Operating systems lab
Gergely Windisch windisch.gergely@nik.bmf.hu
room 4.12
http://nik.bmf.hu/gwindisch/os_2010

• 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 recovered.
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!

• Állítsuk be a BIOSban, hogy bootoljon floppyról.
• grub prompton: Keressük meg a root devicét
• Keressük meg a kernelt! (Fájnévkiegészítés megy
(TAB))
• 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)

• "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)

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 http://www.linuxplanet.com/graphics/screenshots/partitions4.png
http://www.win.tue.nl/~aeb/partitions/partition_types-2.html

• First sector of
HDD
• Contains
– Bootloader
– Partition table

• 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
Terabyte
• Seagate 2000GB-s lemez 59e Ft áfával
• http://www.ipon.hu/webshop/product/seagate_2000gb_lp_5900rpm_32mb_sata2/116689
– MBR stored in only one sector - prone to dataloss

• 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 http://www.ibm.com/developerworks/linux/library/l-gpt/index.html

• Not everyone supports it yet, so don't depend on it
– Hibrid MBR http://www.rodsbooks.com/gdisk/hybrid.html

• 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

• Desktop computer

• Server computer
– boot
– root
– swap
– home
– var
– usr
– tmp
• /home, /usr can also be a network drive
– available from every machine

• Get free disk space: df
• Space occupied by directory: du
• Hard disk info: fdisk
• Mount drives: mount
• Unmount drives: umount

$ df
Filesystem 1K-blocks Used Available Use%
Mounted on
/dev/md2 4881472 793508 4087964 17% /
$ df -h -T
Filesystem Type Size Used Avail Use% Mounted on
/dev/md2 xfs 4.7G 775M 3.9G 17% / df -hT
Filesystem Type Size Used Avail Use% Mounted on
/dev/md2 xfs 4.7G 786M 3.9G 17% /
/dev/md0 ext2 145M 7.2M 130M 6% /boot atlas:/mnt/BIG nfs 465G 306G 160G 66% /.automount/atlas/root/mnt/BIG
//surtr/Files smbfs 254G 140G 115G 55% /mnt/Files

$ du svn/ports
...
32 svn/ports/vnstat/.svn
48 svn/ports/vnstat
6248 svn/ports
$ du -sh svn/ports
6.2M svn/ports
Switches: s: summarize h: human readable
--max-depth=1

• 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

• 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.

• BTRFS
– 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 solution

• 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

• 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

• 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 loop

• 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

#device mount point file system options dump check
/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 http://www.linuxquestions.org/linux/answers/Hardware/etc_fstab_broken_down_and_explained

Opciók: 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

• 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 directory.
– The program should take a look at the available users and their home directories

• Move the /home directory to another disk
• Resize the / partition
• Use the newly freed space to create a swap partition

• Logical Volume Management
– Virtual space on disks
PE: Physical extent

• 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

• Try under SUSE.
• Try resizing it

• Redundant Array of Inexpensive Disks
• Use multiple disks
– Performance: RAID0
– Data security: RAID1
– Security + disk space: RAID5

• Data is split between the disks
• Combined disk space equals the sum of the parts
• Quite fast
• Quite dangerous (one disk fails - no data)
• Data: 11011000
– 1st disk: 1101
– 2nd disk: 1000

• All the data is present on all the disks
• Combined disk space = size of smallest disk
• Slow write
• Quick read
• Safe

• 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

• RAID6: 2 checksum disks - can survive 2 bad hdd
• RAID0+1: 4 hdd-s: striped and mirrored

• Disk names
– /dev/dsk - filesystem level access
– /dev/rdsk - raw data access