Dissecting and amending an Ubuntu initrd
	Part X. Debian (sometimes also Ubuntu)

Dissecting and amending an Ubuntu initrd

Jurjen Bokma

March 2009

The LWP is going to run Ubuntu, so when we need a special purpose initial RAMdisk (i.c. for a partitioning script), it makes sense to work from the Ubuntu installer initrd.

Fetch the original ramdisk and unpack it
Look in the PXELinux configuration file used by the LWP unattended installer for the location of the initrd, and fetch it into a new directory. Take a look at the file, then unzip and unpack it:

$ file initrd.gz initrd.gz: gzip compressed data, was "initrd", from Unix, last modified: Thu Jun 19 21:28:22 2008, max compression $ gzip -d initrd.gz $ file initrd initrd: ASCII cpio archive (SVR4 with no CRC) $ mkdir ramdisk && cd ramdisk $ sudo cpio -idm < ../initrd 36329 blocks $ ls -trl total 20 drwxr-xr-x 2 root root 6 2008-02-06 17:51 sys drwxr-xr-x 2 root root 6 2008-02-06 17:51 proc drwxr-xr-x 2 root root 6 2008-02-06 17:51 mnt drwxr-xr-x 2 root root 6 2008-02-06 17:51 initrd -rwxr-xr-x 1 root root 472 2008-02-06 17:51 init drwxr-xr-x 2 root root 6 2008-02-06 17:51 floppy drwxr-xr-x 2 root root 6 2008-06-19 21:27 tmp drwxr-xr-x 8 root root 71 2009-03-06 10:19 var drwxr-xr-x 2 root root 4096 2009-03-06 10:19 sbin drwxr-xr-x 12 root root 4096 2009-03-06 10:19 lib drwxr-xr-x 2 root root 4096 2009-03-06 10:19 bin drwxr-xr-x 6 root root 49 2009-03-06 10:19 usr drwxr-xr-x 14 root root 4096 2009-03-06 10:19 etc drwxr-xr-x 2 root root 31 2009-03-06 10:19 dev

sudo is needed here to create the device files that are inside the usual initrd. Their presence also makes it dangerous to open all files inside the ramdisk...
Amend the RAMdisk
Copy whatever you need in the RAMdisk into the unpacked directory tree:
cp /tmp/mkdosfs ./sbin cd sbin ln -s mkdosfs mkfs.msdos

Warning
If you copy binaries, either bring along the libraries they need, or make sure they are statically linked! As for how to do that, see e.g. .

To run a script, I created a parasite script in lib/debian-installer.d/S57repartition:
```
#/bin/sh
anna-install parted-udeb
wget -P /tmp http://osis.service.rug.nl/pub/jb/partitioner.sh
sh /tmp/partitioner.sh -x
reboot
	
```
Re-archive and recompress the ramdisk:
$ find ./ |sudo cpio -H newc -ov > ../initrd <snip> $ gzip -9 ../initrd

	Warning
	If you copy binaries, either bring along the libraries they need, or make sure they are statically linked! As for how to do that, see e.g. .


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