binding by different process on the same port

[Willem Jan Withagen]

On 18-12-2016 20:21, Willem Jan Withagen wrote:
> On 18-12-2016 16:50, Jan Knepper wrote:
>> I would guess 
>>
>>> wjw      ceph-osd-0   93336 43 tcp4   127.0.0.1:6804        *:*
>>
>> was started first. Claiming a specific port on a specific IP address...
> 
> Well I've been trying to recreate the situation with a fake server
> programs, one binds on localhost:port and the other on 0.0.0.0:port. And
> the order does not matter, but I have not (yet) been able to actually
> register on a port for the second time.
> 
> So it needs a bit more than just:
>     socket, bind, listen, accept

'mmmm,

There is SO_REUSEPORT, which explains the exact behaviour of what is
going on:
 SO_REUSEPORT allows completely duplicate bindings by multiple processes
 if they all set SO_REUSEPORT before binding the port.  This option
 permits multiple instances of a program to each receive UDP/IP  i
 multicast or broadcast datagrams destined for the bound port.

But the flag is not explicitly set in the code.

But there is a LOT of info on the net about Linux <> FreeBSD on
SO_REUSE{ADRR,PORT}.
So i guess that I have enough things to read over X-mas.

--WjW