Learn Kubernetes Basics

Concepts

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Service

An abstract way to expose an application running on a set of PodsThe smallest and simplest Kubernetes object. A Pod represents a set of running containers on your cluster. as a network service.

No need to modify your application to use an unfamiliar service discovery mechanism. Kubernetes gives pods their own IP addresses and a single DNS name for a set of pods, and can load-balance across them.

Motivation

Kubernetes PodsThe smallest and simplest Kubernetes object. A Pod represents a set of running containers on your cluster. are mortal. They are born and when they die, they are not resurrected. If you use a DeploymentAn API object that manages a replicated application. to run your app, it can create and destroy Pods dynamically (e.g. when scaling out or in).

Each Pod gets its own IP address, however the set of Pods for a Deployment 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 those 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?

Enter Services.

Service resources

In Kubernetes, a Service is an abstraction which defines a logical set of Pods and a policy by which to access them (you’ll sometimes see this pattern called a micro-service). The set of Pods targeted by a Service is usually determined by a selectorAllows users to filter a list of resources based on labels. (see below for why you might want a Service without a selector).

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.

Cloud-native service discovery

If you’re able to use Kubernetes APIs for service discovery in your application, you can query the API serverComponent on the master that exposes the Kubernetes API. It is the front-end for the Kubernetes control plane. for Endpoints, that will be updated 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.

Defining a service

A Service in Kubernetes is a REST object, similar to a Pod. Like all of the REST objects, you can POST a Service definition to the API server to create a new instance.

For example, suppose you have a set of Pods that each listen on TCP port 9376 and carry a label "app=MyApp":

apiVersion: v1
kind: Service
metadata:
  name: my-service
spec:
  selector:
    app: MyApp
  ports:
  - protocol: TCP
    port: 80
    targetPort: 9376

This specification will create a new Service object named “my-service” which targets TCP port 9376 on any Pod with the "app=MyApp" label.

This Service will also be assigned an IP address (sometimes called the “cluster IP”), which is used by the service proxies (see Virtual IPs and service proxies below).

The controller for the Service selector will continuously scan for Pods that match its selector, and will then POST any updates to an Endpoint object also named “my-service”.

Note: A Service can map any incoming port to a targetPort. By default, and for convenience, the targetPort will be set to the same value as the port field.

Port definitions in Pods have names, and you can reference these names in the targetPort attribute of a Service. This will work even if there are a mixture of Pods in the Service, with the same network protocol available via different port numbers but a single configured name. This offers a lot of flexibility for deploying and evolving your Services. For example, you can change the port number 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.

As many Services need to expose more than one port, Kubernetes supports multiple port definitions on a Service object. Each port definition can have the same protocol, or a different one.

Services without selectors

Services most commonly abstract access to Kubernetes Pods, but they can also abstract other kinds of backends. For example:

In any of these scenarios you can define a service without a Pod selector. 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 Endpoint object will not be created automatically. You can manually map the service to the network address and port where it’s running, by adding an Endpoint object manually:

apiVersion: v1
kind: Endpoints
metadata:
  name: my-service
subsets:
  - addresses:
      - ip: 192.0.2.42
    ports:
      - port: 9376
Note:

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 224.0.0.0/24 for IPv4, fe80::/64 for IPv6).

Endpoint IP addresses also cannot be the cluster IPs of other Kubernetes services, because kube-proxykube-proxy is a network proxy that runs on each node in the cluster. doesn’t support virtual IPs as a destination.

Accessing a Service without a selector works the same as if it had a selector. In the example above, traffic will be routed to the single endpoint defined in the YAML: 192.0.2.42:9376 (TCP).

An ExternalName Service is a special case of service that does not have selectors and uses DNS names instead. For more information, see the ExternalName section later in this document.

Virtual IPs and service proxies

Every node in a Kubernetes cluster runs a kube-proxy. kube-proxy is responsible for implementing a form of virtual IP for Services of type other than ExternalName.

Why not use round-robin DNS?

A question that pops up every now and then is why Kubernetes relies on proxying to forward inbound traffic to backends. What about other approaches? For example, would it be possible to configure DNS records that have multiple A values (or AAAA for IPv6), and rely on round-robin name resolution?

There are a few reasons for using proxying for Services:

Version compatibility

Since Kubernetes v1.0 you have been able to use the userspace proxy mode. Kubernetes v1.1 added iptables mode proxying, and in Kubernetes v1.2 the iptables mode for kube-proxy became the default. Kubernetes v1.8 added ipvs proxy mode.

User space proxy mode

In this mode, kube-proxy watches the Kubernetes master for the addition and removal of Service and Endpoint objects. For each Service it opens a port (randomly chosen) on the local node. Any connections to this “proxy port” will be proxied to one of the Service’s backend Pods (as reported via Endpoints). kube-proxy takes the SessionAffinity setting of the Service into account when deciding which backend Pod to use.

Lastly, the user-space proxy installs iptables rules which capture traffic to the Service’s clusterIP (which is virtual) and port. The rules redirect that traffic to the proxy port which proxies the backend Pod.

By default, kube-proxy in userspace mode chooses a backend via a round-robin algorithm.

Services overview diagram for userspace proxy

iptables proxy mode

In this mode, kube-proxy watches the Kubernetes control plane for the addition and removal of Service and Endpoint objects. For each Service, it installs iptables rules which capture traffic to the Service’s clusterIP (which is virtual) and port and redirects that traffic to one of the Service’s backend sets. For each Endpoint object, it installs iptables rules which select a backend Pod.

By default, kube-proxy in iptables mode chooses a backend at random.

Using iptables to handle traffic has a lower system overhead, because traffic is handled by Linux netfilter without the need switch between userspace and the kernel space. This approach is also likely to be more reliable.

If kube-proxy is running in iptables mode and the first Pod that’s selected does not respond, the connection will fail. This is different from userspace mode: in that scenario, kube-proxy would detect that the connection to the first Pod had failed and would automatically retry with a different backend Pod.

You can use Pod readiness probes to verify that backend Pods are working OK, so that kube-proxy in iptables mode only sees backends that test out as healthy. Doing this means you avoid having traffic sent via kube-proxy to a Pod that’s known to have failed.

Services overview diagram for iptables proxy

IPVS proxy mode

FEATURE STATE: Kubernetes v1.11 stable
This feature is stable, meaning:

  • The version name is vX where X is an integer.
  • Stable versions of features will appear in released software for many subsequent versions.

In ipvs mode, kube-proxy watches Kubernetes Services and Endpoints, calls netlink interface to create IPVS rules accordingly and synchronizes IPVS rules with Kubernetes Services and Endpoints periodically. This control loop ensures that IPVS status matches the desired state. When accessing a Service, IPVS will direct traffic to one of the backend Pods.

The IPVS proxy mode is based on netfilter hook function that is similar to iptables mode, but uses hash table as the underlying data structure and works in the kernel space. That means kube-proxy in IPVS mode redirects traffic with a lower latency than kube-proxy in iptables mode, with much better performance when synchronising proxy rules. Compared to the other proxy modes, IPVS mode also supports a higher throughput of network traffic.

IPVS provides more options for balancing traffic to backend Pods; these are:

Note:

To run kube-proxy in IPVS mode, you must make the IPVS Linux available on the node before you starting kube-proxy.

When kube-proxy starts in IPVS proxy mode, it will verify whether IPVS kernel modules are available, and if those are not detected then kube-proxy fall back to running in iptables proxy mode.

Services overview diagram for IPVS proxy

In any of these proxy models, any traffic bound for the Service’s IP:Port is proxied to an appropriate backend without the clients knowing anything about Kubernetes or Services or Pods.

If you want to make sure that connections from a particular client are passed to the same Pod each time, you can select session affinity based the on client’s IP addresses by setting service.spec.sessionAffinity to “ClientIP” (the default is “None”). You can then also set the maximum session sticky time by setting service.spec.sessionAffinityConfig.clientIP.timeoutSeconds appropriately. (the default value is 10800, which works out to be 3 hours).

Multi-Port Services

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: MyApp
  ports:
  - name: http
    protocol: TCP
    port: 80
    targetPort: 9376
  - name: https
    protocol: TCP
    port: 443
    targetPort: 9377
Note:

As with Kubernetes namesA client-provided string that refers to an object in a resource URL, such as /api/v1/pods/some-name. 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 123-abc and web are valid, but 123_abc and -web are not.

Choosing your own IP address

You can specify your own cluster IP address as part of a Service creation 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 returns a 422 HTTP status code to indicate that there’s a problem.

Discovering services

Kubernetes supports 2 primary modes of finding a Service - environment variables and DNS.

Environment variables

When a Pod is run on a Node, the kubelet adds a set of environment variables for each active Service. It supports both Docker links compatible variables (see makeLinkVariables) and simpler {SVCNAME}_SERVICE_HOST and {SVCNAME}_SERVICE_PORT variables, where the Service name is upper-cased and dashes are converted to underscores.

For example, the Service "redis-master" which exposes TCP port 6379 and has been allocated cluster IP address 10.0.0.11 produces the following environment variables:

REDIS_MASTER_SERVICE_HOST=10.0.0.11
REDIS_MASTER_SERVICE_PORT=6379
REDIS_MASTER_PORT=tcp://10.0.0.11:6379
REDIS_MASTER_PORT_6379_TCP=tcp://10.0.0.11:6379
REDIS_MASTER_PORT_6379_TCP_PROTO=tcp
REDIS_MASTER_PORT_6379_TCP_PORT=6379
REDIS_MASTER_PORT_6379_TCP_ADDR=10.0.0.11
Note:

When you have a Pod that might need to acccess 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.

DNS

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 Namespace "my-ns", the control plane and the DNS service acting together will create a DNS record for "my-service.my-ns". Pods in the "my-ns" Namespace should be able to find it by simply doing a name lookup for my-service ("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 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 ExternalName Services. You can find more information about ExternalName resolution in DNS Pods and Services.

Headless services

Sometimes you don’t need or want load-balancing and a single service IP. In this case, you can create what are termed “headless” Services, by explicitly specifying "None" for the cluster IP (.spec.clusterIP).

You can use a headless Service to interface with other service discovery mechanisms, without being tied to Kubernetes’ implementation. For example, you could implement a custom [Operator]( be built upon this API.

For such 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.

With selectors

For headless services that define selectors, the endpoints controller creates Endpoints records in the API, and modifies the DNS configuration to return A records (addresses) that point directly to the Pods backing the Service.

Without selectors

For headless services that do not define selectors, the endpoints controller does not create Endpoints records. However, the DNS system looks for and configures either:

Publishing services (ServiceTypes)

For some parts of your application (e.g. frontends) you may want to expose a Service onto an external IP address, one that’s outside of your cluster.

Kubernetes ServiceTypes allow you to specify what kind of service you want. The default is ClusterIP.

Type values and their behaviors are:

Note: You need CoreDNS version 1.7 or higher to use the ExternalName type.

Type NodePort

If you set the type field to NodePort, the Kubernetes control plane will allocate a port from a range specified by --service-node-port-range flag (default: 30000-32767). Each node will proxy that port each (the same port number on every Node) into your Service. Your service will report that allocated port in its .spec.ports[*].nodePort field.

If you want to specify particular IP(s) to proxy the port, you can set the --nodeport-addresses flag in kube-proxy to particular IP block(s); this is supported since Kubernetes v1.10. This flag takes a comma-delimited list of IP blocks (e.g. 10.0.0.0/8, 192.0.2.0/25) to specify IP address ranges that kube-proxy should consider as local to this node.

For example, if you start kube-proxy with flag --nodeport-addresses=127.0.0.0/8, kube-proxy will select only the loopback interface for NodePort Services. The default for --nodeport-addresses is an empty list, and means that kube-proxy should consider all available network interfaces for NodePort. (That’s also compatible with earlier Kubernetes releases).

If you want a specific port number, you can specify a value in the nodePort 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 about possible port collisions yourself). You also have to use a valid port number, one that’s inside the range configured for NodePort use.

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 just expose one or more nodes’ IPs directly.

Note that this Service will be visible as both <NodeIP>:spec.ports[*].nodePort and .spec.clusterIP:spec.ports[*].port. (If the --nodeport-addresses flag in kube-proxy is set, would be filtered NodeIP(s).)

Type LoadBalancer

On cloud providers which support external load balancers, setting the type field to LoadBalancer will provision a load balancer for your Service. The actual creation of the load balancer happens asynchronously, and information about the provisioned balancer will be published in the Service’s .status.loadBalancer field. For example:

apiVersion: v1
kind: Service
metadata:
  name: my-service
spec:
  selector:
    app: MyApp
  ports:
  - protocol: TCP
    port: 80
    targetPort: 9376
  clusterIP: 10.0.171.239
  loadBalancerIP: 78.11.24.19
  type: LoadBalancer
status:
  loadBalancer:
    ingress:
    - ip: 146.148.47.155

Traffic from the external load balancer will be directed at the backend Pods, though exactly how that works depends on the cloud provider.

Some cloud providers allow you to specify the loadBalancerIP. In those cases, the load-balancer will be created with the user-specified loadBalancerIP. If the loadBalancerIP field is not specified, the loadBalancer will be set up with an ephemeral IP address. If you specify a loadBalancerIP but your cloud provider does not support the feature, the loadbalancerIP field that you set will be ignored.

Note: If you’re using SCTP, see the caveat below about the LoadBalancer Service type.
Note:

On Azure, if you want to use a user-specified public type loadBalancerIP, you first need to create a static type public IP address resource. This public IP address resource should be in the same resource group of the other automatically created resources of the cluster. For example, MC_myResourceGroup_myAKSCluster_eastus.

Specify the assigned IP address as loadBalancerIP. Ensure that you have updated the securityGroupName in the cloud provider configuration file. For information about troubleshooting CreatingLoadBalancerFailed permission issues see, Use a static IP address with the Azure Kubernetes Service (AKS) load balancer or CreatingLoadBalancerFailed on AKS cluster with advanced networking.

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.

You can achieve this by adding one the following annotations to a Service. The annotation to add depends on the cloud service provider you’re using.

Select one of the tabs.

[...]
metadata:
    name: my-service
    annotations:
        cloud.google.com/load-balancer-type: "Internal"
[...]

Use cloud.google.com/load-balancer-type: "internal" for masters with version 1.7.0 to 1.7.3. For more information, see the docs.

[...]
metadata:
    name: my-service
    annotations:
        service.beta.kubernetes.io/aws-load-balancer-internal: 0.0.0.0/0
[...]

[...]
metadata:
    name: my-service
    annotations:
        service.beta.kubernetes.io/azure-load-balancer-internal: "true"
[...]

[...]
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"
[...]

TLS support on AWS

For partial TLS / SSL support on clusters running on AWS, you can add three annotations to a LoadBalancer service:

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 will expect the Pod to authenticate itself over the encrypted connection, using a certificate.

HTTP and HTTPS will select layer 7 proxying: the ELB will terminate the connection with the user, parse headers and inject the X-Forwarded-For header with the user’s IP address (Pods will only see the IP address of the ELB at the other end of its connection) when forwarding requests.

TCP and SSL will select layer 4 proxying: the ELB will forward 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, 80, 443, and 8443, then 443 and 8443 would use the SSL certificate, but 80 would just be proxied HTTP.

From Kubernetes v1.9 onwrds 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 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 “service.beta.kubernetes.io/aws-load-balancer-ssl-negotiation-policy“ 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.

The annotation service.beta.kubernetes.io/aws-load-balancer-access-log-enabled controls whether access logs are enabled.

The annotation service.beta.kubernetes.io/aws-load-balancer-access-log-emit-interval controls the interval in minutes for publishing the access logs. You can specify an interval of either 5 or 60 minutes.

The annotation service.beta.kubernetes.io/aws-load-balancer-access-log-s3-bucket-name controls the name of the Amazon S3 bucket where load balancer access logs are stored.

The annotation service.beta.kubernetes.io/aws-load-balancer-access-log-s3-bucket-prefix specifies the logical hierarchy you created for your Amazon S3 bucket.

    metadata:
      name: my-service
      annotations:
        service.beta.kubernetes.io/aws-load-balancer-access-log-enabled: "true"
        # Specifies whether access logs are enabled for the load balancer
        service.beta.kubernetes.io/aws-load-balancer-access-log-emit-interval: "60"
        # 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-s3-bucket-name: "my-bucket"
        # The name of the Amazon S3 bucket where the access logs are stored
        service.beta.kubernetes.io/aws-load-balancer-access-log-s3-bucket-prefix: "my-bucket-prefix/prod"
        # The logical hierarchy you created for your Amazon S3 bucket, for example `my-bucket-prefix/prod`

Connection Draining on AWS

Connection draining for Classic ELBs can be managed with the annotation service.beta.kubernetes.io/aws-load-balancer-connection-draining-enabled set to the value of "true". The annotation service.beta.kubernetes.io/aws-load-balancer-connection-draining-timeout can 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:
        service.beta.kubernetes.io/aws-load-balancer-connection-idle-timeout: "60"
        # 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-cross-zone-load-balancing-enabled: "true"
        # Specifies whether cross-zone load balancing is enabled for the load balancer

        service.beta.kubernetes.io/aws-load-balancer-additional-resource-tags: "environment=prod,owner=devops"
        # 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-healthcheck-healthy-threshold: ""
        # 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-unhealthy-threshold: "3"
        # 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-interval: "20"
        # 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-timeout: "5"
        # 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-extra-security-groups: "sg-53fae93f,sg-42efd82e"
        # A list of additional security groups to be added to the ELB

Network Load Balancer support on AWS [alpha]

Warning: This is an alpha feature and is not yet recommended for production clusters.

Starting from Kubernetes v1.9.0, you can use AWS Network Load Balancer (NLB) with Services. To use a Network Load Balancer on AWS, use the annotation service.beta.kubernetes.io/aws-load-balancer-type with the value set to nlb.

    metadata:
      name: my-service
      annotations:
        service.beta.kubernetes.io/aws-load-balancer-type: "nlb"
Note: NLB only works with certain instance classes; see the AWS documentation on Elastic Load Balancing for a list of supported instance types.

Unlike Classic Elastic Load Balancers, Network Load Balancers (NLBs) forward the client’s IP address through to the node. If a service’s .spec.externalTrafficPolicy is set to Cluster, the client’s IP address will not be propagated to the end pods.

By setting .spec.externalTrafficPolicy to Local, client IP addresses will be 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:

Rule Protocol Port(s) IpRange(s) IpRange Description
Health Check TCP NodePort(s) (.spec.healthCheckNodePort for .spec.externalTrafficPolicy = Local) VPC CIDR kubernetes.io/rule/nlb/health=<loadBalancerName>
Client Traffic TCP NodePort(s) .spec.loadBalancerSourceRanges (defaults to 0.0.0.0/0) kubernetes.io/rule/nlb/client=<loadBalancerName>
MTU Discovery ICMP 3,4 .spec.loadBalancerSourceRanges (defaults to 0.0.0.0/0) kubernetes.io/rule/nlb/mtu=<loadBalancerName>

In order to limit which client IP’s can access the Network Load Balancer, specify loadBalancerSourceRanges.

spec:
  loadBalancerSourceRanges:
  - "143.231.0.0/16"
Note: If .spec.loadBalancerSourceRanges is not set, Kubernetes will allow traffic from 0.0.0.0/0 to 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.

Type ExternalName

Services of type ExternalName map a service to a DNS name, not to a typical selector such as my-service or cassandra. You specify these services with the spec.externalName parameter.

This Service definition, for example, maps the my-service Service in the prod namespace to my.database.example.com:

apiVersion: v1
kind: Service
metadata:
  name: my-service
  namespace: prod
spec:
  type: ExternalName
  externalName: my.database.example.com
Note: ExternalName accepts an IPv4 address string, but as a DNS names comprised of digits, not as an IP address. ExternalNames that resemble IPv4 addresses are not resolved by CoreDNS or ingress-nginx because ExternalName is intended to specify a canonical DNS name. To hardcode an IP address, consider using headless services.

When looking up the host my-service.prod.svc.cluster.local, the cluster DNS service will return a CNAME record with the value my.database.example.com. Accessing 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 Service’s type.

Note: This section is indebted to the Kubernetes Tips - Part 1 blog post from Alen Komljen.

External IPs

If there are external IPs that route to one or more cluster nodes, Kubernetes services can be exposed on those externalIPs. Traffic that ingresses into the cluster with the external IP (as destination IP), on the service port, will be routed to one of the service endpoints. externalIPs are not managed by Kubernetes and are the responsibility of the cluster administrator.

In the Service spec, externalIPs can be specified along with any of the ServiceTypes. In the example below, “my-service” can be accessed by clients on “80.11.12.10:80” (externalIP:port)

apiVersion: v1
kind: Service
metadata:
  name: my-service
spec:
  selector:
    app: MyApp
  ports:
  - name: http
    protocol: TCP
    port: 80
    targetPort: 9376
  externalIPs:
  - 80.11.12.10

Shortcomings

Using the userspace proxy for VIPs will work at small to medium scale, but will not scale to very large clusters with thousands of Services. The original design proposal for portals has more details on this.

Using the userspace proxy obscures the source IP address of a packet accessing a Service. This makes some kinds of network filtering (firewalling) impossible. The iptables proxy mode does not obscure in-cluster source IPs, but it does still impact clients coming through a load balancer or node-port.

The Type field is designed as nested functionality - each level adds to the previous. This is not strictly required on all cloud providers (e.g. Google Compute Engine does not need to allocate a NodePort to make LoadBalancer work, but AWS does) but the current API requires it.

Virtual IP implementation

The previous information should be sufficient for many people who just want to use Services. However, there is a lot going on behind the scenes that may be worth understanding.

Avoiding collisions

One of the primary philosophies of Kubernetes is that you should not be exposed to situations that could cause your actions to fail through no fault of your own. For the design of the Service resource, this means not making you choose your own port number for a if that choice might collide with someone else’s choice. That is an isolation failure.

In order to allow you to choose a port number for your Services, we must ensure that no two Services can collide. Kubernetes does that by allocating each Service its own IP address.

To ensure each service receives a unique IP, an internal allocator atomically updates a global allocation map in etcdConsistent and highly-available key value store used as Kubernetes’ backing store for all cluster data. prior to creating each Service. The map object must exist in the registry for Services to get IP address assignments, otherwise creations will fail with a message indicating an IP address could not be allocated.

In the control plane, a background controller is responsible for creating that map (needed to support migrating from older versions of Kubernetes that used in-memory locking). Kubernetes also uses controllers to checking for invalid assignments (eg due to administrator intervention) and for cleaning up allocated IP addresses that are no longer used by any Services.

Service IP addresses

Unlike Pod IP addresses, which actually route to a fixed destination, Service IPs are not actually answered by a single host. Instead, kube-proxy uses iptables (packet processing logic in Linux) to define virtual IP addresses which are transparently redirected as needed. When clients connect to the VIP, their traffic is automatically transported to an appropriate endpoint. The environment variables and DNS for Services are actually populated in terms of the Service’s virtual IP address (and port).

kube-proxy supports three proxy modes—userspace, iptables and IPVS—which each operate slightly differently.

Userspace

As an example, consider the image processing application described above. When the backend Service is created, the Kubernetes master assigns a virtual IP address, for example 10.0.0.1. Assuming the Service port is 1234, the Service is observed by all of the kube-proxy instances in the cluster. When a proxy sees a new Service, it opens a new random port, establishes an iptables redirect from the virtual IP address to this new port, and starts accepting connections on it.

When a client connects to the Service’s virtual IP address, the iptables rule kicks in, and redirects the packets to the proxy’s own port. The “Service proxy” chooses a backend, and starts proxying traffic from the client to the backend.

This means that Service owners can choose any port they want without risk of collision. Clients can simply connect to an IP and port, without being aware of which Pods they are actually accessing.

iptables

Again, consider the image processing application described above. When the backend Service is created, the Kubernetes control plane assigns a virtual IP address, for example 10.0.0.1. Assuming the Service port is 1234, the Service is observed by all of the kube-proxy instances in the cluster. When a proxy sees a new Service, it installs a series of iptables rules which redirect from the virtual IP address to per-Service rules. The per-Service rules link to per-Endpoint rules which redirect traffic (using destination NAT) to the backends.

When a client connects to the Service’s virtual IP address the iptables rule kicks in. A backend is chosen (either based on session affinity or randomly) and packets are redirected to the backend. Unlike the userspace proxy, packets are never copied to userspace, the kube-proxy does not have to be running for the virtual IP address to work, and Nodes see traffic arriving from the unaltered client IP address.

This same basic flow executes when traffic comes in through a node-port or through a load-balancer, though in those cases the client IP does get altered.

IPVS

iptables operations slow down dramatically in large scale cluster e.g 10,000 Services. IPVS is designed for load balancing and based on in-kernel hash tables. So you can achieve performance consistency in large number of services from IPVS-based kube-proxy. Meanwhile, IPVS-based kube-proxy has more sophisticated load balancing algorithms (least conns, locality, weighted, persistence).

API Object

Service is a top-level resource in the Kubernetes REST API. You can find more details about the API object at: Service API object.

Supported protocols

TCP

FEATURE STATE: Kubernetes v1.0 stable
This feature is stable, meaning:

  • The version name is vX where X is an integer.
  • Stable versions of features will appear in released software for many subsequent versions.

You can use TCP for any kind of service, and it’s the default network protocol.

UDP

FEATURE STATE: Kubernetes v1.0 stable
This feature is stable, meaning:

  • The version name is vX where X is an integer.
  • Stable versions of features will appear in released software for many subsequent versions.

You can use UDP for most services. For type=LoadBalancer services, UDP support depends on the cloud provider offering this facility.

HTTP

FEATURE STATE: Kubernetes v1.1 stable
This feature is stable, meaning:

  • The version name is vX where X is an integer.
  • Stable versions of features will appear in released software for many subsequent versions.

If your cloud provider supports it, you can use a Service in LoadBalancer mode to set up external HTTP / HTTPS reverse proxying, forwarded to the Endpoints of the Service.

Note: You can also use IngressAn API object that manages external access to the services in a cluster, typically HTTP. in place of Service to expose HTTP / HTTPS services.

PROXY protocol

FEATURE STATE: Kubernetes v1.1 stable
This feature is stable, meaning:

  • The version name is vX where X is an integer.
  • Stable versions of features will appear in released software for many subsequent versions.

If your cloud provider supports it (eg, AWS), you can use a Service in LoadBalancer mode to configure a load balancer outside of Kubernetes itself, that will forward connections prefixed with PROXY protocol.

The load balancer will send an initial series of octets describing the incoming connection, similar to this example

PROXY TCP4 192.0.2.202 10.0.42.7 12345 7\r\n

followed by the data from the client.

SCTP

FEATURE STATE: Kubernetes v1.12 alpha
This feature is currently in a alpha state, meaning:

  • The version names contain alpha (e.g. v1alpha1).
  • Might be buggy. Enabling the feature may expose bugs. Disabled by default.
  • Support for feature may be dropped at any time without notice.
  • The API may change in incompatible ways in a later software release without notice.
  • Recommended for use only in short-lived testing clusters, due to increased risk of bugs and lack of long-term support.

Kubernetes supports SCTP as a protocol value in Service, Endpoint, NetworkPolicy and Pod definitions as an alpha feature. To enable this feature, the cluster administrator needs to enable the SCTPSupport feature gate on the apiserver, for example, --feature-gates=SCTPSupport=true,….

When the feature gate is enabled, you can set the protocol field of a Service, Endpoint, NetworkPolicy or Pod to SCTP. Kubernetes sets up the network accordingly for the SCTP associations, just like it does for TCP connections.

Warnings

Support for multihomed SCTP associations
Warning:

The support of multihomed SCTP associations requires that the CNI plugin can support the assignment of multiple interfaces and IP addresses to a Pod.

NAT for multihomed SCTP associations requires special logic in the corresponding kernel modules.

Service with type=LoadBalancer
Warning: You can only create a Service with type LoadBalancer plus protocol SCTP if the cloud provider’s load balancer implementation supports SCTP as a protocol. Otherwise, the Service creation request is rejected. The current set of cloud load balancer providers (Azure, AWS, CloudStack, GCE, OpenStack) all lack support for SCTP.
Windows
Warning: SCTP is not supported on Windows based nodes.
Userspace kube-proxy
Warning: The kube-proxy does not support the management of SCTP associations when it is in userspace mode.

Future work

In the future, the proxy policy for Services can become more nuanced than simple round-robin balancing, for example master-elected or sharded. We also envision that some Services will have “real” load balancers, in which case the virtual IP address will simply transport the packets there.

The Kubernetes project intends to improve support for L7 (HTTP) Services.

The Kubernetes project intends to have more flexible ingress modes for Services which encompass the current ClusterIP, NodePort, and LoadBalancer modes and more.

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