How to Diagnose and Fix Common Linux Boot, Network, and File‑System Failures

Linux System Boot Failure

Common boot problems stem from four main causes:

Incorrect or missing /etc/fstab – mis‑configured file‑system entries prevent the system from mounting partitions. Recover the file by booting into rescue mode and rebuilding it.

Root‑filesystem corruption after illegal shutdown – sudden power loss can leave the ext3/ext4 file system inconsistent. The system drops to a maintenance shell showing messages such as

checking root filesystem

and

UNEXPECTED INCONSISTENCY; RUN fsck MANUALLY

. Run

fsck -y /dev/sdX

after unmounting the affected partition.

Missing kernel or initramfs files – the boot loader cannot find

vmlinuz

or

initrd.img

in the

/boot

partition. Restore the files from installation media or a backup and update

grub.cfg

.

Hardware failure – defective motherboard, power supply, or disk can also stop booting; replace the faulty component.

<code># umount /dev/sdb5
# fsck.ext3 -y /dev/sdb5</code>

Always unmount the partition before running

fsck

.

Linux Network Failure

Network issues are tackled in four steps:

Verify physical hardware (NIC, cables, switch, router) and replace if defective.

Check that the NIC driver is loaded using

ifconfig

or

ip a

. Use

ethtool

to inspect link speed and status.

Confirm correct IP configuration and ensure no address conflicts.

Inspect the routing table; a wrong default route (e.g., pointing to a LAN‑only subnet) can block external access. Adjust with

route delete default

and

route add default gw 10.10.1.254

.

<code># route delete default
# route add default gw 10.10.1.254</code>

Check DNS resolution by reviewing

/etc/host.conf

and

/etc/nsswitch.conf

(e.g.,

hosts: files dns

).

MBR Sector Failure

Symptoms include “Operating system not found” and boot interruption. Recovery steps:

Add a new virtual disk, partition it (e.g.,

fdisk -l

,

fdisk /dev/sdb

), format, and mount it.

Backup the MBR with

dd if=/dev/sda of=~/mbr.backup bs=512 count=1

.

In rescue mode, restore the MBR using the backup file.

GRUB Boot Failure

Two typical causes:

Incorrect

grub.cfg

entries – the kernel or initramfs paths are wrong, leading to “error: you need to load the kernel first”. Fix by editing

/boot/grub2/grub.cfg

to point to the correct files.

Missing GRUB files – the boot loader cannot locate its modules. Reinstall GRUB with

grub2-install /dev/sda

and regenerate the config.

<code># grub2-install /dev/sda
# grub2-mkconfig -o /boot/grub2/grub.cfg</code>

Forgot Linux Root Password

Two recovery methods:

Rescue mode reset – boot from rescue media, chroot into the system (

chroot /mnt/sysimage

) and run

passwd root

to set a new password.

Single‑user mode – at the GRUB menu press

e

, append

single

to the kernel line, boot, then change the password with

passwd

.

<code># chroot /mnt/sysimage
# passwd root
# exit
# reboot</code>

Read‑Only File System Error

When commands such as

cp

,

mv

, or

chmod

fail with “Read‑only file system”, possible causes are file‑system damage, disk errors, or a wrong

/etc/fstab

entry.

Remount the filesystem read‑write:

mount -o rw,remount /system

.

If the FS is corrupted, run

fsck

on the unmounted device.

Hardware failure may require disk replacement.

<code># mount -o rw,remount /system
# fsck -y /dev/VolGroup00/LogVol00</code>