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Chapter 20. Setting read-only permissions for the root file system

Sometimes, you need to mount the root file system (/) with read-only permissions. Example use cases include enhancing security or ensuring data integrity after an unexpected system power-off.

20.1. Files and directories that always retain write permissions

For the system to function properly, some files and directories need to retain write permissions. When the root file system is mounted in read-only mode, these files are mounted in RAM using the tmpfs temporary file system.

The default set of such files and directories is read from the /etc/rwtab file, which contains:

dirs	/var/cache/man
dirs	/var/gdm
<content truncated>

empty	/tmp
empty	/var/cache/foomatic
<content truncated>

files	/etc/adjtime
files	/etc/ntp.conf
<content truncated>

Entries in the /etc/rwtab file follow this format:

copy-method    path

In this syntax:

  • Replace copy-method with one of the keywords specifying how the file or directory is copied to tmpfs.
  • Replace path with the path to the file or directory.

The /etc/rwtab file recognizes the following ways in which a file or directory can be copied to tmpfs:

empty

An empty path is copied to tmpfs. For example:

empty /tmp
dirs

A directory tree is copied to tmpfs, empty. For example:

dirs /var/run
files

A file or a directory tree is copied to tmpfs intact. For example:

files /etc/resolv.conf

The same format applies when adding custom paths to /etc/rwtab.d/.

20.2. Configuring the root file system to mount with read-only permissions on boot

With this procedure, the root file system is mounted read-only on all following boots.

Procedure

  1. In the /etc/sysconfig/readonly-root file, set the READONLY option to yes:

    # Set to 'yes' to mount the file systems as read-only.
    READONLY=yes
  2. Add the ro option in the root entry (/) in the /etc/fstab file:

    /dev/mapper/luks-c376919e...  /  xfs  x-systemd.device-timeout=0,ro  1  1
  3. Add the ro option to the GRUB_CMDLINE_LINUX directive in the /etc/default/grub file and ensure that the directive does not contain rw:

    GRUB_CMDLINE_LINUX="rhgb quiet... ro"
  4. Recreate the GRUB2 configuration file:

    # grub2-mkconfig -o /boot/grub2/grub.cfg
  5. If you need to add files and directories to be mounted with write permissions in the tmpfs file system, create a text file in the /etc/rwtab.d/ directory and put the configuration there.

    For example, to mount the /etc/example/file file with write permissions, add this line to the /etc/rwtab.d/example file:

    files /etc/example/file
    Important

    Changes made to files and directories in tmpfs do not persist across boots.

  6. Reboot the system to apply the changes.

Troubleshooting

  • If you mount the root file system with read-only permissions by mistake, you can remount it with read-and-write permissions again using the following command:

    # mount -o remount,rw /