netdev - Re: ports beeing reused too fast

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Date:	Sat, 9 May 2009 15:45:15 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Bill Fink <billfink@...dspring.com>
Cc:	Eric Dumazet <dada1@...mosbay.com>,
	Octavian Purdila <opurdila@...acom.com>, netdev@...r.kernel.org
Subject: Re: ports beeing reused too fast

On Sat, 9 May 2009 15:31:35 -0400
Bill Fink <billfink@...dspring.com> wrote:

> On Sat, 09 May 2009, Eric Dumazet wrote:
> 
> > Octavian Purdila a écrit :
> > > On Saturday 09 May 2009 09:58:25 Eric Dumazet wrote:
> > > 
> > >>> I've looked over the  code and it looks right, so maybe net_random() is
> > >>> not random enough? Or maybe there are side effects because of the %
> > >>> port_range?
> > >> Random is random :)
> > >> Probability a port can be reused pretty fast is not nul.
> > >>
> > > 
> > > Thinking again about it... you are right :)
> > > 
> > >> So yes, behavior you discovered is expected, when we switched port
> > >> selection from a sequential one (not very secure btw) to a random one.
> > >>
> > >> Any strong reason why a firewall would drop a SYN because ports were used
> > >> in a previous session ?
> > > 
> > > We don't know why the firewall (Cisco FWSM) is dropping the packets, may be a 
> > > bug, limitation or miss-configuration. We are trying to track this down with 
> > > the firewall vendor.
> > 
> > Normally, the client machine should not reuse a port during the TIME_WAIT duration
> > (TCP_TIMEWAIT_LEN being 60 seconds on linux). Port selection being random or sequential,
> > it should avoid all ports recently used.
> > 
> > Maybe this firewall has a longer TIME_WAIT enforcement (something like 2 minutes)
> 
> But he had 19 ports being reused after only 1000 connect()s, which
> with his stated ~360 (I'm assuming per second) connection rate,
> would only take about 3 seconds.
> 
> 						-Bill

This the same thing as the Birthday paradox

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