[lwip-users] Sharing TCP port

[Danish Ali]

I would like to be able to have multiple simultaneous TCP links to a 
single port (in my case 5060 for SIP).

I have in lwipopts.h

> #define SO_REUSE                        1

I can receive incoming links fairly easily by way of:
>     err_t tcp_err = ERR_CLSD;
>     siptcp_accept = netconn_new_with_callback(NETCONN_TCP, sipconn_callback);
>     if (siptcp_accept != nullptr) {
>         ip_set_option(siptcp_accept->pcb.tcp, SOF_REUSEADDR);
>         tcp_err = netconn_bind(siptcp_accept, IP_ADDR_ANY, 5060);
>     }
>     if (tcp_err == ERR_OK) {
>         logString("Listening for SIP TCP on port 5060\n");
>         netconn_listen(siptcp_accept);

And then in my event loop

>         if (siptcp_accept->acceptmbox->n > 0) {
>             netconn *accept = nullptr;
>             err_t accept_err = netconn_accept(siptcp_accept, &accept);
>             if (accept_err == ERR_OK) {

But I am not sure how to make _outgoing_ TCP links such that they will 
share that same port number.

I start with

>     netconn *create = netconn_new(NETCONN_TCP);

how do I tell lwip that I want it to be on 5060 on my local port before 
I tell it the remote address I want to connect to?

If I try

>     err_t tcp_err = netconn_bind(create, &remote.IPAddress, 5060);

I get the error ERR_USE = -8 /** Address in use.          */

And if I skip that and immediately call

>     tcp_err = netconn_connect(create, &remote.IPAddress, remote.port);

it opens with a random port. I want it to come from 5060 on my machine 
because other ports might be blocked by a firewall.

Is this possible in lwip? If so, how might I do it?

Many thanks,
Danish