The last blog post that was neither a roadmap update nor a release announcement was in December 2020. Thus it is time to give you guys a new guide!

This time we will together explore how to use Cobbler to enable a libvirt internal network to boot over the network. The guide will assume you have a running host OS and libvirt installed and running already. As a guest OS I will use openSUSE Tumbleweed but any other guest OS should work equally well.

In case you have libvirt not installed please consult your OS vendors resources to do so. A few examples:

• openSUSE
• Fedora
• Ubuntu

Note:

There will be a second part of this guide where we will utilize a network bridge to also boot real hardware with our VM based Cobbler setup.

Information:

Depending on your setup you may need to prefix the virsh and other commands with sudo.

Prepare the host environment

To limit the impact of this guide to the rest of your current host we will create a dedicated libvirt network for Cobbler (don’t modify the network default that libvirt provides out of the box).

There are multiple options to create a virtual network (XML for virsh can be found below):

• Create one with virsh persistently: virsh net-define
• Create one with virsh temporarily: virsh net-create
• Create one with virt-manager persistently: virt-manager

<network>
    <name>cobbler</name>
    <uuid>6b7f4f36-7895-4b74-8be6-dfa4b2b84500</uuid>
    <forward mode='nat'/>
    <bridge name='virbr2' stp='on' delay='0'/>
    <mac address='52:54:00:fb:72:57'/>
    <domain name='cobbler.local'/>
    <dns enable='no'/>
    <ip family='ipv4' address='10.17.3.1' prefix='24'>
    </ip>
</network>

After the network is successfully created we can switch over to choosing the guest OS that Cobbler will be installed on. This guide will use openSUSE Tumbleweed since this is what Cobbler is tested on primarily. To get started download the ISO and move it to your desired libvirt storage pool (below I show the default location):

wget https://download.opensuse.org/tumbleweed/iso/openSUSE-Tumbleweed-NET-x86_64-Current.iso
mv openSUSE-Tumbleweed-NET-x86_64-Current.iso /var/lib/libvirt/images

Create and prepare the Cobbler VM

Since now we have the host configured and have the ISO for installing the guest available, we can start setting it up. An example guide can be found in the openSUSE Documentation. For this guide to work as intended give the virtual machine a little more storage for the ISOs, 50GB is recommended at the moment to be on the safe side.

After the guest was successfully set up and installed I highly recommend adding your SSH keys to it. This can be done with the following command:

ssh-copy-id -i ~/.ssh/id_rsa <user>@192.168.122.xxx

Please replace the <user> with the username you chose during setup and xxx with the last three digits of the guest IP.

Now you should be able to login passwordless into the guest with the following command from the host:

ssh root@192.168.122.xxx

Before we start configuring the guest we need to shut it down from the host with::

virsh shutdown "<vm name>"

or if you are logged into the guest via SSH use

systemctl poweroff

to power it off. Now add the second network interface to the guest. Use the network defined above named “cobbler”. If virt-manager asks you if you want to start it please confirm this with “Yes”.

Start the VM again with the UI of virt-manager or use the following command to boot it:

virsh start "<vm name>"

Once you are logged in (preferably via SSH) you can switch to the root user with the su command. Once you are root we can actually start the configuration of the VM.

The first step is to install the DHCP server with the following command:

zypper in dhcp-server

In SUSE-based distributions, the DHCP server refuses operation per default because normally you don’t want it operating on all interfaces. This is also the case for us. We want to operate the DHCP server only on our secondary interface which is in the “cobbler” network. To achieve this, edit the file /etc/sysconfig/dhcpd and change DHCPD_INTERFACE="" to the name of your secondary interface. The entry should look similar to this afterwards:

DHCPD_INTERFACE="enp7s0"

Another thing that is missing for our secondary interface is its static configuration. Since we are hosting the DHCP server, there is nothing assigning our interface its IP, gateway, and DNS server. As such we need to configure this statically. openSUSE Tumbleweed uses NetworkManager per default to configure its networks.

nmcli con mod "Wired connection 1" ipv4.addresses "10.17.3.2/24"
nmcli con mod "Wired connection 1" ipv4.gateway "10.17.3.1"
nmcli con mod "Wired connection 1" ipv4.dns "8.8.8.8"
nmcli con mod "Wired connection 1" ipv4.method "manual"
nmcli c up "Wired connection 1"

You will need to adjust the name of the interface Wired Connection 1 to the interface that isn’t online. This will be the interface written in white normally. The other two interfaces should have a green tone.

Install & Configure Cobbler

At the time of writing this guide, the current version of Cobbler in openSUSE Tumbleweed is 3.3.3. This can be installed with:

zypper in cobbler

Confirm the installation with y when prompted and confirm by hitting the enter key. This will also install the TFTP server along with several other required dependencies.

In a default openSUSE Tumbleweed installation firewalld is configured and active. To open the required ports we execute the following commands:

firewall-cmd --permanent --zone=public --add-service=http
firewall-cmd --permanent --zone=public --add-service=https
firewall-cmd --permanent --zone=public --add-service=tftp
firewall-cmd --permanent --zone=public --add-service=dhcp
firewall-cmd --permanent --zone=public --add-service=dhcpv6

Be sure that this is executed on the guest. Opening these ports on the host may have unexpected consequences.

Since clients can access the desired services now, we should start them up.

• Enable the TFTP service with: systemctl enable --now tftp.service
• Enable the webserver with: systemctl enable --now apache2.service
• Enable the DHCP service with: systemctl enable --now dhcpd.service - this will fail but this is expected at this point

Now we can finally start configuring Cobbler itself. To do so edit the central configuration file at /etc/cobbler/settings.yaml and make it match the IP on the guest (ignore the rest of the settings file for now):

server: 10.17.3.2
next_server_v4: 10.17.3.2
manage_dhcp: true
manage_dhcp_v4: true

For this to take effect restart Cobbler:

systemctl restart cobblerd

Now that Cobbler knows its own IP address, we can modify the DHCP template that Cobbler ships to match our requirements. To do so adjust the file /etc/cobbler/dhcp.template. The important part of the file should look like this after modifications:

subnet 10.17.3.0 netmask 255.255.255.0 {
        option routers             10.17.3.1;
        option domain-name-servers 8.8.8.8;
        option subnet-mask         255.255.255.0;
        range dynamic-bootp        10.17.3.100 10.17.3.254;

The rest of the template can be left as is. Cobbler per-default uses Cheetah as a template language. It supports Jinja via a shebang as well.

Importing your first distribution to Cobbler

Now, at this point Cobbler doesn’t do much. It doesn’t hand out DHCP addresses since we haven’t defined a DHCP pool, the TFTP tree at /srv/tftpboot is empty and Cobbler itself doesn’t contain any records that the API could return.

To change this we will import our first distribution. For the sake of this guide, we will use openSUSE Leap. Get started with downloading the openSUSE Leap image inside the Cobbler VM:

wget https://download.opensuse.org/distribution/leap/15.4/iso/openSUSE-Leap-15.4-DVD-x86_64-Media.iso

Mount the downloaded ISO to a sensible directory that cannot be accidentally modified or deleted. It is recommended to add an entry to /etc/fstab in case you want to reboot the VM without problems:

mkdir -p /mnt/cobbler-isos/leap-15-4
mount -o loop /root/openSUSE-Leap-15.4-DVD-x86_64-Media.iso /mnt/cobbler-isos/leap-15-4

Here is the entry in case you want to permanently mount the ISO:

# <device>                                    <dir>                       <type>  <options> <dump> <fsck>
/root/openSUSE-Leap-15.4-DVD-x86_64-Media.iso /mnt/cobbler-isos/leap-15-4 iso9660 loop      0      0

Now we are ready to populate Cobbler with its first entry. For this we will be using cobbler import to automate setting up a distro and profile:

cobbler import --name="openSUSE-Leap-15-4" --path="/mnt/cobbler-isos/leap-15-4/"

Due to the bug cobbler/cobbler#3417 you will need to apply a fix in case the install= kernel option is missing.

To check this report the desired distribution:

cobbler distro report --name="openSUSE-Leap-15-4-x86_64"

In case under Kernel Options: you can’t read something that contains install you may use the following fixup command for this:

cobbler distro edit --name="openSUSE-Leap-15-4-x86_64" --kernel-options="install=http://10.17.3.2/cblr/distro_mirror/openSUSE-Leap-15-4"

Auto-Install your first VM with Cobbler

Now to do our first fully automated installation of a VM we will need to setup an empty VM that uses the cobbler libvirt network - no installation media required. Make note of the MAC address of the VM. Change the boot order so that the network interface is the first boot option.

We now add the system to Cobbler:

cobbler system add --name="testsystem" --profile="openSUSE-Leap-15-4-x86_64" --mac="<your mac here>" --ip-address=="10.17.3.20"

Then we generate the bootloaders and sync them into the TFTP Tree:

cobbler mkloaders && cobbler sync

To utilize the empty VM we created earlier, boot it via the network with virt-manager or virsh. To achieve this please take a look at the libvirt domain XML specification or take a look at the RedHat Documentation that also covers this topic.

Sadly the default autoyast template for Cobbler 3.3.3 is broken as such a hands-free installation is not possible at the moment.

Final thoughts

By now you should have a rough understanding of the workflow that Cobbler automates and how easy it should be to setup new VM guests. We hope you had fun following this guide. If there are any questions feel free to open a GitHub Discussion.

To clean up after your experiments there are only a couple of commands needed:

virsh destroy <vm name> && virsh destroy <second vm name> && virsh net-destroy cobbler

Further topics that we will cover in future blog posts:

• Write autoinstall snippets for your setups
• Setup Windows Guests

P.S.: In case you find typos and/or grammar mistakes just open an Issue or open a PR.