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If the DebPkg:udev rules 75-cd-aliases-generator.rules and 75-persistent-net-generator.rules are enabled, udev might try to rewrite the files 70-persistent-cd.rules and 70-persistent-net.rules in /etc/udev/rules.d/. Currently, I've no idea what to do. A simple solution is to disable the rules, but then the functionality is missing. | If the DebPkg: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/. |
The File Hierarchy Standard (FHS) permits mounting parts of the filesystem readonly. This has some advantages like fewer filesystem checks on bootup or no need to check the whole filesystem after a crash.
Preconditions
The FHS allows mounting all below /bin, /lib, /sbin and /usr. But you can extend this much more by using different filesystems for some trees and take care for special files.
Locations they must be writable are /etc, /home, /tmp, /var. The hierarchies below /dev, /proc 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:
Device file |
Filesystem |
Mount point |
RO/RW ? |
/dev/hda1 |
ext2 |
/ |
RO |
/dev/hda2 |
ext3 |
/var |
RW |
— |
tmpfs |
/tmp |
RW |
/var/local/home |
bind mount |
/home |
RW |
You can use a filesystem without a journal for /, because you don't write there and you don't need the journal.
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 add the option --noadjtime to HWCLOCKPARS in /etc/init.d/hwclockfirst.sh and /etc/init.d/hwclock.sh or in lenny use the options --noadjfile --localtime / --utc
blkid.tab
because it's modified at runtime by libblkid1
Solution: Create a symlink from /etc/blkid.tab to /var/local/blkid.tab
mtab
used by mount
Solution: Create a symlink from /etc/mtab to /proc/mounts
nologin
modified on boot up by the initscripts bootmisc.sh and rmnologin
This should already be a symlink to /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.
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. # # <file system> <mount point> <type> <options> <dump> <pass> /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.
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 /usr Pre-Invoke { "mount -o remount,rw /"; }; Post-Invoke { "check-etc; mount -o remount,ro / || true"; }; };
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)