Bind to a specific IP in C for ubuntu - c

Question

Hi I am trying to make a simple server that takes in an IP address from getaddrinfo() and binds to it. Using ifconfig, I've found that I have an ip address of wlan0 192.168.2.10 which I would like to bind to. Unfortunately the address I seem to be binding to is my lo device. For some reason when I initialize getaddrinfo("192.168.2.10","3490",&hings,&res); res gets returned to a NULL pointer. I will show off my code bellow.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <arpa/inet.h>
#include <signal.h>
#define MAXDATASIZE 500;
int main(int argc, char *argv[]){
// dealing with client socket
struct sockaddr_storage their_addr;
socklen_t addr_size;
// server socket
struct addrinfo serverSide,*serverInfo,*sortIP;
int optValRet;
int listenSock, newSock;
// this is for reading in information
char buf[501];
char point[INET6_ADDRSTRLEN];
char compare[INET6_ADDRSTRLEN] = "192.168.2.10";
// this is for handeling child processes and signals
struct sigaction sa;
sa.sa_handler = NULL;
sigemptyset(&sa.sa_mask);
sa.sa_flags = SA_RESTART;
if(sigaction(SIGCHLD, &sa, NULL) == -1){
printf("We have a problem, sigaction is not working.\n");
perror("\n");
exit(1);
}
// this sets up addrinfo
memset(&serverSide, 0, sizeof serverSide);
serverSide.ai_family = AF_UNSPEC;
serverSide.ai_socktype = SOCK_STREAM;
serverSide.ai_flags = INADDR_ANY;
// set up the address
if(getaddrinfo("192.168.2.10","3490",&serverSide,&serverInfo)!=0){
printf("get addr not success\n");
perror("\n");
return 1;
}
printf("Got address lists\n");
for(sortIP = serverInfo; sortIP = sortIP->ai_next; sortIP != NULL){
if((listenSock = socket(sortIP->ai_family, sortIP->ai_socktype, sortIP->ai_protocol))==-1){
continue;
}
if(setsockopt(listenSock,SOL_SOCKET,SO_REUSEADDR,&optValRet,sizeof(int))==-1){
perror("\n");
exit(1);
}
if(bind(listenSock,sortIP->ai_addr,sortIP->ai_addrlen) == -1 ){
perror("\n");
close(listenSock);
continue;
}
break;
}
if(sortIP == NULL){printf("sort ip is null.");}
inet_ntop(sortIP->ai_family,sortIP->ai_addr,point,sizeof point);
printf("Tell the clients connect to ip address %s on port 3490\n",point);
listen(listenSock, 10);
addr_size = sizeof their_addr;
newSock = accept(listenSock,(struct sockaddr *)&their_addr,&addr_size);
recv(newSock, buf, 500, 0);
printf("%s\n",buf);
close(listenSock);
close(newSock);
freeaddrinfo(serverInfo);
return 0;
}
Now I have some other questions beside the fact that I'm returning null. Since the wifi router has assigned me the ip address 192.168.2.10 for my subnet, how do I find out what my ip address is if I'm outside the network and trying to contact my server? I'm assuming the inside network ip is different from the outside network ip ... am I wrong? Anyways those are my two questions.
Thanks for any help!

Answer 1

This is wrong and is your immediate problem:
for (sortIP = serverInfo; sortIP = sortIP->ai_next; sortIP != NULL)
You want something like:
for (sortIP = serverInfo; sortIP != NULL; sortIP = sortIP->ai_next)
but I would go with a while loop personally.

Answer 2

To your main question, you should just bind to INADDR_ANY. That avoids that whole mess. Also:
recv(newSock, buf, 500, 0);
printf("%s\n",buf);
The %s format specifier is only for C-style strings, it's not for arbitrary binary data. Also, you throw away the return value from recv. There is no other way to know how many bytes you received.
As for finding your dynamic IP address from outside your network, use any of the dozens of IP posting services that assign you a host name and map it to your dynamic IP address.