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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 Servies and Pods documentation for additional information.

The following example consists of a VirtualMchine and a headless Service witch 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: registrydisk
        volumeName: registryvolume
      - disk:
          bus: virtio
        name: cloudinitdisk
        volumeName: cloudinitvolume
    resources:
      requests:
        memory: 1024M
  terminationGracePeriodSeconds: 0
  volumes:
  - name: registryvolume
    registryDisk:
      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.