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

[Willy Tarreau <w@1wt.eu>]

On Mon, Mar 17, 2008 at 01:17:29PM -0500, Nebojsa Miljanovic wrote:
> OK. I see. So, it would have to be some malicious application running together
> with the server (i.e. on the same CPU). I do see now why you said it would be
> very very hard to make this happen.
> 
> Still, it would be nice to introduce SO_REUSEPORT socket options so secure
> servers (who happen to be clients as well) can re-use ports when necessary.
> 
> Another option would be to check if port re-use is happening inside same
> application and allow it. That may make half of the folks happy, so I am not
> sure if I like it as much as I like SO_REUSEPORT option.

It is most often useful for hot-restart without interrupting service.
As an example, when I update a config in haproxy, I restart a new instance
which does everything and binds to the ports, and then signals the old one
it may go. That way, there is a complete continuity of service. This is
possible by default on *BSD, and with a (very) little patch on linux.

And I agree with Alan about the security implications. If a service binds
itself with SO_REUSEPORT, it definitely says "I agree to share my connection".
So this is not to be used by default in any random application. But for
servers, it's really useful.

Willy