install single-node, single-stack IPv6 k3s with a random, private IPv6 address on the loopback interface for host-local communication

k3s-loop-ula.bash

#!/usr/bin/env bash

_gen_ula(){
        # Generate a 40-bit random global ID
        local random_id
        random_id=$(od -An -N5 -tx1 /dev/urandom | xxd -p | tr -d '\n')
        # Construct the ULA address prefix with the generated global ID
        local ula_prefix="fd${random_id:0:2}:${random_id:2:4}:${random_id:6:4}"
        echo "${ula_prefix}::1"
}

_cluster_cidr(){
	echo "$(_gen_ula)/56"
}

_service_cidr(){
	echo "$(_gen_ula)/112"
}

# if executed
if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then

        set -euxo pipefail

        ULA="$(_gen_ula)"
	BIND_SUBNET="${ULA}/64"
        BIND_ADDRESS="${ULA}/128"
        if ! ip -o -6 route get "${BIND_ADDRESS}" | grep -qE '\sdev\slo\s'
        then
                sudo ip -6 addr add "${BIND_SUBNET}" dev lo
        fi
        # Check if there is a default IPv6 route
        if ! ip -6 route show default | grep -q '^default'
        then
                EGRESS_INTERFACE="$(ip -o -4 route show default | awk '{print $5}')"
                ip -6 route add default dev "${EGRESS_INTERFACE}"
        fi
        curl -sfL https://get.k3s.io \
        | sudo INSTALL_K3S_EXEC="server" bash -s - \
        --node-ip "${ULA}" \
        --cluster-cidr "$(_cluster_cidr)" \
        --service-cidr "$(_service_cidr)" \
        --flannel-ipv6-masq

fi