How to install Cozystack using kexec

This guide explains how to boot Talos Linux from a running Linux OS using the kexec ( kernel execute) mechanism. Using kexec is a convenient way of installing Talos when you have machines with another Linux distribution already installed.

Kexec is both a utility kexec and a system call of the same name. It allows you to boot into a new kernel from the existing system without performing a physical reboot of the machine. It’s as if the kernel was loaded by the standard bootloader at startup, only in this case your existing OS acts as the bootloader.

1.1 Install kexec-tools package

Here and further this guide assumes an Ubuntu / Debian system. Commands for other distributions may vary.

apt install kexec-tools -y

1.2. Download Cozystack-specific Talos Distribution

Cozystack developers have made a special Talos Linux distribution, tailored for the requirements of Cozystack. These distributions are released together with Cozystack, and can be downloaded from the Cozystack releases page on GitHub.

Choice of distribution depends on whether you are using VMs or bare metal.

In most cases, it is best to take the latest stable release:

wget -O /tmp/vmlinuz https://github.com/cozystack/cozystack/releases/latest/download/kernel-amd64
wget -O /tmp/initramfs.xz https://github.com/cozystack/cozystack/releases/latest/download/initramfs-metal-amd64.xz

To install a specific version, download matching distributives. Note that the URL is slightly different from the one for latest:

VERSION=v0.34.3
wget -O /tmp/vmlinuz https://github.com/cozystack/cozystack/releases/download/${VERSION}/kernel-amd64
wget -O /tmp/initramfs.xz https://github.com/cozystack/cozystack/releases/download/${VERSION}/initramfs-metal-amd64.xz

Cozystack supports kexec installation since version v0.29.2

1.3. Gather Network Information

Talos Linux needs network configuration to use during boot time. At this step we will gather the required information and pack it into a command line parameter.

The mechanism we’re using is called “Kernel level IP configuration”. It allows the Linux kernel to set up interfaces and assign IP addresses automatically during boot, based on information passed with the kernel cmdline. It’s a built-in kernel feature which is enabled by default in Talos Linux.

All we need is to make a properly formatted network settings parameter. Below is a small script that does all the work:

IP=$(ip -o -4 route get 8.8.8.8 | awk -F"src " '{sub(" .*", "", $2); print $2}')
GATEWAY=$(ip -o -4 route get 8.8.8.8 | awk -F"via " '{sub(" .*", "", $2); print $2}')
ETH=$(ip -o -4 route get 8.8.8.8 | awk -F"dev " '{sub(" .*", "", $2); print $2}')
CIDR=$(ip -o -4 addr show "$ETH" | awk -F"inet $IP/" '{sub(" .*", "", $2); print $2; exit}')
NETMASK=$(echo "$CIDR" | awk '{p=$1;for(i=1;i<=4;i++){if(p>=8){o=255;p-=8}else{o=256-2^(8-p);p=0}printf(i<4?o".":o"\n")}}')
DEV=$(udevadm info -q property "/sys/class/net/$ETH" | awk -F= '$1~/ID_NET_NAME_ONBOARD/{print $2; exit} $1~/ID_NET_NAME_PATH/{v=$2} END{if(v) print v}')

CMDLINE="init_on_alloc=1 slab_nomerge pti=on console=tty0 console=ttyS0 printk.devkmsg=on talos.platform=metal ip=${IP}::${GATEWAY}:${NETMASK}::${DEV}:::::"
echo $CMDLINE

The output should be similar to:

init_on_alloc=1 slab_nomerge pti=on console=tty0 console=ttyS0 printk.devkmsg=on talos.platform=metal ip=10.0.0.131::10.0.0.1:255.255.255.0::eno2np0:::::

If it looks correct, you can proceed to the next step.

For details and deep explanation of this step, see the documentation:

• Kernel level IP autoconfiguration
• Talos Linux documentation on kernel command line.
• Official Linux Kernel documentation

1.4 Boot Talos Linux

Now that we have distributives and the network configuration, it’s time to boot Talos. The first command loads the Talos kernel into RAM, and the second switches the current system to this new kernel:

kexec --load /tmp/vmlinuz --initrd=/tmp/initramfs.xz --command-line="$CMDLINE"
kexec --exec

To monitor how kexec works and debug any problems with installation, run dmesg -w in a separate console session.

# dmesg -w
... (some logs before running `kexec --load`)
... this message is expected after `kexec --load`:
[Wed Jul 23 18:33:56 2025] PEFILE: Unsigned PE binary

From kexec manual pages:

-l (--load) kernel
       Load the specified kernel into the current kernel.
--initrd=file
       Use file as the kernel's initial ramdisk.
--command-line=string
        Set the kernel command line to string.
-e (--exec)
        Run the currently loaded kernel. Note that it will reboot```
        into the loaded kernel without calling shutdown(8).

Next steps

Once the machine has rebooted, Talos Linux will be loaded in memory. To finalize installation, follow with cluster bootstrapping using one of available tools:

Once you apply the configuration with talm apply -i ... or talosctl apply -i, Talos image gets written to disk and installation is complete.