C UDP client and server on remote hosts - c

Question

I wrote a simple C UDP client and server, where the client sends a character to the server. Everything works fine on the same pc. So I wanted to send a character over to a remote pc. Unfortunately, it's not working: the server is not receiving anything but using wireshark I can see that the packet is going out to the correct destination.
Here is the code:
(servip contains the server's address)
//client
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <string.h>
#include <arpa/inet.h>
#include <unistd.h>
#define SERV_PORT 50000
int main(){
int id, s;
char buf;
char servip[] = "...";
id = socket(AF_INET,SOCK_DGRAM,0);
buf = 'a';
struct sockaddr_in servaddr;
memset((void *)&servaddr, 0, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_port = htons(SERV_PORT);
inet_pton(AF_INET,servip, &servaddr.sin_addr); //controlla se minore di 0
s = sendto(id,&buf,sizeof(char),0,(struct sockaddr *) &servaddr,sizeof(servaddr));
if (s == 0)
fprintf(stderr,"errore in sendto");
return EXIT_SUCCESS;
}
//server
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>
#define SERV_PORT 50000
int main(){
int id, s;
unsigned int len;
char buf;
id = socket(AF_INET,SOCK_DGRAM, 0);
struct sockaddr_in addr;
memset((void *)&addr, 0, sizeof(addr));
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = htonl(INADDR_ANY); /* il server accetta pacchetti su
una qualunque delle sue interfacce di rete */
addr.sin_port = htons(SERV_PORT); /* numero di porta del server */
len = sizeof(addr);
bind(id, (struct sockaddr *) &addr, sizeof(addr));
s = recvfrom(id,&buf,sizeof(char),0,(struct sockaddr *) &addr,&len);
printf("%c\n",buf);
if (s==0)
fprintf(stderr,"errore in recvfrom");
return EXIT_SUCCESS;
}
Can anybody help me? What am I doing wrong? Can you please link me an example of a UDP server and client which actually work on remote hosts?
Thanks in advance.

Answer 1

First, you should test every state of "bind","recvfrom" and "sendto", they both return "-1" if an error occured. Both side , client and server.
Also you should check state of variable global variable errno to know the error that occured.
Check your firewall too. And use if you are not using client and server on the same host, try wireshark on the client and on the server.
Add header errno to check errno.
#include <errno.h>
.
.
.
if (sendto(id,&buf,sizeof(char),0,(struct sockaddr *) &servaddr,sizeof(servaddr))== -1) {
int errsv = errno;
printf("sendto() failed\n");
if (errsv == ...) { ... }
}
.

Answer 2

I suspect you will want to bind to a socket to set the source address and port:
struct sockaddr_in myaddr;
memset((char *)&myaddr, 0, sizeof(myaddr));
myaddr.sin_family = AF_INET;
myaddr.sin_addr.s_addr = htonl(INADDR_ANY);
myaddr.sin_port = htons(0);
if (bind(fd, (struct sockaddr *)&myaddr, sizeof(myaddr)) < 0) {
perror("bind failed");
exit (1);
}
If wireshark is showing the packet was transmitted, can you confirm it also shows the packet being received at the server? This should help eliminate firewall and routing problems.