kubernetes

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README

kubernetes

kubernetes enables reading zone data from a kubernetes cluster. Record names are constructed as "myservice.mynamespace.coredns.local" where:

  • "myservice" is the name of the k8s service (this may include multiple DNS labels, such as "c1.myservice"),
  • "mynamespace" is the k8s namespace for the service, and
  • "coredns.local" is the zone configured for kubernetes.

The record name format can be changed by specifying a name template in the Corefile.

Syntax

kubernetes [zones...]
  • zones zones kubernetes should be authorative for. Overlapping zones are ignored.
kubernetes [zones] {
    endpoint http://localhost:8080
}

Examples

This is the default kubernetes setup, with everything specified in full:

# Serve on port 53
.:53 {
    # use kubernetes middleware for domain "coredns.local"
    kubernetes coredns.local {
        # Kubernetes data API resync period
        # Example values: 60s, 5m, 1h
        resyncperiod 5m
        # Use url for k8s API endpoint
        endpoint https://k8sendpoint:8080
	# The tls cert, key and the CA cert filenames
	tls cert key cacert
        # Assemble k8s record names with the template
        template {service}.{namespace}.{zone}
        # Only expose the k8s namespace "demo"
        namespaces demo
        # Only expose the records for kubernetes objects
        # that match this label selector. The label
        # selector syntax is described in the kubernetes
        # API documentation: http://kubernetes.io/docs/user-guide/labels/
        # Example selector below only exposes objects tagged as
        # "application=nginx" in the staging or qa environments.
        labels environment in (staging, qa),application=nginx
    }
    # Perform DNS response caching for the coredns.local zone
    # Cache timeout is specified by an integer in seconds
    #cache 180 coredns.local
}

Defaults:

  • If the namespaces keyword is omitted, all kubernetes namespaces are exposed.
  • If the template keyword is omitted, the default template of "{service}.{namespace}.{zone}" is used.
  • If the resyncperiod keyword is omitted, the default resync period is 5 minutes.
  • The labels keyword is only used when filtering results based on kubernetes label selector syntax is required. The label selector syntax is described in the kubernetes API documentation at: http://kubernetes.io/docs/user-guide/labels/
Template syntax

Record name templates can be constructed using the symbolic elements: | template symbol | description | | {service} | Kubernetes object/service name. | | {namespace} | The kubernetes namespace. | | {type} | The type of the kubernetes object. Supports values 'svc' and 'pod'. | | {zone} | The zone configured for the kubernetes middleware. |

Basic Setup
Launch Kubernetes

Kubernetes is launched using the commands in the contrib/kubernetes/testscripts/00_run_k8s.sh script.

Configure kubectl and test

The kubernetes control client can be downloaded from the generic URL: http://storage.googleapis.com/kubernetes-release/release/${K8S_VERSION}/bin/${GOOS}/${GOARCH}/${K8S_BINARY}

For example, the kubectl client for Linux can be downloaded using the command: curl -sSL "http://storage.googleapis.com/kubernetes-release/release/v1.2.4/bin/linux/amd64/kubectl"

The contrib/kubernetes/testscripts/10_setup_kubectl.sh script can be stored in the same directory as kubectl to setup kubectl to communicate with kubernetes running on the localhost.

Launch a kubernetes service and expose the service

The following commands will create a kubernetes namespace "demo", launch an nginx service in the namespace, and expose the service on port 80:

$ ./kubectl create namespace demo
$ ./kubectl get namespace

$ ./kubectl run mynginx --namespace=demo --image=nginx
$ ./kubectl get deployment --namespace=demo

$ ./kubectl expose deployment mynginx --namespace=demo --port=80
$ ./kubectl get service --namespace=demo

The script contrib/kubernetes/testscripts/20_setup_k8s_services.sh creates a couple of sample namespaces with services running in those namespaces. The automated kubernetes integration tests in test/kubernetes_test.go depend on these services and namespaces to exist in kubernetes.

Launch CoreDNS

Build CoreDNS and launch using this configuration file:

# Serve on port 53
.:53 {
    kubernetes coredns.local {
		resyncperiod 5m
        endpoint http://localhost:8080
        template {service}.{namespace}.{zone}
        namespaces demo
        # Only expose the records for kubernetes objects
        # that matches this label selector. 
        # See http://kubernetes.io/docs/user-guide/labels/
        # Example selector below only exposes objects tagged as
        # "application=nginx" in the staging or qa environments.
        #labels environment in (staging, qa),application=nginx
    }
    #cache 180 coredns.local # optionally enable caching
}

Put it in ~/k8sCorefile for instance. This configuration file sets up CoreDNS to use the zone coredns.local for the kubernetes services.

The command to launch CoreDNS is:

$ ./coredns -conf ~/k8sCorefile

In a separate terminal a DNS query can be issued using dig:

$ dig @localhost mynginx.demo.coredns.local

; <<>> DiG 9.9.4-RedHat-9.9.4-29.el7_2.3 <<>> @localhost mynginx.demo.coredns.local
; (2 servers found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 47614
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;mynginx.demo.coredns.local.    IN  A

;; ANSWER SECTION:
mynginx.demo.coredns.local. 0   IN  A   10.0.0.10

;; Query time: 2 msec
;; SERVER: ::1#53(::1)
;; WHEN: Thu Jun 02 11:07:18 PDT 2016
;; MSG SIZE  rcvd: 71

Implementation Notes/Ideas

Basic Zone Mapping

The middleware is configured with a "zone" string. For example: "zone = coredns.local".

The Kubernetes service "myservice" running in "mynamespace" would map to: "myservice.mynamespace.coredns.local".

The middleware should publish an A record for that service and a service record.

If multiple zone names are specified, the records for kubernetes objects are exposed in all listed zones.

For example:

# Serve on port 53
.:53 {
    # use kubernetes middleware for domain "coredns.local"
    kubernetes coredns.local {
        # Use url for k8s API endpoint
        endpoint http://localhost:8080
    }
    # Perform DNS response caching for the coredns.local zone
    # Cache timeout is specified by an integer argument in seconds
    # (This works for the kubernetes middleware.)
    #cache 20 coredns.local
    #cache 160 coredns.local
}
Internal IP or External IP?
  • Should the Corefile configuration allow control over whether the internal IP or external IP is exposed?
  • If the Corefile configuration allows control over internal IP or external IP, then the config should allow users to control the precedence.

For example a service "myservice" running in namespace "mynamespace" with internal IP "10.0.0.100" and external IP "1.2.3.4".

This example could be published as:

Corefile directive Result
iporder = internal 10.0.0.100
iporder = external 1.2.3.4
iporder = external, internal 10.0.0.100, 1.2.3.4
iporder = internal, external 1.2.3.4, 10.0.0.100
no directive 10.0.0.100, 1.2.3.4
Wildcards

Publishing DNS records for singleton services isn't very interesting. Service names are unique within a k8s namespace, therefore multiple services will be commonly run with a structured naming scheme.

For example, running multiple nginx services under the names:

Service name
c1.nginx
c2.nginx

or:

Service name
nginx.c3
nginx.c4

A DNS query with wildcard support for "nginx" in these examples should return the IP addresses for all services with "nginx" in the service name.

TBD:

  • How does this relate the the k8s load-balancer configuration?

TODO

  • SkyDNS compatibility/equivalency:
    • Kubernetes packaging and execution
      • Automate packaging to allow executing in Kubernetes. That is, add Docker container build as target in Makefile. Also include anything else needed to simplify launch as the k8s DNS service. Note: Dockerfile already exists in coredns repo to build the docker image. This work item should identify how to pass configuration and run as a SkyDNS replacement.
      • Identify any kubernetes changes necessary to use coredns as k8s DNS server. That is, how do we consume the "--cluster-dns=" and "--cluster-domain=" arguments.
      • Work out how to pass CoreDNS configuration via kubectl command line and yaml service definition file.
      • Ensure that resolver in each kubernetes container is configured to use coredns instance.
      • Update kubernetes middleware documentation to describe running CoreDNS as a SkyDNS replacement. (Include descriptions of different ways to pass CoreFile to coredns command.)
      • Remove dependency on healthz for health checking in kubernetes-rc.yaml file.
      • Expose load-balancer IP addresses.
      • Calculate SRV priority based on number of instances running. (See SkyDNS README.md)
    • Functional work
      • (done. '?' not supported yet) Implement wildcard-based lookup. Minimally support *, consider ? as well.
      • (done) Note from Miek on PR 181: "SkyDNS also supports the word any.
      • Implement SkyDNS-style synthetic zones such as "svc" to group k8s objects. (This should be optional behavior.) Also look at "pod" synthetic zones.
      • Implement test cases for SkyDNS equivalent functionality.
    • SkyDNS functionality, as listed in SkyDNS README: https://github.com/kubernetes/kubernetes/blob/release-1.2/cluster/addons/dns/README.md
      • Expose pods and srv objects.
      • A records in form of pod-ip-address.my-namespace.cluster.local. For example, a pod with ip 1.2.3.4 in the namespace default with a dns name of cluster.local would have an entry: 1-2-3-4.default.pod.cluster.local.
      • SRV records in form of _my-port-name._my-port-protocol.my-namespace.svc.cluster.local CNAME records for both regular services and headless services. See SkyDNS README.
      • A Records and hostname Based on Pod Annotations (k8s beta 1.2 feature). See SkyDNS README.
      • Note: the embedded IP and embedded port record names are weird. I would need to know the IP/port in order to create the query to lookup the name. Presumably these are intended for wildcard queries.
    • Performance
      • Improve lookup to reduce size of query result obtained from k8s API. (namespace-based?, other ideas?)
  • Additional features:

    • Reverse IN-ADDR entries for services. (Is there any value in supporting reverse lookup records?) (need tests, functionality should work based on @aledbf's code.)

    • (done) ~~How to support label specification in Corefile to allow use of labels to indicate zone? For example, the following configuration exposes all services labeled for the "staging" environment and tenant "customerB" in the zone "customerB.stage.local":

        kubernetes customerB.stage.local {
  	# Use url for k8s API endpoint
  	endpoint http://localhost:8080
  	labels environment in (staging),tenant=customerB
  }

      Note: label specification/selection is a killer feature for segmenting test vs staging vs prod environments.~~ Need label testing.

    • Implement IP selection and ordering (internal/external). Related to wildcards and SkyDNS use of CNAMES.

    • Flatten service and namespace names to valid DNS characters. (service names and namespace names in k8s may use uppercase and non-DNS characters. Implement flattening to lower case and mapping of non-DNS characters to DNS characters in a standard way.)

    • Expose arbitrary kubernetes repository data as TXT records?

  • DNS Correctness
    • Do we need to generate synthetic zone records for namespaces?
    • Do we need to generate synthetic zone records for the skydns synthetic zones?
  • Test cases
    • Test with CoreDNS caching. CoreDNS caching for DNS response is working using the cache directive. Tested working using 20s cache timeout and A-record queries. Automate testing with cache in place.
    • Automate CoreDNS performance tests. Initially for zone files, and for pre-loaded k8s API cache. With and without CoreDNS response caching.
    • Try to get rid of kubernetes launch scripts by moving operations into .travis.yml file.
    • Find root cause of timing condition that results in no data returned to test client when running k8s integration tests. Current work-around is a nasty hack of waiting 5 seconds after setting up test server before performing client calls. (See hack in test/kubernetes_test.go)

Documentation

Overview

Package kubernetes provides the kubernetes backend.

Index

Constants

This section is empty.

Variables

This section is empty.

Functions

func NormalizeZoneList 

func NormalizeZoneList(zones []string) []string

NormalizeZoneList filters the zones argument to remove array items that conflict with other items in zones. For example, providing the following zones array: 

[ "a.b.c", "b.c", "a", "e.d.f", "a.b" ]

Returns: 

[ "a.b.c", "a", "e.d.f", "a.b" ]

Zones filted out:

  • "b.c" because "a.b.c" and "b.c" share the common top level "b.c". First listed zone wins if there is a conflict.

Note: This may prove to be too restrictive in practice. 

Need to find counter-example use-cases.

Types

type Kubernetes 

type Kubernetes struct {
	Next          middleware.Handler
	Zones         []string
	Proxy         proxy.Proxy // Proxy for looking up names during the resolution process
	APIEndpoint   string
	APICertAuth   string
	APIClientCert string
	APIClientKey  string
	APIConn       *dnsController
	ResyncPeriod  time.Duration
	NameTemplate  *nametemplate.NameTemplate
	Namespaces    []string
	LabelSelector *unversionedapi.LabelSelector
	Selector      *labels.Selector
}

Kubernetes implements a middleware that connects to a Kubernetes cluster.

func (Kubernetes) A 

func (k Kubernetes) A(zone string, state request.Request, previousRecords []dns.RR) (records []dns.RR, err error)

A returns A records from kubernetes or an error.

func (Kubernetes) AAAA 

func (k Kubernetes) AAAA(zone string, state request.Request, previousRecords []dns.RR) (records []dns.RR, err error)

AAAA returns AAAA records from kubernetes or an error.

func (Kubernetes) CNAME 

func (k Kubernetes) CNAME(zone string, state request.Request) (records []dns.RR, err error)

CNAME returns CNAME records from kubernetes. Not implemented!

func (Kubernetes) Err 

func (k Kubernetes) Err(zone string, rcode int, state request.Request) (int, error)

Err writes an error response back to the client.

func (*Kubernetes) Get 

func (k *Kubernetes) Get(namespace string, nsWildcard bool, servicename string, serviceWildcard bool) ([]*api.Service, error)

Get performs the call to the Kubernetes http API.

func (*Kubernetes) InitKubeCache 

func (k *Kubernetes) InitKubeCache() error

InitKubeCache initializes a new Kubernetes cache. TODO(miek): is this correct?

func (Kubernetes) MX 

func (k Kubernetes) MX(zone string, state request.Request) (records []dns.RR, extra []dns.RR, err error)

MX returns MX records from kubernetes. Not implemented!

func (Kubernetes) NS 

func (k Kubernetes) NS(zone string, state request.Request) (records, extra []dns.RR, err error)

NS returns NS records from kubernetes.

func (Kubernetes) PTR 

func (k Kubernetes) PTR(zone string, state request.Request) ([]dns.RR, error)

PTR Record returns PTR records from kubernetes.

func (*Kubernetes) Records 

func (k *Kubernetes) Records(name string, exact bool) ([]msg.Service, error)

Records looks up services in kubernetes. If exact is true, it will lookup just this name. This is used when find matches when completing SRV lookups for instance.

func (Kubernetes) SOA 

func (k Kubernetes) SOA(zone string, state request.Request) *dns.SOA

SOA Record returns a SOA record from kubernetes.

func (Kubernetes) SRV 

func (k Kubernetes) SRV(zone string, state request.Request) (records []dns.RR, extra []dns.RR, err error)

SRV returns SRV records from kubernetes. If the Target is not a name but an IP address, a name is created on the fly and the IP address is put in the additional section.

func (Kubernetes) ServeDNS 

func (k Kubernetes) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error)

ServeDNS implements the middleware.Handler interface.

func (Kubernetes) TXT 

func (k Kubernetes) TXT(zone string, state request.Request) (records []dns.RR, err error)

TXT returns TXT records from kubernetes. Not implemented!

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