Constantly changing /etc/machine-id

Question

Using raspbian lite (2017-04-10) I've noticed that my /etc/machine-id file changes on each boot. This causes me problems with journald.

I've found the log line where this happens:

Apr 24 19:05:59 raspberrypi systemd[1]: Installed transient /etc/machine-id file.

And tracked down the line of code where this happens. But I can't figure out how to stop it. I've tried writing an empty file to /etc/machine-id, changing permissions, but no luck.

Has anyone experienced this? A work around is to manually write a /etc/machine-id file with a UUID in it, but this becomes quite laborious when setting up a number of pis.

I can reproduce this with a fresh raspbian lite 2017-04-10 image, on a pi 2 and a pi zero w.

Answer 1

Same behaviour on a fresh Raspbian Lite install, only change is root on F2FS, but /etc/machine-id is definitely on persistent storage and filesystem is writeable.

However, on the actual filesystem it is empty. Bizarrely, this is in my mtab:

tmpfs on /etc/machine-id type tmpfs (ro,mode=755)

... with a ctime of 1970, so merely appears to have contents - but being tmpfs it is entirely transient. I didn't even know you could mount a single file. This fixed it to be persistent by actually populating the file in the root filesystem with a new machine-id by first unmounting the temp file:

root@raspberrypi:~# umount /etc/machine-id
root@raspberrypi:~# echo $(dd if=/dev/random bs=32 count=1 | sha1sum | cut -b1-32) > /etc/machine-id
0+1 records in
0+1 records out
13 bytes (13 B) copied, 0.0115326 s, 1.1 kB/s
root@raspberrypi:~# cat /etc/machine-id
5a0911beb985160529f48ceaaa5eb581
root@raspberrypi:~# systemctl reboot
...wait a bit
pi@raspberrypi:~ $ cat /etc/machine-id
5a0911beb985160529f48ceaaa5eb581

You could try to capture the existing id instead, but I went for a brand new one.

Answer 2

A more complete solution to capture the machine-id without interrupting the current session, or generate a new one if it is run before systemd has a chance to do it for you (beware race conditions though):

mkdir /tmp/midchange; mount --bind /etc /tmp/midchange
if [ ! -s /tmp/midchange/machine-id ]; then # The persistent file is empty
  if [ -s /etc/machine-id ]; then           # systemd has generated a machine-id already
    cat /etc/machine-id > /tmp/midchange/machine-id
  else                                      # We need to generate a new machine-id
    dd if=/dev/random bs=32 count=1 | sha1sum | cut -b1-32 > /tmp/midchange/machine-id
    fi
umount /tmp/midchange; rmdir /tmp/midchange

• the --bind mount places a copy of the filesystem somewhere else, but without anything mounted on itself or child files/folders, letting us see the underlying file/folder and also very useful for backups.
• Put this in /etc/rc.local so that it can run on a each reboot. Once the machine-id is stored, you can take it out, but it's very low-impact and can stay, especially if you have a large number of devices. In that instance, include this in the base image.
• The test -s returns true if the file has contents.
• bs=32 count=1on the dd command lets us get random without blocking (ie waiting for random data, which isn't always full). Switch these around if you want more uniqueness and are happy for systemd to catch up and generate a different id that will be discarded on the next reboot.