Can multiple apps listen for TCP on the same port?

[Adrian Chadd]

Hi,

I introduced a socket option in -HEAD that lets you bind multiple
things to the same listen ports.

They're only load balanced if you're using RSS and set up RSS socket
options as well; otherwise only one gets the incoming requests.

IP_BINDMULTI and IP6_BINDMULTI.


-a


On 1 December 2014 at 08:14, Someone Somewhere
<somewheresomeoneis at gmail.com> wrote:
> @Yuri , are you sure that the second instance of nc does not accept any
> connection?
> I did a simple test : ->
> #: nc -l 12345 (shell 1)
> #: nc localhost 12345 (shell2)
> at this point netstat shows that there is no one listening on 12345. This
> means any process should not be able to bind over port 12345(over TCP).
> # nc -l 12345 (shell 3, shell 1 , 2 still active)
> this instance of nc starts listening which I could verify via netstat cmd.
> # nc localhost 12345 (shell 4)
> this nc instance connected to the nc started in previous step over shell 3.
>
> Test ran on Fedora 20.
> [will try this on freeBSD VM if you confirm that this is what you are
> trying]
>
>
> Could you verify if your second nc(server) instance is listening on the
> same socket number?
>
>
> -Kunal.
>
>
>
> On 1 December 2014 at 21:07, Karl Denninger <karl at denninger.net> wrote:
>
>> 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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>>
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