KVM RHCOS PXE Boot⌗

We will create a KVM network called mylab and launch a RHCOS instance using PXE boot. We will use KVM’s integrated DHCP and TFTP server.

Prerequisites:

Linux system with kvm/libvirt installed (using Ubuntu 20.04 for this example).
3GB RAM for the VM.

Steps⌗

Create a libvirt network with static DHCP and TFTP settings
Download initramfs, kernel and rootfs
Prepare a TFTP server for hosting initramfs (initrd.img) and the kernel (vmlinuz)
Configure PXE
Prepare a HTTP server for hosting the rootfs (rhcos-live-rootfs.x86_64.img)
Boot the VM

Create a libvirt network⌗

We define static DHCP entries in virsh net configuraion.
Same goes for the TFTP server root dir and bootp file.

<!-- mylab.xml -->
<network>
    <name>mylab</name>
    <forward mode='nat'>
        <nat>
        <port start='1024' end='65535'/>
        </nat>
    </forward>
    <ip address='192.168.132.1' netmask='255.255.255.0'>

        <tftp root='/var/www/tftpboot' />

        <dhcp>
        <range start='192.168.132.100' end='192.168.132.254'/>
        <host name='bootstrap' mac='52:54:00:90:6c:19' ip='192.168.132.19'/>

        <bootp file="pxelinux.0" />
        </dhcp>
    </ip>
</network>

virsh net-define mylab.xml
virsh net-start mylab

Download initramfs, kernel and rootfs⌗

We’ll be using RedHat’s official images.
For this we need a RedHat account (it’s free, so far).
It’s also possible to do this with Fedora CoreOS images, which is an upstream of RHCOS (or any other OS for that matter).

Download initramfs, kernel and rootfs from:
https://console.redhat.com/openshift/install/platform-agnostic

Prepare a TFTP server⌗

While we’re using KVM to serve the contents of /var/www/tftpboot, we still need to populate that dir.

# /var/www/tftpboot# tree
.
├── ldlinux.c32
├── libutil.c32
├── menu.c32
├── pxelinux.0
├── pxelinux.cfg
│   └── default         # <-- boot menu, config for pxelinux.0
└── rhcos
    ├── initrd.img      # <-- initramfs, served by TFTP
    └── vmlinuz         # <-- kernel, served by TFTP

(On Ubuntu), we can get the libraries by installing the following packages:

apt-get install pxelinux syslinux
# /usr/lib/PXELINUX/pxelinux.0
# /usr/lib/syslinux/modules/bios/ldlinux.c32
# /usr/lib/syslinux/modules/bios/libutil.c32
# /usr/lib/syslinux/modules/bios/menu.c32

Configure PXE⌗

mkdir pxelinux.cfg
touch pxelinuc.cfg/default

pxelinux.0 is a lightwaight OS that starts the bootstrapping process.
pxelinux.cfg/default is configuration for that OS.

# pxelinux.cfg/default
DEFAULT menu.c32
PROMPT 0

MENU TITLE ### MyLab boot menu ###
MENU AUTOBOOT Starting SuperOS in # seconds
TIMEOUT 5
TOTALTIMEOUT 10
ONTIMEOUT local

LABEL local
  MENU LABEL rhcos
  MENU DEFAULT
  KERNEL rhcos/vmlinuz
  APPEND initrd=rhcos/initrd.img ip=dhcp coreos.live.rootfs_url=http://192.168.132.1:8080/rhcos/rhcos-live-rootfs.x86_64.img

# In case we want to add an ignition file (RHCOS and Fedora CoreOS)
# coreos.inst.ignition_url=http://<HTTP_server>/bootstrap.ign

Prepare a HTTP server for serving rootfs⌗

Last thing missing; we need to serve the rootfs defined in pxelinux.cfg/default:

coreos.live.rootfs_url=http://192.168.132.1:8080/rhcos/rhcos-live-rootfs.x86_64.img

Ideally, we would like a HTTPS server but this demo focuses on PXE.
We’ll be using NGINX to server our filesystem.

# /etc/nginx/sites-enabled/pxe 
server {
	listen 8080 default_server;

	root /var/www/pxe;

	location / {
		autoindex on;
	}
}

We’re serving the following filesystem image:

/var/www/pxe/rhcos/rhcos-live-rootfs.x86_64.img

Boot the VM⌗

If all is well, we should be able to network boot a RHCOS VM.
Make sure to use at least 3GB of RAM

virt-install \
    --pxe \
    --connect=qemu:///system \
    --name=bootstrap \
    --vcpus=2 \
    --memory=3072 \
    --os-variant=fedora31 \
    --disk=size=10 \
    --network network=mylab,mac=52:54:00:90:6c:19

The above command will get us to the login screen.
We won’t be able to login though - for that we need to configure COREOS with an ignition file (out of scope for this demo).

Bonus: Ignition⌗

We can configure a COREOS machine at boot using Ignition.
Generate an Ignition file, place it on your HTTP server (alongside rootfs), then add the following boot arguments to your PXE config:

# pxelinux.cfg/default 
DEFAULT menu.c32
PROMPT 0

MENU TITLE ### MyLab boot menu ###
MENU AUTOBOOT Starting SuperOS in # seconds
TIMEOUT 5
TOTALTIMEOUT 10
ONTIMEOUT local

LABEL local
  KERNEL rhcos/vmlinuz
  APPEND initrd=rhcos/initrd.img ip=dhcp coreos.live.rootfs_url=http://192.168.132.1:8080/rhcos/rhcos-live-rootfs.x86_64.img ignition.config.url=http://192.168.132.1:8080/ignition/bootstrap_test.ign ignition.firstboot ignition.platform.id=metal

KVM RHCOS PXE Boot