Date: Wed, 15 Sep 2021 18:18:02 -0500 From: Mike Karels <karels@freebsd.org> To: "Bjoern A. Zeeb" <bzeeb-lists@lists.zabbadoz.net> Cc: freebsd-net@freebsd.org Subject: Re: change to deprecate broadcast on host 0 of a subnet Message-ID: <202109152318.18FNI24k082607@mail.karels.net> In-Reply-To: Your message of Wed, 15 Sep 2021 15:57:34 -0000. <E3AF2A1B-A44D-4526-BD8B-7C0003D256EF@lists.zabbadoz.net>
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Bjoern wrote: > On 12 Sep 2021, at 15:25, Mike Karels wrote: > > Long ago (4.2BSD), the IP broadcast address was the lowest address on > > a > > network, the one with a host part of 0. In RFC1122, the broadcast > > address > > was standardized using a host part of all ones. 4.3BSD changed its > > default, and made the broadcast address settable with ifconfig. > > However, > > FreeBSD *still* broadcasts packets sent to the lowest address on a > > subnet. > > > > I have a change in review to stop broadcasting the lowest address on a > > subnet by default, but added a sysctl to revert to the current > > behavior. > > I really doubt that anyone is still using a 0-based broadcast address. > > This change allows host 0 on a subnet to be used as an assigned host > > address, as long as the systems on that network support it (including > > routers). Linux already has this change. > > > > The review is https://reviews.freebsd.org/D31861. See also > > https:/datatracker.ietf.org/draft-schoen-intarea-lowest-address/ and > I think it is: > https://datatracker.ietf.org/doc/draft-schoen-intarea-lowest-address/ Thanks for the correction, I think I had to type this rather than pasting. > > some of the discussion in https://reviews.freebsd.org/D19316. > > > > Comments are welcome on the review. I will wait a couple of days > > for comments before proceeding. I am also interested in comments on > > whether this should be MFC'ed to 13-stable after a suitable delay. > I would have even gone one further step back and put this under > EXPERIMENTAL > in HEAD and wait until this draft has gone anywhere but with your sysctl > I think > it is fine (from reading the email not the recent review). > I would prefer if the current behaviour stayed default (would also MFC > better) > and then flip if this will indeed go anywhere. I considered that, but I think that the current behavior is simply wrong. We broadcast packets to the lowest address on the net, but we don't receive these broadcasts as such. I was surprised to find that we were still broadcasting these packets. I can't think of any reason we should do that. Any other opinions on the default setting of the sysctl? > My personal note on this is: it is riding a dead horse, driven by > economics, > and it feels 30 year too late to still do this and change this historic > behaviour. 30 years ago, one might have been able to find a Vax running 4.2BSD to send these packets to. I agree that this change should have been made earlier, but that's not a good reason not to do it now. Whether or not the change has economic benefit, it may allow people to use an additional host on small networks. But I look at this primarily as a cleanup. Mike
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