Wildcard-socket simplification hits a wall

[Eric S. Raymond]

Head up, Mark!  Policy issue.

I fear the wildcard-socket simplification, last of our pre-1.0 major
ambitions, has just hit a wall.

The problem is not with the code simplification itself.  The problem is
that there is a configuration feature called "NIC rules" that depends
on knowing what actual physical interface a packet arrived on. NIC
rules are address filters applied to individual interfaces.

In order to implement this against a packet flow that is all being
accepted by the wildcard interface, we need a way to back out of each
packet which physical interface it arrived on.

One might expect this to be available via a CMSG lookup into recmvsg's
per-package auxiliary headers, analogously to the way we now get the
packet-arrival timestamp (see ntpd/ntp_packetstamp.c).  It's the only
place for the information to be that has the right locality.

That's our wall.  There appears to be such auxiliary header part under
Linux. It's not easy to say definitively, because the documentation
of this part of the socket API is scanty and confusing.  But it looks
like, other than the values of certain options set by sockopt(2),
arrival time is the *only* out-of-band data you can reliably back out
of a packet.

Thus we can proceed with the simplification only at the cost of
dropping the NIC-rule feature.  As much as I would love to drop the
snarl of code around interface scanning, I feel I must recommend
against this.  It would be the first case in which we take away an
administrator tool without a specific security rationale. I fear it
would not go over well.

My recommendation is that we ship with this code undisturbed, mark
the NIC-rule feature "deprecated" and speak of it being removed in
the future, then see if anybody squawks.
-- 
		<a href="http://www.catb.org/~esr/">Eric S. Raymond</a>

Of all tyrannies, a tyranny exercised for the good of its victims may
be the most oppressive. It may be better to live under robber barons
than under omnipotent moral busybodies. The robber baron's cruelty may
sometimes sleep, his cupidity may at some point be satiated; but those
who torment us for our own good will torment us without end, for they
do so with the approval of their consciences.
	-- C. S. Lewis