Re: ifconfig(8) refactoring -- YACC grammar now online

From: Robert Watson (rwatson_at_freebsd.org)
Date: 11/30/03 

Date: Sun, 30 Nov 2003 17:41:08 -0500 (EST)
To: Bruce M Simpson <bms@spc.org>

On Sun, 30 Nov 2003, Bruce M Simpson wrote:

> On Sun, Nov 30, 2003 at 02:20:50PM -0500, Robert Watson wrote:
> > if_type seems like it will work for high level classes of interfaces, but
> > something more fine-grained will be required for interfaces that implement
> > multiple classes or subclasses (i.e., 802 generally, and also 802.11b).
>
> The idea just now is we look at if_media if we need to get specific with
> physical interfaces.
>
> tap would seem to be missing from my list, actually; I note it's used to
> provide VMware support in the absence of Netgraph, amongst other things.

if_tap is actually quite useful, and in the same general class of
synthetic interfaces as if_tun. I've used both in building tunneling and
topology-manipulation tools, as well as for debugging routing, etc.
if_tap simulates an 802 device, and if_tun simulates a point-to-point
device. VMware is the only application I know of using if_tap, although I
have a fair amount of my own code that uses it. Userland ppp uses if_tun,
as to some of the third party crypto tunneling tools.

> > Or likewise, tap interfaces might implement 802 generally, but also
> > if_tap-specific primitives. Do we need to probe by-name for capabilities
> > using interface ioctls, or return a "list" of implemented
> > interfaces/classes to allow things to be a bit more multidimensional?
>
> That might work well, actually -- I already added a MIB to rtsock to
> deal with our lack of reporting multicast group memberships, I see no
> reason not to add one to enumerate loaded interface classes.
>
> OTOH, for the 'could load kld' case, this falls down, until the instance
> is created, either through cloning or completing ifattach() for a
> physical interface -- but if CREATE is a separate operation this isn't a
> problem, it is a problem if we want to say something like this in one
> go:-
>
> ifconfig gif0 create tunnel 1.2.3.4 5.6.7.8 10.0.0.1 10.0.0.2
>
> Then you do need a means for the ifconfig instance to ask gif0 if it
> speaks 'tunnel-ese' once it's loaded.
>
> I have to find an abstraction to comfortably deal with this stacking of
> properties/methods, simple polymorphism (a la Java 'implements
> interface') springs to mind.

I think that would be a reasonable approach, although it seems to me that
both the "inheritance" and "implements" models might apply in looking at
sets of protocol relationships. a tap interface is a synthetic interface,
it implements synthetic interface controls, as well as implementing 802.
However, it might be neat to hook up 802.11 to a tap-like interface
sometime as well. Question: does 802.11 imply 802? If so, a notion of
inheritence might be quite useful for driver implementors.

Robert N M Watson FreeBSD Core Team, TrustedBSD Projects
robert@fledge.watson.org Senior Research Scientist, McAfee Research

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