You are viewing documentation for Kubernetes version: v1.27
Kubernetes v1.27 documentation is no longer actively maintained. The version you are currently viewing is a static snapshot. For up-to-date information, see the latest version.
DNS for Services and Pods
Kubernetes creates DNS records for Services and Pods. You can contact Services with consistent DNS names instead of IP addresses.
Kubernetes publishes information about Pods and Services which is used to program DNS. Kubelet configures Pods' DNS so that running containers can lookup Services by name rather than IP.
Services defined in the cluster are assigned DNS names. By default, a client Pod's DNS search list includes the Pod's own namespace and the cluster's default domain.
Namespaces of Services
A DNS query may return different results based on the namespace of the Pod making it. DNS queries that don't specify a namespace are limited to the Pod's namespace. Access Services in other namespaces by specifying it in the DNS query.
For example, consider a Pod in a test
namespace. A data
Service is in
the prod
namespace.
A query for data
returns no results, because it uses the Pod's test
namespace.
A query for data.prod
returns the intended result, because it specifies the
namespace.
DNS queries may be expanded using the Pod's /etc/resolv.conf
. Kubelet
configures this file for each Pod. For example, a query for just data
may be
expanded to data.test.svc.cluster.local
. The values of the search
option
are used to expand queries. To learn more about DNS queries, see
the resolv.conf
manual page.
nameserver 10.32.0.10
search <namespace>.svc.cluster.local svc.cluster.local cluster.local
options ndots:5
In summary, a Pod in the test namespace can successfully resolve either
data.prod
or data.prod.svc.cluster.local
.
DNS Records
What objects get DNS records?
- Services
- Pods
The following sections detail the supported DNS 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. For more up-to-date specification, see Kubernetes DNS-Based Service Discovery.
Services
A/AAAA records
"Normal" (not headless) Services are assigned DNS A and/or AAAA records,
depending on the IP family or families of the Service, with a name of the form
my-svc.my-namespace.svc.cluster-domain.example
. This resolves to the cluster IP
of the Service.
Headless Services
(without a cluster IP) Services are also assigned DNS A and/or AAAA records,
with a name of the form my-svc.my-namespace.svc.cluster-domain.example
. Unlike normal
Services, this resolves to the set of IPs of all 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 has the form
_port-name._port-protocol.my-svc.my-namespace.svc.cluster-domain.example
.
For a regular Service, this resolves to the port number and the domain name:
my-svc.my-namespace.svc.cluster-domain.example
.
For a headless Service, this resolves to multiple answers, one for each Pod
that is backing the Service, and contains the port number and the domain name of the Pod
of the form hostname.my-svc.my-namespace.svc.cluster-domain.example
.
Pods
A/AAAA records
In general a Pod has the following DNS resolution:
pod-ip-address.my-namespace.pod.cluster-domain.example
.
For example, if a Pod in the default
namespace has the IP address 172.17.0.3,
and the domain name for your cluster is cluster.local
, then the Pod has a DNS name:
172-17-0-3.default.pod.cluster.local
.
Any Pods exposed by a Service have the following DNS resolution available:
pod-ip-address.service-name.my-namespace.svc.cluster-domain.example
.
Pod's hostname and subdomain fields
Currently when a Pod is created, its hostname (as observed from within the Pod)
is the Pod's metadata.name
value.
The Pod spec has an optional hostname
field, which can be used to specify a
different hostname. When specified, it takes precedence over the Pod's name to be
the hostname of the Pod (again, as observed from within the Pod). For example,
given a Pod with spec.hostname
set to "my-host"
, the Pod will have its
hostname set to "my-host"
.
The Pod spec also has an optional subdomain
field which can be used to indicate
that the pod is part of sub-group of the namespace. For example, a Pod with spec.hostname
set to "foo"
, and spec.subdomain
set to "bar"
, in namespace "my-namespace"
, will
have its hostname set to "foo"
and its fully qualified domain name (FQDN) set to
"foo.bar.my-namespace.svc.cluster.local"
(once more, as observed from within
the Pod).
If there exists a headless Service in the same namespace as the Pod, with the same name as the subdomain, the cluster's DNS Server also returns A and/or AAAA records for the Pod's fully qualified hostname.
Example:
apiVersion: v1
kind: Service
metadata:
name: busybox-subdomain
spec:
selector:
name: busybox
clusterIP: None
ports:
- name: foo # name is not required for single-port Services
port: 1234
---
apiVersion: v1
kind: Pod
metadata:
name: busybox1
labels:
name: busybox
spec:
hostname: busybox-1
subdomain: busybox-subdomain
containers:
- image: busybox:1.28
command:
- sleep
- "3600"
name: busybox
---
apiVersion: v1
kind: Pod
metadata:
name: busybox2
labels:
name: busybox
spec:
hostname: busybox-2
subdomain: busybox-subdomain
containers:
- image: busybox:1.28
command:
- sleep
- "3600"
name: busybox
Given the above Service "busybox-subdomain"
and the Pods which set spec.subdomain
to "busybox-subdomain"
, the first Pod will see its own FQDN as
"busybox-1.busybox-subdomain.my-namespace.svc.cluster-domain.example"
. DNS serves
A and/or AAAA records at that name, pointing to the Pod's IP. Both Pods "busybox1
" and
"busybox2
" will have their own address records.
An EndpointSlice can specify the DNS hostname for any endpoint addresses, along with its IP.
hostname
is required for the Pod's A or AAAA
record to be created. A Pod with no hostname
but with subdomain
will only create the
A or AAAA record for the headless Service (busybox-subdomain.my-namespace.svc.cluster-domain.example
),
pointing to the Pods' IP addresses. Also, the Pod needs to be ready in order to have a
record unless publishNotReadyAddresses=True
is set on the Service.
Pod's setHostnameAsFQDN field
Kubernetes v1.22 [stable]
When a Pod is configured to have fully qualified domain name (FQDN), its
hostname is the short hostname. For example, if you have a Pod with the fully
qualified domain name busybox-1.busybox-subdomain.my-namespace.svc.cluster-domain.example
,
then by default the hostname
command inside that Pod returns busybox-1
and the
hostname --fqdn
command returns the FQDN.
When you set setHostnameAsFQDN: true
in the Pod spec, the kubelet writes the Pod's FQDN into the hostname for that Pod's namespace. In this case, both hostname
and hostname --fqdn
return the Pod's FQDN.
In Linux, the hostname field of the kernel (the nodename
field of struct utsname
) is limited to 64 characters.
If a Pod enables this feature and its FQDN is longer than 64 character, it will fail to start. The Pod will remain in Pending
status (ContainerCreating
as seen by kubectl
) generating error events, such as Failed to construct FQDN from Pod hostname and cluster domain, FQDN long-FQDN
is too long (64 characters is the max, 70 characters requested). One way of improving user experience for this scenario is to create an admission webhook controller to control FQDN size when users create top level objects, for example, Deployment.
Pod's DNS Policy
DNS policies can be set on a per-Pod basis. Currently Kubernetes supports the
following Pod-specific DNS policies. These policies are specified in the
dnsPolicy
field of a Pod Spec.
- "
Default
": The Pod inherits the name resolution configuration from the node that the Pods run on. See related discussion for more details. - "
ClusterFirst
": Any DNS query that does not match the configured cluster domain suffix, such as "www.kubernetes.io
", is forwarded to an upstream nameserver by the DNS server. Cluster administrators may have extra stub-domain and upstream DNS servers configured. See related discussion for details on how DNS queries are handled in those cases. - "
ClusterFirstWithHostNet
": For Pods running with hostNetwork, you should explicitly set its DNS policy to "ClusterFirstWithHostNet
". Otherwise, Pods running with hostNetwork and"ClusterFirst"
will fallback to the behavior of the"Default"
policy.- Note: This is not supported on Windows. See below for details
- "
None
": It allows a Pod to ignore DNS settings from the Kubernetes environment. All DNS settings are supposed to be provided using thednsConfig
field in the Pod Spec. See Pod's DNS config subsection below.
dnsPolicy
is not
explicitly specified, then "ClusterFirst" is used.
The example below shows a Pod with its DNS policy set to
"ClusterFirstWithHostNet
" because it has hostNetwork
set to true
.
apiVersion: v1
kind: Pod
metadata:
name: busybox
namespace: default
spec:
containers:
- image: busybox:1.28
command:
- sleep
- "3600"
imagePullPolicy: IfNotPresent
name: busybox
restartPolicy: Always
hostNetwork: true
dnsPolicy: ClusterFirstWithHostNet
Pod's DNS Config
Kubernetes v1.14 [stable]
Pod's DNS Config allows users more control on the DNS settings for a Pod.
The dnsConfig
field is optional and it can work with any dnsPolicy
settings.
However, when a Pod's dnsPolicy
is set to "None
", the dnsConfig
field has
to be specified.
Below are the properties a user can specify in the dnsConfig
field:
nameservers
: a list of IP addresses that will be used as DNS servers for the Pod. There can be at most 3 IP addresses specified. When the Pod'sdnsPolicy
is set to "None
", the list must contain at least one IP address, otherwise this property is optional. The servers listed will be combined to the base nameservers generated from the specified DNS policy with duplicate addresses removed.searches
: a list of DNS search domains for hostname lookup in the Pod. This property is optional. When specified, the provided list will be merged into the base search domain names generated from the chosen DNS policy. Duplicate domain names are removed. Kubernetes allows for at most 6 search domains.options
: an optional list of objects where each object may have aname
property (required) and avalue
property (optional). The contents in this property will be merged to the options generated from the specified DNS policy. Duplicate entries are removed.
The following is an example Pod with custom DNS settings:
apiVersion: v1
kind: Pod
metadata:
namespace: default
name: dns-example
spec:
containers:
- name: test
image: nginx
dnsPolicy: "None"
dnsConfig:
nameservers:
- 192.0.2.1 # this is an example
searches:
- ns1.svc.cluster-domain.example
- my.dns.search.suffix
options:
- name: ndots
value: "2"
- name: edns0
When the Pod above is created, the container test
gets the following contents
in its /etc/resolv.conf
file:
nameserver 192.0.2.1
search ns1.svc.cluster-domain.example my.dns.search.suffix
options ndots:2 edns0
For IPv6 setup, search path and name server should be set up like this:
kubectl exec -it dns-example -- cat /etc/resolv.conf
The output is similar to this:
nameserver 2001:db8:30::a
search default.svc.cluster-domain.example svc.cluster-domain.example cluster-domain.example
options ndots:5
DNS search domain list limits
Kubernetes 1.26 [beta]
Kubernetes itself does not limit the DNS Config until the length of the search domain list exceeds 32 or the total length of all search domains exceeds 2048. This limit applies to the node's resolver configuration file, the Pod's DNS Config, and the merged DNS Config respectively.
Some container runtimes of earlier versions may have their own restrictions on the number of DNS search domains. Depending on the container runtime environment, the pods with a large number of DNS search domains may get stuck in the pending state.
It is known that containerd v1.5.5 or earlier and CRI-O v1.21 or earlier have this problem.
DNS resolution on Windows nodes
- ClusterFirstWithHostNet is not supported for Pods that run on Windows nodes.
Windows treats all names with a
.
as a FQDN and skips FQDN resolution. - On Windows, there are multiple DNS resolvers that can be used. As these come with
slightly different behaviors, using the
Resolve-DNSName
powershell cmdlet for name query resolutions is recommended. - On Linux, you have a DNS suffix list, which is used after resolution of a name as fully
qualified has failed.
On Windows, you can only have 1 DNS suffix, which is the DNS suffix associated with that
Pod's namespace (example:
mydns.svc.cluster.local
). Windows can resolve FQDNs, Services, or network name which can be resolved with this single suffix. For example, a Pod spawned in thedefault
namespace, will have the DNS suffixdefault.svc.cluster.local
. Inside a Windows Pod, you can resolve bothkubernetes.default.svc.cluster.local
andkubernetes
, but not the partially qualified names (kubernetes.default
orkubernetes.default.svc
).
What's next
For guidance on administering DNS configurations, check Configure DNS Service