Denial of Service on Bro via Scott Crosby and Dan Wallach's method...fixedin bro-pub-0.8a32?

Jim Mellander jmellander at
Mon Jul 14 09:14:35 PDT 2003

Jim Mellander wrote:
> Christopher Jay Manders wrote:
> >
> > Hi,
> >
> > I am interested to do some further testing of this, but does the a32
> > release have the fixes for the hashing issue inside? (I am referring to
> > their paper at:
> >
> > Has this been extensively tested?
> >
> > Tx!
> >
> > Chris
> if you look in, you'll see the use of MD5 as a hashing function,
> although the old hashing function can still be used - it certainly is
> lighter weight & thus retains a performance advantage, less the DOS
> attack.

By the way, it seems to me that the use of MD5 as a hash function is
overkill.  As I understand it, the issue is not the hash function, per
se, but the predictability of the hashing function potentially leading
to excessive chaining by the insertion of crafted packets, and thus
degrading the hashing function to a linear search.

MD5, of course, makes it computationally infeasable to develop this
chaining, but I would argue that a lighter weight solution would also
give good results, to wit:

Since the issue is the predictability of the hash, the problem really
boils down to determing a way to increase the unpredictability.  One
method would be to introduce a reproducable random factor, that varies
each run of Bro.

So, on startup of Bro, precompute a 2^n size table of random numbers
derived from /dev/random, or other non-reproducable source.  Then the
hashing function could select n bits from the hashed value (using the
original hash function), lookup the table & xor the value with the
computed hash.  The chaining problem still exists, of course, but what
is removed is the predictability, since the table (& thus the hash
function) is specific to *that* run of bro.

Any comments?

Jim Mellander
Incident Response Manager
Computer Protection Program
Lawrence Berkeley National Laboratory
(510) 486-7204

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