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Linux 灾难恢复

2012年10月16日 综合技术 暂无评论 阅读 644 次

Linux 发行版本众多,现如今也得到了越来越广泛的应用,同时也面临着系统出现故障的潜在风险,本文将以发行版本 RHEL6 为例详细介绍几种 Linux 灾难恢复技术和方法,以确保 Linux 系统的安全恢复。

在介绍 Linux 灾难恢复方法之前,我们先来了解下 MBR,其全称为 Master Boot Record,即硬盘的主引导记录。它由三个部分组成,主引导程序、硬盘分区表和硬盘有效标志。在总共 512 字节的主引导扇区里主引导程序(Bootloader)占 446 个字节,第二部分是硬盘分区表,占 64 个字节,硬盘有多少分区以及每一分区的大小都记录在其中。第三部分是硬盘有效标志,占 2 个字节。具体如图示:
图 1. MBR
图 1. MBR

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系统硬盘分区表破坏

生产环境中的 Linux 服务器可能会因为病毒或者意外断电而引起硬盘分区表被破坏,通常恢复硬盘分区表需要之前我们先备份其分区表的信息,一般我们使用 USB 外接设备来备份主机硬盘的分区表。

在主机上挂载 USB 设备后我们查看系统当前磁盘设备 :

 [root@FCoE ~]# fdisk -l

 Disk /dev/sda: 43.0 GB, 42991616000 bytes
 255 heads, 63 sectors/track, 5226 cylinders
 Units = cylinders of 16065 * 512 = 8225280 bytes
 Sector size (logical/physical): 512 bytes / 512 bytes
 I/O size (minimum/optimal): 512 bytes / 512 bytes
 Disk identifier: 0x00032735

   Device Boot      Start         End      Blocks   Id  System
 /dev/sda1   *           1          17      131072   83  Linux
 Partition 1 does not end on cylinder boundary.
 /dev/sda2              17         147     1048576   82  Linux swap / Solaris
 Partition 2 does not end on cylinder boundary.
 /dev/sda3             147        5227    40803328   83  Linux

 Disk /dev/sdb: 2147 MB, 2147483648 bytes
 255 heads, 63 sectors/track, 261 cylinders
 Units = cylinders of 16065 * 512 = 8225280 bytes
 Sector size (logical/physical): 512 bytes / 512 bytes
 I/O size (minimum/optimal): 512 bytes / 512 bytes
 Disk identifier: 0x00000000

 Disk /dev/sdb doesn't contain a valid partition table

 

现在我们在 sdb 这个设备上创建一个新的分区 :

 [root@FCoE ~]# fdisk /dev/sdb
 Device contains neither a valid DOS partition table, nor Sun, SGI or OSF disklabel
 Building a new DOS disklabel with disk identifier 0xcdd48395.
 Changes will remain in memory only, until you decide to write them.
 After that, of course, the previous content won't be recoverable.

 Warning: invalid flag 0x0000 of partition table 4 will be corrected by w(rite)

 WARNING: DOS-compatible mode is deprecated. It's strongly recommended to
         switch off the mode (command 'c') and change display units to
         sectors (command 'u').

 Command (m for help): n
 Command action
   e   extended
   p   primary partition (1-4)
 p
 Partition number (1-4): 1
 First cylinder (1-261, default 1):
 Using default value 1
 Last cylinder, +cylinders or +size{K,M,G} (1-261, default 261):
 Using default value 261

 Command (m for help): p

 Disk /dev/sdb: 2147 MB, 2147483648 bytes
 255 heads, 63 sectors/track, 261 cylinders
 Units = cylinders of 16065 * 512 = 8225280 bytes
 Sector size (logical/physical): 512 bytes / 512 bytes
 I/O size (minimum/optimal): 512 bytes / 512 bytes
 Disk identifier: 0xcdd48395

   Device Boot      Start         End      Blocks   Id  System
 /dev/sdb1               1         261     2096451   83  Linux

 Command (m for help): w
 The partition table has been altered!

 Calling ioctl() to re-read partition table.
 Syncing disks.

 

在新分区 sdb1 上创建文件系统:

 [root@FCoE ~]# mkfs.ext3 /dev/sdb1
 mke2fs 1.41.12 (17-May-2010)
 Filesystem label=
 OS type: Linux
 Block size=4096 (log=2)
 Fragment size=4096 (log=2)
 Stride=0 blocks, Stripe width=0 blocks
 131072 inodes, 524112 blocks
 26205 blocks (5.00%) reserved for the super user
 First data block=0
 Maximum filesystem blocks=536870912
 16 block groups
 32768 blocks per group, 32768 fragments per group
 8192 inodes per group
 Superblock backups stored on blocks:
        32768, 98304, 163840, 229376, 294912

 Writing inode tables: done
 Creating journal (8192 blocks): done
 Writing superblocks and filesystem accounting information: done

 This filesystem will be automatically checked every 24 mounts or
 180 days, whichever comes first.  Use tune2fs -c or -i to override.

 

挂载新的文件系统:

 [root@FCoE ~]# mount /dev/sdb1 /mnt/

 

通常我们通过备份硬盘的 MBR 来备份硬盘分区表:

 [root@FCoE ~]# dd if=/dev/sda of=/mnt/sda.mbr bs=512 count=1
 1+0 records in
 1+0 records out
 512 bytes (512 B) copied, 0.000777948 s, 658 kB/s

 

现在我们来写零硬盘分区表来实现类似分区表被破坏的结果:

 [root@FCoE ~]# dd if=/dev/zero of=/dev/sda bs=1 count=64 skip=446 seek=446
 64+0 records in
 64+0 records out
 64 bytes (64 B) copied, 0.00160668 s, 39.8 kB/s

 

查询硬盘 sda 上的分区信息,发现其已不包含任何分区:

 [root@FCoE ~]# fdisk -l

 Disk /dev/sda: 43.0 GB, 42991616000 bytes
 255 heads, 63 sectors/track, 5226 cylinders
 Units = cylinders of 16065 * 512 = 8225280 bytes
 Sector size (logical/physical): 512 bytes / 512 bytes
 I/O size (minimum/optimal): 512 bytes / 512 bytes
 Disk identifier: 0x00032735

   Device Boot      Start         End      Blocks   Id  System

 Disk /dev/sdb: 2147 MB, 2147483648 bytes
 255 heads, 63 sectors/track, 261 cylinders
 Units = cylinders of 16065 * 512 = 8225280 bytes
 Sector size (logical/physical): 512 bytes / 512 bytes
 I/O size (minimum/optimal): 512 bytes / 512 bytes
 Disk identifier: 0xcdd48395

   Device Boot      Start         End      Blocks   Id  System
 /dev/sdb1               1         261     2096451   83  Linux

 

当主机硬盘分区表丢失了之后,再次启动后 GRUB 会因找不到配置文件而进入命令行模式 :
图 2. 分区表丢失
图 2. 分区表丢失

接下来我们挂载 RHEL6 的安装盘,同时也接入我们之前备份的 USB 设备,然后重启主机,选择 CD-ROM 为第一引导设备,启动后选择“Rescue installed system”。
图 3. 选择援救
图 3. 选择援救

按照提示,最终我们选择一个 shell。
图 4. 选择 shell
图 4. 选择 shell

我们查询系统磁盘信息,发现硬盘设备 sda 没有包含任何分区。

 bash-4.1# fdik – l

 Disk /dev/sda: 43.0 GB, 42991616000 bytes
 255 heads, 63 sectors/track, 5226 cylinders
 Units = cylinders of 16065 * 512 = 8225280 bytes
 Sector size (logical/physical): 512 bytes / 512 bytes
 I/O size (minimum/optimal): 512 bytes / 512 bytes
 Disk identifier: 0x00032735

   Device Boot      Start         End      Blocks   Id  System

 Disk /dev/sdb: 2147 MB, 2147483648 bytes
 255 heads, 63 sectors/track, 261 cylinders
 Units = cylinders of 16065 * 512 = 8225280 bytes
 Sector size (logical/physical): 512 bytes / 512 bytes
 I/O size (minimum/optimal): 512 bytes / 512 bytes
 Disk identifier: 0xcdd48395

   Device Boot      Start         End      Blocks   Id  System
 /dev/sdb1               1         261     2096451   83  Linux

 

我们来恢复它的硬盘分区表,创建一个目录并且挂载之前备份的 USB 设备,我们看到它的设备名是 /dev/sdb。

 bash-4.1# mount /dev/sdb1 /usb
 bash-4.1# ls /usb
 lost+found  sda.mbr

 

通过原来备份的 sda.mbr 文件来恢复硬盘设备 sda 的硬盘分区表 :

 bash-4.1# dd if=/usb/sda.mbr of=/dev/sda bs=1 count=64 skip=446 seek=446
 64+0 records in
 64+0 records out
 64 bytes (64 B) copied, 0.038358 s, 4.6 kB/s

 

再次查询系统磁盘信息:

 bash-4.1# fdisk -l
 Disk /dev/sda: 43.0 GB, 42991616000 bytes
 255 heads, 63 sectors/track, 5226 cylinders
 Units = cylinders of 16065 * 512 = 8225280 bytes
 Sector size (logical/physical): 512 bytes / 512 bytes
 I/O size (minimum/optimal): 512 bytes / 512 bytes
 Disk identifier: 0x00032735

   Device Boot      Start         End      Blocks   Id  System
 /dev/sda1   *           1          17      131072   83  Linux
 Partition 1 does not end on cylinder boundary.
 /dev/sda2              17         147     1048576   82  Linux swap / Solaris
 Partition 2 does not end on cylinder boundary.
 /dev/sda3             147        5227    40803328   83  Linux

 Disk /dev/sdb: 2147 MB, 2147483648 bytes
 255 heads, 63 sectors/track, 261 cylinders
 Units = cylinders of 16065 * 512 = 8225280 bytes
 Sector size (logical/physical): 512 bytes / 512 bytes
 I/O size (minimum/optimal): 512 bytes / 512 bytes
 Disk identifier: 0xcdd48395

   Device Boot      Start         End      Blocks   Id  System
 /dev/sdb1               1         261     2096451   83  Linux

 

硬盘设备 sda 的分区表已经恢复,重启后系统便可正常引导。

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系统 GRUB 损坏

类似得我们可以来写零 Bootloader 来实现 GRUB 被破坏的结果:

 [root@FCoE grub]# dd if=/dev/zero of=/dev/sda bs=446 count=1
 1+0 records in
 1+0 records out
 446 bytes (446 B) copied, 0.0017583 s, 254 kB/s

 

重启后系统会因找不到 GRUB 而卡在“Booting from Hard Disk …”

挂载系统安装光盘然后选择进入 Rescue 模式,然后恢复 GRUB:

 bash-4.1# chroot /mnt/sysimage
 sh-4.1# grub
 grub > root hd(0,0)
 grub > setup (hd0)
 grub > quit

图 5. 恢复 GRUB
图 5. 恢复 GRUB

重启主机后,系统可正常引导。

系统内核文件丢失

系统丢失内核 kernel 文件,再次启动后会提示找不到文件。
图 6. 内核丢失
图 6. 内核丢失

挂载系统安装盘进入援救模式,检查 /boot 目录下发现没有 kernel 文件。

 bash-4.1# chroot /mnt/sysimage
 bash-4.1# ls /boot
 ls
 config-2.6.32-71.el6.x86_64     lost+found
 efi                                  symvers-2.6.32-71.el6.x86_64.gz
 grub                                 System.map-2.6.32-71.el6.x86_64
 initramfs-2.6.32-71.el6.x86_64.img

 

从挂载的系统安装盘强制重新安装内核:

 sh-4.1# mount – o loop /dev/sr0 /media
 sh-4.1# cd /media/Server/Packages
 sh-4.1# rpm -ivh --force kernel-2.6.32-71.el6.x86_64.rpm
 warning: kernel-2.6.32-71.el6.x86_64.rpm: Header V3 RSA/SHA256 Signature,
 key ID fd431d51: NOKEY
 Preparing...                ########################################### [100%]
   1:kernel                  ########################################### [100%]

 

在 /boot 目录下已经生成新的 kernel 文件 vmlinuz-2.6.32-71.el6.x86_64

 sh-4.1## ls /boot
 config-2.6.32-71.el6.x86_64            lost+found
 efi                                         symvers-2.6.32-71.el6.x86_64.gz
 grub                                        System.map-2.6.32-71.el6.x86_64
 initramfs-2.6.32-71.el6.x86_64.img    vmlinuz-2.6.32-71.el6.x86_64

 

重启主机后,系统可正常引导。

系统镜像文件丢失

系统丢失镜像文件,主机启动后黑屏。
图 7. 镜像丢失
图 7. 镜像丢失

挂载系统安装盘进入援救模式 , 检查 /boot 目录下发现没有镜像文件。

 bash-4.1# chroot /mnt/sysimage
 sh-4.1# ls /boot
 config-2.6.32-71.el6.x86_64 	 symvers-2.6.32-71.el6.x86_64.gz
 efi                                 System.map-2.6.32-71.el6.x86_64
 grub                                vmlinuz-2.6.32-71.el6.x86_64
 lost+found

 

重新生成镜像文件 initramfs-2.6.32-71.el6.x86_64.img。

 sh-4.1# cd /boot
 sh-4.1# mkinit
 sh-4.1# ls
 config-2.6.32-71.el6.x86_64           lost+found
 efi                                        symvers-2.6.32-71.el6.x86_64.gz
 grub                                       System.map-2.6.32-71.el6.x86_64
 initramfs-2.6.32-71.el6.x86_64.img   vmlinuz-2.6.32-71.el6.x86_64

 

重启主机后 , 系统可正常引导。

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系统 /boot 分区损坏

一般来说系统 /boot 分区损坏,我们会先尝试修复文件系统。如果文件系统损坏不能修复,那么我们可以参照前述的方法来依次新建 /boot 分区,重新安装内核和镜像,然后安装 GURB 再手工编辑引导菜单,以最终来恢复系统可正常引导。通常我们需要按照如下的步骤来恢复。

创建分区

碰到比较严重的情况就是 /boot 分区已经完全损坏,启动时会提示找不到引导设备。
图 8. 引导分区损坏
图 8. 引导分区损坏

挂载安装盘后进入援救模式,查看分区情况,发现分区 /dev/sda1 不存在。

 bash-4.1#
 Disk /dev/sda: 43.0 GB, 42991616000 bytes
 255 heads, 63 sectors/track, 5226 cylinders
 Units = cylinders of 16065 * 512 = 8225280 bytes
 Sector size (logical/physical): 512 bytes / 512 bytes
 I/O size (minimum/optimal): 512 bytes / 512 bytes
 Disk identifier: 0x00000000

   Device Boot      Start         End      Blocks   Id  System
 /dev/sda2              17         147     1048576   82  Linux swap / Solaris
 Partition 2 does not end on cylinder boundary.
 /dev/sda3             147        5227    40803328   83  Linux

 Disk /dev/sdb: 2147 MB, 2147483648 bytes
 255 heads, 63 sectors/track, 261 cylinders
 Units = cylinders of 16065 * 512 = 8225280 bytes
 Sector size (logical/physical): 512 bytes / 512 bytes
 I/O size (minimum/optimal): 512 bytes / 512 bytes
 Disk identifier: 0xcdd48395

   Device Boot      Start         End      Blocks   Id  System
 /dev/sdb1               1         261     2096451   83  Linux

 

新建一个分区并且设置它为启动分区。

 bash-4.1# fdisk /dev/sda

 WARNING: DOS-compatible mode is deprecated. It's strongly recommended to
         switch off the mode (command 'c') and change display units to
         sectors (command 'u').

 Command (m for help): n
 Command action
   e   extended
   p   primary partition (1-4)
 p
 Partition number (1-4): 1
 First cylinder (1-5226, default 1):
 Using default value 1
 Last cylinder, +cylinders or +size{K,M,G} (1-16, default 16):
 Using default value 16

 Command (m for help): a
 Partition number (1-4): 1

 Command (m for help): p

 Disk /dev/sda: 43.0 GB, 42991616000 bytes
 255 heads, 63 sectors/track, 5226 cylinders
 Units = cylinders of 16065 * 512 = 8225280 bytes
 Sector size (logical/physical): 512 bytes / 512 bytes
 I/O size (minimum/optimal): 512 bytes / 512 bytes
 Disk identifier: 0x00000000

   Device Boot      Start         End      Blocks   Id  System
 /dev/sda1   *           1          16      128488+  83  Linux
 /dev/sda2              17         147     1048576   82  Linux swap / Solaris
 Partition 2 does not end on cylinder boundary.
 /dev/sda3             147        5227    40803328   83  Linux

 Command (m for help): w
 The partition table has been altered!

 

重启主机以更新分区表,然后进入援救模式,并在我们新创建的分区上创建文件系统。

 bash-4.1# mkfs.ext4 /dev/sda1
 Filesystem label=
 OS type: Linux
 Block size=1024 (log=0)
 Fragment size=1024 (log=0)
 Stride=0 blocks, Stripe width=0 blocks
 32128 inodes, 128488 blocks
 6424 blocks (5.00%) reserved for the super user
 First data block=1
 Maximum filesystem blocks=67371008
 16 block groups
 8192 blocks per group, 8192 fragments per group
 2008 inodes per group
 Superblock backups stored on blocks:
        8193, 24577, 40961, 57345, 73729

 Writing inode tables: done
 Creating journal (4096 blocks): done
 Writing superblocks and filesystem accounting information: done

 This filesystem will be automatically checked every 38 mounts or
 180 days, whichever comes first.  Use tune2fs -c or -i to override.

 

安装内核镜像文件

通过前述的方法我们安装内核和镜像文件。

 bash-4.1# chroot /mnt/sysimage
 sh-4.1# mount /dev/sda1 /boot
 sh-4.1# mount – o loop /dev/sr0 /media
 sh-4.1# cd /media/Server/Packages
 sh-4.1# rpm -ivh --force kernel-2.6.32-71.el6.x86_64.rpm
 warning: kernel-2.6.32-71.el6.x86_64.rpm:
 Header V3 RSA/SHA256 Signature, key ID fd431d51: NOKEY
 Preparing...                ########################################### [100%]
   1:kernel                  ########################################### [100%]

 

安装 GRUB

我们安装 GRUB 到硬盘设备 sda 上。

 sh-4.1# grub-install /dev/sda
 Installation finished. No error reported.
 This is the contents of the device map /boot/grub/device.map.
 Check if this is correct or not. If any of the lines is incorrect,
 fix it and re-run the script `grub-install'.

 (fd0)   /dev/fd0
 (hd0)   /dev/sda
 (hd1)   /dev/sdb

 

编辑引导菜单

由于我们创建了新的分区,其对应的 UUID 会发生变化,可以通过命令 blkid 来查询分区的 UUID。

 bash-4.1# blkid
 /dev/loop0: TYPE="squashfs"
 /dev/sda2: UUID="7b1e0fac-ff06-492c-848d-497e2a38c54e" TYPE="swap"
 /dev/sda3: UUID="ef89764e-04ff-4f26-ae82-dcab267ecc66" TYPE="ext4"
 /dev/sdb1: UUID="2b824352-df2a-44c6-a547-838d46f526fa" SEC_TYPE="ext2" TYPE="ext3"
 /dev/loop1: LABEL="RHEL_6.0 x86_64 Disc 1" TYPE="iso9660"
 /dev/sda1: UUID="cec964af-1618-48ff-ac33-4ef71b9d3265" TYPE="ext4"

 

上述的 sda3 为根分区,编辑 /boot/grub/grub.conf 文件更新其对应的 UUID,其内容如下。

 title Red Hat Enterprise Linux 6
 root (hd0,0)
 kernel /vmlinuz-2.6.32-71.el6.x86_64
 root=UUID=ef89764e-04ff-4f26-ae82-dcab267ecc66 rhgb quiet
 initrd /initramfs-2.6.32-71.el6.x86_64.img

 

更新 /etc/fstab

类似的我们也需要更新 /etc/fstab 里 /boot 分区对应的新 UUID,其内容如下。

 #
 # /etc/fstab
 # Created by anaconda on Sun Mar 18 04:35:07 2012
 #
 # Accessible filesystems, by reference, are maintained under '/dev/disk'
 # See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info
 #
 UUID=ef89764e-04ff-4f26-ae82-dcab267ecc66 /                  ext4    defaults        1 1
 UUID=cec964af-1618-48ff-ac33-4ef71b9d3265 /boot              ext4    defaults        1 2
 UUID=7b1e0fac-ff06-492c-848d-497e2a38c54e swap               swap    defaults        0 0
 tmpfs                   /dev/shm                tmpfs   defaults        0 0
 devpts                  /dev/pts                devpts  gid=5,mode=620  0 0
 sysfs                   /sys                     sysfs   defaults        0 0
 proc                    /proc                    proc    defaults        0 0

 

现在我们的恢复步骤已经完成,重启主机后 GRUB 中可见我们配置的系统列表。
图 9. GRUB 菜单
图 9. GRUB 菜单

至此 /boot 分区已恢复,系统可正常引导启动。
图 10. 系统启动
图 10. 系统启动

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