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

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354 points timdoug | 2 comments | 12 Jul 11 17:04 UTC | HN request time: 0.523s | 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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1. rektide ◴[13 Jul 11 01:19 UTC] No.2757385[source]▶
The Mac's DHCP client is using the router's mac addresses to verify it's on the right network, and it already knows it still holds the lease. What the fuck more do you want? What are you afraid of?

The only conceivable grief you can put forward is that the ARP check isn't good enough. The client already owns the lease. If there's IP contention, it's because either a) it's not on the right network after all (arp check) or b) the server's fucked up and it's the servers fault (dhcp server allocated an IP twice).

Ok, so assuming the DHCP server isn't faulty may be a bit of a stretch. But coding your client to assume the server can do it's job does not deserve an "Ok, seriously?" hurf-burf post.

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2. saurik ◴[13 Jul 11 01:31 UTC] No.2757415[source]▶
I take it you have not read any of the rest of this thread?... all of the comments about how DHCP in the wild doesn't work that perfectly, or people saying "this happens all the time with my / my wife/friend/boss's Mac"?