In Kubernetes, a Service is a method for exposing a network application that is running as one or more Pods in your cluster.
A key aim of Services in Kubernetes is that you don't need to modify your existing application to use an unfamiliar service discovery mechanism. You can run code in Pods, whether this is a code designed for a cloud-native world, or an older app you've containerized. You use a Service to make that set of Pods available on the network so that clients can interact with it.
If you use a Deployment to run your app, that Deployment can create and destroy Pods dynamically. From one moment to the next, you don't know how many of those Pods are working and healthy; you might not even know what those healthy Pods are named. Kubernetes Pods are created and destroyed to match the desired state of your cluster. Pods are ephemeral resources (you should not expect that an individual Pod is reliable and durable).
Each Pod gets its own IP address (Kubernetes expects network plugins to ensure this). For a given Deployment in your cluster, the set of Pods running in one moment in time could be different from the set of Pods running that application a moment later.
This leads to a problem: if some set of Pods (call them "backends") provides functionality to other Pods (call them "frontends") inside your cluster, how do the frontends find out and keep track of which IP address to connect to, so that the frontend can use the backend part of the workload?
Services in Kubernetes
The Service API, part of Kubernetes, is an abstraction to help you expose groups of Pods over a network. Each Service object defines a logical set of endpoints (usually these endpoints are Pods) along with a policy about how to make those pods accessible.
For example, consider a stateless image-processing backend which is running with 3 replicas. Those replicas are fungible—frontends do not care which backend they use. While the actual Pods that compose the backend set may change, the frontend clients should not need to be aware of that, nor should they need to keep track of the set of backends themselves.
The Service abstraction enables this decoupling.
The set of Pods targeted by a Service is usually determined by a selector that you define. To learn about other ways to define Service endpoints, see Services without selectors.
If your workload speaks HTTP, you might choose to use an Ingress to control how web traffic reaches that workload. Ingress is not a Service type, but it acts as the entry point for your cluster. An Ingress lets you consolidate your routing rules into a single resource, so that you can expose multiple components of your workload, running separately in your cluster, behind a single listener.
The Gateway API for Kubernetes provides extra capabilities beyond Ingress and Service. You can add Gateway to your cluster - it is a family of extension APIs, implemented using CustomResourceDefinitions - and then use these to configure access to network services that are running in your cluster.
Cloud-native service discovery
If you're able to use Kubernetes APIs for service discovery in your application, you can query the API server for matching EndpointSlices. Kubernetes updates the EndpointSlices for a Service whenever the set of Pods in a Service changes.
For non-native applications, Kubernetes offers ways to place a network port or load balancer in between your application and the backend Pods.
Either way, your workload can use these service discovery mechanisms to find the target it wants to connect to.
Defining a Service
A Service is an object
(the same way that a Pod or a ConfigMap is an object). You can create,
view or modify Service definitions using the Kubernetes API. Usually
you use a tool such as
kubectl to make those API calls for you.
For example, suppose you have a set of Pods that each listen on TCP port 9376
and are labelled as
app.kubernetes.io/name=MyApp. You can define a Service to
publish that TCP listener:
apiVersion: v1 kind: Service metadata: name: my-service spec: selector: app.kubernetes.io/name: MyApp ports: - protocol: TCP port: 80 targetPort: 9376
Applying this manifest creates a new Service named "my-service", which
targets TCP port 9376 on any Pod with the
app.kubernetes.io/name: MyApp label.
Kubernetes assigns this Service an IP address (the cluster IP), that is used by the virtual IP address mechanism. For more details on that mechanism, read Virtual IPs and Service Proxies.
The controller for that Service continuously scans for Pods that match its selector, and then makes any necessary updates to the set of EndpointSlices for the Service.
The name of a Service object must be a valid RFC 1035 label name.
targetPort. By default and for convenience, the
targetPortis set to the same value as the
Port definitions in Pods have names, and you can reference these names in the
targetPort attribute of a Service. For example, we can bind the
of the Service to the Pod port in the following way:
apiVersion: v1 kind: Pod metadata: name: nginx labels: app.kubernetes.io/name: proxy spec: containers: - name: nginx image: nginx:stable ports: - containerPort: 80 name: http-web-svc --- apiVersion: v1 kind: Service metadata: name: nginx-service spec: selector: app.kubernetes.io/name: proxy ports: - name: name-of-service-port protocol: TCP port: 80 targetPort: http-web-svc
This works even if there is a mixture of Pods in the Service using a single configured name, with the same network protocol available via different port numbers. This offers a lot of flexibility for deploying and evolving your Services. For example, you can change the port numbers that Pods expose in the next version of your backend software, without breaking clients.
The default protocol for Services is TCP; you can also use any other supported protocol.
Because many Services need to expose more than one port, Kubernetes supports
multiple port definitions for a single Service.
Each port definition can have the same
protocol, or a different one.
Services without selectors
Services most commonly abstract access to Kubernetes Pods thanks to the selector, but when used with a corresponding set of EndpointSlices objects and without a selector, the Service can abstract other kinds of backends, including ones that run outside the cluster.
- You want to have an external database cluster in production, but in your test environment you use your own databases.
- You want to point your Service to a Service in a different Namespace or on another cluster.
- You are migrating a workload to Kubernetes. While evaluating the approach, you run only a portion of your backends in Kubernetes.
In any of these scenarios you can define a Service without specifying a selector to match Pods. For example:
apiVersion: v1 kind: Service metadata: name: my-service spec: ports: - protocol: TCP port: 80 targetPort: 9376
Because this Service has no selector, the corresponding EndpointSlice (and legacy Endpoints) objects are not created automatically. You can map the Service to the network address and port where it's running, by adding an EndpointSlice object manually. For example:
apiVersion: discovery.k8s.io/v1 kind: EndpointSlice metadata: name: my-service-1 # by convention, use the name of the Service # as a prefix for the name of the EndpointSlice labels: # You should set the "kubernetes.io/service-name" label. # Set its value to match the name of the Service kubernetes.io/service-name: my-service addressType: IPv4 ports: - name: '' # empty because port 9376 is not assigned as a well-known # port (by IANA) appProtocol: http protocol: TCP port: 9376 endpoints: - addresses: - "10.4.5.6" # the IP addresses in this list can appear in any order - "10.1.2.3"
When you create an EndpointSlice object for a Service, you can
use any name for the EndpointSlice. Each EndpointSlice in a namespace must have a
unique name. You link an EndpointSlice to a Service by setting the
on that EndpointSlice.
The endpoint IPs must not be: loopback (127.0.0.0/8 for IPv4, ::1/128 for IPv6), or link-local (169.254.0.0/16 and 126.96.36.199/24 for IPv4, fe80::/64 for IPv6).
The endpoint IP addresses cannot be the cluster IPs of other Kubernetes Services, because kube-proxy doesn't support virtual IPs as a destination.
For an EndpointSlice that you create yourself, or in your own code,
you should also pick a value to use for the
If you create your own controller code to manage EndpointSlices, consider using a
value similar to
"my-domain.example/name-of-controller". If you are using a third
party tool, use the name of the tool in all-lowercase and change spaces and other
punctuation to dashes (
If people are directly using a tool such as
kubectl to manage EndpointSlices,
use a name that describes this manual management, such as
"cluster-admins". You should
avoid using the reserved value
"controller", which identifies EndpointSlices
managed by Kubernetes' own control plane.
Accessing a Service without a selector
Accessing a Service without a selector works the same as if it had a selector. In the example for a Service without a selector, traffic is routed to one of the two endpoints defined in the EndpointSlice manifest: a TCP connection to 10.1.2.3 or 10.4.5.6, on port 9376.
kubectl proxy <service-name>where the service has no selector will fail due to this constraint. This prevents the Kubernetes API server from being used as a proxy to endpoints the caller may not be authorized to access.
ExternalName Service is a special case of Service that does not have
selectors and uses DNS names instead. For more information, see the
Kubernetes v1.21 [stable]
EndpointSlices are objects that represent a subset (a slice) of the backing network endpoints for a Service.
Your Kubernetes cluster tracks how many endpoints each EndpointSlice represents. If there are so many endpoints for a Service that a threshold is reached, then Kubernetes adds another empty EndpointSlice and stores new endpoint information there. By default, Kubernetes makes a new EndpointSlice once the existing EndpointSlices all contain at least 100 endpoints. Kubernetes does not make the new EndpointSlice until an extra endpoint needs to be added.
See EndpointSlices for more information about this API.
In the Kubernetes API, an Endpoints (the resource kind is plural) defines a list of network endpoints, typically referenced by a Service to define which Pods the traffic can be sent to.
The EndpointSlice API is the recommended replacement for Endpoints.
Kubernetes limits the number of endpoints that can fit in a single Endpoints object. When there are over 1000 backing endpoints for a Service, Kubernetes truncates the data in the Endpoints object. Because a Service can be linked with more than one EndpointSlice, the 1000 backing endpoint limit only affects the legacy Endpoints API.
In that case, Kubernetes selects at most 1000 possible backend endpoints to store
into the Endpoints object, and sets an
annotation on the
The control plane also removes that annotation if the number of backend Pods drops below 1000.
Traffic is still sent to backends, but any load balancing mechanism that relies on the legacy Endpoints API only sends traffic to at most 1000 of the available backing endpoints.
The same API limit means that you cannot manually update an Endpoints to have more than 1000 endpoints.
Kubernetes v1.20 [stable]
appProtocol field provides a way to specify an application protocol for
each Service port. This is used as a hint for implementations to offer richer behavior for protocols that they understand.
The value of this field is mirrored by the corresponding
Endpoints and EndpointSlice objects.
This field follows standard Kubernetes label syntax. Valid values are one of:
Implementation-defined prefixed names such as
Kubernetes-defined prefixed names:
|HTTP/2 over cleartext as described in RFC 7540|
For some Services, you need to expose more than one port. Kubernetes lets you configure multiple port definitions on a Service object. When using multiple ports for a Service, you must give all of your ports names so that these are unambiguous. For example:
apiVersion: v1 kind: Service metadata: name: my-service spec: selector: app.kubernetes.io/name: MyApp ports: - name: http protocol: TCP port: 80 targetPort: 9376 - name: https protocol: TCP port: 443 targetPort: 9377
As with Kubernetes names in general, names for ports
must only contain lowercase alphanumeric characters and
-. Port names must
also start and end with an alphanumeric character.
For example, the names
web are valid, but
-web are not.
For some parts of your application (for example, frontends) you may want to expose a Service onto an external IP address, one that's accessible from outside of your cluster.
Kubernetes Service types allow you to specify what kind of Service you want.
type values and their behaviors are:
- Exposes the Service on a cluster-internal IP. Choosing this value
makes the Service only reachable from within the cluster. This is the
default that is used if you don't explicitly specify a
typefor a Service. You can expose the Service to the public internet using an Ingress or a Gateway.
- Exposes the Service on each Node's IP at a static port (the
NodePort). To make the node port available, Kubernetes sets up a cluster IP address, the same as if you had requested a Service of
- Exposes the Service externally using an external load balancer. Kubernetes does not directly offer a load balancing component; you must provide one, or you can integrate your Kubernetes cluster with a cloud provider.
- Maps the Service to the contents of the
externalNamefield (for example, to the hostname
api.foo.bar.example). The mapping configures your cluster's DNS server to return a
CNAMErecord with that external hostname value. No proxying of any kind is set up.
type field in the Service API is designed as nested functionality - each level
adds to the previous. This is not strictly required on all cloud providers, but
the Kubernetes API design for Service requires it anyway.
This default Service type assigns an IP address from a pool of IP addresses that your cluster has reserved for that purpose.
Several of the other types for Service build on the
ClusterIP type as a
If you define a Service that has the
.spec.clusterIP set to
Kubernetes does not assign an IP address. See headless Services
for more information.
Choosing your own IP address
You can specify your own cluster IP address as part of a
request. To do this, set the
.spec.clusterIP field. For example, if you
already have an existing DNS entry that you wish to reuse, or legacy systems
that are configured for a specific IP address and difficult to re-configure.
The IP address that you choose must be a valid IPv4 or IPv6 address from within the
service-cluster-ip-range CIDR range that is configured for the API server.
If you try to create a Service with an invalid
clusterIP address value, the API
server will return a 422 HTTP status code to indicate that there's a problem.
Read avoiding collisions to learn how Kubernetes helps reduce the risk and impact of two different Services both trying to use the same IP address.
If you set the
type field to
NodePort, the Kubernetes control plane
allocates a port from a range specified by
--service-node-port-range flag (default: 30000-32767).
Each node proxies that port (the same port number on every Node) into your Service.
Your Service reports the allocated port in its
Using a NodePort gives you the freedom to set up your own load balancing solution, to configure environments that are not fully supported by Kubernetes, or even to expose one or more nodes' IP addresses directly.
For a node port Service, Kubernetes additionally allocates a port (TCP, UDP or
SCTP to match the protocol of the Service). Every node in the cluster configures
itself to listen on that assigned port and to forward traffic to one of the ready
endpoints associated with that Service. You'll be able to contact the
Service, from outside the cluster, by connecting to any node using the appropriate
protocol (for example: TCP), and the appropriate port (as assigned to that Service).
Choosing your own port
If you want a specific port number, you can specify a value in the
field. The control plane will either allocate you that port or report that
the API transaction failed.
This means that you need to take care of possible port collisions yourself.
You also have to use a valid port number, one that's inside the range configured
for NodePort use.
Here is an example manifest for a Service of
type: NodePort that specifies
a NodePort value (30007, in this example):
apiVersion: v1 kind: Service metadata: name: my-service spec: type: NodePort selector: app.kubernetes.io/name: MyApp ports: # By default and for convenience, the `targetPort` is set to the same value as the `port` field. - port: 80 targetPort: 80 # Optional field # By default and for convenience, the Kubernetes control plane will allocate a port from a range (default: 30000-32767) nodePort: 30007
Reserve Nodeport Ranges to avoid collisions when port assigning
Kubernetes v1.27 [alpha]
The policy for assigning ports to NodePort services applies to both the auto-assignment and
the manual assignment scenarios. When a user wants to create a NodePort service that
uses a specific port, the target port may conflict with another port that has already been assigned.
In this case, you can enable the feature gate
ServiceNodePortStaticSubrange, which allows you
to use a different port allocation strategy for NodePort Services. The port range for NodePort services
is divided into two bands. Dynamic port assignment uses the upper band by default, and it may use
the lower band once the upper band has been exhausted. Users can then allocate from the lower band
with a lower risk of port collision.
Custom IP address configuration for
type: NodePort Services
You can set up nodes in your cluster to use a particular IP address for serving node port services. You might want to do this if each node is connected to multiple networks (for example: one network for application traffic, and another network for traffic between nodes and the control plane).
If you want to specify particular IP address(es) to proxy the port, you can set the
--nodeport-addresses flag for kube-proxy or the equivalent
field of the
kube-proxy configuration file
to particular IP block(s).
This flag takes a comma-delimited list of IP blocks (e.g.
to specify IP address ranges that kube-proxy should consider as local to this node.
For example, if you start kube-proxy with the
kube-proxy only selects the loopback interface for NodePort Services.
The default for
--nodeport-addresses is an empty list.
This means that kube-proxy should consider all available network interfaces for NodePort.
(That's also compatible with earlier Kubernetes releases.)
.spec.clusterIP:spec.ports[*].port. If the
--nodeport-addressesflag for kube-proxy or the equivalent field in the kube-proxy configuration file is set,
<NodeIP>would be a filtered node IP address (or possibly IP addresses).
On cloud providers which support external load balancers, setting the
LoadBalancer provisions a load balancer for your Service.
The actual creation of the load balancer happens asynchronously, and
information about the provisioned balancer is published in the Service's
apiVersion: v1 kind: Service metadata: name: my-service spec: selector: app.kubernetes.io/name: MyApp ports: - protocol: TCP port: 80 targetPort: 9376 clusterIP: 10.0.171.239 type: LoadBalancer status: loadBalancer: ingress: - ip: 192.0.2.127
Traffic from the external load balancer is directed at the backend Pods. The cloud provider decides how it is load balanced.
To implement a Service of
type: LoadBalancer, Kubernetes typically starts off
by making the changes that are equivalent to you requesting a Service of
type: NodePort. The cloud-controller-manager component then configures the external
load balancer to forward traffic to that assigned node port.
You can configure a load balanced Service to omit assigning a node port, provided that the cloud provider implementation supports this.
Some cloud providers allow you to specify the
loadBalancerIP. In those cases, the load-balancer is created
with the user-specified
loadBalancerIP. If the
loadBalancerIP field is not specified,
the load balancer is set up with an ephemeral IP address. If you specify a
but your cloud provider does not support the feature, the
loadbalancerIP field that you
set is ignored.
.spec.loadBalancerIP field for a Service was deprecated in Kubernetes v1.24.
This field was under-specified and its meaning varies across implementations. It also cannot support dual-stack networking. This field may be removed in a future API version.
If you're integrating with a provider that supports specifying the load balancer IP address(es) for a Service via a (provider specific) annotation, you should switch to doing that.
If you are writing code for a load balancer integration with Kubernetes, avoid using this field. You can integrate with Gateway rather than Service, or you can define your own (provider specific) annotations on the Service that specify the equivalent detail.
Load balancers with mixed protocol types
Kubernetes v1.24 [beta]
By default, for LoadBalancer type of Services, when there is more than one port defined, all ports must have the same protocol, and the protocol must be one which is supported by the cloud provider.
The feature gate
MixedProtocolLBService (enabled by default for the kube-apiserver as of v1.24) allows the use of
different protocols for LoadBalancer type of Services, when there is more than one port defined.
Disabling load balancer NodePort allocation
Kubernetes v1.24 [stable]
You can optionally disable node port allocation for a Service of
type: LoadBalancer, by setting
false. This should only be used for load balancer implementations
that route traffic directly to pods as opposed to using node ports. By default,
true and type LoadBalancer Services will continue to allocate node ports. If
is set to
false on an existing Service with allocated node ports, those node ports will not be de-allocated automatically.
You must explicitly remove the
nodePorts entry in every Service port to de-allocate those node ports.
Specifying class of load balancer implementation
Kubernetes v1.24 [stable]
For a Service with
type set to
enables you to use a load balancer implementation other than the cloud provider default.
.spec.loadBalancerClass is not set and a
type of Service uses the cloud provider's default load balancer implementation if the
cluster is configured with a cloud provider using the
If you specify
.spec.loadBalancerClass, it is assumed that a load balancer
implementation that matches the specified class is watching for Services.
Any default load balancer implementation (for example, the one provided by
the cloud provider) will ignore Services that have this field set.
spec.loadBalancerClass can be set on a Service of type
Once set, it cannot be changed.
The value of
spec.loadBalancerClass must be a label-style identifier,
with an optional prefix such as "
internal-vip" or "
Unprefixed names are reserved for end-users.
Internal load balancer
In a mixed environment it is sometimes necessary to route traffic from Services inside the same (virtual) network address block.
In a split-horizon DNS environment you would need two Services to be able to route both external and internal traffic to your endpoints.
To set an internal load balancer, add one of the following annotations to your Service depending on the cloud service provider you're using:
Select one of the tabs.
[...] metadata: name: my-service annotations: networking.gke.io/load-balancer-type: "Internal" [...]
[...] metadata: name: my-service annotations: service.beta.kubernetes.io/aws-load-balancer-internal: "true" [...]
[...] metadata: name: my-service annotations: service.beta.kubernetes.io/azure-load-balancer-internal: "true" [...]
[...] metadata: name: my-service annotations: service.kubernetes.io/ibm-load-balancer-cloud-provider-ip-type: "private" [...]
[...] metadata: name: my-service annotations: service.beta.kubernetes.io/openstack-internal-load-balancer: "true" [...]
[...] metadata: name: my-service annotations: service.beta.kubernetes.io/cce-load-balancer-internal-vpc: "true" [...]
[...] metadata: annotations: service.kubernetes.io/qcloud-loadbalancer-internal-subnetid: subnet-xxxxx [...]
[...] metadata: annotations: service.beta.kubernetes.io/alibaba-cloud-loadbalancer-address-type: "intranet" [...]
[...] metadata: name: my-service annotations: service.beta.kubernetes.io/oci-load-balancer-internal: true [...]
TLS support on AWS
For partial TLS / SSL support on clusters running on AWS, you can add three
annotations to a
metadata: name: my-service annotations: service.beta.kubernetes.io/aws-load-balancer-ssl-cert: arn:aws:acm:us-east-1:123456789012:certificate/12345678-1234-1234-1234-123456789012
The first specifies the ARN of the certificate to use. It can be either a certificate from a third party issuer that was uploaded to IAM or one created within AWS Certificate Manager.
metadata: name: my-service annotations: service.beta.kubernetes.io/aws-load-balancer-backend-protocol: (https|http|ssl|tcp)
The second annotation specifies which protocol a Pod speaks. For HTTPS and SSL, the ELB expects the Pod to authenticate itself over the encrypted connection, using a certificate.
HTTP and HTTPS selects layer 7 proxying: the ELB terminates
the connection with the user, parses headers, and injects the
header with the user's IP address (Pods only see the IP address of the
ELB at the other end of its connection) when forwarding requests.
TCP and SSL selects layer 4 proxying: the ELB forwards traffic without modifying the headers.
In a mixed-use environment where some ports are secured and others are left unencrypted, you can use the following annotations:
metadata: name: my-service annotations: service.beta.kubernetes.io/aws-load-balancer-backend-protocol: http service.beta.kubernetes.io/aws-load-balancer-ssl-ports: "443,8443"
In the above example, if the Service contained three ports,
8443 would use the SSL certificate, but
80 would be proxied HTTP.
From Kubernetes v1.9 onwards you can use
predefined AWS SSL policies
with HTTPS or SSL listeners for your Services.
To see which policies are available for use, you can use the
aws command line tool:
aws elb describe-load-balancer-policies --query 'PolicyDescriptions.PolicyName'
You can then specify any one of those policies using the
annotation; for example:
metadata: name: my-service annotations: service.beta.kubernetes.io/aws-load-balancer-ssl-negotiation-policy: "ELBSecurityPolicy-TLS-1-2-2017-01"
PROXY protocol support on AWS
To enable PROXY protocol support for clusters running on AWS, you can use the following service annotation:
metadata: name: my-service annotations: service.beta.kubernetes.io/aws-load-balancer-proxy-protocol: "*"
Since version 1.3.0, the use of this annotation applies to all ports proxied by the ELB and cannot be configured otherwise.
ELB Access Logs on AWS
There are several annotations to manage access logs for ELB Services on AWS.
controls whether access logs are enabled.
controls the interval in minutes for publishing the access logs. You can specify
an interval of either 5 or 60 minutes.
controls the name of the Amazon S3 bucket where load balancer access logs are
specifies the logical hierarchy you created for your Amazon S3 bucket.
metadata: name: my-service annotations: # Specifies whether access logs are enabled for the load balancer service.beta.kubernetes.io/aws-load-balancer-access-log-enabled: "true" # The interval for publishing the access logs. You can specify an interval of either 5 or 60 (minutes). service.beta.kubernetes.io/aws-load-balancer-access-log-emit-interval: "60" # The name of the Amazon S3 bucket where the access logs are stored service.beta.kubernetes.io/aws-load-balancer-access-log-s3-bucket-name: "my-bucket" # The logical hierarchy you created for your Amazon S3 bucket, for example `my-bucket-prefix/prod` service.beta.kubernetes.io/aws-load-balancer-access-log-s3-bucket-prefix: "my-bucket-prefix/prod"
Connection Draining on AWS
Connection draining for Classic ELBs can be managed with the annotation
to the value of
"true". The annotation
also be used to set maximum time, in seconds, to keep the existing connections open before
deregistering the instances.
metadata: name: my-service annotations: service.beta.kubernetes.io/aws-load-balancer-connection-draining-enabled: "true" service.beta.kubernetes.io/aws-load-balancer-connection-draining-timeout: "60"
Other ELB annotations
There are other annotations to manage Classic Elastic Load Balancers that are described below.
metadata: name: my-service annotations: # The time, in seconds, that the connection is allowed to be idle (no data has been sent # over the connection) before it is closed by the load balancer service.beta.kubernetes.io/aws-load-balancer-connection-idle-timeout: "60" # Specifies whether cross-zone load balancing is enabled for the load balancer service.beta.kubernetes.io/aws-load-balancer-cross-zone-load-balancing-enabled: "true" # A comma-separated list of key-value pairs which will be recorded as # additional tags in the ELB. service.beta.kubernetes.io/aws-load-balancer-additional-resource-tags: "environment=prod,owner=devops" # The number of successive successful health checks required for a backend to # be considered healthy for traffic. Defaults to 2, must be between 2 and 10 service.beta.kubernetes.io/aws-load-balancer-healthcheck-healthy-threshold: "" # The number of unsuccessful health checks required for a backend to be # considered unhealthy for traffic. Defaults to 6, must be between 2 and 10 service.beta.kubernetes.io/aws-load-balancer-healthcheck-unhealthy-threshold: "3" # The approximate interval, in seconds, between health checks of an # individual instance. Defaults to 10, must be between 5 and 300 service.beta.kubernetes.io/aws-load-balancer-healthcheck-interval: "20" # The amount of time, in seconds, during which no response means a failed # health check. This value must be less than the service.beta.kubernetes.io/aws-load-balancer-healthcheck-interval # value. Defaults to 5, must be between 2 and 60 service.beta.kubernetes.io/aws-load-balancer-healthcheck-timeout: "5" # A list of existing security groups to be configured on the ELB created. Unlike the annotation # service.beta.kubernetes.io/aws-load-balancer-extra-security-groups, this replaces all other # security groups previously assigned to the ELB and also overrides the creation # of a uniquely generated security group for this ELB. # The first security group ID on this list is used as a source to permit incoming traffic to # target worker nodes (service traffic and health checks). # If multiple ELBs are configured with the same security group ID, only a single permit line # will be added to the worker node security groups, that means if you delete any # of those ELBs it will remove the single permit line and block access for all ELBs that shared the same security group ID. # This can cause a cross-service outage if not used properly service.beta.kubernetes.io/aws-load-balancer-security-groups: "sg-53fae93f" # A list of additional security groups to be added to the created ELB, this leaves the uniquely # generated security group in place, this ensures that every ELB # has a unique security group ID and a matching permit line to allow traffic to the target worker nodes # (service traffic and health checks). # Security groups defined here can be shared between services. service.beta.kubernetes.io/aws-load-balancer-extra-security-groups: "sg-53fae93f,sg-42efd82e" # A comma separated list of key-value pairs which are used # to select the target nodes for the load balancer service.beta.kubernetes.io/aws-load-balancer-target-node-labels: "ingress-gw,gw-name=public-api"
Network Load Balancer support on AWS
Kubernetes v1.15 [beta]
To use a Network Load Balancer on AWS, use the annotation
service.beta.kubernetes.io/aws-load-balancer-type with the value set to
metadata: name: my-service annotations: service.beta.kubernetes.io/aws-load-balancer-type: "nlb"
Unlike Classic Elastic Load Balancers, Network Load Balancers (NLBs) forward the
client's IP address through to the node. If a Service's
is set to
Cluster, the client's IP address is not propagated to the end
Local, the client IP addresses is
propagated to the end Pods, but this could result in uneven distribution of
traffic. Nodes without any Pods for a particular LoadBalancer Service will fail
the NLB Target Group's health check on the auto-assigned
.spec.healthCheckNodePort and not receive any traffic.
In order to achieve even traffic, either use a DaemonSet or specify a pod anti-affinity to not locate on the same node.
You can also use NLB Services with the internal load balancer annotation.
In order for client traffic to reach instances behind an NLB, the Node security groups are modified with the following IP rules:
|Health Check||TCP||NodePort(s) (||Subnet CIDR||kubernetes.io/rule/nlb/health=<loadBalancerName>|
In order to limit which client IP's can access the Network Load Balancer,
spec: loadBalancerSourceRanges: - "188.8.131.52/16"
.spec.loadBalancerSourceRangesis not set, Kubernetes allows traffic from
0.0.0.0/0to the Node Security Group(s). If nodes have public IP addresses, be aware that non-NLB traffic can also reach all instances in those modified security groups.
Further documentation on annotations for Elastic IPs and other common use-cases may be found in the AWS Load Balancer Controller documentation.
Services of type ExternalName map a Service to a DNS name, not to a typical selector such as
cassandra. You specify these Services with the
This Service definition, for example, maps
my-service Service in the
prod namespace to
apiVersion: v1 kind: Service metadata: name: my-service namespace: prod spec: type: ExternalName externalName: my.database.example.com
A Service of
type: ExternalName accepts an IPv4 address string, but treats that string as a DNS name comprised of digits,
not as an IP address (the internet does not however allow such names in DNS). Services with external names that resemble IPv4
addresses are not resolved by DNS servers.
If you want to map a Service directly to a specific IP address, consider using headless Services.
When looking up the host
my-service.prod.svc.cluster.local, the cluster DNS Service
CNAME record with the value
my-service works in the same way as other Services but with the crucial
difference that redirection happens at the DNS level rather than via proxying or
forwarding. Should you later decide to move your database into your cluster, you
can start its Pods, add appropriate selectors or endpoints, and change the
You may have trouble using ExternalName for some common protocols, including HTTP and HTTPS. If you use ExternalName then the hostname used by clients inside your cluster is different from the name that the ExternalName references.
For protocols that use hostnames this difference may lead to errors or unexpected responses.
HTTP requests will have a
Host: header that the origin server does not recognize;
TLS servers will not be able to provide a certificate matching the hostname that the client connected to.
Sometimes you don't need load-balancing and a single Service IP. In
this case, you can create what are termed headless Services, by explicitly
"None" for the cluster IP address (
You can use a headless Service to interface with other service discovery mechanisms, without being tied to Kubernetes' implementation.
For headless Services, a cluster IP is not allocated, kube-proxy does not handle these Services, and there is no load balancing or proxying done by the platform for them. How DNS is automatically configured depends on whether the Service has selectors defined:
For headless Services that define selectors, the endpoints controller creates EndpointSlices in the Kubernetes API, and modifies the DNS configuration to return A or AAAA records (IPv4 or IPv6 addresses) that point directly to the Pods backing the Service.
For headless Services that do not define selectors, the control plane does not create EndpointSlice objects. However, the DNS system looks for and configures either:
- DNS CNAME records for
- DNS A / AAAA records for all IP addresses of the Service's ready endpoints,
for all Service types other than
- For IPv4 endpoints, the DNS system creates A records.
- For IPv6 endpoints, the DNS system creates AAAA records.
For clients running inside your cluster, Kubernetes supports two primary modes of finding a Service: environment variables and DNS.
When a Pod is run on a Node, the kubelet adds a set of environment variables
for each active Service. It adds
where the Service name is upper-cased and dashes are converted to underscores.
It also supports variables (see makeLinkVariables)
that are compatible with Docker Engine's
"legacy container links" feature.
For example, the Service
redis-primary which exposes TCP port 6379 and has been
allocated cluster IP address 10.0.0.11, produces the following environment
REDIS_PRIMARY_SERVICE_HOST=10.0.0.11 REDIS_PRIMARY_SERVICE_PORT=6379 REDIS_PRIMARY_PORT=tcp://10.0.0.11:6379 REDIS_PRIMARY_PORT_6379_TCP=tcp://10.0.0.11:6379 REDIS_PRIMARY_PORT_6379_TCP_PROTO=tcp REDIS_PRIMARY_PORT_6379_TCP_PORT=6379 REDIS_PRIMARY_PORT_6379_TCP_ADDR=10.0.0.11
When you have a Pod that needs to access a Service, and you are using the environment variable method to publish the port and cluster IP to the client Pods, you must create the Service before the client Pods come into existence. Otherwise, those client Pods won't have their environment variables populated.
If you only use DNS to discover the cluster IP for a Service, you don't need to worry about this ordering issue.
Kubernetes also supports and provides variables that are compatible with Docker
Engine's "legacy container links" feature.
You can read
to see how this is implemented in Kubernetes.
You can (and almost always should) set up a DNS service for your Kubernetes cluster using an add-on.
A cluster-aware DNS server, such as CoreDNS, watches the Kubernetes API for new Services and creates a set of DNS records for each one. If DNS has been enabled throughout your cluster then all Pods should automatically be able to resolve Services by their DNS name.
For example, if you have a Service called
my-service in a Kubernetes
my-ns, the control plane and the DNS Service acting together
create a DNS record for
my-service.my-ns. Pods in the
should be able to find the service by doing a name lookup for
my-service.my-ns would also work).
Pods in other namespaces must qualify the name as
my-service.my-ns. These names
will resolve to the cluster IP assigned for the Service.
Kubernetes also supports DNS SRV (Service) records for named ports. If the
my-service.my-ns Service has a port named
http with the protocol set to
TCP, you can do a DNS SRV query for
_http._tcp.my-service.my-ns to discover
the port number for
http, as well as the IP address.
The Kubernetes DNS server is the only way to access
You can find more information about
ExternalName resolution in
DNS for Services and Pods.
Virtual IP addressing mechanism
Read Virtual IPs and Service Proxies explains the mechanism Kubernetes provides to expose a Service with a virtual IP address.
You can set the
to control how Kubernetes routes traffic to healthy (“ready”) backends.
See Traffic Policies for more details.
If you want to make sure that connections from a particular client are passed to the same Pod each time, you can configure session affinity based on the client's IP address. Read session affinity to learn more.
If there are external IPs that route to one or more cluster nodes, Kubernetes Services
can be exposed on those
externalIPs. When network traffic arrives into the cluster, with
the external IP (as destination IP) and the port matching that Service, rules and routes
that Kubernetes has configured ensure that the traffic is routed to one of the endpoints
for that Service.
When you define a Service, you can specify
externalIPs for any
In the example below, the Service named
"my-service" can be accessed by clients using TCP,
"198.51.100.32:80" (calculated from
apiVersion: v1 kind: Service metadata: name: my-service spec: selector: app.kubernetes.io/name: MyApp ports: - name: http protocol: TCP port: 80 targetPort: 49152 externalIPs: - 198.51.100.32
externalIPs; these are the responsibility of the cluster administrator.
Service is a top-level resource in the Kubernetes REST API. You can find more details about the Service API object.
Learn more about Services and how they fit into Kubernetes:
- Follow the Connecting Applications with Services tutorial.
- Read about Ingress, which exposes HTTP and HTTPS routes from outside the cluster to Services within your cluster.
- Read about Gateway, an extension to Kubernetes that provides more flexibility than Ingress.
For more context, read the following: