netdev - Re: SO_REUSEADDR not allowing server and client to use same port

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Date:	Thu, 28 Feb 2008 21:36:17 +0100
From:	Willy Tarreau <w@....eu>
To:	Nebojsa Miljanovic <neb@...atel-lucent.com>
Cc:	linux-kernel@...r.kernel.org,
	"Kittlitz, Edward (Ned)" <nkittlitz@...atel-lucent.com>,
	asweeney@...atel-lucent.com,
	"Polhemus, William (Bart)" <bpolhemus@...atel-lucent.com>,
	netdev@...r.kernel.org
Subject: Re: SO_REUSEADDR not allowing server and client to use same port

[cc'd netdev]

On Thu, Feb 28, 2008 at 12:15:06PM -0600, Nebojsa Miljanovic wrote:
> Hello all,
> I have seen similar complaints about this issue before in the list archive, but
> I have not seen any resolution. So, I am posting the question again in hope of
> getting some replies.
> 
> Currently, Linux does not allow reuse of same local port for both TCP server and
> TCP client. I understand that there may be a need to prevent two servers from
> binding to the same local port, but having 1 server and 1 outgoing client use it
> should be allowed. Other Unix like operating systems do allow it.

I've been encountering the need for SO_REUSEPORT as implemented on some BSD
for instance. It allows any process to explicitly permit another one to bind
to the same IP:port provided that it also sets its socket with SO_REUSEPORT.
This is a real requirement when you need to restart a service without any
service disruption.

In the end, I've written a trivial patch for 2.4 (but most likely 2.6 would
use nearly the same one). I noticed that the conflict detection logic already
distinguished between 3 cases: sk->reuse==0, sk->reuse==1, sk->reuse >1.
sk->reuse was set to 1 with SO_REUSEADDR. I simply had to make SO_REUSEPORT
do sk->reuse |= 2 to get everything working as expected.

I don't know yet if it is as easy to do in 2.6, but if there is a possible
acceptance of including this useful feature into mainline, I can try to
work a patch (possibly cleaner than my existing one for 2.4).

Regards,
Willy

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