I want to launch an app each time new device appears on network. Do options like that exist in dhcp servers? Or what are the possible hacks to do it anyways?
Answering my own question:
udhcpd can do pretty much what's necessary by combining options:
auto_time 1 #in seconds
Script is called each time udhcpd.leases file is written, which is 1 second (as set in auto_time). Then the script can analyze changes.