Get list of devices on LAN in C

Question

I'd like to get a list of available devices (their IP address or hostname should be fine) connected to my local network. I've got some examples in C#, but I couldn't find any sample using C/C++. I'm familiar with the BSD socket API, just to clarify.

I have a "stub" kind of idea: maybe I should determine the range of the IP addresses that the devices on my LAN can potentially belong to, and then determine if they e. g. respond to PING, or something like that. How could I achieve these?

I want my application to run on Linux and iOS. That's why I'd prefer BSD sockets, or at a maximum, Foundation/GNUstep (although they are written in Objective-C).

Answer 1

#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <netinet/ip_icmp.h>
#include <time.h>
#include <fcntl.h>
#include <signal.h>
#include <time.h>

// Define the Packet Constants
// ping packet size
#define PING_PKT_S 64

// Automatic port number
#define PORT_NO 0

// Automatic port number
#define PING_SLEEP_RATE 1000000

// Gives the timeout delay for receiving packets
// in seconds
#define RECV_TIMEOUT 1
 
// Performs a DNS lookup
char *dns_lookup(char *addr_host, struct sockaddr_in *addr_con)
{
    //printf("\nResolving DNS..\n");
    struct hostent *host_entity;
    char *ip=(char*)malloc(NI_MAXHOST*sizeof(char));
    int i;

    if ((host_entity = gethostbyname(addr_host)) == NULL)
    {
        // No ip found for hostname
        return NULL;
    }
    
    //filling up address structure
    strcpy(ip, inet_ntoa(*(struct in_addr *)
                        host_entity->h_addr));

    (*addr_con).sin_family = host_entity->h_addrtype;
    (*addr_con).sin_port = htons (PORT_NO);
    (*addr_con).sin_addr.s_addr = *(long*)host_entity->h_addr;

    return ip;
    
}

// Resolves the reverse lookup of the hostname
char* reverse_dns_lookup(char *ip_addr)
{
    struct sockaddr_in temp_addr;
    socklen_t len;
    char buf[NI_MAXHOST], *ret_buf;

    temp_addr.sin_family = AF_INET;
    temp_addr.sin_addr.s_addr = inet_addr(ip_addr);
    len = sizeof(struct sockaddr_in);

    if (getnameinfo((struct sockaddr *) &temp_addr, len, buf,
                    sizeof(buf), NULL, 0, NI_NAMEREQD))
    {
        //printf("Could not resolve reverse lookup of hostname\n");
        return NULL;
    }
    ret_buf = (char*)malloc((strlen(buf) +1)*sizeof(char) );
    strcpy(ret_buf, buf);
    return ret_buf;
}



// Driver Code
int main(int argc, char *argv[])
{
    int sockfd;
    char *ip_addr, *reverse_hostname;
    struct sockaddr_in addr_con;
    int addrlen = sizeof(addr_con);
    char net_buf[NI_MAXHOST];

 
 
        int i=0;
        for(int i=1;i<255;++i)
       {

        char ip[80];
        sprintf(ip, "192.168.2.%d", i);
        
        
        ip_addr = dns_lookup(ip, &addr_con);
    if(ip_addr==NULL)
    {
        //printf("\nDNS lookup failed! Could not resolve hostname!\n");
         continue;
    }

    reverse_hostname = reverse_dns_lookup(ip_addr);
    
    if(reverse_hostname==NULL)
    {
        //printf("\nDNS lookup failed! Could not resolve hostname!\n");
         continue;
    }
    //printf("\nTrying to connect to '%s' IP: %s\n",ip, ip_addr);
    printf("\nReverse Lookup domain: %s",
                        reverse_hostname);
                        
                        printf("\n %s \n", ip);
      
     }


 
    
    return 0;
}

Answer 2

You can also e.g. send an ICMP echo request packet to 224.0.0.1. This is a special all-nodes multicast address every node should respond to (except if a firewall rule or network policy setting prevents it).