Kubernetes DNS Concepts: Difference between revisions
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=Internal= | =Internal= | ||
* [[Kubernetes_Concepts#Subjects|Kubernetes Concepts]] | * [[Kubernetes_Concepts#Subjects|Kubernetes Concepts]] | ||
* [[Kubernetes Networking Concepts#DNS_Support|Kubernetes Networking Concepts]] | |||
=Overview= | |||
Each [[Kubernetes_Control_Plane_and_Data_Plane_Concepts#Cluster|Kubernetes cluster]] runs its own DNS service. The DNS service is exposed as a [[#The_DNS_Service|kube-dns]] Kubernetes [[Kubernetes_Service_Concepts#Service_.28ClusterIP_Service.29|ClusterIP service]], running in the "kube-system" namespace. The service is backed by two [[#CoreDNS|coredns]] highly-available pods, also deployed in the "kube-system" namespace. | |||
=The DNS Service= | |||
Each Kubernetes [[Kubernetes_Control_Plane_and_Data_Plane_Concepts#Cluster|cluster]] runs an internal DNS service. The DNS service is exposed as a regular Kubernetes [[Kubernetes_Service_Concepts#Service|ClusterIP service]], deployed in the "kube-system" namespace, which in turn exposes the Kubernetes [[#Internal_DNS_Server|internal DNS server]]: | |||
<syntaxhighlight lang='text'> | |||
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE | |||
kube-dns ClusterIP 10.96.0.10 <none> 53/UDP,53/TCP,9153/TCP 49d | |||
</syntaxhighlight> | |||
The static ClusterIP address allocated to the DNS service (10.96.0.10) is hardcoded into every pod in the cluster, allowing the pods to resolve service names. | |||
Every time a new Kubernetes [[Kubernetes_Service_Concepts#Service|service]] is deployed, the Kubernetes cluster automatically updates the internal database with service name to ClusterIP address mappings, making the ClusterIP address of the new service dynamically available to all other pods in the cluster. [[Kubernetes_Higher_Level_Pod_Controllers#StatefulSet|StatefulSets]] and the individual pods managed by a StatefulSet are also registered with the DNS service. | |||
=DNS Operations= | |||
= | |||
==The DNS Service== | |||
The DNS service is built on [[CoreDNS]]. | |||
Testing name resolution: | |||
kubectl run -it --rm --restart=Never --image=infoblox/dnstools:latest dnstools | |||
Also see: {{Internal|Kubernetes_Service_Concepts#Services_and_DNS|Services and DNS}} | |||
=<span id='Internal_DNS_Server></span>DNS Implementation Details= | |||
==CoreDNS== | |||
<syntaxhighlight lang='text'> | |||
NAME READY STATUS RESTARTS AGE | |||
coredns-5644d7b6d9-kcxt6 1/1 Running 0 8h | |||
coredns-5644d7b6d9-mztf9 1/1 Running 0 8h | |||
</syntaxhighlight> | |||
==kube-dns== | |||
Note that <code>kube-dns</code> seems to be both the name of the Kubernetes [[#DNS_Service|DNS service]] and the name of a backing provider. | |||
==SkyDNS== | |||
----------- | |||
<font color=darkgray>Explain default.svc.cluster.local, svc.cluster.local, cluster.local.</font> | |||
TODO: https://medium.com/kubernetes-tutorials/kubernetes-dns-for-services-and-pods-664804211501 | |||
==Name Resolution inside a Pod== | |||
Each pod gets an <code>/etc/resolv.conf</code> with a name server hardcoded to the IP address of the DNS service <code>kube-dns</code>: | |||
<syntaxhighlight lang='text'> | |||
nameserver 10.96.0.10 | |||
search default.svc.cluster.local svc.cluster.local cluster.local | |||
options ndots:5 | |||
</syntaxhighlight> | |||
The local DNS library is thus configured to use by default the name server behind the Kubernetes [[#DNS_Service|DNS service]]. | |||
==Services and Naming== | |||
Revision as of 06:07, 20 September 2020
Internal
Overview
Each Kubernetes cluster runs its own DNS service. The DNS service is exposed as a kube-dns Kubernetes ClusterIP service, running in the "kube-system" namespace. The service is backed by two coredns highly-available pods, also deployed in the "kube-system" namespace.
The DNS Service
Each Kubernetes cluster runs an internal DNS service. The DNS service is exposed as a regular Kubernetes ClusterIP service, deployed in the "kube-system" namespace, which in turn exposes the Kubernetes internal DNS server:
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE
kube-dns ClusterIP 10.96.0.10 <none> 53/UDP,53/TCP,9153/TCP 49d
The static ClusterIP address allocated to the DNS service (10.96.0.10) is hardcoded into every pod in the cluster, allowing the pods to resolve service names.
Every time a new Kubernetes service is deployed, the Kubernetes cluster automatically updates the internal database with service name to ClusterIP address mappings, making the ClusterIP address of the new service dynamically available to all other pods in the cluster. StatefulSets and the individual pods managed by a StatefulSet are also registered with the DNS service.
DNS Operations
The DNS Service
The DNS service is built on CoreDNS.
Testing name resolution:
kubectl run -it --rm --restart=Never --image=infoblox/dnstools:latest dnstools
Also see:
DNS Implementation Details
CoreDNS
NAME READY STATUS RESTARTS AGE
coredns-5644d7b6d9-kcxt6 1/1 Running 0 8h
coredns-5644d7b6d9-mztf9 1/1 Running 0 8h
kube-dns
Note that kube-dns seems to be both the name of the Kubernetes DNS service and the name of a backing provider.
SkyDNS
Explain default.svc.cluster.local, svc.cluster.local, cluster.local.
TODO: https://medium.com/kubernetes-tutorials/kubernetes-dns-for-services-and-pods-664804211501
Name Resolution inside a Pod
Each pod gets an /etc/resolv.conf with a name server hardcoded to the IP address of the DNS service kube-dns:
nameserver 10.96.0.10
search default.svc.cluster.local svc.cluster.local cluster.local
options ndots:5
The local DNS library is thus configured to use by default the name server behind the Kubernetes DNS service.