linux-kernel - Re: Strange connect behavior

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Message-ID: <4545E0D7.6010608@imag.fr>
Date:	Mon, 30 Oct 2006 12:24:07 +0100
From:	Videau Brice <brice.videau@...g.fr>
To:	Eric Dumazet <dada1@...mosbay.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: Strange connect behavior

It all makes sense now!

Thank you very much.

Regards,

Brice



Eric Dumazet wrote:
> On Monday 30 October 2006 11:24, Videau Brice wrote:
>   
>> Hello,
>>
>> While writing some client server application in c, we noticed a strange
>> behavior : if we try to connect endlessly to a given local port where
>> nobody is listening, and if the port is >= to 32768, after several
>> thousands tries ( Connection refused ) connect will return 0.
>> This behavior is not exhibited when port is < 32768.
>>
>>     
>
> Hello Brice
>
> Yes, it's quite possible your attempts are hitting themselves.
>
> Hint :
>
> cat /proc/sys/net/ipv4/ip_local_port_range
>
> When you connect(), TCP stack automatically chose a source port and bind your 
> outgoing socket. If the chosen port happens to be 35489 (your 'destination' 
> port), then the connect succeeds (you're connected to yourself), as specified 
> by TCP specs.
>
>
>   
>> We confirmed this behavior in kernel 2.6.17-10, 2.6.18-1, 2.6.8, on x86
>> and 2.4.21-32 on ia64, on several hardware configurations.
>> Distribution is debian or ubuntu.
>>
>> Attached is a source file that demonstrate this behavior.
>> ./a.out port_number
>>
>> Sample execution :
>>
>> ./a.out 35489
>> Out port : 35489
>> connect try 1 failed : Connection refused
>> connect try 2 failed : Connection refused
>> connect try 3 failed : Connection refused
>> .....
>> connect try 6089 failed : Connection refused
>> connect try 6090 failed : Connection refused
>> Connection success : 6091 try
>> Connection closed
>> last error : Connection refused
>>
>>
>> Is this behavior to be expected?
>> Can it be disabled?
>>
>> Thanks in advance.
>>
>> Regards,
>>
>> Brice Videau
>>     
> -
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