Kubernetes DNS Concepts: Difference between revisions
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=Internal= | =Internal= | ||
* [[Kubernetes Networking Concepts#DNS_Support|Kubernetes Networking Concepts]] | |||
* [[Kubernetes_Concepts#Subjects|Kubernetes Concepts]] | * [[Kubernetes_Concepts#Subjects|Kubernetes Concepts]] | ||
=Overview= | =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. | 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= | =<span id='DNS_Service'></span>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 [[#The_DNS_Server|internal DNS server]]: | |||
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 [[# | |||
<syntaxhighlight lang='text'> | <syntaxhighlight lang='text'> | ||
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE | NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE | ||
kube-dns ClusterIP 10.96.0.10 <none> 53/UDP,53/TCP,9153/TCP 49d | kube-dns ClusterIP 10.96.0.10 <none> 53/UDP,53/TCP,9153/TCP 49d | ||
</syntaxhighlight> | </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. | The static ClusterIP address allocated to the DNS service (10.96.0.10) is [[#Name_Resolution_inside_a_Pod|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. The mechanism is explained in detail in {{Internal|Kubernetes_Service_Concepts#DNS|Services and DNS}} | 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. The mechanism is explained in detail in: {{Internal|Kubernetes_Service_Concepts#DNS|Services and DNS}} | ||
[[ | [[Kubernetes Workload Resources#StatefulSet|StatefulSets]] and the individual pods managed by a StatefulSet are also registered with the DNS service. | ||
=DNS | =Name Resolution inside a Pod= | ||
A pod can be configured to use the [[#DNS_Server|internal DNS server]] or not for DNS queries executed on the pod. This behavior is configured with the pod manifest [[Kubernetes_Pod_Manifest#Example|dnsPolicy]] element. | |||
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]]. | |||
Each name being resolved on a pod is successively looked up, in order, in the following DNS subdomains: "default.svc.cluster.local", "svc.cluster.local" and "cluster.local". For more details see [[/etc/resolv.conf#Search|/etc/resolv.conf search]]. | |||
=cluster.local= | |||
"cluster.local" is a configurable cluster domain suffix used in all cluster local names. | |||
==svc.cluster.local== | |||
All services in the cluster belong to "svc.cluster.local" subdomain. | |||
=TODO= | |||
<font color=darkgray> | |||
* TODO: https://medium.com/kubernetes-tutorials/kubernetes-dns-for-services-and-pods-664804211501 | |||
</font> | |||
=DNS Operations= | |||
* [[Kubernetes DNS Operations#Troubleshooting_DNS_Resolution|Troubleshooting DNS resolution]] | |||
=<span id='Internal_DNS_Server></span>DNS Implementation Details= | =<span id='Internal_DNS_Server></span>DNS Implementation Details= | ||
==CoreDNS== | |||
==<span id='DNS_Server'></span>The DNS Server== | |||
The Kubernetes cluster's DNS server implementation is based on [[#CoreDNS|CoreDNS]], [[#kube-dns|kube-dns]] or [[#SkyDNS|SkyDNS]], depending on version. The pods implementing the DNS support are exposed to the cluster via a [[#DNS_Service|DNS ClusterIP Service]]. | |||
===CoreDNS=== | |||
<syntaxhighlight lang='text'> | <syntaxhighlight lang='text'> | ||
NAME READY STATUS RESTARTS AGE | NAME READY STATUS RESTARTS AGE | ||
| Line 37: | Line 59: | ||
coredns-5644d7b6d9-mztf9 1/1 Running 0 8h | coredns-5644d7b6d9-mztf9 1/1 Running 0 8h | ||
</syntaxhighlight> | </syntaxhighlight> | ||
==kube-dns== | ===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. | 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== | ===SkyDNS=== | ||
=External DNS= | |||
{{Internal|external-dns|external-dns}} | |||
Latest revision as of 23:31, 11 July 2023
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. The mechanism is explained in detail in:
StatefulSets and the individual pods managed by a StatefulSet are also registered with the DNS service.
Name Resolution inside a Pod
A pod can be configured to use the internal DNS server or not for DNS queries executed on the pod. This behavior is configured with the pod manifest dnsPolicy element.
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.
Each name being resolved on a pod is successively looked up, in order, in the following DNS subdomains: "default.svc.cluster.local", "svc.cluster.local" and "cluster.local". For more details see /etc/resolv.conf search.
cluster.local
"cluster.local" is a configurable cluster domain suffix used in all cluster local names.
svc.cluster.local
All services in the cluster belong to "svc.cluster.local" subdomain.
TODO
DNS Operations
DNS Implementation Details
The DNS Server
The Kubernetes cluster's DNS server implementation is based on CoreDNS, kube-dns or SkyDNS, depending on version. The pods implementing the DNS support are exposed to the cluster via a DNS ClusterIP Service.
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.