Re: [lwip-users] Problem re-opening listening pbc

[Keith Rubow]

My problem with limiting the number of connections on a port has taken a 
turn for the worse. I am trying to implement modbus over TCP on port 
502. I must allow up to 3 concurrent connections to port 502, but no 
more. My trick of closing the listening pcb when I have reached my limit 
of 3 connections does not work. The problem is that once I have accepted 
the 3rd connection I can close the listening pcb. This causes subsequent 
connection attempt to be refused (as desired). However, once any one of 
the 3 connections is closed I need to re-open the listening pbc so 
another connection can be accepted. But I cannot bind to port 502 if ANY 
previous connection is still open. Waiting for all connections to close 
before re-opening the listing pbc is not an option. A connection could 
be up for days.

I could call tcp_abort() in my accept callback once the connection limit 
has been exceeded, but this does not cause the connection to be 
rejected. The client seed the connection accepted, and then immediately 
closed. I would prefer to see new connection attempts rejected once I 
have reached my connection limit.

Is there any way to achieve a limit on the number of connections allowed 
on a given port (with further connection attempts rejected)? As stated 
earlier, NO_SYS=1, raw callback API.

Also, why does lwip not allow me to open a listening pcb on a port if 
there are current open connections to that port (even though there is no 
current LISTENING pbc on the port)?

Any help is appreciated.

Keith Rubow

On 5/9/2018 1:11 PM, Keith Rubow wrote:
> I'm using LWIP for the first time (2.0.3, NO_SYS=1, callback API). 
> Mostly things are going well. But I have an issue with allowing only a 
> single TCP connection to a port. I open a listening pcb with 
> tcp_new(), tcp_bind(), tcp_listen() and set an accept callback. In the 
> accept callback I set up a receive callback, and I use tcp_close() to 
> close the listening pcb. This causes any further connection attempts 
> on the port to be refused. This works great.
>
> However, the problem is when the connection I accepted is closed. My 
> receive callback is called with p=NULL, so I then remove all the 
> callbacks and close the active connection with tcp_close(). Then I 
> re-open the listening pcb with tcp_new(), tcp_bind(), and 
> tcp_listen(). HOWEVER, the tcp_bind() fails with ERR_USE, presumably 
> because the pcb for the active connection is not fully closed yet, 
> making the port still in use.
>
> I found I can "fix" the problem by closing the active connection with 
> tcp_abort() instead of tcp_close(), but this seems kind of wrong. If I 
> knew when the pcb for the active connection was really fully closed, I 
> could open the listening pcb then. But I don't see any way to tell 
> when the close is finished. There is no callback for it, and I am not 
> permitted to access the pcb after calling tcp_close().
>
> So how can I re-open a listening pcb on the port after a connection is 
> closed?
>
> Keith Rubow
>
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