How can I monitor the NIC status(up/down) in a C program without polling the kernel?

Question

Now I need to get the status of the NIC(up or down) in the real time. That means I have to catch the kernel interrupt when the NIC up or down in a blocked loop.

The first stupid method from mine is that check on the /sys/class/net/eth0/operstate or use ioctl to get the ifflag every 100ms in a loop. But 100ms is too long for the app to reroute the traffic and also polling kernel every 100ms is not good idea.

Once I notice the inotify function that can monitor the files in a block mode. But unfortunately, it can't monitor the /sys/class/net/eth0/operstate file since /sys is located in the RAM not in the disk.

So, is there any methods except writing a kernel module to catch the NIC interrupt(up/down) in the C program with a block mode?

Answer 1

Yes, open a netlink socket and listen to the RTMGRP_LINK (network interface create/delete/up/down events) multicast groups.

The netlink man page here has a specific example to do this.

Answer 2

After doing a little research/reading on the web, I managed to cook up a working code to monitor NIC status.

#include <asm/types.h>
#include <sys/socket.h>
#include <unistd.h>
#include <errno.h>
#include <stdio.h>
#include <string.h>
#include <net/if.h>
#include <netinet/in.h>
#include <linux/netlink.h>
#include <linux/rtnetlink.h>
#include <stdlib.h>
#include <sys/time.h>
#include <sys/types.h>

int
read_event (int sockint)
{
  int status;
  int ret = 0;
  char buf[4096];
  struct iovec iov = { buf, sizeof buf };
  struct sockaddr_nl snl;
  struct msghdr msg = { (void *) &snl, sizeof snl, &iov, 1, NULL, 0, 0 };
  struct nlmsghdr *h;
  struct ifinfomsg *ifi;

  status = recvmsg (sockint, &msg, 0);

  if (status < 0)
  {
      /* Socket non-blocking so bail out once we have read everything */
      if (errno == EWOULDBLOCK || errno == EAGAIN)
      return ret;

      /* Anything else is an error */
      printf ("read_netlink: Error recvmsg: %d\n", status);
      perror ("read_netlink: Error: ");
      return status;
  }

  if (status == 0)
   {
      printf ("read_netlink: EOF\n");
   }

  // We need to handle more than one message per 'recvmsg'
  for (h = (struct nlmsghdr *) buf; NLMSG_OK (h, (unsigned int) status);
       h = NLMSG_NEXT (h, status))
    {
      //Finish reading 
      if (h->nlmsg_type == NLMSG_DONE)
        return ret;

      // Message is some kind of error 
      if (h->nlmsg_type == NLMSG_ERROR)
    {
          printf ("read_netlink: Message is an error - decode TBD\n");
          return -1;        // Error
        }

      if (h->nlmsg_type == RTM_NEWLINK)
        {
        ifi = NLMSG_DATA (h);
            printf ("NETLINK::%s\n", (ifi->ifi_flags & IFF_RUNNING) ? "Up" : "Down");
    }
    }

  return ret;
}

int
main (int argc, char *argv[])
{
  fd_set rfds, wfds;
  struct timeval tv;
  int retval;
  struct sockaddr_nl addr;

  int nl_socket = socket (AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
  if (nl_socket < 0)
    {
      printf ("Socket Open Error!");
      exit (1);
    }

  memset ((void *) &addr, 0, sizeof (addr));

  addr.nl_family = AF_NETLINK;
  addr.nl_pid = getpid ();
  addr.nl_groups = RTMGRP_LINK | RTMGRP_IPV4_IFADDR | RTMGRP_IPV6_IFADDR;
//  addr.nl_groups = RTMGRP_LINK;

  if (bind (nl_socket, (struct sockaddr *) &addr, sizeof (addr)) < 0)
    {
      printf ("Socket bind failed!");
      exit (1);
    }

  while (1)
    {
      FD_ZERO (&rfds);
      FD_CLR (nl_socket, &rfds);
      FD_SET (nl_socket, &rfds);

      tv.tv_sec = 10;
      tv.tv_usec = 0;

      retval = select (FD_SETSIZE, &rfds, NULL, NULL, &tv);
      if (retval == -1)
        printf ("Error select() \n");
      else if (retval)
        {
          printf ("Event recieved >> ");
          read_event (nl_socket);
        }
      else
        printf ("## Select TimedOut ## \n");
    }
  return 0;
}