Obtaining SubnetMask in C - c

Question

I wanted to get the IP address and the subnet mask. Now the IP part is done, however I couldn't find any socket function that would return a structure with the subnet mask in it.
Does a socket function exist, that returns it in a structure?
Thanks!

Answer 1

In Unix using getifaddrs
struct ifaddrs haves a member named ifa_netmask (Netmask of interface)
#include <arpa/inet.h>
#include <sys/socket.h>
#include <ifaddrs.h>
#include <stdio.h>
int main ()
{
struct ifaddrs *ifap, *ifa;
struct sockaddr_in *sa;
char *addr;
getifaddrs (&ifap);
for (ifa = ifap; ifa; ifa = ifa->ifa_next) {
if (ifa->ifa_addr->sa_family==AF_INET) {
sa = (struct sockaddr_in *) ifa->ifa_netmask;
addr = inet_ntoa(sa->sin_addr);
printf("Interface: %s\tAddress: %s\n", ifa->ifa_name, addr);
}
}
freeifaddrs(ifap);
return 0;
}
Output
Interface: lo Address: 255.0.0.0
Interface: eth0 Address: 255.255.255.0

Answer 2

In windows using IPHelper.
#include <winsock2.h>
#include <iphlpapi.h>
#include <stdio.h>
#include <stdlib.h>
#pragma comment(lib, "IPHLPAPI.lib")
#define MALLOC(x) HeapAlloc(GetProcessHeap(), 0, (x))
#define FREE(x) HeapFree(GetProcessHeap(), 0, (x))
/* Note: could also use malloc() and free() */
int __cdecl main()
{
PIP_ADAPTER_INFO pAdapterInfo;
ULONG ulOutBufLen = sizeof (IP_ADAPTER_INFO);
pAdapterInfo = (IP_ADAPTER_INFO *) MALLOC(sizeof (IP_ADAPTER_INFO));
GetAdaptersInfo(pAdapterInfo, &ulOutBufLen);
printf("\tIP Mask: \t%s\n", pAdapterInfo->IpAddressList.IpMask.String);
}
if (pAdapterInfo)
FREE(pAdapterInfo);
return 0;
}

Answer 3

Borrowed code from Linux Man page and referred to the code from Keine Lust:
#define _GNU_SOURCE /* To get defns of NI_MAXSERV and NI_MAXHOST */
#include <arpa/inet.h>
#include <sys/socket.h>
#include <netdb.h>
#include <ifaddrs.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <linux/if_link.h>
#include <string.h> /* strcasecmp() */
int get_addr_and_netmask_using_ifaddrs(const char* ifa_name,
char *addr, char *netmask)
{
struct ifaddrs *ifap, *ifa;
struct sockaddr_in *sa;
char *s;
int found = 0;
if (getifaddrs(&ifap) == -1) {
perror("getifaddrs");
exit(EXIT_FAILURE);
}
for (ifa = ifap; ifa && !found; ifa = ifa->ifa_next) {
if (ifa->ifa_addr == NULL)
continue;
if (strcasecmp(ifa_name, ifa->ifa_name))
continue;
/* IPv4 */
if (ifa->ifa_addr->sa_family != AF_INET)
continue;
sa = (struct sockaddr_in *) ifa->ifa_addr;
s = inet_ntoa(sa->sin_addr);
strcpy(addr, s);
sa = (struct sockaddr_in *) ifa->ifa_netmask;
s = inet_ntoa(sa->sin_addr);
strcpy(netmask, s);
found = 1;
}
freeifaddrs(ifap);
if (found)
return EXIT_SUCCESS;
return EXIT_FAILURE;
}
int main(void)
{
char *addr = malloc(NI_MAXHOST);
char *netmask = malloc(NI_MAXHOST);
if (!get_addr_and_netmask_using_ifaddrs ("enp6s0", addr, netmask))
printf("[%s]%s %s\n", __func__, addr, netmask);
else
printf("interface error.\n");
free(addr);
free(netmask);
return 0;
}