Chapter 21. Setting read-only permissions for the root file system
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. Entries in the /etc/rwtab file follow this format: 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: dirs A directory tree is copied to tmpfs , empty. For example: files A file or a directory tree is copied to tmpfs intact. For example: 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-root file, set the READONLY option to yes to mount the file systems as read-only: Add the ro option in the root entry ( / ) in the /etc/fstab file: Enable the ro kernel option: Ensure that the rw kernel option is disabled: 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: Important Changes made to files and directories in tmpfs do 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:
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> copy-method path empty /tmp dirs /var/run files /etc/resolv.conf READONLY=yes /dev/mapper/luks-c376919e... / xfs x-systemd.device-timeout=0, ro 1 1 grubby --update-kernel=ALL --args="ro" grubby --update-kernel=ALL --remove-args="rw" files /etc/example/file mount -o remount,rw /