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

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354 points timdoug | 1 comments | 12 Jul 11 17:04 UTC | HN request time: 1.939s | source
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saurik ◴[12 Jul 11 17:52 UTC] No.2755677[source]▶
> 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.)

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kelnos ◴[12 Jul 11 17:56 UTC] No.2755698[source]▶
Why would that be the case? Assuming it only does this if it knows its previous DHCP lease hasn't run out, it's very likely (unless the DHCP server has been rebooted or has otherwise lost its lease table) that no other device is using that IP.

And even if it screws this up, it looks like it does a proper DHCP request about a second after the interface comes up, so the problem will be fixed quickly.

Do you have any evidence that Macs in practice tend to inadvertently boot people off the network when they join?

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saurik ◴[12 Jul 11 18:05 UTC] No.2755728[source]▶
By the time the problem is "fixed" the other computer on the network has already detected that its IP address is in conflict by ARP, telling the user what happened (interrupting them) and has shifted to a new IP address (losing all of its active connections in the process).

(and yes: some routers hold on to only a certain number of inactive leases, and routers actually do get rebooted in home, office, and hotel environments quite often, crossing the boundary of the 1-2 day leases that are often given to clients.)

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tzs ◴[13 Jul 11 02:26 UTC] No.2757534[source]▶
I don't see how this has anything to do with what Apple is doing. If you reboot the router and it starts issuing addresses from the beginning of its range, it is likely to issue addresses that conflict with the existing leases on the non-Apple computers.

If a DHCP server gives out an address, and then gives it out again before the least has expired or the first client has relinquished it, it is going to cause a conflict no matter who made the clients.

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1. saurik ◴[13 Jul 11 02:43 UTC] No.2757574[source]▶
Before assigning an address to a client, a DHCP server will ping the address first to make certain someone isn't already using it (this behavior is even documented in the manual page of ISC dhcpd); therefore, it is only clients that join the network and assume their old lease is still valid (sniping it from one of the new clients that accidentally ended up with the IP address that used to be leased) that will cause this behavior.