3.实验：破坏本机grub stage1，⽽后在救援模式下修复之
开机现象
光标闪烁后，自动跳到救援模式
第一步：查看MBR前512个字节、55 aa结尾
主引导程序（446B），分区表（64B），硬盘有效标识 （2B），一共是（512B）
[root@centos6 ~]# hexdump -C /dev/sda -n 512 #查看MBR前512个字节 55 aa结尾
00000000 eb 48 90 10 8e d0 bc 00 b0 b8 00 00 8e d8 8e c0 |.H..............|
00000010 fb be 00 7c bf 00 06 b9 00 02 f3 a4 ea 21 06 00 |...|.........!..|
00000020 00 be be 07 38 04 75 0b 83 c6 10 81 fe fe 07 75 |....8.u........u|
00000030 f3 eb 16 b4 02 b0 01 bb 00 7c b2 80 8a 74 03 02 |.........|...t..|
00000040 80 00 00 80 c0 0c 05 00 00 08 fa 90 90 f6 c2 80 |................|
00000050 75 02 b2 80 ea 59 7c 00 00 31 c0 8e d8 8e d0 bc |u....Y|..1......|
00000060 00 20 fb a0 40 7c 3c ff 74 02 88 c2 52 f6 c2 80 |. ..@|<.t...R...|
00000070 74 54 b4 41 bb aa 55 cd 13 5a 52 72 49 81 fb 55 |tT.A..U..ZRrI..U|
00000080 aa 75 43 a0 41 7c 84 c0 75 05 83 e1 01 74 37 66 |.uC.A|..u....t7f|
00000090 8b 4c 10 be 05 7c c6 44 ff 01 66 8b 1e 44 7c c7 |.L...|.D..f..D|.|
000000a0 04 10 00 c7 44 02 01 00 66 89 5c 08 c7 44 06 00 |....D...f.\..D..|
000000b0 70 66 31 c0 89 44 04 66 89 44 0c b4 42 cd 13 72 |pf1..D.f.D..B..r|
000000c0 05 bb 00 70 eb 7d b4 08 cd 13 73 0a f6 c2 80 0f |...p.}....s.....|
000000d0 84 f0 00 e9 8d 00 be 05 7c c6 44 ff 00 66 31 c0 |........|.D..f1.|
000000e0 88 f0 40 66 89 44 04 31 d2 88 ca c1 e2 02 88 e8 |..@f.D.1........|
000000f0 88 f4 40 89 44 08 31 c0 88 d0 c0 e8 02 66 89 04 |..@.D.1......f..|
00000100 66 a1 44 7c 66 31 d2 66 f7 34 88 54 0a 66 31 d2 |f.D|f1.f.4.T.f1.|
00000110 66 f7 74 04 88 54 0b 89 44 0c 3b 44 08 7d 3c 8a |f.t..T..D.;D.}<.|
00000120 54 0d c0 e2 06 8a 4c 0a fe c1 08 d1 8a 6c 0c 5a |T.....L......l.Z|
00000130 8a 74 0b bb 00 70 8e c3 31 db b8 01 02 cd 13 72 |.t...p..1......r|
00000140 2a 8c c3 8e 06 48 7c 60 1e b9 00 01 8e db 31 f6 |*....H|`......1.|
00000150 31 ff fc f3 a5 1f 61 ff 26 42 7c be 7f 7d e8 40 |1.....a.&B|..}.@|
00000160 00 eb 0e be 84 7d e8 38 00 eb 06 be 8e 7d e8 30 |.....}.8.....}.0|
00000170 00 be 93 7d e8 2a 00 eb fe 47 52 55 42 20 00 47 |...}.*...GRUB .G|
00000180 65 6f 6d 00 48 61 72 64 20 44 69 73 6b 00 52 65 |eom.Hard Disk.Re|
00000190 61 64 00 20 45 72 72 6f 72 00 bb 01 00 b4 0e cd |ad. Error.......|
000001a0 10 ac 3c 00 75 f4 c3 00 00 00 00 00 00 00 00 00 |..<.u...........|
000001b0 00 00 00 00 00 00 00 00 d9 58 02 00 00 00 80 20 |.........X..... |
000001c0 21 00 83 aa 28 82 00 08 00 00 00 00 20 00 00 aa |!...(....... ...|
000001d0 29 82 83 fe ff ff 00 08 20 00 00 00 35 0c 00 fe |)....... ...5...|
000001e0 ff ff 83 fe ff ff 00 08 55 0c 00 80 1a 06 00 fe |........U.......|
000001f0 ff ff 05 fe ff ff 00 88 6f 12 00 78 90 06 55 aa |........o..x..U.|
第二部：备份MBR、破坏MBR
[root@centos6 ~]# dd if=/dev/sda of=mbr bs=1 count=512 #备份MBR
512+0 records in
512+0 records out
512 bytes (512 B) copied, 0.00184379 s, 311 kB/s
[root@centos6 ~]# dd if=/dev/zero of=/dev/sda bs=1 count=446 #破坏grub stage1（清空MBR前446字节）
446+0 records in
446+0 records out
446 bytes (446 B) copied, 0.00134689 s, 331 kB/s
[root@centos6 ~]# hexdump -C /dev/sda -n 512 ##查看MBR前512个字节、其中前446字节已被破坏
00000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
000001b0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 80 20 |............... |
000001c0 21 00 83 aa 28 82 00 08 00 00 00 00 20 00 00 aa |!...(....... ...|
000001d0 29 82 83 fe ff ff 00 08 20 00 00 00 35 0c 00 fe |)....... ...5...|
000001e0 ff ff 83 fe ff ff 00 08 55 0c 00 80 1a 06 00 fe |........U.......|
000001f0 ff ff 05 fe ff ff 00 88 6f 12 00 78 90 06 55 aa |........o..x..U.|
00000200
[root@centos6 ~]# reboot #重启
第三步：进入救援模式
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选择第三项Rescue installed system
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语言English
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键盘us
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不需要配置网络，这里选择no
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继续Continue
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这里从提示信息可以看出系统根目录将被挂载到/mnt/sysimage目录下，后面想执行相关命令所以得先执行切换根目录操作： chroot /mnt/sysimage
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选择第一项，将开启一个shell
第四步：修复、grub安装
第五步：再次查看MBR的前446字节：
第六步：关闭selinux
或者编辑/etc/selinux/config，修改为SELINUX=disabled也能实现同样的功能，还有⼀种⽅法是在启动时，中⽌启动进⼊gurb菜单，在kernel⾏的内核参数最后写上selinux=0完全可实现相同的功能
第七步：多运行几次sync确保写入磁盘
bash-4.1# sync #写入磁盘
bash-4.1# sync
bash-4.1# sync
bash-4.1# reboot #重启
