Update /etc/hosts with IPv6 addresses given their IPv4 hostname

update_ipv6.sh

#!/bin/sh

zeroize_macaddrs ()
{
    sed -e 's/\(\s\)\([0-9a-f]\):/\10\2:/' -e 's/\([^0-9a-f]\)\([0-9a-f]\):/\10\2:/g' -e 's/:\([0-9a-f]\)$/:0\1/'
}

find_ip4 ()
{
    grep 'on-dhcp-event' /etc/hosts | zeroize_macaddrs | grep "$1" | while read ip dns _ mac ; do
        if ! echo "$ip" | grep "." > /dev/null ; then
            continue
        fi
        if [ "$1" = "$mac" ]; then
            echo "$dns"
            return
        fi
    done
}

find_ip6 ()
{
    foundip=""
    grep 'ip6-neigh' /etc/hosts | { while read ip _ _ mac ; do
        if ! echo "$ip" | grep ":" > /dev/null ; then
            continue
        fi
        if [ "$1" = "$mac" ]; then
            foundip="$ip"
        fi
    done
    echo "$foundip"; }
}

update_hosts()
{
    change="0"
    ip -6 neigh list nud reachable | grep -v "^fe80" | while read addr _ _ _ mac _ ; do
        existing=$(find_ip6 "$mac")
        if [ "$existing" = "$addr" ]; then
            # Already in the hosts file
            continue
        fi
        if ! [ -z "$existing" ]; then
            # Already in the hosts file with a different address, remove it
            sed -zi 's/^$existing.*$//' /etc/hosts
        fi
        name=$(find_ip4 "$mac")
        if ! [ -z "$name" ]; then
            # Add to the hosts file
            echo -e "$addr\t$name\t#ip6-neigh $mac" >> /etc/hosts
            change="1"
        fi
    done
    if [ "$change" = "1" ]; then
        killall -HUP dnsmasq
    fi
}

main()
{
    while true; do
        update_hosts
        sleep 60
    done
}

main &