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DNS Schema
Notes about the SkyDNS record naming scheme. (Copied from SkyDNS project README for reference while hacking on the k8s middleware.)
Services
A Records
"Normal" (not headless) Services are assigned a DNS A record for a name of the form my-svc.my-namespace.svc.cluster.local.
This resolves to the cluster IP of the Service.
"Headless" (without a cluster IP) Services are also assigned a DNS A record for a name of the form my-svc.my-namespace.svc.cluster.local.
Unlike normal Services, this resolves to the set of IPs of the pods selected by the Service.
Clients are expected to consume the set or else use standard round-robin selection from the set.
Pods
A Records
When enabled, pods are assigned a DNS A record in the form of pod-ip-address.my-namespace.pod.cluster.local.
For example, a pod with ip 1.2.3.4 in the namespace default with a dns name of cluster.local would have
an entry: 1-2-3-4.default.pod.cluster.local.
####A Records and hostname Based on Pod Annotations - A Beta Feature in Kubernetes v1.2
Currently when a pod is created, its hostname is the Pod's metadata.name value.
With v1.2, users can specify a Pod annotation, pod.beta.kubernetes.io/hostname, to specify what the Pod's hostname should be.
If the annotation is specified, the annotation value takes precedence over the Pod's name, to be the hostname of the pod.
For example, given a Pod with annotation pod.beta.kubernetes.io/hostname: my-pod-name, the Pod will have its hostname set to "my-pod-name".
v1.2 introduces a beta feature where the user can specify a Pod annotation, pod.beta.kubernetes.io/subdomain, to specify what the Pod's subdomain should be.
If the annotation is specified, the fully qualified Pod hostname will be "...svc.".
For example, given a Pod with the hostname annotation set to "foo", and the subdomain annotation set to "bar", in namespace "my-namespace", the pod will set its own FQDN as "foo.bar.my-namespace.svc.cluster.local"
If there exists a headless service in the same namespace as the pod and with the same name as the subdomain, the cluster's KubeDNS Server will also return an A record for the Pod's fully qualified hostname. Given a Pod with the hostname annotation set to "foo" and the subdomain annotation set to "bar", and a headless Service named "bar" in the same namespace, the pod will see it's own FQDN as "foo.bar.my-namespace.svc.cluster.local". DNS will serve an A record at that name, pointing to the Pod's IP.
With v1.2, the Endpoints object also has a new annotation endpoints.beta.kubernetes.io/hostnames-map. Its value is the json representation of map[string(IP)][endpoints.HostRecord], for example: '{"10.245.1.6":{HostName: "my-webserver"}}'.
If the Endpoints are for a headless service, then A records will be created with the format ...svc.
For the example json, if endpoints are for a headless service named "bar", and one of the endpoints has IP "10.245.1.6", then a A record will be created with the name "my-webserver.bar.my-namespace.svc.cluster.local" and the A record lookup would return "10.245.1.6".
This endpoints annotation generally does not need to be specified by end-users, but can used by the internal service controller to deliver the aforementioned feature.