Using MetalLb with Kind

Preamble:

When using metallb with kind we are going to deploy it in l2-mode. This means that we need to be able to connect to the ip addresses of the node subnet. If you are using kind on Mac or Windows this will require you to add a route to your laptop routing the subnet used by your kind cluster to the vm that is hosting it. If you are using linux to host a kind cluster. You will not need to do this as the kind node ip addresses are directly attached.

Problem Statement:

Kubernetes on bare metal doesn’t come with an easy integration for things like services of type LoadBalancer.

This mechanism is used to expose services inside the cluster using an external Load Balancing mechansim that understands how to route traffic down to the pods defined by that service.

Most implementations of this are relatively naive. They place all of the available nodes behind the load balancer and use tcp port knocking to determine if the node is “healthy” enough to forward traffic to it.

You can define an externalTrafficPolicy on a service of type LoadBalancer and this can help get the behaviour that you want. From the docs:

$ kubectl explain service.spec.externalTrafficPolicy
KIND:     Service
VERSION:  v1

FIELD:    externalTrafficPolicy <string>

DESCRIPTION:
     externalTrafficPolicy denotes if this Service desires to route external
     traffic to node-local or cluster-wide endpoints. "Local" preserves the
     client source IP and avoids a second hop for LoadBalancer and Nodeport type
     services, but risks potentially imbalanced traffic spreading. "Cluster"
     obscures the client source IP and may cause a second hop to another node,
     but should have good overall load-spreading.

And Metallb has a decent write up on what they do when you configure this stuff:

https://metallb.universe.tf/usage/#traffic-policies

With Metallb there are a different set of assumptions.

Metallb can operate in two distinct modes.

A Layer 2 mode that will use vrrp to arp out for the external ip or VIP on the lan. This means that all traffic for the service will be attracted to only one node and dispersed across the pods defined by the service fromt there.

A bgp mode with externalTrafficPolicy: local metallb will announce the external ip or VIP from all of the nodes where at least one pod is running.

the bgp mode relies on ecmp to balance traffic back to the pods. ECMP is a great solution for this problem and I HIGHLY recommend you use this model if you can.

That said I haven’t created a bgp router for my kind cluster so we wil use the l2-mode for this experiment.

Let’s do this thing!

First let’s bring up a 2 node kind cluster with the following config.

kind: Cluster
apiVersion: kind.sigs.k8s.io/v1alpha3
nodes:
- role: control-plane
- role: worker

Then we need to see if we can ping the node ip of the nodes themselves.

At this point we need to determine the network that is being used for the node ip pool. Since kind nodes are associated with the docker network named “bridge” we can inspect that directly.

I am using a pretty neat tool called jid here that is a repl for json.

So we can see that there is an allocated network of 172.17.0.0/16 in my case.

Let’s swipe the last 10 ip addresses from that allocation and use them for the metallb configuration.

Now we are going to deploy a service!

First let’s create a service of type loadbalancer and see what happens before we install metallb.

I am going to use the echo server for this. I prefer the one built by inanimate. Here is the source and image: inanimate/echo-server

We can see that the EXTERNAL-IP field is pending. This is because there is nothing available in the cluster to manage this type of service.

Now on to the metallb part!

First read the docs https://metallb.universe.tf/installation/

Then we can get started on installing this to our cluster.

We can see that metallb is now installed but we aren’t done yet!

now we need to add a configuration that will use a few of the unused ip addresses from the node ip pool (172.17.0.0/16)

Now if we look at our existing service we can see that the EXTERNAL-IP is still pending

This is because we haven’t yet applied the config for metallb.

Here is the config:

apiVersion: v1
kind: ConfigMap
metadata:
  namespace: metallb-system
  name: config
data:
  config: |
    address-pools:
    - name: default
      protocol: layer2
      addresses:
      - 172.17.255.1-172.17.255.250

You can apply this to your cluster with kubectl apply -f https://git.io/km-config.yaml

Let’s see what happens when we apply this.

We can see the svc get’s an ip address immediately.

And we can curl it!

That’s all for now hit me up on twitter or k8s slack with questions!

Shout-out to Jan Guth for the idea on this post!


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