Service

An abstract way to expose an application running on a set of Pods as a network service.

With Kubernetes you don’t 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 Pods are created and destroyed to match the state of your cluster. Pods are nonpermanent resources. If you use a Deployment to run your app, it can create and destroy Pods dynamically.

Each Pod gets its own IP address, however in a Deployment, 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?

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 (sometimes this pattern is called a micro-service). The set of Pods targeted by a Service is usually determined by a selector. To learn about other ways to define Service endpoints, see Services without selectors.

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 server for Endpoints, that get 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. The name of a Service object must be a valid DNS label name.

For example, suppose you have a set of Pods where each listens on TCP port 9376 and contains a label app=MyApp:

  1. apiVersion: v1
  2. kind: Service
  3. metadata:
  4.   name: my-service
  5. spec:
  6.   selector:
  7.     app: MyApp
  8.   ports:
  9.     - protocol: TCP
  10.       port: 80
  11.       targetPort: 9376

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

Kubernetes assigns this Service 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 continuously scans for Pods that match its selector, and then POSTs 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 is 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 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.

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:

  • 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 a Pod selector. For example:

  1. apiVersion: v1
  2. kind: Service
  3. metadata:
  4.   name: my-service
  5. spec:
  6.   ports:
  7.     - protocol: TCP
  8.       port: 80
  9.       targetPort: 9376

Because this Service has no selector, the corresponding Endpoints object is not created automatically. You can manually map the Service to the network address and port where it’s running, by adding an Endpoints object manually:

  1. apiVersion: v1
  2. kind: Endpoints
  3. metadata:
  4.   name: my-service
  5. subsets:
  6.   - addresses:
  7.       - ip: 192.0.2.42
  8.     ports:
  9.       - port: 9376

The name of the Endpoints object must be a valid DNS subdomain name.

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 cannot be the cluster IPs of other Kubernetes Services, because kube-proxy 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 is 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.

Over Capacity Endpoints

If an Endpoints resource has more than 1000 endpoints then a Kubernetes v1.21 (or later) cluster annotates that Endpoints with endpoints.kubernetes.io/over-capacity: warning. This annotation indicates that the affected Endpoints object is over capacity.

EndpointSlices

FEATURE STATE: Kubernetes v1.21 [stable]

EndpointSlices are an API resource that can provide a more scalable alternative to Endpoints. Although conceptually quite similar to Endpoints, EndpointSlices allow for distributing network endpoints across multiple resources. By default, an EndpointSlice is considered “full” once it reaches 100 endpoints, at which point additional EndpointSlices will be created to store any additional endpoints.

EndpointSlices provide additional attributes and functionality which is described in detail in EndpointSlices.

Application protocol

FEATURE STATE: Kubernetes v1.20 [stable]

The appProtocol field provides a way to specify an application protocol for each Service port. The value of this field is mirrored by the corresponding Endpoints and EndpointSlice objects.

This field follows standard Kubernetes label syntax. Values should either be IANA standard service names or domain prefixed names such as mycompany.com/my-custom-protocol.

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:

  • There is a long history of DNS implementations not respecting record TTLs, and caching the results of name lookups after they should have expired.
  • Some apps do DNS lookups only once and cache the results indefinitely.
  • Even if apps and libraries did proper re-resolution, the low or zero TTLs on the DNS records could impose a high load on DNS that then becomes difficult to manage.

User space 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 opens a port (randomly chosen) on the local node. Any connections to this “proxy port” are 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 and port, and redirect 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 to 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 fails. 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]

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 directs 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 a hash table as the underlying data structure and works in the kernel space. That means kube-proxy in IPVS mode redirects traffic with 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:

  • rr: round-robin
  • lc: least connection (smallest number of open connections)
  • dh: destination hashing
  • sh: source hashing
  • sed: shortest expected delay
  • nq: never queue

Note:

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

When kube-proxy starts in IPVS proxy mode, it verifies whether IPVS kernel modules are available. If the IPVS kernel modules are not detected, then kube-proxy falls back to running in iptables proxy mode.

Services overview diagram for IPVS proxy

In these proxy models, the 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 the session affinity based on the client’s IP addresses by setting service.spec.sessionAffinity to “ClientIP” (the default is “None”). You can 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:

  1. apiVersion: v1
  2. kind: Service
  3. metadata:
  4.   name: my-service
  5. spec:
  6.   selector:
  7.     app: MyApp
  8.   ports:
  9.     - name: http
  10.       protocol: TCP
  11.       port: 80
  12.       targetPort: 9376
  13.     - name: https
  14.       protocol: TCP
  15.       port: 443
  16.       targetPort: 9377

Note:

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 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 return 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:

  1. REDIS_MASTER_SERVICE_HOST=10.0.0.11
  2. REDIS_MASTER_SERVICE_PORT=6379
  3. REDIS_MASTER_PORT=tcp://10.0.0.11:6379
  4. REDIS_MASTER_PORT_6379_TCP=tcp://10.0.0.11:6379
  5. REDIS_MASTER_PORT_6379_TCP_PROTO=tcp
  6. REDIS_MASTER_PORT_6379_TCP_PORT=6379
  7. REDIS_MASTER_PORT_6379_TCP_ADDR=10.0.0.11

Note:

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.

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 create a DNS record for my-service.my-ns. Pods in the my-ns namespace should be able to find the service by 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 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 ExternalName Services. You can find more information about ExternalName resolution in DNS Pods and Services.

Headless Services

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 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 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:

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 (IP 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:

  • CNAME records for ExternalName-type Services.
  • A records for any Endpoints that share a name with the Service, for all other types.

Publishing Services (ServiceTypes)

For some parts of your application (for example, frontends) you may want to expose a Service onto an external IP address, 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:

  • ClusterIP: Exposes the Service on a cluster-internal IP. Choosing this value makes the Service only reachable from within the cluster. This is the default ServiceType.
  • NodePort: Exposes the Service on each Node’s IP at a static port (the NodePort). A ClusterIP Service, to which the NodePort Service routes, is automatically created. You’ll be able to contact the NodePort Service, from outside the cluster, by requesting <NodeIP>:<NodePort>.
  • LoadBalancer: Exposes the Service externally using a cloud provider’s load balancer. NodePort and ClusterIP Services, to which the external load balancer routes, are automatically created.

  • ExternalName: Maps the Service to the contents of the externalName field (e.g. foo.bar.example.com), by returning a CNAME record with its value. No proxying of any kind is set up.

    Note: You need either kube-dns version 1.7 or CoreDNS version 0.0.8 or higher to use the ExternalName type.

You can also use Ingress to expose your Service. Ingress is not a Service type, but it acts as the entry point for your cluster. It lets you consolidate your routing rules into a single resource as it can expose multiple services under the same IP address.

Type NodePort

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 .spec.ports[*].nodePort field.

If you want to specify particular IP(s) to proxy the port, you can set the --nodeport-addresses flag for kube-proxy or the equivalent nodePortAddresses field of the kube-proxy configuration file to particular IP block(s).

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 the --nodeport-addresses=127.0.0.0/8 flag, 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).

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 of 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 expose one or more nodes’ IPs directly.

Note that this Service is visible as <NodeIP>:spec.ports[*].nodePort and .spec.clusterIP:spec.ports[*].port. If the --nodeport-addresses flag for kube-proxy or the equivalent field in the kube-proxy configuration file is set, <NodeIP> would be filtered node IP(s).

For example:

  1. apiVersion: v1
  2. kind: Service
  3. metadata:
  4.   name: my-service
  5. spec:
  6.   type: NodePort
  7.   selector:
  8.     app: MyApp
  9.   ports:
  10.       # By default and for convenience, the `targetPort` is set to the same value as the `port` field.
  11.     - port: 80
  12.       targetPort: 80
  13.       # Optional field
  14.       # By default and for convenience, the Kubernetes control plane will allocate a port from a range (default: 30000-32767)
  15.       nodePort: 30007

Type LoadBalancer

On cloud providers which support external load balancers, setting the type field to 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 .status.loadBalancer field. For example:

  1. apiVersion: v1
  2. kind: Service
  3. metadata:
  4.   name: my-service
  5. spec:
  6.   selector:
  7.     app: MyApp
  8.   ports:
  9.     - protocol: TCP
  10.       port: 80
  11.       targetPort: 9376
  12.   clusterIP: 10.0.171.239
  13.   type: LoadBalancer
  14. status:
  15.   loadBalancer:
  16.     ingress:
  17.     - 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.

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 loadBalancer is 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 is ignored.

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.

Load balancers with mixed protocol types

FEATURE STATE: Kubernetes v1.20 [alpha]

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.

If the feature gate MixedProtocolLBService is enabled for the kube-apiserver it is allowed to use different protocols when there is more than one port defined.

Note: The set of protocols that can be used for LoadBalancer type of Services is still defined by the cloud provider.

Disabling load balancer NodePort allocation

FEATURE STATE: Kubernetes v1.20 [alpha]

Starting in v1.20, you can optionally disable node port allocation for a Service Type=LoadBalancer by setting the field spec.allocateLoadBalancerNodePorts to false. This should only be used for load balancer implementations that route traffic directly to pods as opposed to using node ports. By default, spec.allocateLoadBalancerNodePorts is true and type LoadBalancer Services will continue to allocate node ports. If spec.allocateLoadBalancerNodePorts 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. You must enable the ServiceLBNodePortControl feature gate to use this field.

Specifying class of load balancer implementation

FEATURE STATE: Kubernetes v1.21 [alpha]

Starting in v1.21, you can optionally specify the class of a load balancer implementation for LoadBalancer type of Service by setting the field spec.loadBalancerClass. By default, spec.loadBalancerClass is nil and a LoadBalancer type of Service uses the cloud provider’s default load balancer implementation. If spec.loadBalancerClass is specified, 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 LoadBalancer only. 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 “example.com/internal-vip“. Unprefixed names are reserved for end-users. You must enable the ServiceLoadBalancerClass feature gate to use this field.

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.

  1. [...]
  2. metadata:
  3.     name: my-service
  4.     annotations:
  5.         cloud.google.com/load-balancer-type: "Internal"
  6. [...]
  1. [...]
  2. metadata:
  3.     name: my-service
  4.     annotations:
  5.         service.beta.kubernetes.io/aws-load-balancer-internal: "true"
  6. [...]
  1. [...]
  2. metadata:
  3.     name: my-service
  4.     annotations:
  5.         service.beta.kubernetes.io/azure-load-balancer-internal: "true"
  6. [...]
  1. [...]
  2. metadata:
  3.     name: my-service
  4.     annotations:
  5.         service.kubernetes.io/ibm-load-balancer-cloud-provider-ip-type: "private"
  6. [...]
  1. [...]
  2. metadata:
  3.     name: my-service
  4.     annotations:
  5.         service.beta.kubernetes.io/openstack-internal-load-balancer: "true"
  6. [...]
  1. [...]
  2. metadata:
  3.     name: my-service
  4.     annotations:
  5.         service.beta.kubernetes.io/cce-load-balancer-internal-vpc: "true"
  6. [...]
  1. [...]
  2. metadata:
  3.   annotations:
  4.     service.kubernetes.io/qcloud-loadbalancer-internal-subnetid: subnet-xxxxx
  5. [...]
  1. [...]
  2. metadata:
  3.   annotations:
  4.     service.beta.kubernetes.io/alibaba-cloud-loadbalancer-address-type: "intranet"
  5. [...]

TLS support on AWS

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

  1. metadata:
  2.   name: my-service
  3.   annotations:
  4.     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.

  1. metadata:
  2.   name: my-service
  3.   annotations:
  4.     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 X-Forwarded-For 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:

  1.     metadata:
  2.       name: my-service
  3.       annotations:
  4.         service.beta.kubernetes.io/aws-load-balancer-backend-protocol: http
  5.         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 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:

  1. 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:

  1.     metadata:
  2.       name: my-service
  3.       annotations:
  4.         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:

  1.     metadata:
  2.       name: my-service
  3.       annotations:
  4.         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.

  1.     metadata:
  2.       name: my-service
  3.       annotations:
  4.         service.beta.kubernetes.io/aws-load-balancer-access-log-enabled: "true"
  5.         # Specifies whether access logs are enabled for the load balancer
  6.         service.beta.kubernetes.io/aws-load-balancer-access-log-emit-interval: "60"
  7.         # The interval for publishing the access logs. You can specify an interval of either 5 or 60 (minutes).
  8.         service.beta.kubernetes.io/aws-load-balancer-access-log-s3-bucket-name: "my-bucket"
  9.         # The name of the Amazon S3 bucket where the access logs are stored
  10.         service.beta.kubernetes.io/aws-load-balancer-access-log-s3-bucket-prefix: "my-bucket-prefix/prod"
  11.         # 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.

  1.     metadata:
  2.       name: my-service
  3.       annotations:
  4.         service.beta.kubernetes.io/aws-load-balancer-connection-draining-enabled: "true"
  5.         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.

  1.     metadata:
  2.       name: my-service
  3.       annotations:
  4.         service.beta.kubernetes.io/aws-load-balancer-connection-idle-timeout: "60"
  5.         # 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
  7.         service.beta.kubernetes.io/aws-load-balancer-cross-zone-load-balancing-enabled: "true"
  8.         # Specifies whether cross-zone load balancing is enabled for the load balancer
  10.         service.beta.kubernetes.io/aws-load-balancer-additional-resource-tags: "environment=prod,owner=devops"
  11.         # A comma-separated list of key-value pairs which will be recorded as
  12.         # additional tags in the ELB.
  14.         service.beta.kubernetes.io/aws-load-balancer-healthcheck-healthy-threshold: ""
  15.         # The number of successive successful health checks required for a backend to
  16.         # be considered healthy for traffic. Defaults to 2, must be between 2 and 10
  18.         service.beta.kubernetes.io/aws-load-balancer-healthcheck-unhealthy-threshold: "3"
  19.         # The number of unsuccessful health checks required for a backend to be
  20.         # considered unhealthy for traffic. Defaults to 6, must be between 2 and 10
  22.         service.beta.kubernetes.io/aws-load-balancer-healthcheck-interval: "20"
  23.         # The approximate interval, in seconds, between health checks of an
  24.         # individual instance. Defaults to 10, must be between 5 and 300
  26.         service.beta.kubernetes.io/aws-load-balancer-healthcheck-timeout: "5"
  27.         # The amount of time, in seconds, during which no response means a failed
  28.         # health check. This value must be less than the service.beta.kubernetes.io/aws-load-balancer-healthcheck-interval
  29.         # value. Defaults to 5, must be between 2 and 60
  31.         service.beta.kubernetes.io/aws-load-balancer-security-groups: "sg-53fae93f"
  32.         # A list of existing security groups to be configured on the ELB created. Unlike the annotation
  33.         # 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 
  34.         # of a uniquely generated security group for this ELB.
  35.         # 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).
  36.         # 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
  37.         # of those ELBs it will remove the single permit line and block access for all ELBs that shared the same security group ID.
  38.         # This can cause a cross-service outage if not used properly
  40.         service.beta.kubernetes.io/aws-load-balancer-extra-security-groups: "sg-53fae93f,sg-42efd82e"
  41.         #  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
  42.         # has a unique security group ID and a matching permit line to allow traffic to the target worker nodes (service traffic and health checks).
  43.         # Security groups defined here can be shared between services. 
  45.         service.beta.kubernetes.io/aws-load-balancer-target-node-labels: "ingress-gw,gw-name=public-api"
  46.         # A comma separated list of key-value pairs which are used
  47.         # to select the target nodes for the load balancer

Network Load Balancer support on AWS

FEATURE STATE: 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 nlb.

  1.     metadata:
  2.       name: my-service
  3.       annotations:
  4.         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 is not propagated to the end Pods.

By setting .spec.externalTrafficPolicy to 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:

RuleProtocolPort(s)IpRange(s)IpRange Description
Health CheckTCPNodePort(s) (.spec.healthCheckNodePort for .spec.externalTrafficPolicy = Local)Subnet CIDRkubernetes.io/rule/nlb/health=<loadBalancerName>
Client TrafficTCPNodePort(s).spec.loadBalancerSourceRanges (defaults to 0.0.0.0/0)kubernetes.io/rule/nlb/client=<loadBalancerName>
MTU DiscoveryICMP3,4.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.

  1. spec:
  2.   loadBalancerSourceRanges:
  3.     - "143.231.0.0/16"

Note: If .spec.loadBalancerSourceRanges is not set, Kubernetes allows 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.

Other CLB annotations on Tencent Kubernetes Engine (TKE)

There are other annotations for managing Cloud Load Balancers on TKE as shown below.

  1.     metadata:
  2.       name: my-service
  3.       annotations:
  4.         # Bind Loadbalancers with specified nodes
  5.         service.kubernetes.io/qcloud-loadbalancer-backends-label: key in (value1, value2)
  7.         # ID of an existing load balancer
  8.         service.kubernetes.io/tke-existed-lbid：lb-6swtxxxx
  10.         # Custom parameters for the load balancer (LB), does not support modification of LB type yet
  11.         service.kubernetes.io/service.extensiveParameters: ""
  13.         # Custom parameters for the LB listener
  14.         service.kubernetes.io/service.listenerParameters: ""
  16.         # Specifies the type of Load balancer;
  17.         # valid values: classic (Classic Cloud Load Balancer) or application (Application Cloud Load Balancer)
  18.         service.kubernetes.io/loadbalance-type: xxxxx
  20.         # Specifies the public network bandwidth billing method;
  21.         # valid values: TRAFFIC_POSTPAID_BY_HOUR(bill-by-traffic) and BANDWIDTH_POSTPAID_BY_HOUR (bill-by-bandwidth).
  22.         service.kubernetes.io/qcloud-loadbalancer-internet-charge-type: xxxxxx
  24.         # Specifies the bandwidth value (value range: [1,2000] Mbps).
  25.         service.kubernetes.io/qcloud-loadbalancer-internet-max-bandwidth-out: "10"
  27.         # When this annotation is set，the loadbalancers will only register nodes
  28.         # with pod running on it, otherwise all nodes will be registered.
  29.         service.kubernetes.io/local-svc-only-bind-node-with-pod: true

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:

  1. apiVersion: v1
  2. kind: Service
  3. metadata:
  4.   name: my-service
  5.   namespace: prod
  6. spec:
  7.   type: ExternalName
  8.   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 returns 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.

Warning:

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.

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)

  1. apiVersion: v1
  2. kind: Service
  3. metadata:
  4.   name: my-service
  5. spec:
  6.   selector:
  7.     app: MyApp
  8.   ports:
  9.     - name: http
  10.       protocol: TCP
  11.       port: 80
  12.       targetPort: 9376
  13.   externalIPs:
  14.     - 80.11.12.10

Shortcomings

Using the userspace proxy for VIPs works 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 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 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 etcd 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 check 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 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

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

UDP

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

SCTP

FEATURE STATE: Kubernetes v1.20 [stable]

When using a network plugin that supports SCTP traffic, you can use SCTP for most Services. For type=LoadBalancer Services, SCTP support depends on the cloud provider offering this facility. (Most do not).

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.

Windows

Note: 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.

HTTP

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 Ingress in place of Service to expose HTTP/HTTPS Services.

PROXY protocol

If your cloud provider supports it, 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

  1. PROXY TCP4 192.0.2.202 10.0.42.7 12345 7\r\n

followed by the data from the client.

What’s next