Here is a quick overview of the three methods we will explore in detail:

Methods 2 and 3 both require that CONFIG_IKCONFIG was enabled when building your kernel. If you have not enabled it, only method 1 is available. We will explain how to check this and what to do in either case.

The quickest way to confirm you are running the right kernel is the uname command. It stands for unix name and reports information about the running system.

uname -r

The -r flag asks for the kernel release version. On a custom-built kernel 6.x, you might see:

6.8.0-myproject

This string comes directly from how you configured the kernel before building it. Let’s break it down:

The -myproject part comes from the CONFIG_LOCALVERSION setting in your kernel configuration. When you set it to -myproject during make menuconfig, it gets appended to the version string — making it immediately obvious which custom build is running.

For a more complete system info dump, use uname -a which shows everything:

uname -a
Linux myhostname 6.8.0-myproject #1 SMP PREEMPT Sat Jun 1 10:30:00 IST 2024 x86_64 x86_64 x86_64 GNU/Linux

Here is something many developers do not know: Linux can embed a copy of the entire .config file used to build it directly inside the kernel image. When you enable CONFIG_IKCONFIG during kernel configuration, the build system compresses and bakes the configuration file into the vmlinuz image itself.

This means you can always retrieve the exact configuration that was used to build any kernel image, even years later, as long as you have the vmlinuz file.

The tool to extract it is a shell script that comes with the kernel source:

scripts/extract-ikconfig /boot/vmlinuz-6.8.0-myproject

This dumps the full kernel configuration to stdout. Since it can be hundreds or thousands of lines, you typically pipe it to grep to find specific options:

# Check if a specific feature is enabled
scripts/extract-ikconfig /boot/vmlinuz-6.8.0-myproject | grep CONFIG_IKCONFIG

# Expected output:
CONFIG_IKCONFIG=y
CONFIG_IKCONFIG_PROC=y

Let’s look at what some typical options look like:

These three forms are how every configuration option appears. =y means the feature is compiled directly into the kernel binary. =m means it is built as a loadable kernel module. is not set means the feature was disabled.

You do not even need the kernel source tree to use extract-ikconfig. The script can be obtained directly:

# On Ubuntu/Debian — the script is in the linux-source package
# Or you can use the one bundled with kernel-source package

# Alternative: use grep + dd (the old-school way that always works)
grep -a 'CONFIG_' /boot/vmlinuz-6.8.0-myproject | head -30

The third method is the most elegant. When you enable CONFIG_IKCONFIG_PROC (which requires CONFIG_IKCONFIG to also be enabled), the kernel exposes its own configuration as a compressed file in the proc filesystem at /proc/config.gz.

This means you can query the running kernel’s configuration at any time without touching any files on disk:

# Decompress and search for a config option
zcat /proc/config.gz | grep CONFIG_PREEMPT

# Expected output on a preemptible kernel:
CONFIG_PREEMPT=y
# CONFIG_PREEMPT_NONE is not set

You can also dump the entire config to a file for later comparison:

zcat /proc/config.gz > /tmp/running-kernel.config

When you boot a freshly compiled kernel, here is a systematic checklist to verify everything is correct:

uname -r
# Should show: 6.x.y-yourtag

cat /proc/cmdline
# Shows: BOOT_IMAGE=/boot/vmlinuz-6.x.y-yourtag root=UUID=... ro quiet splash

zcat /proc/config.gz | grep CONFIG_IKCONFIG
# Expected:
# CONFIG_IKCONFIG=y
# CONFIG_IKCONFIG_PROC=y

zcat /proc/config.gz | grep -E "CONFIG_PREEMPT|CONFIG_SMP|CONFIG_HZ"
# Verify each option matches what you set during make menuconfig

zcat /proc/config.gz > /home/user/kernel-6.8.0-myproject.config

The concepts above apply to all modern kernels, but here are some specific things that differ between older 5.x kernels (used in many textbooks) and the current 6.x kernel series: