Rapid DHCP: Or, how do Macs get on the network so fast?

> This network recognition technique allows the Mac to very rapidly discover if it is connected to a known network. If the network is recognized (and presumably if the Mac knows that the DHCP lease is still active), it immediately and presumptuously configures its IP interface with the address it knows is good for this network.

Ok, seriously? That isn't a bug in an implementation somewhere, but in fact a feature that Apple actually is proud of? Am I the only one who finds that if you get a room full of people sitting around with Macs at least one person gets their IP address stolen by someone else?

(edit: I just got downvoted, and then asked the people in the room with me, and they seemed to agree with my perceived correlation regarding the "another computer is using 192.1.0.1" issue... instead of just downvoting, maybe reply? It is actually quite common that DHCP leases on a network get reset for various reasons, and if you just jump on the network without revalidating your lease, you are actually quite likely to just "presumptuously" steal someone else's IP address.)

> Ok, seriously? That isn't a bug in an implementation somewhere,

No.

> but in fact a feature that Apple actually is proud of?

Not just Apple, but I imagine also Microsoft and Sun. They were so proud of it that they wrote a standards-track RFC for it: http://www.ietf.org/rfc/rfc4436.txt

And yes, they were aware of this issue:

   One case where DNAv4 does increase the likelihood of an address
   conflict is when:

      o  a DHCP server hands out an address lease,

      o  the host with that lease leaves the network,

      o  the DHCP server is power-cycled or crashes and is rebooted,

      o  the DHCP server, having failed to save leases to stable
         storage, assigns that same address to another host, and

      o  the first host returns and, having a still-valid lease with
         time remaining, proceeds to use its assigned address,
         conflicting with the new host that is now using that same
         address.

   While Section 4 of the DHCP specification [RFC2131] assumes that DHCP
   servers save their leases in persistent storage, almost no consumer-
   grade NAT gateway does so.  Short DHCP lease lifetimes can mitigate
   this risk, though this also limits the operable candidate
   configurations available for DNAv4 to try.

But evidently they thought it was a good trade-off, and I am inclined to agree.