1. Documentation

DNS for Services and VirtualMachineInstances

DNS for Services and VirtualMachineInstances

Creating unique DNS entries per VirtualMachineInstance

In order to create unique DNS entries per VirtualMachineInstance, it is possible to set spec.hostname and spec.subdomain. If a subdomain is set and a headless service with a name, matching the subdomain, exists, kube-dns will create unique DNS entries for every VirtualMachineInstance which matches the selector of the service. Have a look at the DNS for Services and Pods documentation for additional information.

The following example consists of a VirtualMachine and a headless Service which matches the labels and the subdomain of the VirtualMachineInstance:

apiVersion: kubevirt.io/v1alpha2
kind: VirtualMachineInstance
metadata:
  name: vmi-fedora
  labels:
    expose: me
spec:
  hostname: "myvmi"
  subdomain: "mysubdomain"
  domain:
    devices:
      disks:
      - disk:
          bus: virtio
        name: containerdisk
        volumeName: registryvolume
      - disk:
          bus: virtio
        name: cloudinitdisk
        volumeName: cloudinitvolume
    resources:
      requests:
        memory: 1024M
  terminationGracePeriodSeconds: 0
  volumes:
  - name: registryvolume
    containerDisk:
      image: kubevirt/fedora-cloud-registry-disk-demo:latest
  - cloudInitNoCloud:
      userDataBase64: IyEvYmluL2Jhc2gKZWNobyAiZmVkb3JhOmZlZG9yYSIgfCBjaHBhc3N3ZAo=
    name: cloudinitvolume
---
apiVersion: v1
kind: Service
metadata:
  name: mysubdomain
spec:
  selector:
    expose: me
  clusterIP: None
  ports:
  - name: foo # Actually, no port is needed.
    port: 1234
    targetPort: 1234

As a consequence, when we enter the VirtualMachineInstance via e.g. virtctl console vmi-fedora and ping myvmi.mysubdomain we see that we find a DNS entry for myvmi.mysubdomain.default.svc.cluster.local which points to 10.244.0.57, which is the IP of the VirtualMachineInstance (not of the Service):

[fedora@myvmi ~]$ ping myvmi.mysubdomain
PING myvmi.mysubdomain.default.svc.cluster.local (10.244.0.57) 56(84) bytes of data.
64 bytes from myvmi.mysubdomain.default.svc.cluster.local (10.244.0.57): icmp_seq=1 ttl=64 time=0.029 ms
[fedora@myvmi ~]$ ip a
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 0a:58:0a:f4:00:39 brd ff:ff:ff:ff:ff:ff
    inet 10.244.0.57/24 brd 10.244.0.255 scope global dynamic eth0
       valid_lft 86313556sec preferred_lft 86313556sec
    inet6 fe80::858:aff:fef4:39/64 scope link
       valid_lft forever preferred_lft forever

So spec.hostname and spec.subdomain get translated to a DNS A-record of the form <vmi.spec.hostname>.<vmi.spec.subdomain>.<vmi.metadata.namespace>.svc.cluster.local. If no spec.hostname is set, then we fall back to the VirtualMachineInstance name itself. The resulting DNS A-record looks like this then: <vmi.metadata.name>.<vmi.spec.subdomain>.<vmi.metadata.namespace>.svc.cluster.local.