Ethernet mimicry
Suppose you are a station attached to an unswitched Ethernet segment through which traffic is passing. You don't have an IP address.
You can't get one through DHCP, because either there is no DHCP server or there is one, but it isn't configured to give your station an IP address.
The network has no access control (which is pretty obvious when we say "attached to an unswitched Ethernet segment") and it has a default gateway which is willing to route IP traffic to and from the Internet for all local machines with IP addresses appropriate for the local segment.
By observing local traffic on the segment (and perhaps by making non-destructive active probes), how can you identify the gateway's IP address and a valid but unused IP address for yourself (and, preferably, the IP address of a name server which will perform recursive queries on your behalf), and so autoconfigure yourself as an IP node on the network without the benefit of DHCP service?
I think I know a solution to this problem, which I call the "Ethernet mimicry" problem. The short way of phrasing the problem is "how can you autoconfigure yourself on a network which won't give you an address with DHCP"? I talked to Anirvan about this a couple of weeks ago and worked out an approach I think would work.
I talked about this with Dan Kaminsky at CodeCon. He seems more likely than I to be able to implement it. The basic parts of the solution include an ability to recognize gateways (they receive traffic not addressed to them and send traffic not originated by them, whereas ordinary machines receive traffic not originated by them and send traffic not addressed to them) and an ability to tell whether a particular IP address is in use on a local segment (by sending ARP queries for it -- a capability apparently already included in the current MacOS and used when you try to set an IP address manually).
When we told Kragen about this, he revealed that he'd already invented it. Oops!
