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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.

tip

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 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.

  1. Install HAProxy and KeepAlived:
sudo apt-get install haproxy keepalived
  1. 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
  1. 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
    }
}
  1. Restart HAProxy and KeepAlived on lb-1 and lb-2:
systemctl restart haproxy
systemctl restart keepalived
  1. 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