Get information about all the network interfaces in linux

This program lists all active network interfaces in the system with associated IP address, MAC address and the subnetmask.

#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>

#define __KERNEL__
#include <asm/types.h>
#undef __KERNEL__

#include <linux/ethtool.h>
#include <linux/sockios.h>

#include <net/if.h>
#include <sys/ioctl.h>
#include <arpa/inet.h>

/* this is straight from beej's network tutorial. It is a nice wrapper
 * for inet_ntop and helpes to make the program IPv6 ready
 */
char *get_ip_str(const struct sockaddr *sa, char *s, size_t maxlen)
{
  switch(sa->sa_family) {
    case AF_INET:
      inet_ntop(AF_INET, &(((struct sockaddr_in *)sa)->sin_addr), s, maxlen);
      break;

    case AF_INET6:
      inet_ntop(AF_INET6, &(((struct sockaddr_in6 *)sa)->sin6_addr), s, maxlen);
      break;

    default:
      strncpy(s, "Unknown AF", maxlen);
      return NULL;
  }

  return s;
}

int main (int argc, char **argv)
{
  char          buf[1024] = {0};
  struct ifconf ifc = {0};
  struct ifreq *ifr = NULL;
  int           sck = 0;
  int           nInterfaces = 0;
  int           i = 0;
  struct ifreq ifr_subnet;
  struct sockaddr *addr_subnet = NULL;

  /* Get a socket handle. */
  sck = socket(AF_INET, SOCK_DGRAM, 0);
  if(-1 == sck) {
    perror("socket");
    return 1;
  }

  /* Query available interfaces. */
  ifc.ifc_len = sizeof(buf);
  ifc.ifc_buf = buf;
  if(-1 == ioctl(sck, SIOCGIFCONF, &ifc)) {
    perror("ioctl(SIOCGIFCONF)");
    close (sck);
    return 1;
  }

  /* Iterate through the list of interfaces. */
  ifr = ifc.ifc_req;
  nInterfaces = ifc.ifc_len / sizeof(struct ifreq);
  for(i = 0; i < nInterfaces; i++)
  {
    struct ifreq *item = &ifr[i];

    /* Show the device name and IP address */
    struct sockaddr *addr = &(item->ifr_addr);
    char ip[INET6_ADDRSTRLEN];
    printf("%s: IP %s",
           item->ifr_name,
           get_ip_str(addr, ip, INET6_ADDRSTRLEN));

    /* Get the MAC address */
    if(-1 == ioctl(sck, SIOCGIFHWADDR, item)) {
      perror("ioctl(SIOCGIFHWADDR)");
      close (sck);
      return 1;
    }

    /* display result */
    printf(", MAC %.2x:%.2x:%.2x:%.2x:%.2x:%.2x ",
           (unsigned char)item->ifr_hwaddr.sa_data[0],
           (unsigned char)item->ifr_hwaddr.sa_data[1],
           (unsigned char)item->ifr_hwaddr.sa_data[2],
           (unsigned char)item->ifr_hwaddr.sa_data[3],
           (unsigned char)item->ifr_hwaddr.sa_data[4],
           (unsigned char)item->ifr_hwaddr.sa_data[5]);

    /* Reset the memory allocated for ifreq */
    memset (&ifr_subnet, 0, sizeof (struct ifreq));

    /* Update interface name into ifreq */
    strcpy (ifr_subnet.ifr_name, item->ifr_name);

    /* Call ioctl function */
    if(-1 == ioctl (sck, SIOCGIFNETMASK, &ifr_subnet ))
    {
      perror("ioctl(SIOCGIFNETMASK)");
      close (sck);
      return 1;
    }
    addr_subnet = &(ifr_subnet.ifr_addr);
    printf("SubnetMask: %s\n", get_ip_str(addr_subnet, ip, INET6_ADDRSTRLEN));
  }

  close (sck);
  return 0;
}

Reference: http://www.adamrisi.com/?p=84

Simple PING implementation in C

#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/ip_icmp.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/signal.h>
#include <string.h>

#define DEFDATALEN      56
#define MAXIPLEN        60
#define MAXICMPLEN      76

static char *hostname = NULL;

static int in_cksum(unsigned short *buf, int sz)
{
  int nleft = sz;
  int sum = 0;
  unsigned short *w = buf;
  unsigned short ans = 0;
  
  while (nleft > 1) {
    sum += *w++;
    nleft -= 2;
  }
  
  if (nleft == 1) {
    *(unsigned char *) (&ans) = *(unsigned char *) w;
    sum += ans;
  }
  
  sum = (sum >> 16) + (sum & 0xFFFF);
  sum += (sum >> 16);
  ans = ˜sum;
  return (ans);
}

static void noresp(int ign)
{
  printf("No response from %s\n", hostname);
  exit(0);
}

static void ping(const char *host)
{
  struct hostent *h;
  struct sockaddr_in pingaddr;
  struct icmp *pkt;
  int pingsock, c;
  char packet[DEFDATALEN + MAXIPLEN + MAXICMPLEN];
  
  if ((pingsock = socket(AF_INET, SOCK_RAW, 1)) < 0) {       /* 1 == ICMP */
    perror("ping: creating a raw socket");
    exit(1);
  }
  
  /* drop root privs if running setuid */
  setuid(getuid());
  
  memset(&pingaddr, 0, sizeof(struct sockaddr_in));
  
  pingaddr.sin_family = AF_INET;
  if (!(h = gethostbyname(host))) {
    fprintf(stderr, "ping: unknown host %s\n", host);
    exit(1);
  }
  memcpy(&pingaddr.sin_addr, h->h_addr, sizeof(pingaddr.sin_addr));
  hostname = h->h_name;
  
  pkt = (struct icmp *) packet;
  memset(pkt, 0, sizeof(packet));
  pkt->icmp_type = ICMP_ECHO;
  pkt->icmp_cksum = in_cksum((unsigned short *) pkt, sizeof(packet));
  
  c = sendto(pingsock, packet, sizeof(packet), 0,
             (struct sockaddr *) &pingaddr, sizeof(struct sockaddr_in));
  
  if (c < 0 || c != sizeof(packet)) {
    if (c < 0)
      perror("ping: sendto");
    fprintf(stderr, "ping: write incomplete\n");
    exit(1);
  }
  
  signal(SIGALRM, noresp);
  alarm(2);                                     /* give the host 5000ms to respond */
  /* listen for replies */
  while (1) {
    struct sockaddr_in from;
    size_t fromlen = sizeof(from);
    
    if ((c = recvfrom(pingsock, packet, sizeof(packet), 0,
                      (struct sockaddr *) &from, &fromlen)) < 0) {
      if (errno == EINTR)
        continue;
      perror("ping: recvfrom");
      continue;
    }
    if (c >= 76) {                   /* ip + icmp */
      struct iphdr *iphdr = (struct iphdr *) packet;
      
      pkt = (struct icmp *) (packet + (iphdr->ihl << 2));      /* skip ip hdr */
      if (pkt->icmp_type == ICMP_ECHOREPLY)
        break;
    }
  }
  printf("%s is alive!\n", hostname);
  return;
}

int main ()
{
  ping ("192.168.1.2");

}

Reference: http://www.koders.com/c/fid30EA22902AFF5800481BBC6F65DADCDAF92D6E37.aspx

Avahi-autoipd

It is an Zeroconf implementation of Avahi which facilitates configuration of a network element automatically on the local network in the absence of DHCP server. It uses a special address range 169.254.0.0/16 and prefix fe80::/16 for IPV4 and IPV6 link-local addressing respectively. When the autoipd daemon starts, it generates an IP address randomly using the MAC address information and announces its new IP using ARP probe to find address conflict. If the given address is being used by another network element, it generates a new IP and does ARP probe till it successfully claims the address. It stores this IP address in a file for persistence across network restart (file location on linux machine will be "/var/lib/avahi-autoipd/<mac_address>"). To configure this address to be routable on the local link, autoipd adds routing rule for this IP address in the routing table.

Source: RFC3927, Avahi