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Hook Sidecar Container


In KubeVirt, a Hook Sidecar container is a sidecar container (a secondary container that runs along with the main application container within the same Pod) used to apply customizations before the Virtual Machine is initialized. This ability is provided since configurable elements in the VMI specification do not cover all of the libvirt domain XML elements.

The sidecar containers communicate with the main container over a socket with a gRPC protocol. There are two main sidecar hooks:

  1. onDefineDomain: This hook helps to customize libvirt's XML and return the new XML over gRPC for the VM creation.
  2. preCloudInitIso: This hook helps to customize the cloud-init configuration. It operates on and returns JSON formatted cloud-init data.

Enabling Sidecar feature gate

Sidecar feature gate can be enabled by following the steps mentioned in Activating feature gates.

In case of a development cluster created using kubevirtci, follow the steps mentioned in the developer doc to enable the feature gate.

Sidecar-shim container image

To run a VM with custom modifications, the sidecar-shim-image takes care of implementing the communication with the main container.

The image contains the sidecar-shim binary built using sidecar_shim.go which should be kept as the entrypoint of the container. This binary will search in $PATH for binaries named after the hook names (e.g onDefineDomain and preCloudInitIso) and run them. Users must provide the necessary arguments as command line options (flags).

In the case of onDefineDomain, the arguments will be the VMI information as JSON string, (e.g --vmi vmiJSON) and the current domain XML (e.g --domain domainXML). It outputs the modified domain XML on the standard output.

In the case of preCloudInitIso, the arguments will be the VMI information as JSON string, (e.g --vmi vmiJSON) and the CloudInitData (e.g --cloud-init cloudInitJSON). It outputs the modified CloudInitData (as JSON) on the standard ouput.

Shell or python scripts can be used as alternatives to the binary, by making them available at the expected location (/usr/bin/onDefineDomain or /usr/bin/preCloudInitIso depending upon the hook).

Go, Python, Shell - pick any one

Although a binary doesn't strictly need to be generated from Go code, and a script doesn't strictly need to be one among Shell or Python, for the purpose of this guide, we will use those as examples.

Go binary

Example Go code modifiying the SMBIOS system information can be found in the KubeVirt repo. Binary generated from this code, when available under /usr/bin/ondefinedomain in the sidecar-shim-image, is run right before VMI creation and the baseboard manufacturer value is modified to reflect what's provided in the annotation in VMI spec.

Shell or Python script

If you pefer writing a shell or python script instead of a Go program, create a Kubernetes ConfigMap and use annotations to make sure the script is run before the VMI creation. The flow would be as below:

  1. Create a ConfigMap containing the shell or python script you want to run
  2. Create a VMI containing the annotation and mention the ConfigMap information in it.

ConfigMap with shell script

apiVersion: v1
kind: ConfigMap
  name: my-config-map
data: |
    tempFile=`mktemp --dry-run`
    echo $4 > $tempFile
    sed -i "s|<baseBoard></baseBoard>|<baseBoard><entry name='manufacturer'>Radical Edward</entry></baseBoard>|" $tempFile
    cat $tempFile

ConfigMap with python script

apiVersion: v1
kind: ConfigMap
  name: my-config-map
data: |
    #!/usr/bin/env python3

    import xml.etree.ElementTree as ET
    import sys

    def main(s):
        # write to a temporary file
        f = open("/tmp/orig.xml", "w")

        # parse xml from file
        xml = ET.parse("/tmp/orig.xml")
        # get the root element
        root = xml.getroot()
        # find the baseBoard element
        baseBoard = root.find("sysinfo").find("baseBoard")

        # prepare new element to be inserted into the xml definition
        element = ET.Element("entry", {"name": "manufacturer"})
        element.text = "Radical Edward"
        # insert the element
        baseBoard.insert(0, element)

        # write to a new file
        # print file contents to stdout
        f = open("/tmp/new.xml")

    if __name__ == "__main__":

After creating one of the above ConfigMap, create the VMI using the manifest in this example. Of importance here is the ConfigMap information stored in the annotations:

annotations: >
            "args": ["--version", "v1alpha2"],
            "image": "registry:5000/kubevirt/sidecar-shim:devel",
            "configMap": {"name": "my-config-map", "key": "", "hookPath": "/usr/bin/onDefineDomain"}

The name field indicates the name of the ConfigMap on the cluster which contains the script you want to execute. The key field indicates the key in the ConfigMap which contains the script to be executed. Finally, hookPath indicates the path where you want the script to be mounted. It could be either of /usr/bin/onDefineDomain or /usr/bin/preCloudInitIso depending upon the hook you want to execute.

Verify everything works

Whether you used the Go binary or a Shell/Python script from the above examples, you would be able to see the newly created VMI have the modified baseboard manufacturer information. After creating the VMI, verify that it is in the Running state, and connect to its console and see if the desired changes to baseboard manufacturer get reflected:

# Once the VM is ready, connect to its display and login using name and password "fedora"
cluster/ vnc vmi-with-sidecar-hook-configmap

# Check whether the base board manufacturer value was successfully overwritten
sudo dmidecode -s baseboard-manufacturer