Pre-seed SSH host keys so sops-nix secrets decrypt on first boot

sops-nix (in the nixos flake) derives each host's age decryption key
from its own /etc/ssh/ssh_host_ed25519_key at activation time, which
runs before systemd would otherwise generate that key on first boot
(sshd-keygen is a plain systemd service gated behind multi-user.target;
activation scripts run earlier). Without pre-seeding, secrets --
including the login password -- fail to decrypt on a fresh install's
very first boot.

- scripts/prepare-host-key.sh: run on the admin workstation before an
  install, generates the host's ed25519 keypair and prints the exact
  steps to register its derived age key in nixos/.sops.yaml and
  re-encrypt the affected secrets/*.yaml files.
- common.nix's auto-install.sh: after disko mounts /mnt and before
  nixos-install, installs a pre-seeded key from /root/host-keys/ into
  /mnt/etc/ssh/ if present, otherwise warns and asks for confirmation
  before continuing without one.
- installer.nix now imports common.nix (previously only proxmox-lxc.nix
  did), so the ISO/netboot path used for EFI VM installs gets the same
  auto-install.sh and pre-seed check, not just the LXC path.
- Also fixes a pre-existing stray backtick in the disko invocation that
  broke auto-install.sh's bash syntax entirely, independent of this
  change (found while rendering the script to verify the new logic).

README.md documents the new pre-flight workflow.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitignore

@@ -1 +1,5 @@
 result
+
+# Locally-generated SSH host keys staged for transfer to a new machine
+# during install (see scripts/prepare-host-key.sh) — never commit these.
+host-keys/

README.md

@@ -199,7 +199,12 @@ The generated `/etc/auto-install.sh` performs the following steps:
 
 6. Prepare `/mnt` using the Disko-generated mount configuration.
 
-7. Install NixOS:
+7. Check for a pre-seeded SSH host key at `/root/host-keys/<host>_ssh_host_ed25519_key`
+   and install it to `/mnt/etc/ssh/` if present (see "Pre-Seeding SSH Host
+   Keys for sops-nix" below) — prompts for confirmation before continuing
+   without one.
+
+8. Install NixOS:
 
    ```sh
    nixos-install \
@@ -207,9 +212,48 @@ The generated `/etc/auto-install.sh` performs the following steps:
      --no-root-password
    ```
 
-8. Remove temporary installation files.
+9. Remove temporary installation files.
 
-9. Reboot.
+10. Reboot.
+
+## Pre-Seeding SSH Host Keys for sops-nix
+
+The `nixos` flake manages secrets with sops-nix, using an age key derived
+from each host's own `/etc/ssh/ssh_host_ed25519_key`. That key is normally
+generated by a systemd service (`sshd-keygen`) the first time a host boots
+— but sops-nix decrypts secrets (including the login password) earlier
+than that, during system *activation*, which runs before systemd starts
+pursuing the target that `sshd-keygen` is gated behind. On a genuinely
+fresh install, this means secrets fail to decrypt on the very first boot
+unless the host key already exists beforehand.
+
+To avoid this, generate the key ahead of time and register it with the
+`nixos` flake's sops-nix setup **before** installing:
+
+```sh
+./scripts/prepare-host-key.sh <hostname> [path-to-nixos-repo]
+```
+
+This generates `host-keys/<hostname>_ssh_host_ed25519_key(.pub)` locally
+and prints the exact steps to add its derived age key to `nixos/.sops.yaml`,
+re-encrypt the relevant `secrets/*.yaml` files with `sops updatekeys`, and
+commit + push the `nixos` repo.
+
+Once that's done and the target machine is booted into the installer,
+copy the generated key onto it before running (or continuing)
+`/etc/auto-install.sh`:
+
+```sh
+scp host-keys/<hostname>_ssh_host_ed25519_key{,.pub} root@<target-ip>:/root/host-keys/
+```
+
+`auto-install.sh` checks for this file automatically and installs it to
+the target's `/mnt/etc/ssh/` before running `nixos-install`. If it's
+missing, the script warns and asks for confirmation before continuing —
+useful for hosts that don't consume any sops-nix secrets, but skipping it
+for a host that does will lock secrets out of decrypting on first boot.
+
+`host-keys/` is gitignored — never commit private key material.
 
 ## Configuration Notes
 

common.nix

@@ -105,11 +105,35 @@ fi
     
 if nix eval --refresh --json  --extra-experimental-features "flakes nix-command" "''${FLAKE_BASE_URL}#nixosConfigurations.''${choice}.config.disko.devices.disk.main.device" >/dev/null 2>&1; then
     disko --mode destroy,format,mount \
-        --flake "''${FLAKE_BASE_URL}#''${choice}" --yes`
+        --flake "''${FLAKE_BASE_URL}#''${choice}" --yes
 else
     echo "Selected host has no Disko configuration."
 fi
 
+# sops-nix derives this host's decryption key from its own SSH host key
+# at *activation* time, which runs before systemd would otherwise
+# generate one on first boot. Without pre-seeding it here, secrets
+# (including the login password) fail to decrypt on first boot.
+# Generate the key + register it with `nixos`'s sops-nix setup ahead of
+# time (see nix-auto-installer/scripts/prepare-host-key.sh), then scp it
+# to /root/host-keys/ on this machine before continuing.
+mkdir -p /root/host-keys
+if [[ -f "/root/host-keys/''${choice}_ssh_host_ed25519_key" ]]; then
+    echo "Found pre-seeded SSH host key for ''${choice}, installing to target..."
+    install -D -m 0600 "/root/host-keys/''${choice}_ssh_host_ed25519_key" /mnt/etc/ssh/ssh_host_ed25519_key
+    install -D -m 0644 "/root/host-keys/''${choice}_ssh_host_ed25519_key.pub" /mnt/etc/ssh/ssh_host_ed25519_key.pub
+else
+    echo "WARNING: no pre-seeded host key found at /root/host-keys/''${choice}_ssh_host_ed25519_key"
+    echo "sops-nix secrets (including the login password) will NOT decrypt on first boot."
+    echo "Run scripts/prepare-host-key.sh for host ''${choice} on your admin workstation first,"
+    echo "then scp the result here, if this host needs sops-nix secrets."
+    read -rp "Continue without a pre-seeded key anyway? (y/N): " skip_key
+    if [[ ! "$skip_key" =~ ^[Yy]$ ]]; then
+        echo "Aborted."
+        exit 1
+    fi
+fi
+
 mkdir -p /mnt/install-tmp
 export TMPDIR=/mnt/install-tmp
 

installer.nix

@@ -1,54 +1,11 @@
-{ pkgs, modulesPath, ... }:
+{ modulesPath, ... }:
 
 {
   imports = [
     "${modulesPath}/installer/cd-dvd/installation-cd-minimal.nix"
+    ./common.nix
   ];
 
-
-#   networking.useDHCP = true;
-
-  time.timeZone = "Australia/Brisbane";
-
-
-  services.openssh.enable = true;
-
-  services.openssh.settings = {
-    PermitRootLogin = "yes";
-  };
-
-
-  environment.systemPackages = with pkgs; [
-    git
-    curl
-    jq
-    parted
-    e2fsprogs
-    btrfs-progs
-    util-linux
-    disko
-  ];
-
-
-#   environment.etc."auto-install.sh" = {
-#     source = ./auto-install.sh;
-#     mode = "0755";
-#   };
-
-
-#   users.users.root = {
-#     initialPassword = "nixos";
-#   };
-
-
-  users.users.nixos = {
-    isNormalUser = true;
-    extraGroups = [
-      "wheel"
-    ];
-  };
-
-
   # Only for ISO/Linode serial environments if required
   # NOT for LXC
   #
@@ -56,4 +13,11 @@
   #   "console=ttyS0,19200n8"
   # ];
 
+  # Run installer when logging in (same pattern as proxmox-lxc.nix)
+  environment.etc."bash_profile-nixos".text = ''
+    if [ -n "$PS1" ] && [ ! -e ~/.auto_install_ran ]; then
+      sudo /etc/auto-install.sh
+      touch ~/.auto_install_ran
+    fi
+  '';
 }

scripts/prepare-host-key.sh

@@ -0,0 +1,65 @@
+#!/usr/bin/env bash
+# Generates a new machine's SSH host key ahead of installing it, so
+# sops-nix (in the `nixos` flake) has something to derive an age key
+# from before the target ever boots.
+#
+# Why this is needed: sops-nix derives each host's decryption key from
+# its own /etc/ssh/ssh_host_ed25519_key at *activation* time, but that
+# activation runs before systemd would otherwise generate this key on
+# first boot (sshd-keygen is a normal systemd service gated behind
+# multi-user.target; activation scripts run earlier than that). Without
+# pre-seeding, secrets — including the root/nixos login password — fail
+# to decrypt on the machine's very first boot.
+#
+# This script only touches your admin workstation and the `nixos` repo's
+# .sops.yaml (it never contacts the target machine). Run it, follow the
+# printed next steps, then use the resulting key with the auto-install.sh
+# prompt (see common.nix) when you actually install the new machine.
+set -euo pipefail
+
+hostname="${1:?usage: scripts/prepare-host-key.sh <hostname> [path-to-nixos-repo]}"
+nixos_repo="${2:-../nixos}"
+sops_yaml="${nixos_repo}/.sops.yaml"
+
+if [[ ! -f "$sops_yaml" ]]; then
+  echo "ERROR: $sops_yaml not found. Pass the nixos repo path as the 2nd argument." >&2
+  exit 1
+fi
+
+keydir="$(dirname "$0")/../host-keys"
+mkdir -p "$keydir"
+keyfile="${keydir}/${hostname}_ssh_host_ed25519_key"
+
+if [[ -f "$keyfile" ]]; then
+  echo "ERROR: $keyfile already exists. Remove it first if you want to regenerate." >&2
+  exit 1
+fi
+
+nix-shell -p openssh ssh-to-age --run "
+  set -euo pipefail
+  ssh-keygen -t ed25519 -N '' -C '${hostname}' -f '${keyfile}' >/dev/null
+  age_pub=\$(ssh-to-age -i '${keyfile}.pub')
+  cat <<EOF
+
+Generated: ${keyfile}(.pub)
+
+=== 1. Add this line under `keys:` in ${sops_yaml} ===
+  - &${hostname} \${age_pub}
+
+=== 2. Add *${hostname} to whichever creation_rules key_groups this host needs ===
+(e.g. secrets/common.yaml always; add a per-host secrets/${hostname}.yaml
+ block too if this host will get its own secrets, same pattern as
+ nix-cache/server.)
+
+=== 3. Re-encrypt every secrets file you just added it to ===
+  nix-shell -p sops --run 'sops updatekeys ${nixos_repo}/secrets/common.yaml'
+
+=== 4. Commit + push the nixos repo so the flake build picks up the new recipient ===
+
+=== 5. When you boot the installer on the new machine, scp the key in ===
+  scp ${keyfile}{,.pub} root@<target-ip>:/root/host-keys/
+
+Then continue with /etc/auto-install.sh as normal — it will find the
+pre-seeded key and install it before running nixos-install.
+EOF
+"