Linux Initramfs – Is Making /etc Directory Read-Only a Bad Idea

New answer (2015-03-22)

(Note: This answer is simpler than previous, but not more secure. My first answer is stronger because you could keep files read-only by fs mount options before permission flags. So forcing to write a files without permission to write won't work at all.)

Yes, under Debian, there is a package: fsprotect (homepage).

It use aufs (by default, but could use another unionfs tool) to permit live session changes but in RAM by default, so everything is forgotten at reboot.

You could install them by running simply:

apt-get install fsprotect

Once done, from online doc:

After that:

• Edit /boot/grub/menu.lst or /etc/default/grub2 or /etc/lilo.conf and add "fsprotect=1G" to kernel parameters.
• Modify 1G as needed.
• Apply changes (i.e. run update-grub)
• Edit /etc/default/fsprotect if you want to protect filesystems other than /.
• reboot

You may also want to password protect the grub bootloader or forbid any changes to it.

From there, if some file is protected against changes, for sample by

chmod ugo-w myfile

if you use for sample vi myfile and try to write on it with command :w!, this will work and your myfile became changed. You may reboot in order to retrieve unmodified myfile.

That's not even possible with my following first solution:

Old (first) answer:

Yes, it is a strong solution, but powerfull!

Making r/o useable

You have to mount some directories in rw, like /var, /etc and maybe /home. This could by done using aufs or unionfs. I like this another way, using /dev/shm and mount --bind:

cp -a /var /dev/shm/
mount --bind /dev/shm/var /var

You could before, move all directories who have not to change in normal operation in a static-var, than create symlinks in /var:

mkdir /static-var
mkdir /static-var/cache
mkdir /static-var/lib
mv /var/lib/dpkg /static-var/lib/dpkg
ln -s /static-var/lib/dpkg /var/lib/dpkg
mv /var/cache/apt /static-var/cache/apt
ln -s /static-var/cache/apt /var/cache/apt
... # an so on

So when remounting in ro, copying /var in /dev/shm won't take too much space as most files are moved to /static-var and only symlinks are to be copied in ram.

The better way to do this finely is to make a full power-cycle, one day of full work and finely run a command like:

find / -type f -o -type f -mtime -1

So you will see which files needs to be located on read-write partition.

Logging

As in this host no writeable static memory exist, in order to store history and other logs, you have to config a remote syslog server.

echo >/etc/syslog.conf '*.* @mySyslogServer.localdomain'

In this way, if your system break for any reason, everything before is logged.

Upgrading

When running whith some mount --bind in use, for doing such an upgrade while system is in use (whithout the need of running init 1, for reducing down-time), the simplier way is to re-build a clean root, able to do the upgrade:

After remounting '/' in read-write mode:

mount -o remount,rw /

for mpnt in /{,proc,sys,dev{,/pts}};do
    mount --bind $mnpt /$mnt$mpnt;
    done

chroot /mnt

apt-get update && apt-get dist-upgrade

exit

umount /mnt/{dev{/pts,},proc,sys,}

sync
mount -o remount,ro /

And now:

shutdown -r now