netdev - Re: binding UDP port 0 with SO

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Date:	Thu, 02 Aug 2012 18:57:25 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Tobias S. Josefowitz" <t.josefowitz@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: binding UDP port 0 with SO_REUSEADDR

On Thu, 2012-08-02 at 18:33 +0200, Tobias S. Josefowitz wrote:
> Hi again,
> 
> I need to say, after looking at "my" socket(7),
> 
>        SO_REUSEADDR
>               Indicates  that  the rules used in validating addresses supplied
>               in a bind(2) call should allow reuse of  local  addresses.   For
>               AF_INET  sockets  this means that a socket may bind, except when
>               there is an active listening socket bound to the address.   When
>               the listening socket is bound to INADDR_ANY with a specific port
>               then it is not possible to bind  to  this  port  for  any  local
>               address.  Argument is an integer boolean flag.
> 
> I think the surprise-factor of the bind-0-behaviour even increased for
> me. Is there a specific reason for handing out used ports when binding
> port 0 with REUSEADDR?

There is no concept of listening sockets for UDP.

This documentation applies for TCP, and makes no sense for UDP.

There is no value using 'port 0' and REUSEADDR on UDP, really.



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