Give each Reflash board its own persistent SSH host key

bin/prod/ssh-keygen-boot

@@ -0,0 +1,40 @@
+#!/bin/bash
+
+set -euo pipefail
+
+LOG_FILE="${LOG_FILE:-/var/log/reflash.log}"
+KEY_DIR="${REFLASH_SSH_KEY_DIR:-/mnt/usb/ssh_host_keys}"
+
+info() {
+    echo "[info] $1" >> "$LOG_FILE"
+    echo "$1"
+}
+
+# Build-time SSH host key generation (a side effect of installing
+# openssh-server) would bake identical keys into every image and every board
+# flashed from it. Keys are deleted at build time instead, and restored (or
+# generated once and saved) here from the USB drive - the one thing on this
+# board that actually persists across reboots, unlike the initrd root fs.
+
+# On a freshly-flashed drive, /mnt/usb's partition doesn't exist yet - it's
+# normally created by expand-usb, which the Go server calls on its own
+# startup. This unit runs before reflash.service specifically so it has
+# /mnt/usb to itself first, which means that hasn't happened yet here.
+# expand-usb is idempotent (a no-op once the partition already exists), so
+# calling it again from reflash.service afterward is harmless.
+expand-usb
+
+mount-unmount-usb mounted rw
+
+if [ -f "$KEY_DIR/ssh_host_rsa_key" ]; then
+    info "Restoring SSH host keys from USB storage"
+    cp "$KEY_DIR"/ssh_host_* /etc/ssh/
+else
+    info "No saved SSH host keys on USB storage - generating and saving new ones"
+    ssh-keygen -A
+    mkdir -p "$KEY_DIR"
+    cp /etc/ssh/ssh_host_* "$KEY_DIR/"
+fi
+
+mount-unmount-usb unmounted
+info "SSH host keys ready"

mkimage.sh

@@ -180,6 +180,13 @@ RemainAfterExit=yes
 [Install]
 EOF
 
+# Installing openssh-server generated SSH host keys as a side effect - baked
+# into this one build, they'd be identical across every image and every
+# board flashed from it (#80). Delete them; ssh-keygen-boot.service (set up
+# below, outside the chroot) restores or generates them from USB storage
+# on boot instead.
+rm -f /etc/ssh/ssh_host_*
+
 # Clean up
 rm -rf /usr/sbin/policy-rc.d
 rm ./*.deb
@@ -253,6 +260,30 @@ cat <<EOF > "${ROOTFSDIR}"/initrd/etc/systemd/resolved.conf.d/mdns.conf
 MulticastDNS=yes
 EOF
 
+# This board's root fs runs from initrd and doesn't persist writes across
+# reboots, so keys generated straight into /etc/ssh would be regenerated
+# (and thus change) every boot. ssh-keygen-boot restores/saves them against
+# /mnt/usb instead - the one thing that actually persists - so a given board
+# keeps a stable identity while still not sharing a key with every other
+# image/board (#80). Ordered before both ssh.service and reflash.service so
+# it has /mnt/usb to itself - mount-unmount-usb has no locking of its own.
+cat <<EOF >"${ROOTFSDIR}"/initrd/etc/systemd/system/ssh-keygen-boot.service
+[Unit]
+Description=Restore or generate persistent SSH host keys from USB storage (see #80)
+Before=ssh.service reflash.service
+ConditionPathExists=!/etc/ssh/ssh_host_rsa_key
+
+[Service]
+Type=oneshot
+ExecStart=/usr/local/bin/ssh-keygen-boot
+RemainAfterExit=yes
+
+[Install]
+WantedBy=multi-user.target
+EOF
+
+systemctl enable ssh-keygen-boot --root="${ROOTFSDIR}"/initrd
+
 cat <<EOF >"${ROOTFSDIR}"/initrd/etc/systemd/system/reflash.service
 [Unit]
 Description=Refactor flashing server