etc but is absent from the current one — even
  # real (non-symlink) copies.  On every routine nixos-rebuild switch/test that
  # lacks NIXOS_HOST_KEYS_DIR the key is absent from environment.etc, so it
  # gets removed as "obsolete".  sops-nix derives its age decryption key from
  # /etc/ssh/ssh_host_ed25519_key; deletion cascades into every sops secret
  # failing with "Error getting data key: 0 successful groups required, got 0".
  #
  # Fix: two activation scripts that bracket the etc step.
  #   preserveSshHostKey — no deps, runs before etc — saves the live key to
  #                        /run (tmpfs) before etc can delete it.
  #   restoreSshHostKey  — deps=[etc], runs after etc — reinstalls the key via
  #                        `install` (atomic, sets mode) if etc removed it.
  #                        The resulting file is not registered in environment.etc
  #                        for either the previous or current generation, so
  #                        subsequent rebuilds leave it alone permanently.
  system.activationScripts.preserveSshHostKey = ''
    if [ -f /etc/ssh/ssh_host_ed25519_key ]; then
      cp /etc/ssh/ssh_host_ed25519_key /run/sshd-host-key-preserve.tmp
      cp /etc/ssh/ssh_host_ed25519_key.pub /run/sshd-host-key-preserve.pub.tmp
    fi
  '';

  system.activationScripts.restoreSshHostKey = {
    deps = [ "etc" ];
    text = ''
      if [ ! -f /etc/ssh/ssh_host_ed25519_key ] && [ -f /run/sshd-host-key-preserve.tmp ]; then
        install -m 0600 /run/sshd-host-key-preserve.tmp /etc/ssh/ssh_host_ed25519_key
        install -m 0644 /run/sshd-host-key-preserve.pub.tmp /etc/ssh/ssh_host_ed25519_key.pub
      fi
      rm -f /run/sshd-host-key-preserve.tmp /run/sshd-host-key-preserve.pub.tmp
    '';
  };

  # virtualisation/proxmox-lxc.nix (imported above) registers the Nix
  # store DB via a systemd service (register-nix-paths) -- it never runs
  # an activation script at all. Confirmed live this means neither
  # sops-nix's "for users" secrets (password hashes -- installed by the
  # activation script itself, not a systemd service, since they need to
  # exist *before* user creation) nor the user-creation step that
  # consumes them ever run on a real lxc-* boot. Regular secrets
  # (nix-serve's key, beszel's token, etc.) work anyway because sops-nix
  # provides its own systemd service for those.
  #
  # A systemd service, not boot.postBootCommands: tried that first (it's
  # a genuine, generally-invoked hook -- nixos/modules/system/boot/stage-2-init.sh,
  # which becomes this container's actual /sbin/init, unconditionally
  # runs it) but switch-to-configuration behaves differently that early in
  # boot (raw stage-2-init.sh, before systemd itself has even started) --
  # confirmed live it silently failed to rewrite /etc/shadow from there
  # even in "test" mode, despite the exact same command working reliably
  # every time when run post-boot (i.e. as a normal systemd service, which
  # is what this is). Not fully root-caused why the early context
  # specifically breaks it; a real systemd service sidesteps needing to.
  #
  # /etc/shadow already has PLACEHOLDER entries for every declared user
  # baked in at build time (part of constructing the system closure).
  # update-users-groups.pl deliberately never overwrites an *existing*
  # shadow entry -- a correct safety property in general (don't clobber a
  # real user's real password on a config rebuild) -- but on a genuine
  # first boot that only means the real hashedPasswordFile-derived hash
  # never gets the chance to be applied either, since the placeholder is
  # already "seen". Safe to clear here specifically: there is no real
  # password yet to protect on a first boot.
  #
  # "test" mode, not "boot": confirmed live "boot" mode aborts partway
  # through (before rewriting /etc/shadow) on a warning that "/boot" is on
  # a different filesystem -- a real check for a host with a bootloader to
  # update, meaningless for a container that has none
  # (boot.loader.{grub,systemd-boot}.enable are both false above), but it
  # still aborts the script. "test" runs every activation step without
  # touching boot-loader state at all.
  #
  # ConditionPathExists (systemd-native, not a bash-level check) means
  # this only ever runs once, on the genuine first boot -- systemd itself
  # skips even starting it on every later boot once the marker exists.
  # switch-to-configuration is otherwise the operator's call per this
  # repo's own safety rules, not something to run on every boot.
  systemd.services.nixos-lxc-first-boot-activate = {
    description = "Complete first-boot NixOS activation (users, secrets) for this LXC container";
    wantedBy = [ "multi-user.target" ];
    unitConfig.ConditionPathExists = "!/var/lib/nixos-lxc-first-boot-activated";
    serviceConfig = {
      Type = "oneshot";
      RemainAfterExit = true;
    };
    script = ''
      rm -f /etc/shadow
      /run/current-system/bin/switch-to-configuration test
      mkdir -p /var/lib
      touch /var/lib/nixos-lxc-first-boot-activated
    '';
  };
}