net.inet.ip.portrange.randomized=1 hurts

Andre Oppermann andre at
Wed Jun 2 03:41:56 PDT 2004

Dmitry Pryanishnikov wrote:
> Hello!
> > Date:      Tue, 1 Jun 2004 19:07:35 -0500 (CDT)
> > From:      Mike Silbersack <silby at>
> >
> > 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.


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