netdev - Re: ports beeing reused too fast

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Message-ID: <4A059E75.7060008@cosmosbay.com>
Date:	Sat, 09 May 2009 17:17:09 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Octavian Purdila <opurdila@...acom.com>
CC:	netdev@...r.kernel.org
Subject: Re: ports beeing reused too fast

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)

> 
>> 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().
> 

Yes, but its obviously complex to handle at application side...


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