Linux: How to use rescue system to fix server problem


Step 1: boot into rescue mode

Some server provider provide feature Reload Operating System

rescue system

Step 2: find mount root filesystem

root@sysresccd /root % fdisk -l

Disk /dev/sda: 119.2 GiB, 128035676160 bytes, 250069680 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x000613a7

Device    Boot     Start       End    Blocks  Id System
/dev/sda1           2048      6143      2048  83 Linux
/dev/sda2 *         6144   1030143    512000  83 Linux
/dev/sda3        1030144 250068991 124519424  8e Linux LVM


Disk /dev/sdb: 1.8 TiB, 2000398934016 bytes, 3907029168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0x00006199

Device    Boot Start        End     Blocks  Id System
/dev/sdb1       2048 3907029167 1953513560  83 Linux


Disk /dev/mapper/vg-swap: 7.8 GiB, 8321499136 bytes, 16252928 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

Disk /dev/mapper/vg-tmp: 1 GiB, 1073741824 bytes, 2097152 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

Disk /dev/mapper/vg-root: 110 GiB, 118107406336 bytes, 230678528 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

Step 3: mount root filesystem

root@sysresccd / % mkdir /old
root@sysresccd / % mount -t ext4 /dev/mapper/vg-root /old

Step 4: do your fix

Example edit file /old/etc/ssh/sshd_config

Step 5: reboot into regular mode

Done!

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