> I don't believe it's possible to have a user log in to get an IP.
It is possible, in hotels that have broadband in rooms, and on some
university campuses I've been too they have a DHCP server setup to serve
addresses from a private block. On that network there is a webserver
setup to intercept any http request coming from a client in the private
block and redirect the user to a page where he/she has to login. On
login a cgi (or some such) makes a change in the DHCP database to
allocate the user a "real" IP. The user gets instructions on the
ensuing webpage to do a release/renew and boom they are setup.
Sorry, I don't know of any opensource packages to do this, but it
shouldn't be too hard.
Of course, unless you setup your routers to block packets based on MAC
address this won't prevent someone from "guessing" a valid IP and
setting it up static.