https://pressers.name/2024/05/18/per-client-nsupdate/rfc2136-dns-updates-when-dnsmasq-issues-a-lease/

nsupdate.sh

#!/bin/sh

# to use:
# write me to /usr/local/bin/dnsmasq-nsupdate.sh
# chmod +x /usr/local/bin/dnsmasq-nsupdate.sh
# append to /etc/dnsmasq.conf:
# dhcp-script=/usr/local/bin/dnsmasq-nsupdate.sh
# in /etc/init.d/dnsmasq, find the blob of lines that begin "procd_add_jail_mount" and add:
#         procd_add_jail_mount /usr/local/bin/dnsmasq-nsupdate.sh /usr/bin/logger /usr/bin/nsupdate /lib /usr/lib /etc/dnsmasq-nsupdate
# restart dnsmasq

# Borrowed from https://askubuntu.com/questions/557098/bash-command-to-convert-ip-addresses-into-their-reverse-form
reverseip () {
    local IFS
    IFS=.
    set -- $1
    echo $4.$3.$2.$1
}

# Add, old, or del, determines mode
AOD=$1
# Mac address of the lease
MAC=$2
# New IP address
IP_ADDR=$3
RIP=$(reverseip $3)
# Hostname
L_HOSTNAME=$4

FQDN=${L_HOSTNAME}.${DNSMASQ_DOMAIN}
TTL="3600"
KEYFILE=/etc/dnsmasq-nsupdate/keyfile.key

if [ $AOD == "add" ] || [ $AOD == "old" ]
then
    echo "
    update delete ${FQDN} A
    update add ${FQDN} ${TTL} A ${IP_ADDR}

    update delete ${RIP}.in-addr.arpa. PTR
    update add ${RIP}.in-addr.arpa. $TTL PTR ${FQDN}
    send
    " | nsupdate -k $KEYFILE | logger -t dnsmasq-nsupdate
else
    # clear old addresses only
    echo "
    update delete ${FQDN} A

    update delete ${RIP}.in-addr.arpa. PTR
    send
    " | nsupdate -k $KEYFILE | logger -t dnsmasq-nsupdate
fi