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Inspecting the content of an initrd file
If you want to look what is inside of an initrd file (don’t ask me why I needed this ;)), here you can find some information on how you can do that. Depending on what kernel you are using you might encounter various initrd type of files.
Uncompressed initrd/cramfs type of file
Older kernels will use a initrd/cramfs type of file. If we want to look inside this file all we have to do is to mount it (as a loop device). Example showing how to mount a file called initrd.img-2.6.8-2-686 (from a default Debian Sarge kernel):
# All work is done in a temporary directory
mkdir /tmp/initrdmount
# Mount the image file directly
mount -o loop /boot/initrd.img-2.6.8-2-686 /tmp/initrdmount
#Investigate its content, etc.
cd /tmp/initrdmount
#Once done un-mount it:
umount /tmp/initrdmountSeen on: Debian Sarge, kernel 2.6.8
Compressed initrd/cramfs type of file
This is the same as above just that the file is compressed (with gzip). If we want to look inside this file we need to uncompress it first and then mount it (as a loop device). Example showing how to mount a file called initrd-2.4.21-40.EL.img (from a default RHEL3 kernel):
# All work is done in a temporary directory
mkdir /tmp/initrdmount
# Copy the image, uncompress it
cp /boot/initrd-2.4.21-40.EL.img /tmp/initrd.img.gz
gunzip -v /tmp/initrd.img.gz
# Mount the image file
mount -o loop /tmp/initrd.img /tmp/initrdmount
#Investigate its content, etc.
cd /tmp/initrdmount
#Once done un-mount it:
umount /tmp/initrdmountSeen on: RHEL3, Centos3, kernels 2.4.21
initramfs type of file
Newer kernels will use this type of file. The initramfs is an cpio archive so all we have to do is to uncompress it to a temporary directory. The example from bellow uses the file initrd.img-2.6.15-1-686-smp (from a default Debian Etch kernel):
# All work is done in a temporary directory
mkdir /tmp/initrdmount
# Copy the image, uncompress it
cp /boot/initrd.img-2.6.15-1-686-smp /tmp/initrd.img.gz
gunzip -v /tmp/initrd.img.gz
# Extract the content of the cpio archive
cd /tmp/initrdmount
cpio -i < /tmp/initrd.imgSeen on: Debian Etch, kernel 2.6.15; RHEL4, Centos4, kernel 2.6.9.
Tags: cramfs, initramfs, initrd, kernel
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28th June 2006, 01:01
Excellent article covering all the details
14th December 2006, 09:45
A little more depth (e.g., how the kernel uses the init ram disk, how to create a ram disk) would be useful but, otherwise good info.
5th January 2007, 15:27
In response to Eric P’s request. This article covers the in-depth details of ramfs:
http://linuxdevices.com/articles/AT4017834659.html
20th June 2008, 13:09
Excellent and timeless.
2 years later and it is still the simplest exposition of what needs to be done to accomplish the task.
No mumbo jumbo. Just the straight goods.
Thanks!