Acredito que /var/lib/misc/dnsmasq.leases é o arquivo que você deseja
Se não, use o dhcp-script = e escreva o seu próprio manpage
At startup, dnsmasq reads /etc/dnsmasq.conf, The format of this file consists of one option per line, exactly as the long options detailed in the OPTIONS section but without the leading "--".
--dhcp-script= Whenever a new DHCP lease is created, or an old one destroyed, or a TFTP file transfer completes, the executable specified by this option is run. must be an absolute pathname, no PATH search occurs. The arguments to the process are "add", "old" or "del", the MAC address of the host (or DUID for IPv6) , the IP address, and the hostname, if known. "add" means a lease has been created, "del" means it has been destroyed, "old" is a notification of an existing lease when dnsmasq starts or a change to MAC address or hostname of an existing lease (also, lease length or expiry and client-id, if leasefile-ro is set). If the MAC address is from a network type other than ethernet, it will have the network type prepended, eg "06-01:23:45:67:89:ab" for token ring. The process is run as root (assuming that dnsmasq was originally run as root) even if dnsmasq is configured to change UID to an unprivileged user.