I mean, ever?

dhclient(8) hints at "The -w flag can be used to  cause  the
client  not  to  exit  when  it doesn't find any such interfaces.   The
omshell (1) program can then be used to notify the client when  a  net-
work  interface  has  been  added  or  removed," but leaves HOW to
do that to the reader's imagination.

For about three years I've occasionally thought "gee, one day I should
really sit down and figure that out, but, not today." I finally sat
down and ... didn't figure it out.  And a bit of googling shows any
number of people who have reached the same point, without turning up a
single example of someone actually doing it.

AFAICT:

1. The (documented) omshell ... control ... state=<n>
   for changing the global client state works as described. n=2 is
   release on all managed interfaces and exit; n=3 and 4 are prepare
   all managed interfaces for sleep and reconfig them after sleep,
   respectively (attention ACPI folks!).

2. The (undocumented) omshell ... interface ... name=<n> ... remove
   does work, and makes the client release and unmanage the specified
   interface n.

3. For a new interface n that the client has not seen before, the
   undocumented omshell ... interface ... name=<n> ... create
   is accepted without error, and has no effect. But it cannot be
   used on any interface that has been seen before (even if previously
   unmanaged with remove) or is mentioned in any saved lease; the
   create fails because the object already exists. In that case, you
   need (the undocumented)

4. omshell ... interface ... name=<n> ... state="up" ... update
   which produces the file dhclient.core. :/

I think I've found the lines in the code responsible for (3) not working,
and the rough neighborhood where the (4) problem has to be, but:

These lines are in code that's unmodified all the way back to our tree's
r1.1, so if I'm right, this functionality has simply never worked.
Or, equivalently, if anyone has been using it with success, I'm on the
wrong track.

Has anyone ever seen it work?  Does Ted Lemon read this list?

-Chap