Date: Thu, 16 Sep 2021 08:33:42 -0700 (PDT) From: "Rodney W. Grimes" <freebsd-rwg@gndrsh.dnsmgr.net> To: karels@FreeBSD.org Cc: "Bjoern A. Zeeb" <bzeeb-lists@lists.zabbadoz.net>, freebsd-net@FreeBSD.org Subject: Re: change to deprecate broadcast on host 0 of a subnet Message-ID: <202109161533.18GFXg9Q025919@gndrsh.dnsmgr.net> In-Reply-To: <202109152318.18FNI24k082607@mail.karels.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. I agree with Mike here, though the IETF/RFC process is lacking, the use of all 0 host as a broadcast address is dead and has been dead for decades. If anyone anyplace can point me to a piece of equipment or software that can even recieve these without special configuration I would gladly recant my possition. > > 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. There has been nothing pushing for a change here, the thinking in the network community has been that you can not use a host of 0 on a subnet, and left at that. Again, the process to correct the RFC's has been lacking. > Mike -- Rod Grimes rgrimes@freebsd.org
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