UDP recvfrom call returns wrong port number. Why?

Question

Im playing with 'recvfrom' and 'sendto' calls using UDP. I have a client that just broadcast UDP packets on port 6000. I have a server that binds a socket on port 6000, and do a single recvfrom.

The problem is that sin_port member of struct sockaddr returned from recvfrom is always incorrect. Why?

I would post some source code but someone already posted that question (with no answers) and I'm using almost the same code as he. Besides, you can get further information about this problem reading his post: FORUM POST.

Thanks in advance

EDIT: I really think 'cause number 2' from 'nos' answer might be the problem. How can I check it?

Answer 1

Are you converting from network byte order?

Answer 2

Here's 2 likely causes:

1. You're not converting the sin_port to host order before you inspect/display it. sin_port comes in network endian. That is you might be displaying the port as big endian on a little endian machine.

2. Your client uses a random source port. So while your destination port is 6000, the source port of the client is randomly chosen. recvfrom gives you the source IP address and the source port no. Not the destination port.

If it's neither of these, please provide some relevant test code, and the actual values you are seeing. It's possible you e.g. have some buffer overflows, or something other fishy going on.

EDIT, looking at the code in your link, you hardcode the buffer size(udpPkg.udpRecvBuf) as 1024. Is the buffer really atleast 1024 big ? If not, you're probably overflowing udpPkg.udpRecvBuf.