Linux with / mounted read-only

By Mikael Ståldal

(This post has been edited since it was first published.)

I wondered why you usually mount / (the root file system) read-write in Linux and decided to do some experiments to find out if it is possible to have it mounted read-only.

So why do you want to do that? Perhaps you have the root file system on a read-only media, such as CD-ROM. Or on a writable media which can only handle a limited number of writes, such as a CD-RW or flash disk. It would also increase security since it will be more difficult (though not impossible) for some malware to infect your system.

I found out that it is possible to mount / read-only, but only after some tweaking. Here is how I did it in Ubuntu 8.04 (hardy) desktop.

The first obvious step is to change the mount options to “ro” for / in /etc/fstab and reboot. But the tweaking has to be done first, so don’t reboot yet!

There are some locations in the file system which has to be writeable, the solution is to mount them as tmpfs. After some experiments, I found out that I had to mount the following locations as tmpfs (assuming that /dev is already mounted in an appropriate way):

  • /tmp
  • /media
  • /var/run
  • /var/lock
  • /var/tmp
  • /var/crash
  • /var/log
  • /var/lib/xkb
  • /var/lib/gdm
  • /var/lib/dhcp3 (only if you use DHCP client)
  • /var/lib/nfs (only if you use NFS client)
  • /var/spool/cups

Ubuntu mounts /var/run and /var/lock as tmpfs by default, and this is done in /etc/init.d/mountkernfs.sh and /etc/init.d/mtab.sh.

Add this to /etc/init.d/mountkernfs.sh after the mounting of /var/lock:

domount tmpfs "" /var/tmp -omode=1777,nodev,noexec,nosuid
domount tmpfs "" /var/crash -omode=0755,nodev,noexec,nosuid

domount tmpfs "" /var/spool/cups -omode=0710,nodev,noexec,nosuid
chgrp lp /var/spool/cups
mkdir /var/spool/cups/tmp
chmod 1770 /var/spool/cups/tmp
chgrp lp /var/spool/cups/tmp

domount tmpfs "" /var/log -omode=0755,nodev,noexec,nosuid
mkdir /var/log/apparmor
mkdir /var/log/apt
mkdir /var/log/cups
mkdir /var/log/dist-upgrade
mkdir /var/log/fsck
mkdir /var/log/gdm
mkdir /var/log/news
mkdir /var/log/samba
mkdir /var/log/unattended-upgrades

domount tmpfs "" /var/lib/dhcp3 -omode=0755,nodev,noexec,nosuid

domount tmpfs "" /var/lib/xkb -omode=0755,nodev,noexec,nosuid

domount tmpfs "" /var/lib/gdm -omode=0755,nodev,noexec,nosuid
mkdir /var/lib/gdm/.fontconfig

domount tmpfs "" /var/lib/nfs -omode=0755,nodev,noexec,nosuid
mkdir /var/lib/nfs/sm
mkdir /var/lib/nfs/sm.bak
mkdir /var/lib/nfs/rpc_pipefs

domount tmpfs "" /tmp -omode=1777,nodev,exec,nosuid
touch /tmp/resolv.conf
touch /tmp/adjtime

# this is necessary to avoid that the above files are removed later in the boot process
touch /tmp/.clean

domount tmpfs "" /media -omode=0755,nodev,noexec,nosuid
# The following lines are not necessary in 9.04, perhaps not in 8.10 either
mkdir /media/cdrom0
ln -s /media/cdrom0 /media/cdrom
mkdir /media/floppy0
ln -s /media/floppy0 /media/floppy
mkdir /media/usbdisk

And add this to /etc/init.d/mtab.sh after the handling of /var/lock:

domtab tmpfs /var/log "varlog" -omode=0755,nodev,noexec,nosuid
domtab tmpfs /var/tmp "vartmp" -omode=1777,nodev,noexec,nosuid
domtab tmpfs /var/crash "varcrash" -omode=0755,nodev,noexec,nosuid
domtab tmpfs /var/spool/cups "varspoolcups" -omode=0710,nodev,noexec,nosuid
domtab tmpfs /var/lib/dhcp3 "varlibdhcp3" -omode=0755,nodev,noexec,nosuid
domtab tmpfs /var/lib/xkb "varlibxkb" -omode=0755,nodev,noexec,nosuid
domtab tmpfs /var/lib/gdm "varlibgdm" -omode=0755,nodev,noexec,nosuid
domtab tmpfs /var/lib/nfs "varlibnfs" -omode=0755,nodev,noexec,nosuid

domtab tmpfs /tmp "tmp" -omode=1777,nodev,exec,nosuid
domtab tmpfs /media "media" -omode=0755,nodev,noexec,nosuid

(I am not really sure what the actual purpose of /etc/init.d/mtab.sh is, perhaps it’s not necessary to modify it.)

There are some files in /etc which have to be writeable:

  • /etc/mtab
  • /etc/adjtime
  • /etc/resolv.conf (only if you use DHCP client and let it set DNS configuration)

I handle /etc/mtab by symlink it to /proc/mounts, that has some minor side-effects but I can live with it. I handle /etc/adjtime and /etc/resolv.conf by symlinking them to /tmp. In order for this to work, you have to patch the DHCP client (dhcp3-client) accodring to this bug report.

You also have to mount /home read-write somewhere, and I would not recommend using tmpfs. You can use a separate hard disk partition or NFS.

Finally it might be a good idea to set a password for the root account, this enables you to switch to a virtual console (Ctrl–Alt–F1) and login as root if something goes wrong.

If you then do want to change anything, such as edit a file in /etc or install or upgrade a package, you can just remount / as read-write temporary (assuming that the media actually is writeable):
sudo mount -o rw,noatime,remount /

and revert to read-only when finished:
sudo mount -o ro,noatime,remount /

Note that this setup is for a desktop system, it’s probably not appropriate for a server.

Update:
If you have plenty of RAM (such as at least 1 GB), then you can also mount /var/cache/apt as tmpfs. That helps if you have limited free space on / and want to to a distribution upgrade.

Add this to /etc/init.d/mountkernfs.sh:

domount tmpfs "" /var/cache/apt -omode=0755,nodev,noexec,nosuid,size=1g
mkdir -p /var/cache/apt/archives/partial

And add this to /etc/init.d/mtab.sh:

domtab tmpfs /var/cache/apt "varcacheapt" -omode=0755,nodev,noexec,nosuid,size=1g