External networking for Kubernetes services

Tue 10 February 2015 by Lars Kellogg-Stedman Tags docker kubernetes

I have recently started running some "real" services (that is, "services being consumed by someone other than myself") on top of Kubernetes (running on bare metal), which means I suddenly had to confront the question of how to provide external access to Kubernetes hosted services. Kubernetes provides two solutions to this problem, neither of which is particularly attractive out of the box:

  1. There is a field createExternalLoadBalancer that can be set in a service description. This is meant to integrate with load balancers provided by your local cloud environment, but at the moment there is only support for this when running under GCE.

  2. A service description can have a list of public IP addresses associated with it in the publicIPS field. This will cause kube-proxy to create rules in the KUBE-PROXY chain of your nat table to direct traffic inbound to those addresses to the appropriate local kube-proxy port.

The second option is a good starting point, since if you were to simply list the public IP addresses of your Kubernetes minions in the publicIPs field, everything would Just Work. That is, inbound traffic to the appropriate port on your minions would get directed to kube-proxy by the nat rules. That's great for simple cases, but in practice it means that you cannot have more that N services exposed on a given port where N is the number of minions in your cluster. That limit is difficult if you -- like I do -- have an all-in-one (e.g., on a single host) Kubernetes deployment on which you wish to host multiple web services exposed on port 80 (and even in a larger environment, you really don't want "number of things on port XX" tightly coupled to "number of minions").

Introducing Kiwi

To overcome this problem, I wrote Kiwi, a service that listens to Kubernetes for events concerning new/modified/deleted services, and in response to those events manages (a) the assignment of IP addresses to network interfaces on your minions and (b) creating additional firewall rules to permit traffic inbound to your services to pass a default-deny firewall configuration.

Kiwi uses etcd to coordinate ownership of IP addresses between minions in your Kubernetes cluster.

How it works

Kiwi listens to event streams from both Kubernetes and Etcd.

On the Kubernetes side, Kiwi listens to /api/v1beta/watch/services, which produces events in response to new, modified, or deleted services. The Kubernetes API uses a server-push model, in which a client makes a single HTTP request and then receives a series of events over the same connection. A event looks something like:

{
  "type": "ADDED",
  "object": {
    "portalIP": "10.254.93.176",
    "containerPort": 80,
    "publicIPs": [
      "192.168.1.100"
    ],
    "selector": {
      "name": "test-web"
    },
    "protocol": "TCP",
    "port": 8080,
    "kind": "Service",
    "id": "test-web",
    "uid": "72bc1286-a440-11e4-b83e-20cf30467e62",
    "creationTimestamp": "2015-01-24T22:15:43-05:00",
    "selfLink": "/api/v1beta1/services/test-web",
    "resourceVersion": 245,
    "apiVersion": "v1beta1",
    "namespace": "default"
  }
}

I am using the Python requests library, which it turns out has a bug in its handling of streaming server responses, but I was able to work around that issue once I realized what was going on.

On the Etcd side, Kiwi uses keys under the /kiwi/publicips prefix to coordinate address ownership among Kiwi instances. It listens to events from Etcd regarding key create/delete/set/etc operations in this prefix by calling /v2/keys/kiwi/publicips?watch=true&recursive=true. This is a long-poll request, rather than a streaming request: that means that a request will only ever receive a single event, but it may need to wait for a while before it receives that response. This model worked well with the requests library out of the box.

After receiving an event from Kubernetes, Kiwi iterates over the public IP addresses in the publicIPs key, and for any address that is not already being manged by the local instance it makes a claim on that address by attempting to atomically create a key in etcd under /kiwi/publicips/ (such as /kiwi/publicips/192.168.1.100). If this attempt succeeds, Kiwi on the local minion has claimed that address and proceeds to assign it to the local interface. If the attempt to set that key does not succeed, it means the address is already being managed by Kiwi on another minion.

The address keys are set with a TTL of 20 seconds, after which they will be expired. If an address expires, other Kiwi instances will receive notification from Etcd and ownership of that address will transfer to another Kiwi instance.

Getting started with Kiwi

The easiest way to get started with Kiwi is to use the larsks/kiwi Docker image that is automatically built from the Git repository. For example, if you want to host public ip addresses on eth0 in the range 192.168.1.32/28, you would start it like this:

docker run --privileged --net=host larsks/kiwi \
  --interface eth0 \
  --range 192.168.1.32/28

You need both --privileged and --net=host in order for Kiwi to assign addresses to your host interfaces and to manage the iptables configuration.

An Example

Start Kiwi as described above. Next, plae the following content in a file called service.yaml:

kind: Service
apiVersion: v1beta1
id: test-web
port: 8888
selector:
  name: test-web
containerPort: 80
publicIPs:
  - 192.168.1.100

Create the service using kubectl:

kubectl create -f service.yaml

After a short pause, you should see the address show up on interface eth0; the entry will look something like:

inet 192.168.1.100/32 scope global dynamic eth0:kube
       valid_lft 17sec preferred_lft 17sec

The eth0:kube is a label applied to the address; this allows Kiwi to clean up these addresses at startup (by getting a list of Kiwi-configured addresses with ip addr show label eth0:kube).

The valid_lft and preferred_lft fields control the lifetime of the interface. When these counters reach 0, the addresses are removed by the kernel. This ensure that if Kiwi dies, the addresses can successfully be re-assigned on another node.


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