From: John Bashinski <jbash@cisco.com>
Date: Tue, 17 Sep 1996 11:32:38 +0800
To: jfricker@vertexgroup.com (John F. Fricker)
Subject: Re: J'accuse!: Whitehouse and NSA vs. Panix and VTW
In-Reply-To: <2.2.32.19960916190033.010773d0@vertexgroup.com>
Message-ID: <199609162039.NAA10136@mort>
MIME-Version: 1.0
Content-Type: text/plain


> Well IPSec provides for authentication of endpoints which would identify the
> syn attacker.

Only if the attacker were so stupid as to put in valid authentication
data identifying herself. 

I think IPSEC would allow you to throw away the SYNs without processing
them and without putting anything in your incoming connection queue. On the
other hand, you'd have to do all the authentication protocol and
computation for each packet in order to determine that it was bogus. I can
see where that could lead to a still worse denial-of-service attack if your
IPSEC code wasn't properly written.

> What amazes me is that routers happily pass packets with foreign IP return
> addresses.

Defining what a "foreign IP return address" is quickly becomes complicated.

> I guess there is some valid utility to being able to originate a
> connection that actually goes somewhere else for intiating a many to many
> protocol. But I can't think of any practical application that would
> necessarily be that way.

As far as I know, nothing does that.

> So why do routers let packets leave local networks that do not appear to
> originate from said local network?

Because routers don't know which networks are "local networks" and which
networks are transit networks. When a router gets a packet from one of
its interfaces, it has no way of knowing whether that packet originated
on the local network, or was forwarded on by some other router... possibly
from an original source a dozen network hops away.

> Doesn't routing work "both ways" so to speak?

Um, "both ways"? No, not really, if you mean what I think you mean.  I
think you're saying that, if a router receives a packet claiming to be from
host A, and that packet doesn't come from the direction of host A, as
defined by the direction in which the router itself would send a packet
which was destined for host A, it should drop the packet.

The problem with that is that, if host A sends a packet to host B, there's
no guarantee that the path that packet takes is the same as the path a
packet would take from host B to host A. It usually is, but not
always. Transient routing assymetries are very common in routing protocols,
and it's possible, and even occasionally useful, to set up networks where
there are permanent asymmetries.

It's a pretty basic part of the architecture of IP networks that routers
forward based only on destination addresses. Changing this would break a
lot of existing systems. Keeping both "to" and "from" route information
for each destination would entail redesigning all the routing protocols now
in use, as well as doubling the associated memory and computation
requirements. It won't happen soon, if ever.

It may happen that router vendors will start adding configurable options to
discard suspicious packets in the (pretty common) case where routing is
expected to be symmetric. Such options would have to be used with great
care, by network administrators who were very sure they knew what they were
doing. They couldn't be made the defaults without breaking the universe, so
there'd always be people who should turn them on, but wouldn't.

As it stands today, it's possibly to manually configure a router to reject
packets that don't come from addresses expected on the interface the
packets arrive on. Such filters are entirely static, and don't respond to
changes in the network. It's reasonable to set them up on a "stub" link
that forms the only path leading to a reasonably well-defined segment of
the network...  like a LAN, or a small site. It's much less reasonable on a
router in the middle of a complex network, and more or less impossible in
Internet "core" routers... unless you can anticipate every possible dynamic
network change, your filters are going to get it wrong sometimes.

Myself, I always configure routers to filter out bogus source
addresses... when they're being installed at points in the network where
it's obvious which addresses those are. Most ISPs don't do it even when
it's easy, and that's one of the sources of the problem.

					-- John B.