BLOG08a: Linux GRUB Parameters

BLOG18: Linux GRUB Parameters (Initialization), Runtime Behavior (sysctl Tunables), Dynamic Modules (modprobe)

What are the differences between GRUB kernel boot parameters, sysctl parameters, and modprobe kernel modules?

This document explains the differences between three types of kernel configuration:

• Kernel boot parameters in GRUB (cmdline parameters)

• sysctl parameters

• modprobe kernel modules

The following sections provide a clear explanation of the differences between all three.

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1. Kernel Parameters in GRUB (Boot-time Kernel Arguments)

These are kernel command-line parameters passed at boot via GRUB.

They modify how the kernel initializes itself before modules load and before sysctl is applied.

Examples:

cgroup_enable=memory
systemd.unified_cgroup_hierarchy=1
iptables=legacy
swapaccount=1
ipv6.disable=1

Where they live?

/etc/default/grub

When applied?

• At boot time

• Before the kernel starts

• Changes require reboot

Purpose:

• Enable/disable kernel subsystems

• Change cgroup modes

• Disable swap, IPv6

• Configure low-level memory handling

• Control security features (AppArmor/SELinux startup mode)

These settings cannot be changed at runtime.

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2. sysctl Parameters (Runtime Kernel Tunables)

These modify kernel behavior while the system is running.

Location:

/proc/sys/* /etc/sysctl.conf /etc/sysctl.d/*

When applied?

• At runtime (immediate effect)

• Can be persistent with sysctl.d

• Does not require reboot (unless certain networking options)

Purpose:

Adjust runtime networking, memory limits, routing, etc.

Examples:

net.ipv4.ip_forward = 1
net.core.somaxconn = 1024
vm.swappiness = 10

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3. modprobe (Kernel Modules)

Loads or unloads kernel modules (.ko files) — similar to drivers.

Examples:

modprobe br_netfilter
modprobe overlay
modprobe ip_tables

Purpose:

• Add features to the kernel dynamically

• Allow Kubernetes networking or storage drivers

• Implement packet filtering modules

When applied?

• At runtime

• No reboot required

• Must persist using /etc/modules-load.d/*.conf

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How All Three Interact

GRUB Parameters

• Affect kernel core behavior

• Set things that cannot be changed later

• Example: cgroup v1 vs v2 mode

modprobe Kernel Modules

• Add extra "plugin" features

• Loadable anytime after boot

sysctl Runtime Parameters

• Configure behavior of already-loaded kernel components

• Change limits instantly

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Example to Understand the Difference

The following example uses cgroup memory accounting (important for Kubernetes):

1. GRUB parameter (boot-time)

cgroup_enable=memory
swapaccount=1

• Enables memory cgroups/full accounting

• Must be set before boot

• Required by kubelet or containerd runtime features

• Cannot be changed with sysctl

2. modprobe (module load)

modprobe br_netfilter

• Loads the module to inspect packets on Linux bridges

3. sysctl parameter (runtime behavior)

sysctl -w net.ipv4.ip_forward=1

• Enables routing (only works if networking modules exist)

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Simple Analogy

Category	Analogy
GRUB kernel arguments	BIOS settings — set before the system starts
modprobe modules	Plugging in new hardware/driver while PC is running
sysctl	Changing Windows registry/network tuning at runtime

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Kubernetes Example

To run Kubernetes properly, you may need all three:

Layer	Example	Why
GRUB	cgroup_enable=memory, swapaccount=1	Allow kubelet/containerd to manage memory
modprobe	br_netfilter, overlay	Allow bridge filtering + overlay fs
sysctl	net.ipv4.ip_forward=1, bridge-nf-call-iptables=1	Enable pod networking

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Summary Table

Feature	GRUB Kernel Args	modprobe	sysctl
When applied	At boot	Runtime	Runtime
Requires reboot	Yes	No	No
Controls	Kernel core behavior	Kernel modules (drivers)	Kernel behavior tuning
Examples	cgroup_enable, swapaccount	overlay, br_netfilter	ip_forward, swappiness
Who needs it?	kubelet runtime, memory cgroups	CNI, container runtime	kube-proxy, routing
Can change dynamically?	No	Yes	Yes

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The following sections provide a complete, practical, production-grade list of commands to show:

• Loaded kernel modules

• Available (but NOT loaded) modules

• All GRUB kernel parameters

• All sysctl tunables (current + default + available)

Everything works on Ubuntu, Debian, RHEL/CentOS, Rocky, Amazon Linux, etc.

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1. List All LOADED Kernel Modules

These are modules currently active in the running kernel:

Command

lsmod

Better readable output:

lsmod | sort

Check if a specific module is loaded:

lsmod | grep <module>

Example:

lsmod | grep br_netfilter

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2. List All AVAILABLE Kernel Modules (Installed but NOT loaded)

These are modules present under /lib/modules/$kernel/ but not loaded.

Command

find /lib/modules/$(uname -r)/kernel/ -type f -name "*.ko*"

To see only module names (clean list):

find /lib/modules/$(uname -r)/kernel/ -type f -name "*.ko*" | sed 's|.*/||' | sed 's/\.ko.*//'

To compare:

Loaded vs Not loaded

comm -13 <(lsmod | awk '{print $1}' | sort) \
         <(find /lib/modules/$(uname -r)/kernel/ -type f -name "*.ko*" | sed 's|.*/||' | sed 's/\.ko.*//' | sort)

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3. List All GRUB Kernel Parameters

These are parameters passed to the kernel at boot.

See parameters used by the currently running kernel

cat /proc/cmdline

See configured GRUB file (what will apply after reboot)

cat /etc/default/grub

See all menu entries (grub.cfg)

grep linux /boot/grub/grub.cfg

To get ONLY kernel parameters from grub.cfg

grep -oP 'linux.* \K.*' /boot/grub/grub.cfg

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4. List ALL sysctl tunables

A. Show all CURRENT sysctl kernel settings

sysctl -a

B. Show only network sysctl values

sysctl -a | grep net

C. Show only modified (active) sysctl values

sysctl -a --pattern '.*'

Or:

cat /proc/sys/*/*/* 2>/dev/null

D. Show sysctl configurations applied from files

(Defaults + overrides)

1. Global config:

cat /etc/sysctl.conf

2. Drop-in configs:

ls -1 /etc/sysctl.d/

Example:

cat /etc/sysctl.d/*.conf

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5. List All sysctl Parameters Available in the Kernel

These are every kernel parameter that exists under /proc/sys/.

Directory listing:

find /proc/sys -type f

Clean list:

find /proc/sys -type f | sed 's|/proc/sys/||' | tr '/' '.'

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SUMMARY (For Quick Use)

Category	Command
Loaded modules	lsmod
Available modules	find /lib/modules/$(uname -r)/kernel -name "*.ko*"
GRUB parameters (running)	cat /proc/cmdline
GRUB parameters (configured)	cat /etc/default/grub
All sysctl tunables (active)	sysctl -a
Sysctl configs	/etc/sysctl.conf, /etc/sysctl.d/*.conf
All sysctl available	find /proc/sys

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