coredns/middleware/kubernetes
Michael Richmond 289f53d386 k8s middleware cleanup, testcases, basic SRV (#181)
* Removing unnecessary gitignore pattern

* Updating Makefile to run unittests for subpackages

* Adding Corefile validation to ignore overlapping zones

* Fixing SRV query handling

* Updating README.md now that SRV works

* Fixing debug message, adding code comment

* Clarifying implementation of zone normalization

* "Overlapping zones" is ill-defined. Reimplemented zone overlap/subzone
  checking to contain these functions in k8s middleware and provide
  better code comments explaining the normalization.

* Separate build verbosity from test verbosity

* Cleaning up comments to match repo code style

* Merging warning messages into single message
* Moving function docs to before function declaration

* Adding test cases for k8sclient connector

* Tests cover connector create and setting base url
* Fixed bugs in connector create and setting base url functions

* Updaing README to group and order development work

* Priority focused on achieving functional parity with SkyDNS.

* Adding work items to README and cleaning up formatting

* More README format cleaning

* List formating

* Refactoring k8s API call to allow dependency injection

* Add test cases for data parsing from k8s into dataobject structures

* URL is dependency-injected to allow replacement with a mock http
  server during test execution

* Adding more data validation for JSON parsing tests

* Adding test case for GetResourceList()

* Adding notes about SkyDNS embedded IP and port record names

* Marked test case implemented.

* Fixing formatting for example command.

* Fixing formatting

* Adding notes about Docker image building.

* Adding SkyDNS work item

* Updating TODO list

* Adding name template to Corefile to specify how k8s record names are assembled

* Adding template support for multi-segment zones

* Updating example CoreFile for k8s with template comment

* Misc whitespace cleanup

* Adding SkyDNS naming notes

* Adding namespace filtering to CoreFile config

* Updating example k8sCoreFile to specify namespaces

* Removing unused codepath

* Adding check for valid namespace

* More README TODO restructuring to focus effort

* Adding template validation while parsing CoreFile

* Record name template is considered invalid if it contains a symbol of the form ${bar} where the symbol
  "${bar}" is not an accepted template symbol.

* Refactoring generation of answer records

* Parse typeName out of query string
* Refactor answer record creation as operation over list of ServiceItems

* Moving k8s API caching into SkyDNS equivalency segment

* Adding function to assemble record names from template

* Warning: This commit may be broken. Syncing to get laptop code over to dev machine.

* More todo notes

* Adding comment describing sample test data.

* Update k8sCorefile

* Adding comment

* Adding filtering support for kubernetes "type"

* Required refactoring to support reuse of the StringInSlice function.

* Cleaning up formatting

* Adding note about SkyDNS supporting word "any".

* baseUrl -> baseURL

* Also removed debug statement from core/setup/kubernetes.go

* Fixing test breaking from Url -> URL naming changes

* Changing record name template language ${...} -> {...}

* Fix formatting with go fmt

* Updating all k8sclient data getters to return error value

* Adding error message to k8sclient data accessors

* Cleaning up setup for kubernetes

* Removed verbose nils in initial k8s middleware instance
* Set reasonable defaults if CoreFile has no parameters in the
kubernetes block. (k8s endpoint, and name template)

* Formatting cleanup -- go fmt
2016-07-07 09:40:58 +01:00
..
k8sclient k8s middleware cleanup, testcases, basic SRV (#181) 2016-07-07 09:40:58 +01:00
msg BK8s datasource middleware -- PoC for A records (#153) 2016-06-06 20:49:53 +01:00
nametemplate k8s middleware cleanup, testcases, basic SRV (#181) 2016-07-07 09:40:58 +01:00
util k8s middleware cleanup, testcases, basic SRV (#181) 2016-07-07 09:40:58 +01:00
handler.go k8s middleware cleanup, testcases, basic SRV (#181) 2016-07-07 09:40:58 +01:00
kubernetes.go k8s middleware cleanup, testcases, basic SRV (#181) 2016-07-07 09:40:58 +01:00
lookup.go k8s middleware cleanup, testcases, basic SRV (#181) 2016-07-07 09:40:58 +01:00
README.md k8s middleware cleanup, testcases, basic SRV (#181) 2016-07-07 09:40:58 +01:00
SkyDNS.md k8s middleware cleanup, testcases, basic SRV (#181) 2016-07-07 09:40:58 +01:00
subzone.go k8s middleware cleanup, testcases, basic SRV (#181) 2016-07-07 09:40:58 +01:00
subzone_test.go k8s middleware cleanup, testcases, basic SRV (#181) 2016-07-07 09:40:58 +01:00

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.

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 {
        # Use url for k8s API endpoint
        endpoint http://localhost:8080
    }
#    cache 160 coredns.local
}

Basic Setup

Launch Kubernetes

Kubernetes is launched using the commands in the following run_k8s.sh script:

#!/bin/bash

# Based on instructions at: http://kubernetes.io/docs/getting-started-guides/docker/

#K8S_VERSION=$(curl -sS https://storage.googleapis.com/kubernetes-release/release/latest.txt)
K8S_VERSION="v1.2.4"

ARCH="amd64"

export K8S_VERSION
export ARCH

#DNS_ARGUMENTS="--cluster-dns=10.0.0.10 --cluster-domain=cluster.local"
DNS_ARGUMENTS=""

docker run -d \
    --volume=/:/rootfs:ro \
    --volume=/sys:/sys:ro \
    --volume=/var/lib/docker/:/var/lib/docker:rw \
    --volume=/var/lib/kubelet/:/var/lib/kubelet:rw \
    --volume=/var/run:/var/run:rw \
    --net=host \
    --pid=host \
    --privileged \
    gcr.io/google_containers/hyperkube-${ARCH}:${K8S_VERSION} \
    /hyperkube kubelet \
    --containerized \
    --hostname-override=127.0.0.1 \
    --api-servers=http://localhost:8080 \
    --config=/etc/kubernetes/manifests \
    ${DNS_ARGUMENTS} \
    --allow-privileged --v=2

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 following setup_kubectl.sh script can be stored in the same directory as kubectl to setup kubectl to communicate with kubernetes running on the localhost:

#!/bin/bash

BASEDIR=`realpath $(dirname ${0})`

${BASEDIR}/kubectl config set-cluster test-doc --server=http://localhost:8080
${BASEDIR}/kubectl config set-context test-doc --cluster=test-doc
${BASEDIR}/kubectl config use-context test-doc

alias kubctl="${BASEDIR}/kubectl"

Verify that kubectl is working by querying for the kubernetes namespaces:

$ ./kubectl get namespaces
NAME      STATUS    AGE
default   Active    8d
test      Active    7d

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

Launch CoreDNS

Build CoreDNS and launch using the configuration file in conf/k8sCorefile. This configuration file sets up CoreDNS to use the zone coredns.local for the kubernetes services.

The command to launch CoreDNS is:

$ ./coredns -conf 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 (implemented)

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.

Initial implementation just performs the above simple mapping. Subsequent revisions should allow different namespaces to be published under different 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 provided by the 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 precidence.

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?
  • Do wildcards search across namespaces?
  • Initial implementation assumes that a namespace maps to the first DNS label below the zone managed by the kubernetes middleware. This assumption may need to be revised.

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.)
      • Expose load-balancer IP addresses.
      • Calculate SRV priority based on number of instances running. (See SkyDNS README.md)
    • Functional work
      • Implement wildcard-based lookup. Minimally support *, consider ? as well.
      • 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
      • 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?)
      • Caching of k8s API dataset.
      • DNS response caching is good, but we should also cache at the http query level as well. (Take a look at https://github.com/patrickmn/go-cache as a potential expiring cache implementation for the http API queries.)
      • Push notifications from k8s for data changes rather than pull via API?
  • Additional features:
    • Implement namespace filtering to different zones. That is, zone "a.b" publishes services from namespace "foo", and zone "x.y" publishes services from namespaces "bar" and "baz". (Basic version implemented -- need test cases.)

    • Reverse IN-ADDR entries for services. (Is there any value in supporting reverse lookup records?

    • How to support label specification in Corefile to allow use of labels to indicate zone? (Is this even useful?) 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
        	label "environment" : "staging", "tenant" : "customerB"
        }
      

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

    • 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?

    • Support custom user-provided templates for k8s names. A string provided in the middleware configuration like {service}.{namespace}.{type} defines the template of how to construct record names for the zone. This example would produce myservice.mynamespace.svc.cluster.local. (Basic template implemented. Need to slice zone out of current template implementation.)

  • 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
    • Implement test cases for http data parsing using dependency injection for http get operations.
    • 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.
    • Automate integration testing with kubernetes.