I used to have this on a PIX:
dhcpd enable guests
dhcpd auto_config outside interface guests
dhcpd address 10.10.10.21-10.10.10.29 guests
dhcpd domain guests.whatever.com interface guests
... where guests was one of my inside interfaces and outside is, well, you guessed it, the outside interface configured as ip address dhcp setroute
so everytime a guest hooked on this guests interface it was automatically asigned the DNS servers and the gateway of last resort currently set on my outside interface freeing me to manually set them, and, more important, to constantly tracking the ever-changing DNS of my ISP.
so question is ... how do I do this on a router ?
I mean, what is the auto_config equivalent in IOS to import these settings from an interface acting as DHCP client to my ISP ?