You are right, it definetely looked like an OS or
library bug, but that was too unlikely to me at the
moment.

Anyway, as it turns out to be, this is a by-design
behaviour of FreeBSD with system config
net.inet.ip.portrange.randomized turned on, so that port
numbers are picked at random. I can see the tradeoffs, but to
me this looks like a bug, because as a client I can't do
anything to prevent random numbers from matching. What
difference does it make if it's happening more frequently
under higher loads, there is still a chance that any particular
connection fails for no particular reason, like in:

if (rand() == rand()) fail();

I can switch net.inet.ip.portrange.randomized off, but then it
doesn't make switching it on sane at all. I'm somewhat
displeased with FreeBSD in this matter. Do you happen to know
if similar is true with Linux ?

Sincerely,
Dmitry Dvoinikov
http://www.targeted.org/
[...]

On Thursday 02 September 2004 09.49, Dmitry Dvoinikov wrote:[...]

Never heard of such behaviour, but it definitely sounds like a bug in FreeBSD. 
You should probably contact them for it.

Never heard of it happening on any other system - or on FreeBSD for that 
matter. Which version is this on?[...]

It's version 4.10 RELEASE and I have submitted a report here:
http://www.freebsd.org/cgi/query-pr.cgi?pr=71274

And it looks like it's not a bug specific to FreeBSD, but rather
a common feature which just occassionally breaks in FreeBSD because
of it's simplistic implementation.

Sincerely,
Dmitry Dvoinikov
http://www.targeted.org/
[...]