Depending on your system, you'll find it in any one of these:

1. /proc/config.gz
2. /boot/config
3. /boot/config-$(uname -r)

and possibly more places.

For an actual running kernel, one way to get the config file this is to

cat /proc/config.gz | gunzip > running.config

or,

zcat /proc/config.gz > running.config

Then running.config will contain the configuration of the running linux kernel.

However this is only possible if your running linux kernel was configured to have /proc/config.gz. The configuration for this is found in

• General setup
  • [*] Kernel .config support
    • [*] Enable access to .config through /proc/config.gz

Most distributions do not have this configuration set. They provide kernel config files in their kernel packages and is usually found in /boot/ directory.

A Little bit late but maybe it helps someone. I didn't have /proc/config.gz nor /boot/config nor /boot/config-$(uname -r) on my Computer. I had to run modprobe configs as root. Then, /proc/config.gz was present

Regardless of the distribution, you can run: cat /lib/modules/$(uname -r)/build/.config

Source: proc(5) man page (search for /proc/config.gz).

If you couldn't find kernel configuration in /boot/ nor in /proc/config.gz, you can try extracting this information from the kernel itself.

Inside any kernel source code there is a script for extracting config located in scripts/extract-ikconfig, pass the kernel you want its configuration as parameter to this script.

This solution will only work if Kernel .config support was enabled in the compiled kernel.

For RedHat-based distributions, the .config file of the off-the-shelf kernel can be found with the command cat /lib/modules/$(uname -r)/build/.config that's available after the package kernel-devel is installed using the command:

yum -y install kernel-devel

Note that with the real Red Hat Enterprise Linux distribution, you will need to enable the source-repository to get this package. On RHEL8, use the following command to do that:

subscription-manager repos --enable=rhel-8-for-x86_64-baseos-source-rpms

If you can't find any of the suggested files and you are able to modprobe you should almost always be able to get a copy of the current config this way.

modprobe configs # might need `sudo modprobe configs`

# This will create /proc/config.gz
zcat /proc/config.gz

# Or if you are looking for whether a specific option was set
zgrep USBIP /proc/config.gz