Why UDP socket doesn't receive udp traffic from nc -u host port?

Question

 int main(int argc, char *argv[])
 {
     struct sockaddr_in src = { .sin_family=AF_INET, .sin_addr.s_addr=INADDR_ANY, .sin_port=htons(90) };

     int fd = socket(AF_INET, SOCK_DGRAM, IPPROTO_IP);

     bind(fd, (struct sockaddr *)&src, sizeof(src));

     char buf[1024];
     ssize_t res = recvfrom(fd, buf, sizeof(buf), 0, NULL, 0);
     printf("res=%zi\n", res);

     return 0;
 }

After compiling and execute this program. On another terminal I execute

nc -u localhost 90

to test if I'm actually receiving some udp traffic from "any interface" as I would expect since I've used INADDR_ANY. But the programs just hangs. What am I missing?

Answer 1

port 90 is less than 1024 so a process requires privileges in order to bind a socket to it.

You have two options:

1. choose a port above 1024
2. run your program with privileges (sudo...)

One should consider testing the result of bind() (as any other system call in general) to easily detect such situations.

Answer 2

I wasn't providing enough data through nc . I was facing two situation:

nc -u localhost 1025

The program was hanging....obviously because I didn't pass nothing except from '\\n' which wasn't received by the socket.

nc -u -v localhost 1025

I was still passing just '\\n' but in this case the socket was receiving it.

Writing a bunch of characters worked in both cases.