Hostname patterns and machine-id

`?` placeholders in `/etc/hostname`

Put this in the rootfs overlay’s etc/hostname:

garbanzo-????????

When systemd applies the hostname, it replaces each ? with a hex digit derived from the machine ID by cryptographic hashing. The same machine ID always gives the same hostname, but you can’t work backwards from the hostname to the machine ID. The result looks like garbanzo-7aaf846c.

This arrived in systemd 258, which is what Avocado’s 2024 rootfs ships. Older systemd treats ? as an invalid character and filters it out.
Newer systemd, not yet in any Avocado release as of this writing, adds $ placeholders that pick words from word lists ($-$-???? → sunny-red-92a9). Don’t use $ on a 258 system.
This is a systemd feature, not an Avocado one. Avocado just copies your /etc/hostname.

Avocado’s machine-id is stateless

Unless you set your own post_install, the rootfs image step does this:

# "Empty /etc/machine-id for stateless systemd on read-only rootfs."
touch "$ROOTFS_WORK/etc/machine-id"

The rootfs, including /etc, is read-only EROFS. When systemd finds an empty /etc/machine-id that it can’t write to, it makes up a new random ID for that boot and mounts it over the file. Nothing in Avocado saves it. systemd-machine-id-commit needs a writable /etc to do that, and there isn’t one.

So the machine ID is different on every boot, and so is everything derived from it. The boot log says so directly:

systemd[1]: Initializing machine ID from random generator.
systemd[1]: Installed transient '/etc/machine-id' file.
systemd[1]: Hostname set to <garbanzo-f90761a7>.

systemd does look for a saved ID in /var/lib/dbus/machine-id first, and /var is persistent. But it ignores that file when it’s a symlink, and the D-Bus package’s tmpfiles rule (L /var/lib/dbus/machine-id - - - - /etc/machine-id) creates exactly that symlink. So nothing survives a reboot.

Everything derived from it moves:

Derived from machine-id	Effect
`?` characters in `/etc/hostname`	The hostname changes every boot. mDNS names, DHCP hostnames and log labels all move.
Your own uses, such as a USB gadget serial number taken from `cut -c1-16 /etc/machine-id`	Changes every boot. Windows treats each boot as a new USB device.
systemd-networkd DHCP client identifier, for `.network` files that don’t set `ClientIdentifier=`	Defaults to a DUID derived from machine-id, so you get a different lease and IP each boot. The stock `80-wired.network` sets `ClientIdentifier=mac`, which is stable.
Persistent journal directory (`/var/log/journal/<machine-id>/`), if the journal is persistent	Each boot writes to a new directory, and plain `journalctl` reads only the current one. Use `journalctl --merge` to see earlier boots.

Also: machine-id is meant to be confidential (see machine-id(5)). Don’t put the raw value anywhere a host or network can read it, such as USB descriptors or mDNS TXT records.

Check your device

Run this, reboot, and run it again:

cat /etc/machine-id; findmnt /etc/machine-id; hostnamectl --static; hostname

If the ID differs between boots, everything above applies. If findmnt shows a mount on /etc/machine-id, systemd generated a temporary ID.

Getting a stable per-device identity

A few options, depending on what you need:

Hostname and serial numbers: derive them from a stable hardware ID at runtime instead of from machine-id, and hash it rather than exposing it raw. On Jetson, the module serial is usually in /proc/device-tree/serial-number. Check that it exists on your carrier board. A small oneshot service can set a transient hostname (hostnamectl set-hostname --transient ...) or feed the value to your gadget setup script.
Values that must be stable, unique and not reveal machine-id: systemd-id128 machine-id --app-specific=<your UUID> gives a keyed hash per purpose. It’s on the 2024 rootfs. It’s only as stable as machine-id itself.
Keeping the random machine-id: replace the /var/lib/dbus/machine-id symlink once with a regular file holding the current ID, from an early oneshot service. systemd should then log Initializing machine ID from D-Bus machine ID. on later boots. This follows from systemd’s source; it hasn’t been tried on a device.
A fixed machine-id: systemd accepts systemd.machine_id=<32 hex chars> on the kernel command line. That needs a per-device command line, which Avocado has no mechanism for.
DHCP: set ClientIdentifier=mac in your own .network files, as the stock 80-wired.network does.

If you replace the rootfs post_install, you take over these defaults, including the empty machine-id. See the config schema.