Cluster Load Balancer

This section describes how to install an external load balancer in front of a High Availability (HA) K3s cluster’s server nodes. Two examples are provided: Nginx and HAProxy.

External load-balancers should not be confused with the embedded ServiceLB, which is an embedded controller that allows for use of Kubernetes LoadBalancer Services without deploying a third-party load-balancer controller. For more details, see Service Load Balancer.

External load-balancers can be used to provide a fixed registration address for registering nodes, or for external access to the Kubernetes API Server. For exposing LoadBalancer Services, external load-balancers can be used alongside or instead of ServiceLB, but in most cases, replacement load-balancer controllers such as MetalLB or Kube-VIP are a better choice.

Prerequisites

All nodes in this example are running Ubuntu 20.04.

For both examples, assume that a HA K3s cluster with embedded etcd has been installed on 3 nodes.

Each k3s server is configured with:

# /etc/rancher/k3s/config.yaml
token: lb-cluster-gd
tls-san: 10.10.10.100

The nodes have hostnames and IPs of:

server-1: 10.10.10.50
server-2: 10.10.10.51
server-3: 10.10.10.52

Two additional nodes for load balancing are configured with hostnames and IPs of:

lb-1: 10.10.10.98
lb-2: 10.10.10.99

Three additional nodes exist with hostnames and IPs of:

agent-1: 10.10.10.101
agent-2: 10.10.10.102
agent-3: 10.10.10.103

Setup Load Balancer

HAProxy
Nginx

HAProxy is an open source option that provides a TCP load balancer. It also supports HA for the load balancer itself, ensuring redundancy at all levels. See HAProxy Documentation for more info.

Additionally, we will use KeepAlived to generate a virtual IP (VIP) that will be used to access the cluster. See KeepAlived Documentation for more info.

Install HAProxy and KeepAlived:
```
sudo apt-get install haproxy keepalived
```

Add the following to /etc/haproxy/haproxy.cfg on lb-1 and lb-2:

frontend k3s-frontend
    bind *:6443
    mode tcp
    option tcplog
    default_backend k3s-backend

backend k3s-backend
    mode tcp
    option tcp-check
    balance roundrobin
    default-server inter 10s downinter 5s
    server server-1 10.10.10.50:6443 check
    server server-2 10.10.10.51:6443 check
    server server-3 10.10.10.52:6443 check

Add the following to /etc/keepalived/keepalived.conf on lb-1 and lb-2:

global_defs {
  enable_script_security
  script_user root
}

vrrp_script chk_haproxy {
    script 'killall -0 haproxy' # faster than pidof
    interval 2
}

vrrp_instance haproxy-vip {
    interface eth1
    state <STATE> # MASTER on lb-1, BACKUP on lb-2
    priority <PRIORITY> # 200 on lb-1, 100 on lb-2

    virtual_router_id 51

    virtual_ipaddress {
        10.10.10.100/24
    }

    track_script {
        chk_haproxy
    }
}

Restart HAProxy and KeepAlived on lb-1 and lb-2:

systemctl restart haproxy
systemctl restart keepalived

On agent-1, agent-2, and agent-3, run the following command to install k3s and join the cluster:

curl -sfL https://get.k3s.io | K3S_TOKEN=lb-cluster-gd sh -s - agent --server https://10.10.10.100:6443

You can now use kubectl from server node to interact with the cluster.

root@server-1 $ k3s kubectl get nodes -A
NAME       STATUS   ROLES                       AGE     VERSION
agent-1    Ready    <none>                      32s     v1.27.3+k3s1
agent-2    Ready    <none>                      20s     v1.27.3+k3s1
agent-3    Ready    <none>                      9s      v1.27.3+k3s1
server-1   Ready    control-plane,etcd,master   4m22s   v1.27.3+k3s1
server-2   Ready    control-plane,etcd,master   3m58s   v1.27.3+k3s1
server-3   Ready    control-plane,etcd,master   3m12s   v1.27.3+k3s1

Nginx does not natively support a High Availability (HA) configuration. If setting up an HA cluster, having a single load balancer in front of K3s will reintroduce a single point of failure.

Nginx Open Source provides a TCP load balancer. See Using nginx as HTTP load balancer for more info.

Create a nginx.conf file on lb-1 with the following contents:

events {}

stream {
  upstream k3s_servers {
    server 10.10.10.50:6443;
    server 10.10.10.51:6443;
    server 10.10.10.52:6443;
  }

  server {
    listen 6443;
    proxy_pass k3s_servers;
  }
}

Run the Nginx load balancer on lb-1:

Using docker:

docker run -d --restart unless-stopped \
    -v ${PWD}/nginx.conf:/etc/nginx/nginx.conf \
    -p 6443:6443 \
    nginx:stable

Or install nginx and then run:

cp nginx.conf /etc/nginx/nginx.conf
systemctl start nginx

On agent-1, agent-2, and agent-3, run the following command to install k3s and join the cluster:

curl -sfL https://get.k3s.io | K3S_TOKEN=lb-cluster-gd sh -s - agent --server https://10.10.10.98:6443

You can now use kubectl from server node to interact with the cluster.

root@server1 $ k3s kubectl get nodes -A
NAME       STATUS   ROLES                       AGE     VERSION
agent-1    Ready    <none>                      30s     v1.27.3+k3s1
agent-2    Ready    <none>                      22s     v1.27.3+k3s1
agent-3    Ready    <none>                      13s     v1.27.3+k3s1
server-1   Ready    control-plane,etcd,master   4m49s   v1.27.3+k3s1
server-2   Ready    control-plane,etcd,master   3m58s   v1.27.3+k3s1
server-3   Ready    control-plane,etcd,master   3m16s   v1.27.3+k3s1