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Re: tap(4) interface management

On Wed, 13 May 2009, Quentin Garnier wrote:

> On Wed, May 13, 2009 at 10:16:58AM -0400, der Mouse wrote:
> > > Extending the tap driver would be less invasive but unfortunately the
> > > tap IO is carried out on a file descriptor so is no way (?) to do out
> > > of band communication (socket control messages would have been great
> > > for this), the best I can come up with is to have an ioctl enable
> > > (eg) SIGUSR to be raised on informational changes and have the daemon
> > > check.
> >
> > > thoughts and suggestions?
> >
> > I would be inclined to do it in the tap driver.  I would probably do
> > this by providing a tap-specific ioctl which puts it into a mode where
> > data, at least tap->user, has a header that (possibly among other
> > things) allows userland to distinguish between packets and other
> > ancillary data.  The default, of course, would be no header and no
> > out-of-band data.

Mm, this sounds interesting and it would actually be quite simple to
process a header (in btpand) as its already doing the ethernet header.

> > Whether the mode applies to the fd or to the tap device is an open
> > question.  The former makes more sense; I would expect the latter to be
> > easier to do.  In practice I don't expect there's much difference.

I think there is no difference because there can be only one fd per
device with tap anyway

> > Another possibility would be a (tap-specific) ioctl which opens and
> > returns another file descriptor for the out-of-band data.  I'm not sure
> > which of these two I prefer - probably whichever looks easier to
> > implement.

I didn't think of that either, it would be easier to pass complete
information on a separate descriptor (and would save a bunch of fiddling
around trying to fetch information) though it starts to look just like
route(4) but private to tap(4)

> I think you can do all that using tap-specific kqueue filters.  (See
> kqueue(2), kfilter_register(9) and knote(9).)

I guess so, though I'm not desperately keen on that as btpand is using the
event(3) API and although that does internally use kqueue (by default), I
don't think there is a way to add events that would trigger from a custom


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