Linux with / mounted read-only

(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 (CtrlAltF1) 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
This entry was posted in Linux. Bookmark the permalink.

14 Responses to Linux with / mounted read-only

  1. Added /var/crash and /var/spool/cups

  2. YeTr2 says:

    /etc/printcap

  3. Lieven says:

    nice work, I was looking for something like this. I’ll check if this will work with an nfs mounted system. The problem was that I needed to export the nfs system as read only as it is shared by multiple computers. Then startxfc4 couldn’t write it’s lock so I’ll try if your approach with the symlinks will work.
    Thanks again for the info.

  4. David Burgess says:

    Any plan to update this for 8.10 or 9.04? I’ve tried your method on 8.04 but it doesn’t look like any of the changes to /etc/init.d/mountkernfs.sh are being effected.

  5. David Burgess says:

    Re: 8.10

    I got it figured. I simply added a variable to each ‘domount’ line in /etc/init.d/mountkernfs.sh. For example, I changed the first line from

    domount tmpfs “” /var/tmp -omode=1777,nodev,noexec,nosuid

    to

    domount tmpfs “” /var/tmp vartmp -omode=1777,nodev,noexec,nosuid

    adding ‘vartmp’. Works great. Thank you for a very practical guide.

    db

  6. Pingback: User links about "mkdir" on iLinkShare

  7. starfry says:

    Hi, good information.
    I have problem with network manager not knowing network is complete (presume it needs to write to a state file that is now read-only). Any ideas what the file might be?

    nm-applet constantly shows “finishing connection to wired network” and firefox starts in offline mode.

    cheers.

  8. Added /var/cache/apt.

  9. distrol says:

    Greeeeat!!!!

    This was exactly what I’ve been looking for at some time now.

    Thank you Mikael!

  10. David Burgess says:

    Can’t find mountkernfs.sh in karmic 🙁
    I love this blog entry; still come back here any time I install a new system. I guess change is good and I’ll have to figure out how the new Ubuntu mounts its filesystems at boot.

  11. I have found out that it in Ubuntu 9.04, it is not necessary to create any directories in /media, HAL will do that automatically.

    I’m not sure about Ubuntu 8.10.

  12. David,

    I haven’t tried karmic yet. But try searching for “domount” in all files in /etc/init.d:

    grep domount /etc/init.d/*

  13. I have now upgraded to karmic, there you have to do it in /etc/fstab. See new post about this.

Leave a Reply

Your email address will not be published. Required fields are marked *



(this is a captcha)