Linux read only rootfs

How to make rootfs read-only?

I try to make my rootfs read-only. I thought that I better use overlay filesystem for this purpose (I don’t have other alternatives now). Yet, after many trials and changing fstab, I can’t manage to find any fstab which manage to start ubuntu with rootfs as readonly. I also googled itm but found no examples. This is what I tried, and I also tried many variations (options column), but still get error in overlay mount. As you can see, the main changes are adding «ro» to the rootfs, and adding overlay for /etc /var and /home.

# /etc/fstab: static file system information. # # Use 'blkid' to print the universally unique identifier for a # device; this may be used with UUID= as a more robust way to name devices # that works even if disks are added and removed. See fstab(5). # #      # / was on /dev/sda1 during installation UUID=075d905c-5c56-4eae-9402-5a880d108a8c / ext4 ro,errors=remount-ro 0 1 /dev/sdb /media/store ext4 defaults 1 1 # swap was on /dev/sda5 during installation UUID=e190d8d1-5490-4ef5-9547-a38aefa44b3e none swap sw 0 0 /root/myswapfile swap swap defaults 0 0 overlay /var overlay rw,noatime,lowerdir=/var,upperdir=/media/store /1,workdir=/media/store /11 0 2 overlay /etc overlay rw,noatime,lowerdir=/etc,upperdir=/media/store /2,workdir=/media/store /22 0 2 overlay /home overlay rw,noatime,lowerdir=/home,upperdir=/media/store /3,workdir=/media/store /33 0 2 

Overlay is supported in my kernel, and things does works if I change my rootfs option in ftab from ro to writebale, but I need it read-only! Thanks for any idea, ranran

Источник

Preconditions

The FHS allows mounting all underneath /bin, /lib, /sbin and /usr read-only. But you can extend this much more by using different filesystems for some trees and take care for special files.

Locations that must be writable are /etc, /home, /srv, /tmp, /var. The hierarchies below /dev, /proc, /selinux and /sys are already handled by special filesystems.

For /tmp you can use a tmpfs filesystem or its own filesystem. For /var it’s prefered to use its own filesystem. An example can look like this:

You can use a filesystem without a journal for /, because you don’t write there and you don’t need the journal. This can be an ext4, too, hence you can take advantage of the improvements of ext4. Create the filesystem with mke2fs -t ext4 -O ^has_journal /dev/sda1 or remove the journal with tune2fs -O ^has_journal /dev/sda1.

Special files in /etc

You have to take care for some files in /etc. These are

adjtime

because it’s modified on boot up; see bug 156489

Solution: Create a symlink from /etc/adjtime to /var/local/adjtime and

(1) add the option —noadjfile to HWCLOCKPARS in /etc/init.d/hwclockfirst.sh and /etc/init.d/hwclock.sh

(2) fix /etc/init.d/hwclockfirst.sh by replacing -f by -L in “if [ -w /etc ] && [ ! -f /etc/adjtime ] && [ ! -e /etc/adjtime ]; then”; see 520606.

alsa: init.d/alsa-utils

All versions before alsa-utils/1.0.27.2-1 (@2013-10-25 concerns wheezy version) of alsa-utils package startup script creates /.pulse files, leading to multiple error messages «Failed to create secure directory» when pulseaudio is installed.

blkid.tab

because it’s modified at runtime by libblkid1

Solution: You can’t create a symlink from /etc/blkid.tab to /var/local/blkid.tab because, unfortunately, libblkid1 will not honor this symlink. It will replace it on every write by a file, if the filesystem is mounted for writing (e.g. while doing an apt-get install). To work around this you must set the environement variable BLKID_FILE to /var/local/blkid.tab. You should do this in /etc/environment to set the variable for everybody who might do mounting.

courier imap

Courier IMAP uses a text file (/etc/courier/shared/index) for fast user lookups if running as a mail server for virtual mailboxes (The default configuration of authenticating against pam is unaffected by this).

If using virtual mailboxes with shared accounts the file will need to be moved elseware, the directory /var/cache/courier/shared/ would be suitable but will need to be manually created.

Once that is done update /etc/courier/imapd and change IMAP_SHAREDINDEXFILE to IMAP_SHAREDINDEXFILE=/etc/courier/shared/index .

cups

CUPS stores any kind of state files under /etc (classes.conf, cupsd.conf, printers.conf subscriptions.conf) and upstream is against any modification.

lvm

Lvm stores a backup of current and archives of previous metadata in /etc/lvm/. That causes any operation altering the metadata (vgreduce, vgextend, lvcreate, lvremove, lvresize, . ) to fail if / is not remounted read-write during the operation.

Solution: The location of the backup and archives is specified in /etc/lvm/lvm.conf. Set backup_dir = «/var/backups/lvm/backup» and archive_dir = «/var/backups/lvm/archive», create /var/backups/lvm and move /etc/lvm/backup and /etc/lvm/archive there.

Note: Lvm normally creates a backup during boot. This no longer happens as it is smart enough to see that /var is not yet mounted (or still read-only). But unless you use cluster lvm you will always already have a current backup from the last time you changed the metadata. So no harm done.

Relevant bugs: 372207 562234 (for etckeeper behavior WRT LVM files see 462355)

network/run

Used by ifupdown up to Squeeze

Solution: ifupdown links /etc/network/run to /run/network in postinst if /etc/network/run is not a directory.

rm -rf /etc/network/run dpkg-reconfigure ifupdown

Alternatively: Create a symlink from /etc/network/run to /lib/init/rw/etc-network-run (network/run is accessed by ifupdown init scripts before /var might be mounted, therefore, the abuse of /lib/init/rw)

Systems running Wheezy will be automatically moved to using /run/network no matter what their existing configuration was.

nologin

modified on boot up by the initscripts bootmisc.sh and rmnologin

This should already be a symlink to /var/lib/initscripts/nologin

In wheezy the init scripts directly modify /var/lib/initscripts/nologin

resolv.conf

If you have only a static nameserver configuration, then there’s no problem. Otherwise you should use the package resolvconf.

passwd, shadow

These files might be modified by the user with the tools chfn, chsh and passwd. If you are the only user of you system, you can remount the filesystem read/write before using these tools. Otherwise you might think about using NIS or LDAP.

samba/dhcp.conf

If the dhcp3-client (AKA isc-dhcp-client) package is installed, every time a DHCP connection is established, /etc/dhcp3/dhclient-enter-hooks.d/samba creates /etc/samba/dhcp.conf, no matter if it is used or not in /etc/samba/smb.conf.

suck

• move /etc/suck/sucknewsrc* to a new directory /var/local/suck,
• create a symlink /etc/suck/suckkillfile to /var/local/suck/suckkillfile
• and set etcdir in get-news.conf to /var/local/suck (this sets the -dd option of suck)

udev

If the udev rules 75-cd-aliases-generator.rules and 75-persistent-net-generator.rules are enabled, udev will try to update the files 70-persistent-cd.rules and 70-persistent-net.rules in /etc/udev/rules.d/ if needed. It is recommended to create the files once with all the rules needed and then disable the /etc/init.d/udev-mtab init script. While the root is readonly new rules are added to /dev/.udev/rules.d/.

Enable readonly root

To make your root filesystem is mounted readonly you must edit your /etc/fstab and set the mount option ro.

# /etc/fstab: static file system information. # #      /dev/hda1 / ext2 defaults,noatime,ro,errors=remount-ro 0 1 /dev/hda4 /var ext3 defaults 0 2

The option noatime is useful while the disk is mounted read/write while updates.

Enable readonly root during installation

Note: Tested with Debian Squeeze build from 2010-10-20

In the Debian installer, when configuring the filesystem and mountpoint for a partition, once can also select extra flags for the mount. If you want a readonly root then you can select that there and the installer will create the fstab corretcly for you. This gives you a system that almost works.

You will have to correct /etc/mtab, ifupdown and, if you have no /tmp partition, add tmpfs to /etc/fstab to get a clean boot. Do that at the shell prompt at the second console (Ctrl-Alt-F2) in the installer or first before any other fixes from above and reboot. Before you fix ifupdown you will have no networking so don’t go this way when installing remote.

After the second boot you might get an error from exim4 that the panic log isn’t empty. This is because the network didn’t come up on the first boot (you fixed ifupdown now, right?). Check that it is just that, remove the file (see , section 2.5.1. “The daily cron job”, esp. logrotate -f /etc/logrotate.d/exim4-paniclog) and restart exim4.

Tips and tricks

Make apt-get remount / if needed

To make apt-get remounts the filesystem automatically read/write before calling dpkg and remounting it readonly after dpkg finished, put these lines in /etc/apt/apt.conf

DPkg // Auto re-mounting of a readonly / Pre-Invoke < "mount -o remount,rw /"; >; Post-Invoke < "test $= yes || mount -o remount,ro / || true"; >; >;

The environment variable NO_APT_REMOUNT can be set to yes to prevent apt remounting the filesystem read-only. This is handy, if you plan to configure the installed package or make other changes in /etc.

apt-daily / unattended-upgrades

The unattended-upgrades periodically checks for upgradable packages. These are stored in /var/lib/apt/periodic and additionally the lock file /var/lib/apt/daily_lock is used. In order to allow temporary write access, pre- and post-execute hooks can be assigned to the apt-daily-upgrade service by running systemctl edit apt-daily-upgrade and entering the following lines:

[Service] ExecStartPre=-mount -o remount,rw / ExecStartPost=-sh -c 'if [ "$" != yes ]; then mount -o remount,ro /; fi'

Find processes blocking the remount readonly

After an upgrade of packages you might be faced with the problem that mount refuses to remount the filesystem readonly telling you “/ is busy.” This is caused by deleted files they are still used by a process. To find out which processes use deleted files use the tool checkrestart(1) from the package debian-goodies or use the following command. Often these are daemons using upgraded libraries. You have to restart them to make the files are released.

% < lsof +L1; lsof | sed -n '/SYSV/d; /DEL\|(path /p;'; >| grep -Ev '/(dev|home|tmp|var)' COMMAND PID USER FD TYPE DEVICE SIZE NLINK NODE NAME login 1546 root 4r REG 3,3 1331 0 66165 /etc/passwd (deleted) startx 1587 joerg 10r REG 3,3 4491 0 295122 /usr/bin/startx xinit 1609 joerg txt REG 3,3 19084 0 295565 /usr/bin/xinit zsh-beta 5058 joerg txt REG 3,3 628968 0 458849 /bin/zsh-beta zsh-beta 5058 joerg 12r REG 3,3 174728 0 205450 /usr/share/zsh-beta/functions/Completion.zwc zsh-beta 5058 joerg 13r REG 3,3 2221256 0 205405 /usr/share/zsh-beta/functions/Completion/Unix.zwc zsh-beta 5058 joerg 14r REG 3,3 237528 0 205398 /usr/share/zsh-beta/functions/Completion/Base.zwc udevd 458 root mem REG 3,3 131417 /lib/libnss_files-2.7.so (path inode=140638) udevd 458 root mem REG 3,3 131431 /lib/libnss_nis-2.7.so (path inode=140653) udevd 458 root mem REG 3,3 131389 /lib/libnsl-2.7.so (path inode=140616) udevd 458 root mem REG 3,3 131401 /lib/libnss_compat-2.7.so (path inode=140623) udevd 458 root mem REG 3,3 131212 /lib/libdl-2.7.so (path inode=140598) udevd 458 root mem REG 3,3 131159 /lib/libc-2.7.so (path inode=140581) udevd 458 root mem REG 3,3 131089 /lib/ld-2.7.so (path inode=140572) syslog-ng 1406 root mem REG 3,3 131417 /lib/libnss_files-2.7.so (path inode=140638) syslog-ng 1406 root mem REG 3,3 131431 /lib/libnss_nis-2.7.so (path inode=140653) syslog-ng 1406 root mem REG 3,3 131389 /lib/libnsl-2.7.so (path inode=140616) syslog-ng 1406 root mem REG 3,3 131401 /lib/libnss_compat-2.7.so (path inode=140623) syslog-ng 1406 root mem REG 3,3 131159 /lib/libc-2.7.so (path inode=140581) syslog-ng 1406 root mem REG 3,3 131089 /lib/ld-2.7.so (path inode=140572)

Read only Root The AUFS Way

Источник