Linux can store a gzip’ed copy of the kernel configuration file used to build the kernel in the kernel itself, and make it available to users via /proc/config.gz.
This file is not always present in a Linux distribution. It is only present when /proc support is enabled and CONFIG_IKCONFIG and CONFIG_IKCONFIG_PROC are set to Y in the kernel configuration file. Most distributions do not enable it by default; instead they include the kernel configuration file under /boot.
If in-kernel configuration support is built as a kernel module, then before you can use /proc/config.gz, the configs module needs to be loaded into the kernel using insmod or modprobe.
To quickly check a configuration setting, you could use zcat or zgrep:
# zcat /proc/config.gz | grep IKCONFIG CONFIG_IKCONFIG=y CONFIG_IKCONFIG_PROC=y
To extract a copy of the kernel configuration file you could do something like the following:
# zcat /proc/config.gz > /boot/config-$(uname -r).new
Finally, if CONFIG_IKCONFIG is set but CONFIG_IKCONFIG_PROC was not set, the extract-ikconfig script can be used to extract the configuration file from a kernel image file.