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45 lines
3.3 KiB
Markdown
45 lines
3.3 KiB
Markdown
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## DNS Schema
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Notes about the SkyDNS record naming scheme. (Copied from SkyDNS project README for reference while
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hacking on the k8s middleware.)
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### Services
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#### A Records
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"Normal" (not headless) Services are assigned a DNS A record for a name of the form `my-svc.my-namespace.svc.cluster.local.`
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This resolves to the cluster IP of the Service.
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"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.`
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Unlike normal Services, this resolves to the set of IPs of the pods selected by the Service.
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Clients are expected to consume the set or else use standard round-robin selection from the set.
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### Pods
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#### A Records
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When enabled, pods are assigned a DNS A record in the form of `pod-ip-address.my-namespace.pod.cluster.local.`
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For example, a pod with ip `1.2.3.4` in the namespace default with a dns name of `cluster.local` would have
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an entry: `1-2-3-4.default.pod.cluster.local.`
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####A Records and hostname Based on Pod Annotations - A Beta Feature in Kubernetes v1.2
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Currently when a pod is created, its hostname is the Pod's `metadata.name` value.
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With v1.2, users can specify a Pod annotation, `pod.beta.kubernetes.io/hostname`, to specify what the Pod's hostname should be.
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If the annotation is specified, the annotation value takes precendence over the Pod's name, to be the hostname of the pod.
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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".
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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.
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If the annotation is specified, the fully qualified Pod hostname will be "<hostname>.<subdomain>.<pod namespace>.svc.<cluster domain>".
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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"
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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.
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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.
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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"}}'.
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If the Endpoints are for a headless service, then A records will be created with the format <hostname>.<service name>.<pod namespace>.svc.<cluster domain>
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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".
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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.
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