Dual stack hotspot/captive portal

[Ben Jencks]

Does anyone have experience setting up dual stack captive portal systems, e.g. for wireless hotspots? The difficulty is in tying the user's identity (as they log into the portal) to *all* of their IP addresses. With v4 it's easy, they only have one address and it's the one they use to log into the captive portal. With dual stack they have at least two: v4 and v6, plus possibly v6 privacy addresses that change over time.

The only option seems to be identifying users by MAC address post-login, but that's still imperfect. With v4 you can use the DHCP lease table to tie MACs to IPs, but with v6 the best I can think of is monitoring the neighbor table. Has anyone come up with any better solutions?

DHCPv6 is not an option; there are too many operating systems in wide use that have no support for it.

-Ben