netdev - Re: bind()/inet_csk_get_port() fails when no port is requested

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Message-ID: <1326908219.4910.14.camel@edumazet-HP-Compaq-6005-Pro-SFF-PC>
Date:	Wed, 18 Jan 2012 18:36:59 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Flavio Leitner <fbl@...hat.com>
Cc:	netdev <netdev@...r.kernel.org>,
	Marcelo Leitner <mleitner@...hat.com>
Subject: Re: bind()/inet_csk_get_port() fails when no port is requested

Le mercredi 18 janvier 2012 à 15:11 -0200, Flavio Leitner a écrit :
> Hi folks,
> 
> It has been reported to me that bind() fails when you leave
> the port up to the kernel to choose and succeed when you
> request a certain port in the same conditions.
> 
> For example, let's restrict the ephemeral port range to 3 ports only:
> # echo "32768 32770" > /proc/sys/net/ipv4/ip_local_port_range
> 
> Assuming the system has two IP addresses: 172.31.1.6/24 and
> 192.168.100.6/24 then run the following python script which
> allocates all ephemeral ports using one IP address and then
> try to bind another one using another IP address.
> 
...
> Conclusion: When using ephemeral ports, inet_csk_get_port()
> fails without checking if a conflict had happened. When using
> fixed ports on the other hand, inet_csk_get_port() works
> as expected.
> 
> I will attach a quick hack to illustrate what I am thinking.
> The idea is to check all ports first and if it fails, then
> try again looking for a port that doesn't conflict. So, for
> most cases, the algorithm is the same, but when the system
> ran out of ports, there is a hope :-)
> 
> Is there a reason to behave like that? or is this a real bug?
> Sounds like a FAQ, but I am not finding an explanation for this
> on the net yet.
> 

Hi Flavio

This seems a very good idea.

Only drawback is when table is really full, we'll scan it twice.



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