LSR and packet filters

Sean M. Doran smd at
Sun Sep 14 17:42:32 UTC 1997

Vadim Antonov <avg at> writes:

> Hey, LSR is useful for all kinds of very interesting
> denial of service attacks.  A clever combination of LSR and
> forged source addresses may make the attack virtually
> untraceable.

The denials-of-service are generally driven by poor
implementation of networking software, much of which has
been corrected.  Moreover, in the case of LSR-using
source-forged attacks, tracing becomes *easier* because
you need only trap on LSR traffic and work backwards.

What is *hard* is source-forged attacks which are in
profile and option-free.

> Useful for what?  traceroute -g  is the _only_ useful
> application for LSR.  Disabling LSR and adding an application
> level service for tracing back would be just as useful.

There are several people here who have mentioned on and
off that LSR telnet is extremely handy to them.  

If you could send traffic using LSR and pay less severely
for using the option in older routers, then I can think of
several applications for sending lots of traffic with the
LSR option toggled at the source.  I can equally think of
useful applications for SSR.
> Encryption is an overkill for 99% of all applications. 

No argument here. :)

> Disabling LSR and doing SA filtering can take care of
> _most_ security problems.  And it is computationally
> cheap.

SA filtering is more useful than disabling LSR.
What does the additional disabling of LSR on top of
ubiquitous source address filtering buy you really?

> This will not make the network absolutely secure (there ain't
> no such thing as absolute security), but it definitely will
> make it _more_ secure.

So will turning it all off, the ultimate in the utility vs
security trade-off.

> How'd you like to get a stream of nasty bogons aimed at
> your router(s) and arriving from virtually all directions?
> There's a number of ways to kill ciscos with pretty low-rate
> streams.

If I had a BFR up and running, this would make an
interesting test to prove the design point of handling
fully-decorated micropackets at line rate across fully
half of all the interfaces in a fully-decked-out box.
Talk to Peter about beating on one of his, or come back to
me and BC in a couple weeks.

(It would be equally interesting to beat on a fully decked
out 75xx box with modern VIPs and dFIB, I guess.  We both
already know what happens when you even sneeze funny in the
presence of an RP, although SPD is pretty cool at avoiding
the "gee your router is unavailable" problem.)


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