How to create a udp connection with defined source port?

[jcorporation]

Is it possible to define the source port of an outgoing udp connection? I can not find any hint in the documentation or tutorials.

I know that this is unusual, but I want to talk with an API endpoint that responds only to udp connections from a specific port, 7090 in this case. The api endpoint is from a Keba Wallbox.

[scaprile]

Not that unusual, it would be like a communication between servers. Open a listener on that port and use that connection to send.
You have to effectively bind to that port so no one else uses it, so in the case of UDP it makes total sense to open a listener there.

[jcorporation]

I have another question: How I send data to udp destination with a listening connection? I thought I can use mg_send, but there is no option for a destination. Must I set the struct mg_addr rem manually in the listening connection?

[scaprile]

Good point, looks like you start the conversation so you don't have the other end data already filled in for you... I have the socket version in my mind (sendto()), let me think a Mongoose-ish way without digging too deep, but basically that would be providing those parameters sendto() would use,

[jcorporation]

This works for me:

//set wallbox uri
mgr->conns->rem.port = mg_htons(7090);
mgr->conns->rem.is_ip6 = false;
mg_aton(mg_str("192.168.93.199"), (struct mg_addr *)mgr->conns->rem.ip);

[scaprile]

Yeah, a bit low-level but simple. You should do that using c-> in your event handler, catching the OPEN event. That code above works because you have only one connection, mgr->conns is actually a linked list...
Something like:

  if (ev == MG_EV_OPEN && c->is_listening) {
      mg_aton(mg_str("192.168.93.199"), &c->rem); // this one marks "not ip6"
      c->rem.port = mg_htons(7090);
  } else ...

[jcorporation]

Thanks for the hints!