netdev - Re: ports beeing reused too fast

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Message-Id: <200905091611.20321.opurdila@ixiacom.com>
Date:	Sat, 9 May 2009 16:11:19 +0300
From:	Octavian Purdila <opurdila@...acom.com>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	netdev@...r.kernel.org
Subject: Re: ports beeing reused too fast

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.

> Previous mode can be restored by application itself, using a bind() before
> connect(), if this application knows it has a very high rate of connections
> from a particular host to a particular host. (source ports range being
> small anyway, so your firewall could complain again)

Do you mean bind() with port != 0 ? Because I am already using bind() before 
connect().

Thanks,
tavi

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