Can multiple apps listen for TCP on the same port?

[Karl Denninger]

The second bind() call does fail but if the application ignores the return code...‎. Are you sure all the associated system call return codes are being checked?

The right way to do this Imho  is to have a parent process that calls bind and listen, gets the notification of an incoming connection via select() (allowing detection of exceptions as well) and then calls accept() and, now having a connected file handle, fork()s and executes whatever is to handle the connection with the parent closing the handle so as to not orphan the handle when the child exits.
‎
-- Karl
(On Passport PDA)‎


  Original Message  
From: Yuri‎
Sent: Monday, December 1, 2014 10:26
To: Daniel Corbe
Cc: freebsd-net at freebsd.org
Subject: Re: Can multiple apps listen for TCP on the same port?

On 12/01/2014 07:02, Daniel Corbe wrote:
> Generally the answer to your question is no. Two applications cannot
> occupy the same port on the same protocol at the same time.
>
> To expand on this answer and to hopefully shed some light on why the
> behavior you're observing with your application is absolutely correct;
> the calling application (in this case, nc) has to explicitly call bind(2)
> before it can begin accepting connections. If that port is already in
> use then the call to bind(2) will fail. And in your case I suspect nc
> is simply choosing to silently fail.

Here the question is what does it mean "occupy the port"? The first 
instance isn't listening any more. The listening socket was closed. Why 
the presence of the socket that was accepted from (now closed) listening 
socket in the first instance is considered "occupying it"?

Actually no system call in the second instance ever fails.

Yuri
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