r228700 can't dhclient em0

[Gleb Smirnoff]

  Brooks,

On Tue, Dec 20, 2011 at 10:51:34AM -0600, Brooks Davis wrote:
B> We almost certainly need to back r228571 out.  This is not an acceptable
B> upgrade path that would be acceptable historically.  Specially, we have
B> effectively promised users that an X.Y world will work on an X+1.0
B> kernel for most of history.  There are obvious exceptions to this, but
B> we have never allowed ifconfig to be one of them (I broke it many years
B> ago with my first attempt to add if_epoch to if_data and had to back
B> that out).

Pardon, where did we promise that? The applications in jail should work,
but not kernel configuration tools. The network facilities like ipfw
and pf has changed their ABI numerous times, making a new kernel
with older world inaccessible via network after boot.

Considering r228571: we need to specify vhid as additional address
attribute in atomic manner, via one ioctl(). Address can't be first
configured, and then made redundant, that would lead it to being
static for a short period, sending gratutious ARP announcement, etc.

An assumption that we are not allowed to change ABI for our own tools
strongly discourages bringing in new features :(

-- 
Totus tuus, Glebius.