change to deprecate broadcast on host 0 of a subnet

From: Mike Karels <karels_at_FreeBSD.org>
Date: Sun, 12 Sep 2021 10:25:56 -0500
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
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.

		Mike
Received on Sun Sep 12 2021 - 15:25:56 UTC

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