# A simple Kubernetes load balancer

Configures nginx to forward connections to your node IPs.
Services should be declared as NodePort, which means that they
open a port on all nodes. When the request lands on any node,
it is forwarded to the correct pod via the network mesh kubernetes
is using. In theory, there is one a hop penalty.

But lets be honest. You're running with a single LB, probably on a
GCE free tier N1 VM. That extra hop doesn't matter.

## Config

Configure nginx to do what you want, test it. Use any Node IP for your testing.
This will become the 'template_dir' in the argument to the LB.

Move that directory (i.e., `/etc/nginx`) to somewhere new,
(i.e. `/etc/nginx-template/`). Make a symlink from that
new directory to the old one (i.e., `ln -s /etc/nginx-template /etc/nginx`).

Make a workspace directory for this tool; it will write configs to this folder
before updating the symlink you created above. It needs to be persistent so on
server reboot the service starts ok (i.e., `mkdir /var/skubelb/`).

Make sure the user running the tool has read access to the template folder, read-write
access to the workspace folder and config symlink.

Run the server with a command like:

```sh
skubelb --needle some_node_ip \
    --workspace_dir /var/skubelb \
    --config_symlink /etc/nginx \
    --template_dir /etc/nginx-template
    --listen 0.0.0.0:8888
```

Replacing `some_node_ip` with the node IP you used during the initial setup.

Next, configure the Kubernetes nodes to POST `http://loadbalancer:8888/register` when
they started, and DELETE `http://loadbalancer:8888/register` when they shutdown.

#### Running as a system service

Setup a user to run the service; make that user
with `useradd -M skubelb`, Prevent logins with `usermod -L skubelb`.

Make a workspace dir, `mkdir /var/skubelb/`, and give access to the
daemon user, `chown skubelb:skubelb /var/skubelb/`.

Add the systemd config to `/etc/systemd/system/skubelb.service`:

```toml
[Unit]
Description=Simple Kubernetes Load Balancer
After=network.target
StartLimitIntervalSec=0

[Service]
Type=simple
Restart=always
RestartSec=1
User=skubelb
ExecStart=/usr/local/bin/skubelb --needle some_node_ip \
    --workspace-dir /var/skubelb \
    --config-symlink /etc/nginx \
    --template-dir /etc/nginx-template \
    --listen 0.0.0.0:8888 \
    --reload-cmd '/usr/bin/sudo systemctl reload nginx'

[Install]
WantedBy=multi-user.target
```

Make sure you update `--needle some_node_ip` with something
like `--needle 123.44.55.123`. The IP of node you tested with.

### Sample Kubernets configuration

Deploy this [daemon set](https://kubernetes.io/docs/concepts/workloads/controllers/daemonset/)
to your cluster, replacing `lb_address` with the address of your load balancer.

```yaml
apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: skubelb
  namespace: skubelb
  labels:
    k8s-app: skubelb
spec:
  selector:
    matchLabels:
      name: skubelb
  template:
    metadata:
      labels:
        name: skubelb
    spec:
      tolerations:
      # these tolerations are to have the daemonset runnable on control plane nodes
      # remove them if your control plane nodes should not run pods
      - key: node-role.kubernetes.io/control-plane
        operator: Exists
        effect: NoSchedule
      - key: node-role.kubernetes.io/master
        operator: Exists
        effect: NoSchedule
      containers:
      - name: skubelb
        image: alpine/curl:latest
        command: ['sh', '-c', 'echo "Wait for heat death of universe" && sleep 999999d']
        lifecycle:
          preStop:
            exec:
              command: ['curl', '-X', 'DELETE', '34.56.7.198:8888/register']
          preStart:
            exec:
              command: ['curl', '-X', 'POST', '34.56.7.198:8888/register']
        resources:
          limits:
            memory: 200Mi
          requests:
            cpu: 10m
            memory: 100Mi
      terminationGracePeriodSeconds: 30
```

NOTE: you should need to make an entry in the firewall to allow this request through. It is very important that the firewall entry has a source filter; it should only be allowed from the Kubernetes cluster. Nginx will forward traffic to any host that registers, and this could easily become a MitM vulnerability.