service.go

/*
Copyright 2017 The Kubernetes Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package source

import (
	"context"
	"fmt"
	"net"
	"sort"
	"strings"
	"text/template"

	log "github.com/sirupsen/logrus"
	v1 "k8s.io/api/core/v1"
	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
	"k8s.io/apimachinery/pkg/labels"
	kubeinformers "k8s.io/client-go/informers"
	coreinformers "k8s.io/client-go/informers/core/v1"
	"k8s.io/client-go/kubernetes"
	"k8s.io/client-go/tools/cache"

	"sigs.k8s.io/external-dns/endpoint"
)

const (
	defaultTargetsCapacity = 10
)

// serviceSource is an implementation of Source for Kubernetes service objects.
// It will find all services that are under our jurisdiction, i.e. annotated
// desired hostname and matching or no controller annotation. For each of the
// matched services' entrypoints it will return a corresponding
// Endpoint object.
type serviceSource struct {
	client           kubernetes.Interface
	namespace        string
	annotationFilter string

	// process Services with legacy annotations
	compatibility                  string
	fqdnTemplate                   *template.Template
	combineFQDNAnnotation          bool
	ignoreHostnameAnnotation       bool
	publishInternal                bool
	publishHostIP                  bool
	alwaysPublishNotReadyAddresses bool
	resolveLoadBalancerHostname    bool
	serviceInformer                coreinformers.ServiceInformer
	endpointsInformer              coreinformers.EndpointsInformer
	podInformer                    coreinformers.PodInformer
	nodeInformer                   coreinformers.NodeInformer
	serviceTypeFilter              map[string]struct{}
	labelSelector                  labels.Selector
}

// NewServiceSource creates a new serviceSource with the given config.
func NewServiceSource(ctx context.Context, kubeClient kubernetes.Interface, namespace, annotationFilter string, fqdnTemplate string, combineFqdnAnnotation bool, compatibility string, publishInternal bool, publishHostIP bool, alwaysPublishNotReadyAddresses bool, serviceTypeFilter []string, ignoreHostnameAnnotation bool, labelSelector labels.Selector, resolveLoadBalancerHostname bool) (Source, error) {
	tmpl, err := parseTemplate(fqdnTemplate)
	if err != nil {
		return nil, err
	}

	// Use shared informers to listen for add/update/delete of services/pods/nodes in the specified namespace.
	// Set resync period to 0, to prevent processing when nothing has changed
	informerFactory := kubeinformers.NewSharedInformerFactoryWithOptions(kubeClient, 0, kubeinformers.WithNamespace(namespace))
	serviceInformer := informerFactory.Core().V1().Services()
	endpointsInformer := informerFactory.Core().V1().Endpoints()
	podInformer := informerFactory.Core().V1().Pods()
	nodeInformer := informerFactory.Core().V1().Nodes()

	// Add default resource event handlers to properly initialize informer.
	serviceInformer.Informer().AddEventHandler(
		cache.ResourceEventHandlerFuncs{
			AddFunc: func(obj interface{}) {
			},
		},
	)
	endpointsInformer.Informer().AddEventHandler(
		cache.ResourceEventHandlerFuncs{
			AddFunc: func(obj interface{}) {
			},
		},
	)
	podInformer.Informer().AddEventHandler(
		cache.ResourceEventHandlerFuncs{
			AddFunc: func(obj interface{}) {
			},
		},
	)
	nodeInformer.Informer().AddEventHandler(
		cache.ResourceEventHandlerFuncs{
			AddFunc: func(obj interface{}) {
			},
		},
	)

	informerFactory.Start(ctx.Done())

	// wait for the local cache to be populated.
	if err := waitForCacheSync(context.Background(), informerFactory); err != nil {
		return nil, err
	}

	// Transform the slice into a map so it will
	// be way much easier and fast to filter later
	serviceTypes := make(map[string]struct{})
	for _, serviceType := range serviceTypeFilter {
		serviceTypes[serviceType] = struct{}{}
	}

	return &serviceSource{
		client:                         kubeClient,
		namespace:                      namespace,
		annotationFilter:               annotationFilter,
		compatibility:                  compatibility,
		fqdnTemplate:                   tmpl,
		combineFQDNAnnotation:          combineFqdnAnnotation,
		ignoreHostnameAnnotation:       ignoreHostnameAnnotation,
		publishInternal:                publishInternal,
		publishHostIP:                  publishHostIP,
		alwaysPublishNotReadyAddresses: alwaysPublishNotReadyAddresses,
		serviceInformer:                serviceInformer,
		endpointsInformer:              endpointsInformer,
		podInformer:                    podInformer,
		nodeInformer:                   nodeInformer,
		serviceTypeFilter:              serviceTypes,
		labelSelector:                  labelSelector,
		resolveLoadBalancerHostname:    resolveLoadBalancerHostname,
	}, nil
}

// Endpoints returns endpoint objects for each service that should be processed.
func (sc *serviceSource) Endpoints(ctx context.Context) ([]*endpoint.Endpoint, error) {
	services, err := sc.serviceInformer.Lister().Services(sc.namespace).List(sc.labelSelector)
	if err != nil {
		return nil, err
	}
	services, err = sc.filterByAnnotations(services)
	if err != nil {
		return nil, err
	}

	// filter on service types if at least one has been provided
	if len(sc.serviceTypeFilter) > 0 {
		services = sc.filterByServiceType(services)
	}

	endpoints := []*endpoint.Endpoint{}

	for _, svc := range services {
		// Check controller annotation to see if we are responsible.
		controller, ok := svc.Annotations[controllerAnnotationKey]
		if ok && controller != controllerAnnotationValue {
			log.Debugf("Skipping service %s/%s because controller value does not match, found: %s, required: %s",
				svc.Namespace, svc.Name, controller, controllerAnnotationValue)
			continue
		}

		svcEndpoints := sc.endpoints(svc)

		// process legacy annotations if no endpoints were returned and compatibility mode is enabled.
		if len(svcEndpoints) == 0 && sc.compatibility != "" {
			svcEndpoints, err = legacyEndpointsFromService(svc, sc)
			if err != nil {
				return nil, err
			}
		}

		// apply template if none of the above is found
		if (sc.combineFQDNAnnotation || len(svcEndpoints) == 0) && sc.fqdnTemplate != nil {
			sEndpoints, err := sc.endpointsFromTemplate(svc)
			if err != nil {
				return nil, err
			}

			if sc.combineFQDNAnnotation {
				svcEndpoints = append(svcEndpoints, sEndpoints...)
			} else {
				svcEndpoints = sEndpoints
			}
		}

		if len(svcEndpoints) == 0 {
			log.Debugf("No endpoints could be generated from service %s/%s", svc.Namespace, svc.Name)
			continue
		}

		log.Debugf("Endpoints generated from service: %s/%s: %v", svc.Namespace, svc.Name, svcEndpoints)
		sc.setResourceLabel(svc, svcEndpoints)
		endpoints = append(endpoints, svcEndpoints...)
	}

	// this sorting is required to make merging work.
	// after we merge endpoints that have same DNS, we want to ensure that we end up with the same service being an "owner"
	// of all those records, as otherwise each time we update, we will end up with a different service that gets data merged in
	// and that will cause external-dns to recreate dns record due to different service owner in TXT record.
	// if new service is added or old one removed, that might cause existing record to get re-created due to potentially new
	// owner being selected. Which is fine, since it shouldn't happen often and shouldn't cause any disruption.
	if len(endpoints) > 1 {
		sort.Slice(endpoints, func(i, j int) bool {
			return endpoints[i].Labels[endpoint.ResourceLabelKey] < endpoints[j].Labels[endpoint.ResourceLabelKey]
		})
		// Use stable sort to not disrupt the order of services
		sort.SliceStable(endpoints, func(i, j int) bool {
			if endpoints[i].DNSName != endpoints[j].DNSName {
				return endpoints[i].DNSName < endpoints[j].DNSName
			}
			return endpoints[i].RecordType < endpoints[j].RecordType
		})
		mergedEndpoints := []*endpoint.Endpoint{}
		mergedEndpoints = append(mergedEndpoints, endpoints[0])
		for i := 1; i < len(endpoints); i++ {
			lastMergedEndpoint := len(mergedEndpoints) - 1
			if mergedEndpoints[lastMergedEndpoint].DNSName == endpoints[i].DNSName &&
				mergedEndpoints[lastMergedEndpoint].RecordType == endpoints[i].RecordType &&
				mergedEndpoints[lastMergedEndpoint].SetIdentifier == endpoints[i].SetIdentifier &&
				mergedEndpoints[lastMergedEndpoint].RecordTTL == endpoints[i].RecordTTL {
				mergedEndpoints[lastMergedEndpoint].Targets = append(mergedEndpoints[lastMergedEndpoint].Targets, endpoints[i].Targets[0])
			} else {
				mergedEndpoints = append(mergedEndpoints, endpoints[i])
			}
		}
		endpoints = mergedEndpoints
	}

	for _, ep := range endpoints {
		sort.Sort(ep.Targets)
	}

	return endpoints, nil
}

// extractHeadlessEndpoints extracts endpoints from a headless service using the "Endpoints" Kubernetes API resource
func (sc *serviceSource) extractHeadlessEndpoints(svc *v1.Service, hostname string, ttl endpoint.TTL) []*endpoint.Endpoint {
	var endpoints []*endpoint.Endpoint

	labelSelector, err := metav1.ParseToLabelSelector(labels.Set(svc.Spec.Selector).AsSelectorPreValidated().String())
	if err != nil {
		return nil
	}
	selector, err := metav1.LabelSelectorAsSelector(labelSelector)
	if err != nil {
		return nil
	}

	endpointsObject, err := sc.endpointsInformer.Lister().Endpoints(svc.Namespace).Get(svc.GetName())
	if err != nil {
		log.Errorf("Get endpoints of service[%s] error:%v", svc.GetName(), err)
		return endpoints
	}

	pods, err := sc.podInformer.Lister().Pods(svc.Namespace).List(selector)
	if err != nil {
		log.Errorf("List pods of service[%s] error: %v", svc.GetName(), err)
		return endpoints
	}

	endpointsType := getEndpointsTypeFromAnnotations(svc.Annotations)

	targetsByHeadlessDomainAndType := make(map[endpoint.EndpointKey]endpoint.Targets)
	for _, subset := range endpointsObject.Subsets {
		addresses := subset.Addresses
		if svc.Spec.PublishNotReadyAddresses || sc.alwaysPublishNotReadyAddresses {
			addresses = append(addresses, subset.NotReadyAddresses...)
		}

		for _, address := range addresses {
			// find pod for this address
			if address.TargetRef == nil || address.TargetRef.APIVersion != "" || address.TargetRef.Kind != "Pod" {
				log.Debugf("Skipping address because its target is not a pod: %v", address)
				continue
			}
			var pod *v1.Pod
			for _, v := range pods {
				if v.Name == address.TargetRef.Name {
					pod = v
					break
				}
			}
			if pod == nil {
				log.Errorf("Pod %s not found for address %v", address.TargetRef.Name, address)
				continue
			}

			headlessDomains := []string{hostname}
			if pod.Spec.Hostname != "" {
				headlessDomains = append(headlessDomains, fmt.Sprintf("%s.%s", pod.Spec.Hostname, hostname))
			}

			for _, headlessDomain := range headlessDomains {
				targets := getTargetsFromTargetAnnotation(pod.Annotations)
				if len(targets) == 0 {
					if endpointsType == EndpointsTypeNodeExternalIP {
						node, err := sc.nodeInformer.Lister().Get(pod.Spec.NodeName)
						if err != nil {
							log.Errorf("Get node[%s] of pod[%s] error: %v; not adding any NodeExternalIP endpoints", pod.Spec.NodeName, pod.GetName(), err)
							return endpoints
						}
						for _, address := range node.Status.Addresses {
							if address.Type == v1.NodeExternalIP || (address.Type == v1.NodeInternalIP && suitableType(address.Address) == endpoint.RecordTypeAAAA) {
								targets = append(targets, address.Address)
								log.Debugf("Generating matching endpoint %s with NodeExternalIP %s", headlessDomain, address.Address)
							}
						}
					} else if endpointsType == EndpointsTypeHostIP || sc.publishHostIP {
						targets = endpoint.Targets{pod.Status.HostIP}
						log.Debugf("Generating matching endpoint %s with HostIP %s", headlessDomain, pod.Status.HostIP)
					} else {
						targets = endpoint.Targets{address.IP}
						log.Debugf("Generating matching endpoint %s with EndpointAddress IP %s", headlessDomain, address.IP)
					}
				}
				for _, target := range targets {
					key := endpoint.EndpointKey{
						DNSName:    headlessDomain,
						RecordType: suitableType(target),
					}
					targetsByHeadlessDomainAndType[key] = append(targetsByHeadlessDomainAndType[key], target)
				}
			}
		}
	}

	headlessKeys := []endpoint.EndpointKey{}
	for headlessKey := range targetsByHeadlessDomainAndType {
		headlessKeys = append(headlessKeys, headlessKey)
	}
	sort.Slice(headlessKeys, func(i, j int) bool {
		if headlessKeys[i].DNSName != headlessKeys[j].DNSName {
			return headlessKeys[i].DNSName < headlessKeys[j].DNSName
		}
		return headlessKeys[i].RecordType < headlessKeys[j].RecordType
	})
	for _, headlessKey := range headlessKeys {
		allTargets := targetsByHeadlessDomainAndType[headlessKey]
		targets := []string{}

		deduppedTargets := map[string]struct{}{}
		for _, target := range allTargets {
			if _, ok := deduppedTargets[target]; ok {
				log.Debugf("Removing duplicate target %s", target)
				continue
			}

			deduppedTargets[target] = struct{}{}
			targets = append(targets, target)
		}

		if ttl.IsConfigured() {
			endpoints = append(endpoints, endpoint.NewEndpointWithTTL(headlessKey.DNSName, headlessKey.RecordType, ttl, targets...))
		} else {
			endpoints = append(endpoints, endpoint.NewEndpoint(headlessKey.DNSName, headlessKey.RecordType, targets...))
		}
	}

	return endpoints
}

func (sc *serviceSource) endpointsFromTemplate(svc *v1.Service) ([]*endpoint.Endpoint, error) {
	hostnames, err := execTemplate(sc.fqdnTemplate, svc)
	if err != nil {
		return nil, err
	}

	providerSpecific, setIdentifier := getProviderSpecificAnnotations(svc.Annotations)

	var endpoints []*endpoint.Endpoint
	for _, hostname := range hostnames {
		endpoints = append(endpoints, sc.generateEndpoints(svc, hostname, providerSpecific, setIdentifier, false)...)
	}

	return endpoints, nil
}

// endpointsFromService extracts the endpoints from a service object
func (sc *serviceSource) endpoints(svc *v1.Service) []*endpoint.Endpoint {
	var endpoints []*endpoint.Endpoint
	// Skip endpoints if we do not want entries from annotations
	if !sc.ignoreHostnameAnnotation {
		providerSpecific, setIdentifier := getProviderSpecificAnnotations(svc.Annotations)
		var hostnameList []string
		var internalHostnameList []string

		hostnameList = getHostnamesFromAnnotations(svc.Annotations)
		for _, hostname := range hostnameList {
			endpoints = append(endpoints, sc.generateEndpoints(svc, hostname, providerSpecific, setIdentifier, false)...)
		}

		internalHostnameList = getInternalHostnamesFromAnnotations(svc.Annotations)
		for _, hostname := range internalHostnameList {
			endpoints = append(endpoints, sc.generateEndpoints(svc, hostname, providerSpecific, setIdentifier, true)...)
		}
	}
	return endpoints
}

// filterByAnnotations filters a list of services by a given annotation selector.
func (sc *serviceSource) filterByAnnotations(services []*v1.Service) ([]*v1.Service, error) {
	labelSelector, err := metav1.ParseToLabelSelector(sc.annotationFilter)
	if err != nil {
		return nil, err
	}
	selector, err := metav1.LabelSelectorAsSelector(labelSelector)
	if err != nil {
		return nil, err
	}

	// empty filter returns original list
	if selector.Empty() {
		return services, nil
	}

	filteredList := []*v1.Service{}

	for _, service := range services {
		// convert the service's annotations to an equivalent label selector
		annotations := labels.Set(service.Annotations)

		// include service if its annotations match the selector
		if selector.Matches(annotations) {
			filteredList = append(filteredList, service)
		}
	}

	return filteredList, nil
}

// filterByServiceType filters services according their types
func (sc *serviceSource) filterByServiceType(services []*v1.Service) []*v1.Service {
	filteredList := []*v1.Service{}
	for _, service := range services {
		// Check if the service is of the given type or not
		if _, ok := sc.serviceTypeFilter[string(service.Spec.Type)]; ok {
			filteredList = append(filteredList, service)
		}
	}

	return filteredList
}

func (sc *serviceSource) setResourceLabel(service *v1.Service, endpoints []*endpoint.Endpoint) {
	for _, ep := range endpoints {
		ep.Labels[endpoint.ResourceLabelKey] = fmt.Sprintf("service/%s/%s", service.Namespace, service.Name)
	}
}

func (sc *serviceSource) generateEndpoints(svc *v1.Service, hostname string, providerSpecific endpoint.ProviderSpecific, setIdentifier string, useClusterIP bool) []*endpoint.Endpoint {
	hostname = strings.TrimSuffix(hostname, ".")
	ttl, err := getTTLFromAnnotations(svc.Annotations)
	if err != nil {
		log.Warn(err)
	}

	epA := &endpoint.Endpoint{
		RecordTTL:  ttl,
		RecordType: endpoint.RecordTypeA,
		Labels:     endpoint.NewLabels(),
		Targets:    make(endpoint.Targets, 0, defaultTargetsCapacity),
		DNSName:    hostname,
	}

	epAAAA := &endpoint.Endpoint{
		RecordTTL:  ttl,
		RecordType: endpoint.RecordTypeAAAA,
		Labels:     endpoint.NewLabels(),
		Targets:    make(endpoint.Targets, 0, defaultTargetsCapacity),
		DNSName:    hostname,
	}

	epCNAME := &endpoint.Endpoint{
		RecordTTL:  ttl,
		RecordType: endpoint.RecordTypeCNAME,
		Labels:     endpoint.NewLabels(),
		Targets:    make(endpoint.Targets, 0, defaultTargetsCapacity),
		DNSName:    hostname,
	}

	var endpoints []*endpoint.Endpoint
	var targets endpoint.Targets

	targets = getTargetsFromTargetAnnotation(svc.Annotations)

	if len(targets) == 0 {
		switch svc.Spec.Type {
		case v1.ServiceTypeLoadBalancer:
			if useClusterIP {
				targets = extractServiceIps(svc)
			} else {
				targets = extractLoadBalancerTargets(svc, sc.resolveLoadBalancerHostname)
			}
		case v1.ServiceTypeClusterIP:
			if svc.Spec.ClusterIP == v1.ClusterIPNone {
				endpoints = append(endpoints, sc.extractHeadlessEndpoints(svc, hostname, ttl)...)
			} else if sc.publishInternal {
				targets = extractServiceIps(svc)
			}
		case v1.ServiceTypeNodePort:
			// add the nodeTargets and extract an SRV endpoint
			targets, err = sc.extractNodePortTargets(svc)
			if err != nil {
				log.Errorf("Unable to extract targets from service %s/%s error: %v", svc.Namespace, svc.Name, err)
				return endpoints
			}
			endpoints = append(endpoints, sc.extractNodePortEndpoints(svc, hostname, ttl)...)
		case v1.ServiceTypeExternalName:
			targets = extractServiceExternalName(svc)
		}
	}

	for _, t := range targets {
		switch suitableType(t) {
		case endpoint.RecordTypeA:
			epA.Targets = append(epA.Targets, t)
		case endpoint.RecordTypeAAAA:
			epAAAA.Targets = append(epAAAA.Targets, t)
		case endpoint.RecordTypeCNAME:
			epCNAME.Targets = append(epCNAME.Targets, t)
		}
	}

	if len(epA.Targets) > 0 {
		endpoints = append(endpoints, epA)
	}
	if len(epAAAA.Targets) > 0 {
		endpoints = append(endpoints, epAAAA)
	}
	if len(epCNAME.Targets) > 0 {
		endpoints = append(endpoints, epCNAME)
	}
	for _, endpoint := range endpoints {
		endpoint.ProviderSpecific = providerSpecific
		endpoint.SetIdentifier = setIdentifier
	}
	return endpoints
}

func extractServiceIps(svc *v1.Service) endpoint.Targets {
	if svc.Spec.ClusterIP == v1.ClusterIPNone {
		log.Debugf("Unable to associate %s headless service with a Cluster IP", svc.Name)
		return endpoint.Targets{}
	}
	return endpoint.Targets{svc.Spec.ClusterIP}
}

func extractServiceExternalName(svc *v1.Service) endpoint.Targets {
	return endpoint.Targets{svc.Spec.ExternalName}
}

func extractLoadBalancerTargets(svc *v1.Service, resolveLoadBalancerHostname bool) endpoint.Targets {
	if len(svc.Spec.ExternalIPs) > 0 {
		return svc.Spec.ExternalIPs
	}

	// Create a corresponding endpoint for each configured external entrypoint.
	var targets endpoint.Targets
	for _, lb := range svc.Status.LoadBalancer.Ingress {
		if lb.IP != "" {
			targets = append(targets, lb.IP)
		}
		if lb.Hostname != "" {
			if resolveLoadBalancerHostname {
				ips, err := net.LookupIP(lb.Hostname)
				if err != nil {
					log.Errorf("Unable to resolve %q: %v", lb.Hostname, err)
					continue
				}
				for _, ip := range ips {
					targets = append(targets, ip.String())
				}
			} else {
				targets = append(targets, lb.Hostname)
			}
		}
	}

	return targets
}

func (sc *serviceSource) extractNodePortTargets(svc *v1.Service) (endpoint.Targets, error) {
	var (
		internalIPs endpoint.Targets
		externalIPs endpoint.Targets
		ipv6IPs     endpoint.Targets
		nodes       []*v1.Node
		err         error
	)

	switch svc.Spec.ExternalTrafficPolicy {
	case v1.ServiceExternalTrafficPolicyTypeLocal:
		nodesMap := map[*v1.Node]struct{}{}
		labelSelector, err := metav1.ParseToLabelSelector(labels.Set(svc.Spec.Selector).AsSelectorPreValidated().String())
		if err != nil {
			return nil, err
		}
		selector, err := metav1.LabelSelectorAsSelector(labelSelector)
		if err != nil {
			return nil, err
		}
		pods, err := sc.podInformer.Lister().Pods(svc.Namespace).List(selector)
		if err != nil {
			return nil, err
		}

		for _, v := range pods {
			if v.Status.Phase == v1.PodRunning {
				node, err := sc.nodeInformer.Lister().Get(v.Spec.NodeName)
				if err != nil {
					log.Debugf("Unable to find node where Pod %s is running", v.Spec.Hostname)
					continue
				}
				if _, ok := nodesMap[node]; !ok {
					nodesMap[node] = *new(struct{})
					nodes = append(nodes, node)
				}
			}
		}
	default:
		nodes, err = sc.nodeInformer.Lister().List(labels.Everything())
		if err != nil {
			return nil, err
		}
	}

	for _, node := range nodes {
		for _, address := range node.Status.Addresses {
			switch address.Type {
			case v1.NodeExternalIP:
				externalIPs = append(externalIPs, address.Address)
			case v1.NodeInternalIP:
				internalIPs = append(internalIPs, address.Address)
				if suitableType(address.Address) == endpoint.RecordTypeAAAA {
					ipv6IPs = append(ipv6IPs, address.Address)
				}
			}
		}
	}

	access := getAccessFromAnnotations(svc.Annotations)
	if access == "public" {
		return append(externalIPs, ipv6IPs...), nil
	}
	if access == "private" {
		return internalIPs, nil
	}
	if len(externalIPs) > 0 {
		return append(externalIPs, ipv6IPs...), nil
	}
	return internalIPs, nil
}

func (sc *serviceSource) extractNodePortEndpoints(svc *v1.Service, hostname string, ttl endpoint.TTL) []*endpoint.Endpoint {
	var endpoints []*endpoint.Endpoint

	for _, port := range svc.Spec.Ports {
		if port.NodePort > 0 {
			// following the RFC 2782, SRV record must have a following format
			// _service._proto.name. TTL class SRV priority weight port
			// see https://en.wikipedia.org/wiki/SRV_record

			// build a target with a priority of 0, weight of 0, and pointing the given port on the given host
			target := fmt.Sprintf("0 50 %d %s", port.NodePort, hostname)

			// take the service name from the K8s Service object
			// it is safe to use since it is DNS compatible
			// see https://kubernetes.io/docs/concepts/overview/working-with-objects/names/#dns-label-names
			serviceName := svc.ObjectMeta.Name

			// figure out the protocol
			protocol := strings.ToLower(string(port.Protocol))
			if protocol == "" {
				protocol = "tcp"
			}

			recordName := fmt.Sprintf("_%s._%s.%s", serviceName, protocol, hostname)

			var ep *endpoint.Endpoint
			if ttl.IsConfigured() {
				ep = endpoint.NewEndpointWithTTL(recordName, endpoint.RecordTypeSRV, ttl, target)
			} else {
				ep = endpoint.NewEndpoint(recordName, endpoint.RecordTypeSRV, target)
			}

			endpoints = append(endpoints, ep)
		}
	}

	return endpoints
}

func (sc *serviceSource) AddEventHandler(ctx context.Context, handler func()) {
	log.Debug("Adding event handler for service")

	// Right now there is no way to remove event handler from informer, see:
	// https://github.com/kubernetes/kubernetes/issues/79610
	sc.serviceInformer.Informer().AddEventHandler(eventHandlerFunc(handler))
}