LSR and packet filters

Sean M. Doran smd at
Sun Sep 14 18:16:35 UTC 1997

"Alex \"Mr. Worf\" Yuriev" <alex at> writes:

> It is called KISS principle. In computer security it is also called
> minimizing possible risk.

Cool, as I just recently said, I love learning brand new
things from people older and wiser than me.

> Well, then he is *WRONG*.

Dr Perlman's excellent book, _Interconnections_, ISBN
0-201-56332-0, Addison-Wesley, 1992, is probably the
definitive text on routing to that date, and subsequently
(désolé, Christian :) )

I would look forward to a new version which would take
recent developments in integrated IS-IS for IP (and in
IP), BGP with all its many new features, IDRP, and the
specification of NIMROD into account, since Dr Perlman is
not only very informative but also a fun read.

> I think it is funny that network operators say "It must be done at the
> application layer because otherwise my network won't scale" while
> people that deal with applied crypto say "Are you nuts? Why do you want to
> make every application utilize its own cryptographic method? You are
> creating a weakness".

The latter group are lazy.

The former group has lazy people too, but it also has
people who use rather neat encryption on the physical
transmissions layers to prevent people with vampire taps,
microwave interceptors, or little satellite dishes
eavesdropping or forging traffic.

However, security at the physical layer, as with security
at the next two or so transport layers, is non-transitive.
You do not become more secure because you scramble the
bits inside your IP datagram than when you scramble the
bits in your TCP segment, and you make it harder to scale
using NATs and ALGs, which are the fundamental building
blocks of the growing Internet.

You also make it less possible to be fair using Van
Jacobson's excellent pleas to implement profile
enforcement towards the edges, or indeed, to implement
anything like RED/WRED/WFQ along parts of the path between
two endpoints scrambling everything inside IP datagrams.

Moreover, scrambling *all* communications or
authenticating *all* communications (thus preventing
things like Vixie's and Cisco's various
web-query-redirection software from working correctly) is
an insane waste of CPU time.

Consequently, encryption and authentication *is* something
you want explicitly turned on by applications, and if
possible it should generate sane (if not necessarily
transparently descriptive) TCP headers.

Implementation details are up to you host people types.
Throwing it back at network operators is silly.  Our
market is in getting traffic around, not in solving
people's security problems (as if any solution proposed or
implmented by any ISP or set of ISPs would really
seriously be believed to be secure anyway).

> Face it, the security of any chain is equal to security of its weakest
> link and currently that link is not host security.

Yah, multiuser systems are really secure.

> > The lesson of Kerberos and SSH and so forth is that
> > ES-to-ES security is useless if one of the ESes itself is
> > compromised.
> Since when is that the case for Kerberos? Only if you compromise the KDC
> you break security of the model.

Ticket stealing is an old game.  All you need is the right
file permissions and decent timing (it's usually a big window).

Ran Atkinson mentioned that K5 doesn't give him
particularly warm fuzzies.  I'm sure you could ask him
some of his reasons, and get a lucid explanation of them.
He's bright, even if I don't find myself liking the IPSEC
model very much.

> While disabling LSR does not make network equipment more robust, it
> prevents a series of very interesting attacks including DOS attacks
> against that equipment.

Which of those denials-of-service are not implementation
dependent, where the implementation generally is now fully
end-of-line Cisco 7000+RP or RSP-only equipment?

> > Sure it is.  Or rather, it is useful to be able to infer a
> > number of path characteristics between two communicating
> > endpoints for such things as flow control and route
> > selection.
> So why don't we all have at least SNMP access to the routers of those
> networks? A lot of people would surely want to what is going on inside
> there? 

Mostly because SNMP is a bad misdesign compared to earlier
efforts before SNMP's standardization, and the ability to
dig out what you really need to know is sometimes more
challenging using SNMP than observing TCP timings and
using hacks like provoking ttl exceededs.

> I think the answer to this question is simple - such ability would
> conflict with a security policy

I have trouble understanding why infering things about the
path, for instance, the amount of congestion, the round
trip times, the behaviour of queues, and so forth gets in
the way of security policies, but people who believe that
are, of course, more than welcome to use ALGs and
firewalling techniques to prevent access to their

Remember, the point is that only people authorized to use
your network should have access to the things you seem to
think are possibly heavy secrets.  If you don't want
random people finding these things out, you don't let
random people use your network.  It's quite simple.

However, if you do let random people use your network, you
probably should not cripple your own use of that
connectivity by doing stupid things in the name of some
slightly additional warm fuzzy security.


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