UDP client source port in C?

Question

I am writing a UDP client, and I need to specify the source port of my UDP packet in my data to send.

• How can my program get the random port number generated by kernal that the udp client uses to send data to the voip server. so
• How can I specify a specific UDP source port before sending data.

I will be very grateful. Please reply to me as soon as possible. At the moment, my project is stopped.

Answer 1

Use bind to bind your socket to port 0, which allows you to use getsockname to get the port. You can also bind your socket to a specific port if you want.

for example (assuming the IPv4 socket, error checking):

 struct sockaddr_in sin = {}; socklen_t slen; int sock; short unsigned int port; sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP); sin.sin_family = AF_INET; sin.sin_addr.s_addr = htonl(INADDR_ANY); sin.sin_port = 0; bind(sock, (struct sockaddr *)&sin, sizeof(sin)); /* Now bound, get the address */ slen = sizeof(sin); getsockname(sock, (struct sockaddr *)&sin, &slen); port = ntohs(sin.sin_port); 

Alternatively, if you are communicating with the same server, you can use connect in your UDP socket (which also gives you a convenient side effect that allows you to use send instead of sendto and the UDP socket only accepts datagrams from your “connected” peer), then use getsockname to get the local port / address. You can still bind your socket before using connect .

eg:

 struct sockaddr_in sin = {}; socklen_t slen; int sock; short unsigned int port; sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP); /* set sin to your target host... */ ... connect(sock, (struct sockaddr *)&sin, sizeof(sin)); /* now retrieve the address as before */ slen = sizeof(sin); getsockname(sock, (struct sockaddr *)&sin, &slen); port = ntohs(sin.sin_port); 

Answer 2

You must bind(2) your socket to the port of your choice. See also man 7 ip and man 7 udp .