Using DNS Pods and Services
Introduction
As of Kubernetes 1.3, DNS is a built-in service launched automatically using the addon manager cluster add-on.
Kubernetes DNS schedules a DNS Pod and Service on the cluster, and configures the kubelets to tell individual containers to use the DNS Service’s IP to resolve DNS names.
What things get DNS names?
Every Service defined in the cluster (including the DNS server itself) is assigned a DNS name. By default, a client Pod’s DNS search list will include the Pod’s own namespace and the cluster’s default domain. This is best illustrated by example:
Assume a Service named foo in the Kubernetes namespace bar. A Pod running in namespace bar can look up this service by simply doing a DNS query for foo. A Pod running in namespace quux can look up this service by doing a DNS query for foo.bar.
Supported DNS schema
The following sections detail the supported record types and layout that is supported. Any other layout or names or queries that happen to work are considered implementation details and are subject to change without warning.
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.
SRV records
SRV Records are created for named ports that are part of normal or Headless Services. For each named port, the SRV record would have the form _my-port-name._my-port-protocol.my-svc.my-namespace.svc.cluster.local. For a regular service, this resolves to the port number and the CNAME: my-svc.my-namespace.svc.cluster.local. For a headless service, this resolves to multiple answers, one for each pod that is backing the service, and contains the port number and a CNAME of the pod of the form auto-generated-name.my-svc.my-namespace.svc.cluster.local.
Backwards compatibility
Previous versions of kube-dns made names of the form my-svc.my-namespace.cluster.local (the ‘svc’ level was added later). This is no longer supported.
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’s hostname and subdomain fields
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. The Pod annotation, if specified, 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”.
With v1.3, the PodSpec has a hostname field, which can be used to specify the Pod’s hostname. This field value takes precedence over thepod.beta.kubernetes.io/hostname annotation value.
v1.2 introduces a beta feature where the user can specify a Pod annotation, pod.beta.kubernetes.io/subdomain, to specify the Pod’s subdomain. The final domain will be “ ...svc.". For example, a Pod with the hostname annotation set to "foo", and the subdomain annotation set to "bar", in namespace "my-namespace", will have the FQDN "foo.bar.my-namespace.svc.cluster.local"
With v1.3, the PodSpec has a subdomain field, which can be used to specify the Pod’s subdomain. This field value takes precedence over the pod.beta.kubernetes.io/subdomain annotation value.
Example:
apiVersion: v1
kind: Service
metadata:
name: default-subdomain
spec:
selector:
name: busybox
clusterIP: None
ports:
- name: foo # Actually, no port is needed.
port: 1234
targetPort: 1234
---
apiVersion: v1
kind: Pod
metadata:
name: busybox1
labels:
name: busybox
spec:
hostname: busybox-1
subdomain: default-subdomain
containers:
- image: busybox
command:
- sleep
- "3600"
name: busybox
---
apiVersion: v1
kind: Pod
metadata:
name: busybox2
labels:
name: busybox
spec:
hostname: busybox-2
subdomain: default-subdomain
containers:
- image: busybox
command:
- sleep
- "3600"
name: busybox
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 also returns an A record for the Pod’s fully qualified hostname. Given a Pod with the hostname set to “busybox-1” and the subdomain set to “default-subdomain”, and a headless Service named “default-subdomain” in the same namespace, the pod will see it’s own FQDN as “busybox-1.default-subdomain.my-namespace.svc.cluster.local”. DNS serves an A record at that name, pointing to the Pod’s IP. Both pods “busybox1” and “busybox2” can have their distinct A records.
As of Kubernetes v1.2, the Endpoints object also has the 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, an A record is 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", an A record is 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.
With v1.3, The Endpoints object can specify the hostname for any endpoint, along with its IP. The hostname field takes precedence over the hostname value that might have been specified via the endpoints.beta.kubernetes.io/hostnames-map annotation.
With v1.3, the following annotations are deprecated: pod.beta.kubernetes.io/hostname, pod.beta.kubernetes.io/subdomain, endpoints.beta.kubernetes.io/hostnames-map
How do I test if it is working?
Create a simple Pod to use as a test environment
Create a file named busybox.yaml with the following contents:
apiVersion: v1
kind: Pod
metadata:
name: busybox
namespace: default
spec:
containers:
- image: busybox
command:
- sleep
- "3600"
imagePullPolicy: IfNotPresent
name: busybox
restartPolicy: Always
Then create a pod using this file:
kubectl create -f busybox.yaml
Wait for this pod to go into the running state
You can get its status with:
kubectl get pods busybox
You should see:
NAME READY STATUS RESTARTS AGE
busybox 1/1 Running 0 <some-time>
Validate that DNS is working
Once that pod is running, you can exec nslookup in that environment:
kubectl exec -ti busybox -- nslookup kubernetes.default
You should see something like:
Server: 10.0.0.10
Address 1: 10.0.0.10
Name: kubernetes.default
Address 1: 10.0.0.1
If you see that, DNS is working correctly.
Troubleshooting Tips
If the nslookup command fails, check the following:
Check the local DNS configuration first
Take a look inside the resolv.conf file. (See “Inheriting DNS from the node” and “Known issues” below for more information)
kubectl exec busybox cat /etc/resolv.conf
Verify that the search path and name server are set up like the following (note that search path may vary for different cloud providers):
search default.svc.cluster.local svc.cluster.local cluster.local google.internal c.gce_project_id.internal
nameserver 10.0.0.10
options ndots:5
Quick diagnosis
Errors such as the following indicate a problem with the kube-dns add-on or associated Services:
$ kubectl exec -ti busybox -- nslookup kubernetes.default
Server: 10.0.0.10
Address 1: 10.0.0.10
nslookup: can't resolve 'kubernetes.default'
or
$ kubectl exec -ti busybox -- nslookup kubernetes.default
Server: 10.0.0.10
Address 1: 10.0.0.10 kube-dns.kube-system.svc.cluster.local
nslookup: can't resolve 'kubernetes.default'
Check if the DNS pod is running
Use the kubectl get pods command to verify that the DNS pod is running.
kubectl get pods --namespace=kube-system -l k8s-app=kube-dns
You should see something like:
NAME READY STATUS RESTARTS AGE
...
kube-dns-v19-ezo1y 3/3 Running 0 1h
...
If you see that no pod is running or that the pod has failed/completed, the DNS add-on may not be deployed by default in your current environment and you will have to deploy it manually.
Check for Errors in the DNS pod
Use kubectl logs command to see logs for the DNS daemons.
kubectl logs --namespace=kube-system $(kubectl get pods --namespace=kube-system -l k8s-app=kube-dns -o name) -c kubedns
kubectl logs --namespace=kube-system $(kubectl get pods --namespace=kube-system -l k8s-app=kube-dns -o name) -c dnsmasq
kubectl logs --namespace=kube-system $(kubectl get pods --namespace=kube-system -l k8s-app=kube-dns -o name) -c healthz
See if there is any suspicious log. W, E, F letter at the beginning represent Warning, Error and Failure. Please search for entries that have these as the logging level and use kubernetes issues to report unexpected errors.
Is DNS service up?
Verify that the DNS service is up by using the kubectl get service command.
kubectl get svc --namespace=kube-system
You should see:
NAME CLUSTER-IP EXTERNAL-IP PORT(S) AGE
...
kube-dns 10.0.0.10 <none> 53/UDP,53/TCP 1h
...
If you have created the service or in the case it should be created by default but it does not appear, see this debugging services page for more information.
Are DNS endpoints exposed?
You can verify that DNS endpoints are exposed by using the kubectl get endpoints command.
kubectl get ep kube-dns --namespace=kube-system
You should see something like:
NAME ENDPOINTS AGE
kube-dns 10.180.3.17:53,10.180.3.17:53 1h
If you do not see the endpoints, see endpoints section in the debugging services documentation.
For additional Kubernetes DNS examples, see the cluster-dns examples in the Kubernetes GitHub repository.
Kubernetes Federation (Multiple Zone support)
Release 1.3 introduced Cluster Federation support for multi-site Kubernetes installations. This required some minor (backward-compatible) changes to the way the Kubernetes cluster DNS server processes DNS queries, to facilitate the lookup of federated services (which span multiple Kubernetes clusters). See the Cluster Federation Administrators’ Guide for more details on Cluster Federation and multi-site support.
How it Works
The running Kubernetes DNS pod holds 3 containers - kubedns, dnsmasq and a health check called healthz. The kubedns process watches the Kubernetes master for changes in Services and Endpoints, and maintains in-memory lookup structures to service DNS requests. The dnsmasq container adds DNS caching to improve performance. The healthz container provides a single health check endpoint while performing dual healthchecks (for dnsmasq and kubedns).
The DNS pod is exposed as a Kubernetes Service with a static IP. Once assigned the kubelet passes DNS configured using the --cluster-dns=10.0.0.10 flag to each container.
DNS names also need domains. The local domain is configurable, in the kubelet using the flag --cluster-domain=<default local domain>
The Kubernetes cluster DNS server (based off the SkyDNS library) supports forward lookups (A records), service lookups (SRV records) and reverse IP address lookups (PTR records).
Inheriting DNS from the node
When running a pod, kubelet will prepend the cluster DNS server and search paths to the node’s own DNS settings. If the node is able to resolve DNS names specific to the larger environment, pods should be able to, also. See “Known issues” below for a caveat.
If you don’t want this, or if you want a different DNS config for pods, you can use the kubelet’s --resolv-conf flag. Setting it to “” means that pods will not inherit DNS. Setting it to a valid file path means that kubelet will use this file instead of /etc/resolv.conf for DNS inheritance.
Known issues
Kubernetes installs do not configure the nodes’ resolv.conf files to use the cluster DNS by default, because that process is inherently distro-specific. This should probably be implemented eventually.
Linux’s libc is impossibly stuck (see this bug from 2005) with limits of just 3 DNS nameserver records and 6 DNS search records. Kubernetes needs to consume 1 nameserver record and 3 search records. This means that if a local installation already uses 3 nameservers or uses more than 3 searches, some of those settings will be lost. As a partial workaround, the node can run dnsmasqwhich will provide more nameserver entries, but not more search entries. You can also use kubelet’s --resolv-conf flag.
If you are using Alpine version 3.3 or earlier as your base image, DNS may not work properly owing to a known issue with Alpine. Checkhere for more information.
References
https://kubernetes.io/docs/admin/dns/