UPD client and UDP server - c

Question

How can I use different machines to send messages using UPD client and server? I understand that using these two on the same machine, I just need to compile them and messages go through. But with different machines how?
This code is for the UDP client.
/*
Simple udp client
Silver Moon (m00n.silv3r#gmail.com)
*/
#include<stdio.h> //printf
#include<string.h> //memset
#include<stdlib.h> //exit(0);
#include<arpa/inet.h>
#include<sys/socket.h>
#define SERVER "127.0.0.1"
#define BUFLEN 512 //Max length of buffer
#define PORT 8888 //The port on which to send data
void die(char *s)
{
perror(s);
exit(1);
}
int main(void)
{
struct sockaddr_in si_other;
int s, i, slen=sizeof(si_other);
char buf[BUFLEN];
char message[BUFLEN];
if ( (s=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1)
{
die("socket");
}
memset((char *) &si_other, 0, sizeof(si_other));
si_other.sin_family = AF_INET;
si_other.sin_port = htons(PORT);
if (inet_aton(SERVER , &si_other.sin_addr) == 0)
{
fprintf(stderr, "inet_aton() failed\n");
exit(1);
}
while(1)
{
printf("Enter message : ");
gets(message);
//send the message
if (sendto(s, message, strlen(message) , 0 , (struct sockaddr *) &si_other, slen)==-1)
{
die("sendto()");
}
//receive a reply and print it
//clear the buffer by filling null, it might have previously received data
memset(buf,'\0', BUFLEN);
//try to receive some data, this is a blocking call
if (recvfrom(s, buf, BUFLEN, 0, (struct sockaddr *) &si_other, &slen) == -1)
{
die("recvfrom()");
}
puts(buf);
}
close(s);
return 0;
}
This code is for the UDP server.
/* Simple udp server
Silver Moon (m00n.silv3r#gmail.com) */
#include<stdio.h> //printf
#include<string.h> //memset
#include<stdlib.h> //exit(0);
#include<arpa/inet.h>
#include<sys/socket.h>
#define BUFLEN 512 //Max length of buffer
#define PORT 8888 //The port on which to listen for incoming data
void die(char *s)
{
perror(s);
exit(1);
}
int main(void)
{
struct sockaddr_in si_me, si_other;
int s, i, slen = sizeof(si_other) , recv_len;
char buf[BUFLEN];
//create a UDP socket
if ((s=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1)
{
die("socket");
}
// zero out the structure
memset((char *) &si_me, 0, sizeof(si_me));
si_me.sin_family = AF_INET;
si_me.sin_port = htons(PORT);
si_me.sin_addr.s_addr = htonl(INADDR_ANY);
//bind socket to port
if( bind(s , (struct sockaddr*)&si_me, sizeof(si_me) ) == -1)
{
die("bind");
}
//keep listening for data
while(1)
{
printf("Waiting for data...");
fflush(stdout);
//try to receive some data, this is a blocking call
if ((recv_len = recvfrom(s, buf, BUFLEN, 0, (struct sockaddr *) &si_other, &slen)) == -1)
{
die("recvfrom()");
}
//print details of the client/peer and the data received
printf("Received packet from %s:%d\n", inet_ntoa(si_other.sin_addr),
ntohs(si_other.sin_port));
printf("Data: %s\n" , buf);
//now reply the client with the same data
if (sendto(s, buf, recv_len, 0, (struct sockaddr*) &si_other, slen) == -1)
{
die("sendto()");
}
}
close(s);
return 0;
}

Answer 1

127.0.0.1 is a local IP address (assigned to loopback interface see below IP for lo0 interface). Its good for communicating across difference process acting in client server roles on the same host/system.
What you need is the assigned IP of the primary interface (like eth0/ en0 etc.) on your server system. On a MAC this looks as below
#ifconfig -au
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
..
inet 127.0.0.1 netmask 0xff000000
nd6 options=1<PERFORMNUD>
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
..
inet 192.168.1.2 netmask 0xffffff00 broadcast 192.168.1.255
nd6 options=1<PERFORMNUD>
media: autoselect
status: active
In the above output I'd use the IP address on en0 which is 192.168.1.2 (for server to bind) and then the clients can connect to that IP over the network.