netdev - Re: ports beeing reused too fast

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Message-Id: <20090509153135.04874f72.billfink@mindspring.com>
Date:	Sat, 9 May 2009 15:31:35 -0400
From:	Bill Fink <billfink@...dspring.com>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Octavian Purdila <opurdila@...acom.com>, netdev@...r.kernel.org
Subject: Re: ports beeing reused too fast

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
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