Recover port from Boost Asio udp::endpoint

Question

I am programming a server and client program to communicate between a windows PC using the Boost libraries and a Linux ARM beagleboard using the asio stand alone libraries. I have for a while had successful UDP communication between the two devices but now I want to recover the port from the endpoint the server discovers when the client connects. The way the client connects is via query:

udp::resolver resolver(io_service);
udp::resolver::query query_tx(udp::v4(), hostIP, "43210");
udp::endpoint receiver_endpoint_tx = *resolver.resolve(query_tx);

where host IP is a string and this works fine. Upon debugging though I notice that when i check the value returned by:

receiver_endpoint_tx.port()

This returns 51880. Now don't jump the guns and yell out network byte order and host byte order. I AM AWARE. The strange part is that this number 51880 sometimes is a different number and when i check what the server has stored in its endpoint it is a completely different number: 21743. Now I know I must be doing something wrong with the byte orders but i tried:

//unsigned long port_long       = boost::asio::detail::socket_ops::host_to_network_long(receiver_endpoint_tx.port());
//unsigned long port_short      = boost::asio::detail::socket_ops::host_to_network_short(receiver_endpoint_tx.port());

And they do not give me back my original port: 43210. Neither does network to host. So what am i missing and how can I on both ends recover my 43210 port? Obviously it must be there somewhere because they are successfully communicating. Thanks in advance, sorry if noob mistake :)

Ralf Ralf · Accepted Answer · 2013-03-05T07:28:25

Fistly, UDP is connectionless, there is no connection.

I'm not sure if I understand you correctly, but it sound too me like you want to bind to specific port numbers. If you want the client to send a packet from port x to port y on the server, and the server should respond from port y to port x, then you need to bind the sockets to the desired ports. Alternatively you can use the constructor to bind. Not doing so will result in the OS using ephemeral ports.

Further, to get the remote endpoint that a packet was received from the async_receive_from takes the sender_endpoint reference parameter. When the read handler is called, you can retrieve host and port from it.

Recover port from Boost Asio udp::endpoint

1 Answers