ipvs-txt-lb

#!/bin/sh
set -e

if [ -z "$IPVSTXTLB_ADDRESS" ]; then
    >&2 echo IPVSTXTLB_ADDRESS not set, must be set to IP:PORT to use for load balancer
    exit 1
fi

if [ -z "$IPVSTXTLB_POLL_INTERVAL" ]; then
    >&2 echo IPVSTXTLB_POLL_INTERVAL not set, must be set to num seconds between polls
    exit 1
fi

if [ -z "$IPVSTXTLB_RECORD" ]; then
    >&2 echo IPVSTXTLB_RECORD not set, must be set to location of the TXT record
    exit 1
fi

get_hosts() {
    dig -t TXT +short "$IPVSTXTLB_RECORD" | sed -e 's/"//g' -e 's/;/\n/g'
}

update_hosts() {
    >&2 echo Updating hosts

    current="$(ipvsadm --save | tail -n+2 | cut --delimiter=' ' --field=5 | cut --delimiter=':' --field=1)"
    next="$(get_hosts)"

    for host in $next; do
        if ! echo "$current" | grep "$host" - >/dev/null && [ "$host" != "$(hostname)" ]; then
            >&2 echo "Discovered new host: $host"
            ipvsadm --add-server --tcp-service "$IPVSTXTLB_ADDRESS" --real-server "$host" --masquerading
            >&2 echo "Added $host to pool"
	fi
    done

    for host in $current; do
        if ! echo "$next" | grep "$host" - >/dev/null; then
            >&2 echo "Removing host: $host"
            ipvsadm --delete-server --tcp-service "$IPVSTXTLB_ADDRESS" --real-server "$host"
	    >&2 echo "Removed $host from pool"
	fi
    done

    >&2 echo Hosts up to date
}

start_lb() {
    >&2 echo "Starting on $IPVSTXTLB_ADDRESS"
    ipvsadm --add-service --tcp-service "$IPVSTXTLB_ADDRESS" --scheduler rr
}

stop_lb() {
    >&2 echo "Stopping $IPVVSTXTLB_ADDRESS"
    ipvsadm --delete-service --tcp-service "$IPVSTXTLB_ADDRESS"
    return 0
}

check_err_exit() {
    exit_code=$?
    if [ $exit_code -ne 0 ]; then
        >&2 echo Error, exiting
        stop_lb || true
	exit $exit_code
    fi
    exit 0
}

trap 'check_err_exit' EXIT
trap 'stop_lb' TERM

start_lb

while true; do
    update_hosts
    sleep "$IPVSTXTLB_POLL_INTERVAL"
done