UDP catchall

[Jeremie Le Hen]

On Mon, Oct 29, 2007 at 07:49:47PM +0000, Bruce M. Simpson wrote:
>  Brooks Davis wrote:
> > While I think this idea has some merit, I think we specifically want
> > the current wildcard ability to allow for a system that requires
> > minimal configuration.  The problem with a range is that it doesn't
> > allow disjoint sets and it requires that if you really do want all the
> > ports you need to produce a list of currently allocated ports to avoid
> > allocating.  A more (over)engineered solution holds some attraction, but
> > I'm not yet convinced the fact that it could exist precludes the current
> > implementation.
> 
>  Actually I concur with you on this point, based solely on the disjoint sets 
>  point.
> 
>  Another vector of attack would be to put the relay functionality into PF, 
>  which can do the packet matching. However this of course suffers from the 
>  problem that if you just want a plain old UDP socket for mtund, you won't 
>  get that unless you go to the inpcb layer anyway.
> 
>  But who says mtund needs to use sockets for its traffic relay? There is 
>  definite appeal in *not* doing it in the socket layer at all -- an 
>  adaptation of pf's log socket may suffice...

I can think of a possible implementation of mtund(8) without kernel
patching.  The next pf(4) import from OpenBSD will likely allow to log
to some particular pflog(4) interface (instead of the default pflog0).

It will then be possible to create a couple of rules matching one or
more ranges of ports and logging to, say, pflog1.  Reading on the
latter, mtund(8) will immediately open a socket bound to the
corresponding port.  This is a kind of port knocking.  Thanks to TCP
retransmission algorithm or mtunc(1)'s cleverness in case of UDP socket,
the second packet should hit mtund(8).

One downside is that it requires a bunch of configuration in pf.conf(5),
so it may not be as straightforward to set up as one may have expected.

I don't know TCP internals, it may affect TCP slow start or have some
other minor drawbacks.  But hey, we're talking about bypassing firewall
:-)...

My 2 cents.
Regards,
-- 
Jeremie Le Hen
< jeremie at le-hen dot org >< ttz at chchile dot org >