HEADS UP: 15.0-CURRENT, change t o bridge(4) might break some network configurations with “Invalid argument”

hello,

although it's possible everyone who is affected by this is already aware
of the change, i thought i should send a heads up anyway, if only to
have a single place to discuss this (since there was quite a lot of
discussion).

in short, following this commit...

b61850c4e6f "bridge(4): default net.link.bridge.member_ifaddrs to false"
https://cgit.freebsd.org/src/commit/?id=b61850c4e6f6b0f21b36da7238db969d9090309e

...it is now impossible to use a network interface which has an IP
address assigned to it as a bridge member, or to configure an IP
address on an interface which is a member of a bridge.

the immediate, "oh shit, my network is broken" fix for this issue is to
set the sysctl net.link.bridge.member_ifaddrs=1.  this will restore the
previous behaviour of bridge(4).

however, the preferred fix is that if you are doing something like this
in /etc/rc.conf:

	cloned_interfaces="bridge0"
	ifconfig_ix0="1.1.1.1/24"
	ifconfig_bridge0="addm ix0"

you should do this instead:

	cloned_interfaces="bridge0"
	ifconfig_ix0="up"
	ifconfig_bridge0"1.1.1.1/24 addm ix0"

in other words, instead of assigning the IP address to the member
interface, assign it to the bridge instead.

i am aware that there are some configurations which currently cannot be
done this way.  in that case, please set member_ifaddrs=1 and i hope to
have resolved all of these cases prior to 16.0-RELEASE, at which point i
intend to remove the member_ifaddrs sysctl.

i do *not* intend to revert this commit, but i *do* want to work with
people who are negatively affected by this change to address their
use-case prior to the removal of the aforementioned sysctl.

specific known issues:

- ifconfig_bridge0="SYNCDHCP" may be broken, in which case try "DHCP"
  instead.

- automatic dhclient invocation on a bridge member via devd when the
  link comes up is broken.

in both cases, setting member_ifaddrs=1 will restore the previous
behaviour.  again, i intend to fix or provide alternatives for all
known breakages caused by this commit.

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