The dhcpcd man page states that dhcpcd will execute
/etc/dhcpc/dhcpcd-<interface>.exe "whenever it detects a change in IP
address". It would be very useful for firewall purposes if pump had similar
functionality. Rather than executing a script when the IP changed, I would
have it execute the script when a lease was obtained or renewed (in case
the new lease changed something other than the IP address).

-Peter