Unix operációs rendszer 0001 -

Unix/Linux basics
0110 - Partitioning, FS, LVM, RAID
Operating systems lab
Gergely Windisch
[email protected]
room 4.12
Hacking lost root password exercise
• We have a linux machine that has some problems. It won't boot.
The system installed on it is very important, it needs to be
All the passwords are forgotten
• Exercise:
Start the system
Mend GRUB so that it can start automatically
Change the GRUB admin password to rootpw.
Change the root user's password to rootpw
• Hints
There's a floppy disk in the drive
To make changes, the / partition needs to be remounted as rw
/etc/fstab holds the name of the root partition
name of the kernel: linux24
• Good luck!
Exercise hints hintek
• Állítsuk be a BIOSban, hogy bootoljon floppyról.
• grub prompton: Keressük meg a root devicét
• Keressük meg a kernelt! (Fájnévkiegészítés megy
• Keressük ki, hogy mi a root fájlrendszer neve
linuxban - /etc/fstab fájl
• A fájlrendszer most még read-only, újra kell
mountolni rwnek: mount -o remount,rw /
• Szerkeszteni kell a /boot/grub/menu.lst -t:
• passwd root
• passwd dsluser (talán ez a neve)
Exercise solution
"Elfelejtett" jelszavak (ezek nem kellenek a megoldáshoz)
– root: elfelejt
– dsluser: dslaaa
– grub jelszó: elfelejt
Visszaállítás lépései
– Állítsuk be a BIOSban, hogy bootoljon floppyról.
bootgrubimg.img nevű image betöltődik, egy üres grubot tartalmaz
– grub prompton: find /sbin/init - root devicét megkeressük
– root (hd0,1) - vagy amit az előző parancs adott
– Keressük meg a kernelt!
asd /boot (TAB), nálunk linux24
– Keressük ki, hogy mi a root fájlrendszer neve linuxban
cat /etc/fstab - és itt nézzük meg, hogy mi van a / mellett. (/dev/hda2)
– Grub parancs: kernel /boot/linux24 root=/dev/hda2 init=/bin/bash
a single nem megy, mivel az is jelszavazott. init=/bin/bash szükséges
– A fájlrendszer most még read-only, újra kell mountolni rwnek: mount -o remount,rw /
– Szerkeszteni kell a /boot/grub/menu.lst -t: vim /boot/grub/menu.lst - módosítsunk
Írjuk át a DSL bejegyzést a jó kernelfájlra és a jó meghajtónévre
Írjuk át a jelszót, ehhez md5sum segítségével generálhatunk jelszót (vagy jelszó nélkül is mehet)
– passwd root
– passwd dsluser (talán ez a neve)
Hard disks (reminder)
Az ábrák nagy szeretettel lopva innen: http://kac.duf.hu/~balage/szakdoga/hdd.htm
• Filesystems are created on partitions (at least one)
• In the beginning: 4 partition / disk
• 15 partition/disk using extended partitions
• First sector of
• Contains
– Bootloader
– Partition table
Limits of MBR
• Legacy system - always updated (=hacked)
– 4 primary partitions
– can have 15 with extended partitions
– addresses: 24 bits (CHS), later 32 bits (LBA)
– CHS: 8 GB limit - sometimes even today (boot)
– LBA: 32 bit address --> maximum disk capacity: 2
• Seagate 2000GB-s lemez 59e Ft áfával
– MBR stored in only one sector - prone to dataloss
GUID Partition Table (GPT)
• Intel: Extended Firmware Interface (EFI)
– To get rid of the BIOS
• GTP properties
only LBA addressing
64 bit pointers - 9.4 zettabyte (9.4*10^21)
128 partitions (all primary)
GTP data structure is stored at multiple locations more secure
– important parts are CRCd
– partitions can have names
GPT layout
GPT support
• Not everyone supports it yet, so don't depend
on it
– Hibrid MBR http://www.rodsbooks.com/gdisk/hybrid.html
Disk management in linux
• hard disk
– IDE disks: hd
• /dev/hda
• /dev/hdb …
– SCSI disks: sd (sata as well)
• /dev/sda
• /dev/sdb
• hdaX, where X: 1..15 (128)
• sdaX
Partitioning scheme
• Desktop computer
Partitioning scheme (2)
• Server computer
• /home, /usr can also be a network drive
– available from every machine
Drive management
Get free disk space: df
Space occupied by directory: du
Hard disk info: fdisk
Mount drives: mount
Unmount drives: umount
df: disk free
$ df
Mounted on
$ df -h -T
df -hT
atlas:/mnt/BIG nfs
//surtr/Files smbfs
Used Available Use%
17% /
Used Avail Use% Mounted on
775M 3.9G 17% /
Used Avail Use% Mounted on
786M 3.9G 17% /
7.2M 130M
6% /boot
306G 160G 66% /.automount/atlas/root/mnt/BIG
140G 115G 55% /mnt/Files
du: disk utilization
$ du svn/ports
$ du -sh svn/ports
s: summarize
h: human readable
Partition types under linux
• ext2 - exists since 1993.
– http://en.wikipedia.org/wiki/Ext2
• ext3 - journaling version of ext2
– file operations go into the journal.
– in case of a system failure, the system knows
where to look for inconsistencies.
– (Which is good because there is no need for a full
system check - the computer can boot faster.
Part of the kernel since 2.4.15
File systems(2)
• ext4 - Extended version of ext3.
– More files in directories
– more secure journaling (crc)
– nanoseconds in time stamps
• ReiserFS
– V3
• Journaling
• Optimized for small files (store database natively)
– V4: Upgrade to ReiserFS.
– Development is uncertain.
File systems (3)
– New generation file system for linux
– Similar to Sun ZFS
– pools, snapshots, crc, resizability, etc.
– online filesystem check
– available in the testing branch since 2.6.29 promised to be ready by the end of 2008
– It is said to be the future - ext4 is just a temporary
Filesystem creation
• fdisk -l : check which disks are available
• partitioning tools
– fdisk
– cfdisk
– parted
– gparted
– gui tools
Create a bunch of disks on SuSE (4 of them)
Make it ready for use using the tools
sdb: keep it in one part, reiserfs
sdc: split it to three parts, create two filesystems
for the first two, leave the third untouched
– use a different tool to create a filesystem on the third
• sdd: create an ntfs partition (used by windows)
– resize it with gparted (if available), split it, create
filesystems on it
mount, umount
• The file systems need to be attached to be
used (hdds, CDs, floppys)
• They need to be removed after use
• attach disk: mount
– Attached fs goes into the / file system
• Layout of physical disks is hidden
• mount -o remount,rw / - RW mód
mount usage
• What are the currently mounted disks?
– mount
• Mount a disk
– mount -t ext3 /dev/sdb1 /mnt/backuphd
• umount /mnt/backuphd
• Attached fs hides previous content
• mount -t ext3 /dev/sdb1 /mnt/backuphd
• mount -t ext3 /dev/sdc1 /mnt/backuphd
• Remount read-only filesystem as rw:
– mount -t <file_system_type> </dev/yourdevice> /
-o remount -o rw
• Mount CD image
– mount -t auto Britney.iso /home/hallgato/Britni -o
Automatic mounting
• Currently mounted volumes
– /etc/mtab
• Volumes to be mounted
– /etc/fstab
• He who is not present in fstab needs to be
mounted manually
• He who is, doesn't
– or can be mounted easier
fstab layout
#device mount point file system options dump
/dev/hda1 / reiserfs defaults 0 1
/dev/hdb1 /home ext2 auto,notail 1 1
/dev/hdc /mnt/cdrom iso9660 defaults 0 0
/dev/hdd /mnt/dvdrw iso9660 ro,users,noauto 0 0
/dev/sda1 /usr reiserfs ro, owner 0 1
/dev/fd0 /mnt/floppy auto user 0 0
/dev/hda5 swap swap defaults 0 0
host:/var /root/var nfs uid=0,gid=0 1 0
proc /proc proc proc defaults 0 0
Fstab options
auto: bootláskor automatikus csatlakoztatás
noauto: nem csatlakozik magától (de nem kell mindenét megadni)
owner: csak root és a tulajdonos mountolhatja
user: bármelyik felhasználó mountolhat, csak root és a tulaj umountolhat
users: umount is mehet bárkinek
ro: csak olvasás
defaults: alapértelmezett opciók
uid=x: x felhasználó lesz a csatolt fájlok tulajdonosa
gid=x: x csoport lesz a csatolt fájlok tulajdonosa
noexec: a fájlrendszer fájljai nem futtathatóak
Fstab options
• auto: automount at boot
• noauto: won't boot automatically (de nem kell
mindenét megadni)
• owner: only root and the owner can mount it
• user: every user can mount, only root can unmount
• users: every user can mount and unmount
• ro: read only
• defaults: default options
• uid=x: set the owner of the files on the volume
• gid=x: set the group owner of the files on the volume
• noexec: the files cannot be executed
Put your newly created filesystems in fstab
All but the last should be mounted automatically
One should only be readable
The last one should be rw, but should not be mounted
automatically. Every user needs to be able to mount it.
• Create a symbolic link in the users home directories
that point to the new drives
• Extra exercise: create a script that creates symbolic
links to all the drives in each and every user's home
– The program should take a look at the available users and
their home directories
Exercise (optional)
• Move the /home directory to another disk
• Resize the / partition
• Use the newly freed space to create a swap
• Logical Volume Management
– Virtual space on disks
PE: Physical extent
Properties of LVM
• Pros
– More flexible
– can be resized easier
– somewhat independent of hdd structure
• Filesystems can span across multiple disks
• Cons
– Disaster recovery is hard
– online resizing is not possible
Creating LVM
• Try under SUSE.
• Try resizing it
• Redundant Array of Inexpensive Disks
• Use multiple disks
– Performance: RAID0
– Data security: RAID1
– Security + disk space: RAID5
RAID 0 - Striping
• Data is split between the disks
• Combined disk space equals the sum of the
• Quite fast
• Quite dangerous (one disk fails - no data)
• Data: 11011000
– 1st disk: 1101
– 2nd disk: 1000
RAID 1 - mirror
All the data is present on all the disks
Combined disk space = size of smallest disk
Slow write
Quick read
• At least three disks
• data on 2, checksum (XOR) on the third
• for example:
– Disk A 1st byte: 10010110 - data 1
– Disk B 1st byte: 11101100 - data 2
– Disk C 1st byte: 01111010 - checksum
• Either one fails - no problem
Further RAID variants
• RAID6: 2 checksum disks - can survive 2 bad
• RAID0+1: 4 hdd-s: striped and mirrored
OpenSolaris file management
• Disk names
– /dev/dsk - filesystem level access
– /dev/rdsk - raw data access
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