Master Linux Kernel Essentials: Modules, Commands, and System Configuration

1. Kernel Overview

The Linux kernel is an open‑source, monolithic Unix‑like operating system core that acts as the central interface between hardware and processes, managing resources efficiently. Its main responsibilities include memory management, process scheduling, device driver mediation, and handling system calls and security.

2. Kernel Modules

2.1 uname command

Usage: uname [OPTION]... Options:

-n display node name

-r display VERSION‑RELEASE

-s display kernel name

-v display kernel version

-m display hardware name

-i display hardware platform

-p display processor type

-o display operating system

# uname -m
i686

# uname -r
2.6.32-573.22.1.el6.i686

# uname -a
Linux MyServer 2.6.32-573.22.1.el6.i686 ... i686 i386 GNU/Linux

2.2 lsmod command

Shows currently loaded kernel modules.

Definition: Data is read from /proc/modules. Typical usage:

lsmod | grep -i ext4

# cat /proc/modules
iptable_filter 2173 0 - Live 0xed9b2000
ip_tables 9567 1 iptable_filter, Live 0xed9a9000
ext3 203718 1 - Live 0xed962000
jbd 65315 1 ext3, Live 0xed904000
…

# lsmod | grep ext4
ext4               339812  2
jbd2                75927  1 ext4
mbcache             6017  2 ext3,ext4

Field meanings:

Column 1: module name

Column 2: module size

Column 3: number of dependent modules

Column 4: list of dependent modules

2.3 modinfo command

Displays detailed information about a kernel module.

Definition: modinfo queries information for a specified module, even if it is not currently loaded.

# modinfo ext4
filename:       /lib/modules/2.6.32-573.22.1.el6.i686/kernel/fs/ext4/ext4.ko
license:        GPL
description:    Fourth Extended Filesystem
author:         Remy Card, Stephen Tweedie, Andrew Morton, Andreas Dilger, Theodore and others
srcversion:     CB1B990F5A758DFB0FB12F1
depends:        mbcache,jbd2
vermagic:       2.6.32-573.22.1.el6.i686 SMP mod_unload modversions 686

# modinfo btrfs
filename:       /lib/modules/2.6.32-573.22.1.el6.i686/kernel/fs/btrfs/btrfs.ko
license:        GPL
alias:          devname:btrfs-control
alias:          char-major-10-234
srcversion:     B412C18B0F5BF7F1B3C941A
depends:        libcrc32c,zlib_deflate,lzo_compress,lzo_decompress
vermagic:       2.6.32-573.22.1.el6.i686 SMP mod_unload modversions 686

2.4 modprobe command

Loads or unloads kernel modules.

Definition: Uses configuration files ( /etc/modprobe.conf or /etc/modprobe.d/*.conf) and dependency information from modules.dep (generated by depmod) to insert or remove modules.

# modules.dep is the dependency file; modules.dep.bin is its binary form
# ls /lib/modules/2.6.32-358.6.1.el6.i686/
build      modules.block    modules.ieee1394map  modules.ofmap     modules.symbols.bin  weak-updates
extra      modules.ccwmap   modules.inputmap     modules.order     modules.usbmap
kernel     modules.dep      modules.isapnpmap    modules.pcimap    source
modules.alias     modules.dep.bin  modules.modesetting  modules.seriomap  updates
modules.alias.bin  modules.drm      modules.networking   modules.symbols   vdso

Syntax examples:

modprobe -c

modprobe -l -t dirname wildcard

modprobe -r -v -n -i modulename …

Common options:

-v verbose output

-C reload configuration

-c print configuration and exit

-n dry‑run (often with -v)

-i ignore install/remove directives in config files

-q quiet (suppress errors)

-r remove module instead of inserting

-f force (dangerous)

-l list all modules

-a insert all modules from the command line

-t force display of modules in a directory

-s log errors to syslog

2.5 depmod command

Generates kernel module dependency files and system‑information mapping files.

Syntax: depmod [-adeisvV] [-m <file>] [--help] [module_name] Key options:

-a analyze all available modules

-d debug mode

-e output unresolved symbols

-i ignore symbol version checks

-m<file> use specified symbol file

-s record errors in system log

-v verbose output

-V show version

--help display help

2.6 insmod and rmmod commands

Insert or remove kernel modules directly.

insmod: Inserts a module file into the kernel. Typical usage: insmod [filename] [module options...]. Most users prefer modprobe because it resolves dependencies automatically.

rmmod: Removes a module from the kernel. Usually invoked via modprobe -r. Options include:

-f force removal (requires CONFIG_MODULE_FORCE_UNLOAD)

-w wait until the module is no longer in use

-s log errors to syslog instead of stderr

# rmmod modulename

3. /proc Filesystem

The kernel exposes internal state, statistics, and configurable parameters through the proc pseudo‑filesystem.

Typical entries include /proc/cpuinfo, /proc/meminfo, /proc/sys, and many others. Example to list the directory:

# ls /proc/
1 1173 22 29855 35 47 60 973 filesystems loadavg scsi version
10 12 23 3 36 48 600 buddyinfo fs locks self vmallocinfo
…

3.1 sysctl command

Syntax: sysctl [options] [parameter] Common options:

-n print only values

-e ignore unknown keys

-N print only names

-w write a new value

-p load settings from /etc/sysctl.conf -a display all parameters

-A display all parameters in a table

Examples:

# sysctl -w kernel.sysrq=0
# sysctl -p
# sysctl -a

3.2 Modifying configuration files

# cat /etc/sysctl.conf
# Kernel sysctl configuration file for Red Hat Linux
net.ipv4.ip_forward = 0
net.ipv4.conf.default.rp_filter = 1
net.ipv4.conf.default.accept_source_route = 0
kernel.sysrq = 0
kernel.core_uses_pid = 1
net.ipv4.tcp_syncookies = 1
…

3.3 Practical usage

# sysctl -a
# sysctl -w kernel.sysrq=0
# sysctl -w net.ipv4.conf.default.accept_redirects=0
# echo 1 > /proc/sys/net/ipv4/ip_forward
# /sbin/sysctl -p
# /sbin/sysctl -w net.ipv4.route.flush=1

4. /sys Filesystem

sysfs presents hardware device attributes and configurable kernel parameters; many entries are writable for tuning.

4.1 udev

udev runs in user space and creates device nodes under /dev based on information from /sys.

It processes hardware add/remove events and loads required firmware.

Rules are read from /etc/udev/rules.d and /usr/lib/udev/rules.d.

4.2 Creating a ramdisk

Method 1 (mkinitrd):

# mkinitrd /boot/initramfs-$(uname -r).img $(uname -r)

Method 2 (dracut):

# dracut /boot/initramfs-$(uname -r).img $(uname -r)

4.3 Inspecting a ramdisk

# file /boot/initramfs-2.6.32-504.el6.x86_64.img
# gzip -d initramfs-2.6.32-504.el6.x86_64.img.gz
# cpio -id < ../initramfs-2.6.32-504.el6.x86_64.img
# cat init
# ls sbin

5. Compiling the Kernel

5.1 Preparation

Install development groups (e.g., "Server Platform Development", "Development Tools") on CentOS 6.

Gather hardware information (CPU, PCI, USB, block devices) using cat /proc/cpuinfo, lspci, lsusb, lsblk, etc.

Obtain the kernel source from www.kernel.org.

5.2 Simple kernel installation

Copy the current kernel config as a starting point:

# cp /boot/config-$(uname -r) .config
# make menuconfig   # adjust options graphically
# make -j$(nproc)    # compile using all cores
# make modules_install
# make install       # installs bzImage, initramfs and updates GRUB

5.3 Detailed compilation steps

Configuration options: make config CLI‑based configuration make menuconfig curses UI make gconfig GTK UI make xconfig Qt UI make defconfig default configuration for the target make allnoconfig all options set to "no"

Compilation:

# make -j4          # compile with 4 parallel jobs
# make dir/         # compile only a specific subdirectory
# make drivers/net/ethernet/intel/e1000/e1000.ko   # compile a single module

Cross‑compilation: make ARCH=arm64 (replace arm64 with the target architecture).

Cleaning the source tree: make clean remove most generated files make mrproper remove all generated files plus config make distclean remove everything including patches and backup files