Kickstart driven CentOS 7 install from USB

None of what is written below is particularly original, however, I was unable to find a method documented on the internet at the time of writing that successfully created a kickstart driven CentOS 7 USB installer.

My interest was in doing this manually as I require this USB (image) to be created from a script. Therefore, I did not look into using ISO to USB applications - in addition, these typically do not allow custom kickstart files to be used.

References

Much of the process described below was found on the CentOS Wiki page on Installing from USB key, and from the Softpanorama page on the same subject. I thoroughly recommend reading all of the latter as it highlights the shortcomings/dangers associated with the steps below.

USB key preparation

Partition USB

This can probably be done as a disk image too, though I haven't tried this yet. Below I will use /dev/sdX for the USB device.

Create two partitions, one of type W95 FAT32 (LBA) (assigned code "c" in fdisk) of ~250MB, make this partition bootable. Create an ext3 partition from the remaining space.

sudo fdisk /dev/sdX
n (create partition, accept defaults for type, number, and first sector)
+250M (defined size as 250MB)
t
c (change type to W95 FAT32 (LBA) - other FAT types may work, but I have not tried)
a (make bootable)
n (create partition, accept defaults for type, number, first sector, and size)
w (write changes to device)

Format partitons

sudo mkfs -t vfat -n "BOOT" /dev/sdX1
sudo mkfs -L "DATA" /dev/sdX2

Write MBR data to device

sudo dd conv=notrunc bs=440 count=1 if=/usr/share/syslinux/mbr.bin of=/dev/sdX

Install syslinux to first parition

sudo syslinux /dev/sdX1

Copy files to USB

Mount the partitions

mkdir BOOT && sudo mount /dev/sdX1 BOOT
mkdir DATA && sudo mount /dev/sdX2 DATA
mkdir DVD && sudo mount /path/to/centos/dvd.iso DVD

Copy DVD isolinux contents to BOOT

sudo cp DVD/isolinux/* BOOT

rename isolinux.cfg to syslinux.cfg

sudo mv BOOT/isolinux.cfg BOOT/syslinux.cfg

I also deleted a few bits from BOOT I didn't think were required, e.g. isolinux.bin, TRANS.TBL, upgrade.img, grub.conf.
I then copied my kickstart file to the BOOT directory and the CentOS 7 ISO to the DATA partition.

The final file structure looked something like this:

BOOT/
├── boot.cat
├── boot.msg
├── initrd.img
├── ks.cfg
├── ldlinux.sys
├── memtest
├── splash.png
├── syslinux.cfg
├── upgrade.img
├── vesamenu.c32
└── vmlinuz
DATA/
└── CentOS-7.0-1406-x86_64-Minimal.iso

Edit the syslinux.cfg

So that it points to the ISO and the kickstart

Here is the install CentOS 7 entry from the Minimal ISO isolinux.cfg (which we renamed syslinux.cfg):

label linux                                                                     
  menu label ^Install CentOS 7                                                  
  kernel vmlinuz                                                                
  append initrd=initrd.img inst.stage2=hd:LABEL=CentOS\x207\x20x86_64 quiet

The append line is changed to read the following:

append initrd=initrd.img inst.stage2=hd:sdb2:/ ks=hd:sdb1:/ks.cfg

I suspect LABEL could be used here, rather than the enumerated device, which would make it safer, but I haven't tried this yet. Assuming the system you are installing on only has a single HD the USB key will be enumerated as sdb more information about this can be found in the Softpanorama article.

When you boot from the USB and select Install CentOS 7, it now installs the system as described by your kickstart.