Execute command upon interface initialization?

[Ido Admon]

Hello list,
I couldn't find any way to execute an arbitrary command upon network
interface initialization. My situations is that I have and ethernet card
that takes DHCP from a cables provider (not a fixed IP address) and I
need to update a DynDNS hostname everytime it does (everytime the IP
changes).
I couldn't find a way to execute shell commands using the rc system or
dhclient, what did I miss?
Sure I could just use cron but that's not very elegant.

Thanks and please cc me as I'm not on the list,
Ido Admon