Chapter 21. 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.
21.1. 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. Note that the readonly-root package is required to have this file present in your system.
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:
emptyAn empty path is copied to
tmpfs. For example:empty /tmp
dirsA directory tree is copied to
tmpfs, empty. For example:dirs /var/run
filesA file or a directory tree is copied to
tmpfsintact. For example:files /etc/resolv.conf
The same format applies when adding custom paths to /etc/rwtab.d/.
21.2. 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
In the
/etc/sysconfig/readonly-rootfile, set theREADONLYoption toyesto mount the file systems as read-only:READONLY=yes
Add the
rooption in the root entry (/) in the/etc/fstabfile:/dev/mapper/luks-c376919e... / xfs x-systemd.device-timeout=0,ro 1 1Enable the
rokernel option:# grubby --update-kernel=ALL --args="ro"
Ensure that the
rwkernel option is disabled:# grubby --update-kernel=ALL --remove-args="rw"
If you need to add files and directories to be mounted with write permissions in the
tmpfsfile system, create a text file in the/etc/rwtab.d/directory and put the configuration there.For example, to mount the
/etc/example/filefile with write permissions, add this line to the/etc/rwtab.d/examplefile:files /etc/example/file
ImportantChanges made to files and directories in
tmpfsdo not persist across boots.- 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 /
