Dmitry Pryanishnikov wrote: > > Hello! > > > Date: Tue, 1 Jun 2004 19:07:35 -0500 (CDT) > > From: Mike Silbersack <[EMAIL PROTECTED]> > > > > On Tue, 1 Jun 2004, Andre Oppermann wrote: > > > >> A port should not be reused this fast. Maybe the randomness isn't > >> so random after all and choses the same port over again and again? > > > >We use arc4random, so I don't think that's likely, but it is possible. > > OK, I would like to provide some statistics based on FTP server log. > In the following table, first column is the total number of PORT commands > per FTP session, second is the number of PORT commands between the first and > second occurence of reused port (which is the cause of "425" error), third > column is the interval between those occurences in secons: > > Total # of PORT comm. Interval, # of PORT Interval, sec > > 558 35 50 > 336 50 20 > 165 160 55 > > So, it doesn't seem to me that random number generator works badly, but any > randomness doesn't _guarantee_ that port number won't repeat within 2*MSL > seconds, does it? Also I have heard of algorithms (but can't recollect now) > that actually guarantee non-repeatness of the large portion (up > to the interval range) of pseudo-random sequence. If we had such an algorihm > for random port allocation, we won't get reused ports so often (by default, > portrange.hilast=65535 and portrange.hifirst=49152, so theoretically we would > have 16383 non-repeated port numbers before the first repeat).
The random generator indeed works badly. If it was truely random it should generate a collision only every (1/range) on average. Maybe the arc4random function reuses the same or small number of initial vectors all over again leading to the same small set of 'randomized' ports. -- Andre _______________________________________________ [EMAIL PROTECTED] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-net To unsubscribe, send any mail to "[EMAIL PROTECTED]"
