[CentOS] How is initrd.img packed and compressed?

Wed Jan 31 22:48:13 UTC 2018
Mircea Husz <mirceahusz at yahoo.com>

On Wed, 2018-01-31 at 19:46 +0000, James Pearson wrote:
> Mircea Husz wrote:
> > In order to work around a known upstream bug I needed to add a udev
> > rule to pxeboot initrd.img on CentOS 7.
> > 
> > The process is straightforward:
> > 1 - extract the pxeboot initrd.img  to a new directory
> > 2 - add the udev rule needed to fix the bug
> > 3 - pack and compress it back in initrd.img format
> Instead of re-creating a new initrd.img, why not just create an 
> 'updates' image that contains your new udev rule and use the 
> 'inst.updates=' pxeboot cmdline option?
> I use this to add things to the install image - no need to alter the 
> existing initrd image - if you need more info, let me know
> James Pearson

I would very much like your idea to work for us. I just tried it and
found that inst.updates 
gets loaded later in the boot process than when it's in initrd.img, and
for this particular bug
 that's too late. The udev rule we're adding deals with renaming the
network interface, which 
happens earlier in the boot process.