Getting netmask from a network interface

Simple sample of getting netmask information

#include <stdio.h>

#include <string.h> /* for strncpy */

#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <netinet/in.h>
#include <net/if.h>

int
main()
{
 int fd;
 struct ifreq ifr;

 fd = socket(AF_INET, SOCK_DGRAM, 0);

 /* I want an IPv4 netmask */
 ifr.ifr_addr.sa_family = AF_INET;

 /* I want netmask attached to "eth0" */
 strncpy(ifr.ifr_name, "eth0", IFNAMSIZ-1);

 ioctl(fd, SIOCGIFNETMASK, &ifr);

 close(fd);

 /* display result */
 printf("%s\n", inet_ntoa(((struct sockaddr_in *)&ifr.ifr_addr)->sin_addr));

 return 0;
}

On Linux platform, you can use ioctl to obtain the netmask information. To use ioctl, you must create a socket. The important thing is that the socket family used is AF_INET. You can use either TCP(SOCK_STREAM) or UDP(SOCK_DGRAM) socket. This sample uses an UDP socket.

The socket used to get the netmask information can be closed after obtaining the netmask information. You can use it as a normal socket to communicate too.