Undertstanding UDP broadcast sending and receiving in C (system functions) - c

Question

I'm trying to learn how sockets and networks work. For example in C in Linux. I have two simple programs. Both take ip and port as parameters. The first program is a server that broadcasts a message every second.
int main(int argc, char** argv){
struct sockaddr_in server;
if (argc < 3){ printf("\nToo few arguments. You need to pass ipv4 and port number!\n"); return 1; }
int port = -1;
if (isNumber(argv[2])) port = atoi(argv[2]);
if (port < 0){
printf("Error value for port!\n");
return 1;
}
/** prepare server */
server.sin_port = htons(port);
server.sin_family = AF_INET;
inet_aton(argv[1], &server.sin_addr);
int serverSocket = socket(AF_INET, SOCK_DGRAM, 0);
int broadcast=1;
setsockopt(serverSocket, SOL_SOCKET, SO_BROADCAST,
&broadcast, sizeof broadcast);
bind(serverSocket, (struct sockaddr*) &server, sizeof(server));
/** Client to send */
struct sockaddr_in client;
client.sin_port = htons(port);
client.sin_family = AF_INET;
client.sin_addr.s_addr = INADDR_BROADCAST;
char buf[128] = "Test from server";
while(1){
sleep(1);
sendto(serverSocket, buf, 18, 0, (struct sockaddr*) &client, sizeof(client));
}
close(serverSocket);
return 0;
}
The second program is a simple client that receives a message, prints it out, and outputs the sender's ip.
int main(int argc, char** argv){
struct sockaddr_in currUser;
if (argc < 3){ printf("\nToo few arguments. You need to pass ipv4 and port number!\n"); return 1; }
int port = -1;
if (isNumber(argv[2])) port = atoi(argv[2]);
if (port < 0){
printf("Error value for port!\n");
return 1;
}
currUser.sin_port = htons(port);
currUser.sin_family = AF_INET;
inet_aton(argv[1], &currUser.sin_addr);
int userSocket = socket(AF_INET, SOCK_DGRAM, 0);
bind(userSocket, (struct sockaddr*) &currUser, sizeof(currUser));
/** listen */
char buf[128];
struct sockaddr_in brd;
unsigned slen = sizeof(brd);
while(1){
int readCount = recvfrom(userSocket, buf, 128, 0, (struct sockaddr *)&brd, &slen);
buf[readCount] = 0;
printf("Get %s\n", buf);
printf("Ip who send: %s\n", inet_ntoa(brd.sin_addr));
}
close(userSocket);
return 0;
}
I expect that recvfrom get the source address of the message in brd . If I send directly to the client from the server (i.e. not broadcast, but in the client parameter for sendto specifying directly ip ), it works. However, for the broadcast case, the address is different. Here is an example:
The output for $route is as follows:
Destination Gateway Genmask Flags Metric Ref Use Iface
default _gateway 0.0.0.0 UG 600 0 0 wlp3s0
link-local 0.0.0.0 255.255.0.0 U 1000 0 0 br-29e12aa6bbb8
172.17.0.0 0.0.0.0 255.255.0.0 U 0 0 0 docker0
172.18.0.0 0.0.0.0 255.255.0.0 U 0 0 0 br-29e12aa6bbb8
192.168.0.0 0.0.0.0 255.255.255.0 U 600 0 0 wlp3s0
Tell me please, I don't understand something in the operation of the recvfrom function or how the packet is transmitted over the network within these two programs?