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Services

Kubernetes Pods are mortal. They are born and when they die, they are not resurrected. ReplicaSets in particular create and destroy Pods dynamically (e.g. when scaling out or in). While each Pod gets its own IP address, even those IP addresses cannot be relied upon to be stable over time. This leads to a problem: if some set of Pods (let’s call them backends) provides functionality to other Pods (let’s call them frontends) inside the Kubernetes cluster, how do those frontends find out and keep track of which backends are in that set?

Enter Services.

A Kubernetes Service is an abstraction which defines a logical set of Pods and a policy by which to access them - sometimes called a micro-service. The set of Pods targeted by a Service is (usually) determined by a Label Selector (see below for why you might want a Service without a selector).

As an example, consider an 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 or keep track of the list of backends themselves. The Service abstraction enables this decoupling.

For Kubernetes-native applications, Kubernetes offers a simple Endpoints API that is updated whenever the set of Pods in a Service changes. For non-native applications, Kubernetes offers a virtual-IP-based bridge to Services which redirects to the backend Pods.

• Defining a service
• Virtual IPs and service proxies
• Multi-Port Services
• Choosing your own IP address
• Discovering services
• Headless services
• Publishing services - service types
• Shortcomings
• Future work
• The gory details of virtual IPs
• API Object
• Supported protocols
• What's next

Defining a service

A Service in Kubernetes is a REST object, similar to a Pod. Like all of the REST objects, a Service definition can be POSTed to the apiserver to create a new instance. For example, suppose you have a set of Pods that each expose port 9376 and carry a label "app=MyApp".

kind: Service
apiVersion: v1
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 below). The Service’s selector will be evaluated continuously and the results will be POSTed to an Endpoints object also named “my-service”.

Note that a Service can map an incoming port to any targetPort. By default the targetPort will be set to the same value as the port field. Perhaps more interesting is that targetPort can be a string, referring to the name of a port in the backend Pods. The actual port number assigned to that name can be different in each backend Pod. 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.

TCP is the default protocol for services, and you can also use any other supported protocol. At the moment, you can only set a single port and protocol for a Service.

Services without selectors

Services generally 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 test you use your own databases.
• You want to point your service to a service in another Namespace or on another cluster.
• You are migrating your workload to Kubernetes and some of your backends run outside of Kubernetes.

In any of these scenarios you can define a service without a selector:

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

Because this service has no selector, the corresponding Endpoints object will not be created. You can manually map the service to your own specific endpoints:

kind: Endpoints
apiVersion: v1
metadata:
  name: my-service
subsets:
  - addresses:
      - ip: 1.2.3.4
    ports:
      - port: 9376

Note: The endpoint IPs may not be loopback (127.0.0.0/8), link-local (169.254.0.0/16), or link-local multicast (224.0.0.0/24). They cannot be the cluster IPs of other Kubernetes services either because the kube-proxy component doesn’t support virtual IPs as destination yet.

Accessing a Service without a selector works the same as if it had a selector. The traffic will be routed to endpoints defined by the user (1.2.3.4:9376 in this example).

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.

In Kubernetes v1.0, Services are a “layer 4” (TCP/UDP over IP) construct, the proxy was purely in userspace. In Kubernetes v1.1, the Ingress API was added (beta) to represent “layer 7”(HTTP) services, iptables proxy was added too, and became the default operating mode since Kubernetes v1.2. In Kubernetes v1.8.0-beta.0, ipvs proxy was added.

Proxy-mode: userspace

In this mode, kube-proxy watches the Kubernetes master for the addition and removal of Service and Endpoints 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 in Endpoints). Which backend Pod to use is decided based on the SessionAffinity of the Service. Lastly, it installs iptables rules which capture traffic to the Service’s clusterIP (which is virtual) and Port and redirects that traffic to the proxy port which proxies the backend Pod. By default, the choice of backend is round robin.

Proxy-mode: iptables

In this mode, kube-proxy watches the Kubernetes master for the addition and removal of Service and Endpoints 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 Endpoints object, it installs iptables rules which select a backend Pod. By default, the choice of backend is random.

Obviously, iptables need not switch back between userspace and kernelspace, it should be faster and more reliable than the userspace proxy. However, unlike the userspace proxier, the iptables proxier cannot automatically retry another Pod if the one it initially selects does not respond, so it depends on having working readiness probes.

Proxy-mode: ipvs

In this mode, kube-proxy watches Kubernetes Services and Endpoints, calls netlink interface to create ipvs rules accordingly and syncs ipvs rules with Kubernetes Services and Endpoints periodically, to make sure ipvs status is consistent with the expectation. When Service is accessed, traffic will be redirected to one of the backend Pods.

Similar to iptables, Ipvs is based on netfilter hook function, but uses hash table as the underlying data structure and works in the kernel space. That means ipvs redirects traffic much faster, and has much better performance when syncing proxy rules. Furthermore, ipvs provides more options for load balancing algorithm, such as:

• rr: round-robin
• lc: least connection
• dh: destination hashing
• sh: source hashing
• sed: shortest expected delay
• nq: never queue

Note: ipvs mode assumes IPVS kernel modules are installed on the node before running kube-proxy. When kube-proxy starts with ipvs proxy mode, kube-proxy would validate if IPVS modules are installed on the node, if it’s not installed kube-proxy will fall back to iptables proxy mode.

In any of these proxy model, 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. Client-IP based session affinity can be selected by setting service.spec.sessionAffinity to “ClientIP” (the default is “None”), and you can set the max session sticky time by setting the field service.spec.sessionAffinityConfig.clientIP.timeoutSeconds if you have already set service.spec.sessionAffinity to “ClientIP” (the default is “10800”).

Multi-Port Services

Many Services need to expose more than one port. For this case, Kubernetes supports multiple port definitions on a Service object. When using multiple ports you must give all of your ports names, so that endpoints can be disambiguated. For example:

kind: Service
apiVersion: v1
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 that the port names must only contain lowercase alphanumeric characters and -, and must begin & end with an alphanumeric character. 123-abc and web are valid, but 123_abc and -web are not valid names.

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 a user chooses must be a valid IP address and within the service-cluster-ip-range CIDR range that is specified by flag to the API server. If the IP address value is invalid, the apiserver returns a 422 HTTP status code to indicate that the value is invalid.

Why not use round-robin DNS?

A question that pops up every now and then is why we do all this stuff with virtual IPs rather than just use standard round-robin DNS. There are a few reasons:

• There is a long history of DNS libraries not respecting DNS TTLs and caching the results of name lookups.
• Many apps do DNS lookups once and cache the results.
• Even if apps and libraries did proper re-resolution, the load of every client re-resolving DNS over and over would be difficult to manage.

We try to discourage users from doing things that hurt themselves. That said, if enough people ask for this, we may implement it as an alternative.

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

This does imply an ordering requirement - any Service that a Pod wants to access must be created before the Pod itself, or else the environment variables will not be populated. DNS does not have this restriction.

DNS

An optional (though strongly recommended) cluster add-on is a DNS server. The DNS server watches the Kubernetes API for new Services and creates a set of DNS records for each. If DNS has been enabled throughout the cluster then all Pods should be able to do name resolution of Services automatically.

For example, if you have a Service called "my-service" in a Kubernetes Namespace called "my-ns", a DNS record for "my-service.my-ns" is created. Pods which exist in the "my-ns" Namespace should be able to find it by simply doing a name lookup for "my-service". Pods which exist in other Namespaces must qualify the name as "my-service.my-ns". The result of these name lookups is the cluster IP.

Kubernetes also supports DNS SRV (service) records for named ports. If the "my-service.my-ns" Service has a port named "http" with protocol TCP, you can do a DNS SRV query for "_http._tcp.my-service.my-ns" to discover the port number for "http".

The Kubernetes DNS server is the only way to access services of type ExternalName. More information is available in the 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 “headless” services by specifying "None" for the cluster IP (.spec.clusterIP).

This option allows developers to reduce coupling to the Kubernetes system by allowing them freedom to do discovery their own way. Applications can still use a self-registration pattern and adapters for other discovery systems could easily 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:

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

Publishing services - service types

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

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 will route, 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 will route, 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. This requires version 1.7 or higher of kube-dns.

Type NodePort

If you set the type field to NodePort, the Kubernetes master will allocate a port from a range specified by --service-node-port-range flag (default: 30000-32767), and each Node will proxy that port (the same port number on every Node) into your Service. That port will be reported in your Service’s .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) (which is supported since Kubernetes v1.10). A comma-delimited list of IP blocks (e.g. 10.0.0.0/8, 1.2.3.4/32) is used to filter addresses 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 --nodeport-addresses is defaulted to empty ([]), which means select all available interfaces and is in compliance with current NodePort behaviors.

If you want a specific port number, you can specify a value in the nodePort field, and the system will allocate you that port or else the API transaction will fail (i.e. you need to take care about possible port collisions yourself). The value you specify must be in the configured range for node ports.

This gives developers the freedom to set up their own load balancers, 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:

kind: Service
apiVersion: v1
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 the loadBalancerIP to be specified. In those cases, the load-balancer will be created with the user-specified loadBalancerIP. If the loadBalancerIP field is not specified, an ephemeral IP will be assigned to the loadBalancer. If the loadBalancerIP is specified, but the cloud provider does not support the feature, the field will be ignored.

Special notes for Azure: To use user-specified public type loadBalancerIP, a static type public IP address resource needs to be created first, and it 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.

Note: The support of SCTP in the cloud provider’s load balancer is up to the cloud provider’s load balancer implementation. If SCTP is not supported by the cloud provider’s load balancer the Service creation request is accepted but the creation of the load balancer fails.

Internal load balancer

In a mixed environment it is sometimes necessary to route traffic from services inside the same VPC.

In a split-horizon DNS environment you would need two services to be able to route both external and internal traffic to your endpoints.

This can be achieved by adding the following annotations to the service based on cloud provider.

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

[...]
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"
[...]

SSL support on AWS

For partial SSL support on clusters running on AWS, starting with 1.3 three annotations can be added 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.

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, the following annotations may be used:

    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.

Beginning in 1.9, services can use predefined AWS SSL policies for any HTTPS or SSL listeners. To see which policies are available for use, run the awscli command:

aws elb describe-load-balancer-policies --query 'PolicyDescriptions[].PolicyName'

Any one of those policies can then be specified 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.

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 ELB

Network Load Balancer support on AWS [alpha]

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

Starting in version 1.9.0, Kubernetes supports Network Load Balancer (NLB). 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"

Unlike Classic Elastic Load Balancers, Network Load Balancers (NLBs) forward the client’s IP 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 pods on the same node.

NLB can also be used 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:

Be aware that 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.

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

spec:
  loadBalancerSourceRanges:
  - "143.231.0.0/16"

Note: NLB only works with certain instance classes, see the AWS documentation for supported instance types.

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:

kind: Service
apiVersion: v1
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 name 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 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 ServiceSpec, 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)

kind: Service
apiVersion: v1
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. See the original design proposal for portals for more details.

Using the userspace proxy obscures the source-IP of a packet accessing a Service. This makes some kinds of firewalling impossible. The iptables proxier 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.

Future work

In the future we envision that the proxy policy 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 VIP will simply transport the packets there.

We intend to improve our support for L7 (HTTP) Services.

We intend to have more flexible ingress modes for Services which encompass the current ClusterIP, NodePort, and LoadBalancer modes and more.

The gory details of virtual IPs

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 users should not be exposed to situations that could cause their actions to fail through no fault of their own. In this situation, we are looking at network ports - users should not have to choose a port number if that choice might collide with another user. That is an isolation failure.

In order to allow users to choose a port number for their Services, we must ensure that no two Services can collide. We do 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 IPs, otherwise creations will fail with a message indicating an IP could not be allocated. A background controller is responsible for creating that map (to migrate from older versions of Kubernetes that used in memory locking) as well as checking for invalid assignments due to administrator intervention and cleaning up any IPs that were allocated but which no service currently uses.

IPs and VIPs

Unlike Pod IP addresses, which actually route to a fixed destination, Service IPs are not actually answered by a single host. Instead, we use 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 VIP and port.

We support three proxy modes - userspace, iptables and ipvs which 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 VIP to this new port, and starts accepting connections on it.

When a client connects to the VIP the iptables rule kicks in, and redirects the packets to the Service 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 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 installs a series of iptables rules which redirect from the VIP to per-Service rules. The per-Service rules link to per-Endpoint rules which redirect (Destination NAT) to the backends.

When a client connects to the VIP 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 VIP to work, and the client IP is not altered.

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 we 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. More details about the API object can be found 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.

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 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

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

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, users can set the protocol field of a Service, Endpoint, NetworkPolicy and 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

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

A Service with type LoadBalancer and protocol SCTP can be created only 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) do not support SCTP.

Windows

SCTP is not supported on Windows based nodes.

Userspace kube-proxy

The kube-proxy does not support the management of SCTP associations when it is in userspace mode.

What's next

Read Connecting a Front End to a Back End Using a Service.

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