Date: Tue, 7 Apr 2015 15:24:17 -0700 From: Charles Swiger <cswiger@mac.com> To: Yuri <yuri@rawbw.com> Cc: Brooks Davis <brooks@freebsd.org>, net@freebsd.org Subject: Re: [BUG?] dhclient sends packets with source IP address that has been deleted Message-ID: <B1957BA9-80A5-404B-99F3-85AAFA521C4E@mac.com> In-Reply-To: <5524472C.1050905@rawbw.com> References: <55234B74.5020506@rawbw.com> <20150407145354.GA9746@spindle.one-eyed-alien.net> <5524472C.1050905@rawbw.com>
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On Apr 7, 2015, at 2:07 PM, Yuri <yuri@rawbw.com> wrote: > On 04/07/2015 07:53, Brooks Davis wrote: >> I suppose that since dhclient has been killed and restarted it can't >> know it's on the same network, but in practice you want to try to get >> the same lease again and fall back if it turns out you've moved or = your dhcp >> server is broken and lost state. I don't see how this would hurt = anything. >=20 > Let's say dhclient is restarted after a while (ex. after the reboot), = when some other host already has that same IP address. dhclient sends = the broadcast with it, and the response will be sent to another host, = which currently has that address, and that other host will discard this = response. dhclient keeps trying for many seconds, doesn't get any = response. Then it falls back to sending from 0.0.0.0->255.255.255.255 = (as it should have done in the first place), and immediately gets the = valid response. The problem delays DHCP handshake, this is how this can = hurt. In point of fact, the IP used for the source address doesn't matter too = much for DHREQUESTs because they are subnet local (by definition), and = the replies will reach the original sender because they are addressed to = the layer-2 MAC address of that host. DHCP operates as much on layer-2 = as layer-3 and dhclient, ISC dhcpd, and other DHCP software should = handle such cases. There is a specific protocol coming from Zeroconf defined here: http://www.ietf.org/rfc/rfc5227.txt = <http://www.ietf.org/rfc/rfc5227.txt>; ...which talks about how to handle potential IP conflicts via ARP = probing. Regards, --=20 -Chuck
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