nlink_123.c

#include <errno.h>
#include <unistd.h>
#include <stdio.h>
#include <memory.h>
#include <net/if.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <linux/rtnetlink.h>

// little helper to parsing message using netlink macroses
void parseRtattr(struct rtattr *tb[], int max, struct rtattr *rta, int len)
{
	memset(tb, 0, sizeof(struct rtattr *) * (max + 1));

	while (RTA_OK(rta, len))    // while not end of the message
	{
		if (rta->rta_type <= max)
		{
			tb[rta->rta_type] = rta; // read attr
		}
		rta = RTA_NEXT(rta,len);    // get next attr
	}
}

int main()
{
	int fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);   // create netlink socket

	if (fd < 0)
	{
		printf("Failed to create netlink socket: %s\n", (char*)strerror(errno));
		return 1;
	}

	struct sockaddr_nl  local;  // local addr struct
	char buf[8192];             // message buffer
	struct iovec iov;           // message structure
	iov.iov_base = buf;         // set message buffer as io
	iov.iov_len = sizeof(buf);  // set size

	memset(&local, 0, sizeof(local));

	local.nl_family = AF_NETLINK; // set protocol family
	local.nl_groups = RTMGRP_LINK | RTMGRP_IPV4_IFADDR | RTMGRP_IPV4_ROUTE; // set groups we interested in
	local.nl_pid = getpid();    // set out id using current process id

	// initialize protocol message header
	struct msghdr msg;
	{
		msg.msg_name = &local;                  // local address
		msg.msg_namelen = sizeof(local);        // address size
		msg.msg_iov = &iov;                     // io vector
		msg.msg_iovlen = 1;                     // io size
	}

	if (bind(fd, (struct sockaddr*)&local, sizeof(local)) < 0)       // bind socket
	{
		printf("Failed to bind netlink socket: %s\n", (char*)strerror(errno));
		close(fd);
		return 1;
	}

	// read and parse all messages from the
	while (1)
	{
		ssize_t status = recvmsg(fd, &msg, MSG_DONTWAIT);

		//  check status
		if (status < 0)
		{
			if (errno == EINTR || errno == EAGAIN)
			{
				usleep(250000);
				continue;
			}

			printf("Failed to read netlink: %s", (char*)strerror(errno));
			continue;
		}

		if (msg.msg_namelen != sizeof(local))   // check message length, just in case
		{
			printf("Invalid length of the sender address struct\n");
			continue;
		}

		// message parser
		struct nlmsghdr *h;

		for (h = (struct nlmsghdr*)buf; status >= (ssize_t)sizeof(*h);)      // read all messagess headers
		{
			int len = h->nlmsg_len;
			int l = len - sizeof(*h);
			char *ifName = NULL;

			if ((l < 0) || (len > status))
			{
				printf("Invalid message length: %i\n", len);
				continue;
			}

			// now we can check message type
			if ((h->nlmsg_type == RTM_NEWROUTE) || (h->nlmsg_type == RTM_DELROUTE))   // some changes in routing table
			{
				printf("Routing table was changed\n");
			}
			else        // in other case we need to go deeper
			{
				char *ifUpp;
				char *ifRunn;
				struct ifinfomsg *ifi;  // structure for network interface info
				struct rtattr *tb[IFLA_MAX + 1];

				ifi = (struct ifinfomsg*) NLMSG_DATA(h);    // get information about changed network interface

				parseRtattr(tb, IFLA_MAX, IFLA_RTA(ifi), h->nlmsg_len);  // get attributes

				if (tb[IFLA_IFNAME])    // validation
				{
					ifName = (char*)RTA_DATA(tb[IFLA_IFNAME]); // get network interface name
				}

				if (ifi->ifi_flags & IFF_UP)   // get UP flag of the network interface
				{
					ifUpp = (char*)"UP";
				}
				else
				{
					ifUpp = (char*)"DOWN";
				}

				if (ifi->ifi_flags & IFF_RUNNING)   // get RUNNING flag of the network interface
				{
					ifRunn = (char*)"RUNNING";
				}
				else
				{
					ifRunn = (char*)"NOT RUNNING";
				}

				char ifAddress[256];    // network addr
				struct ifaddrmsg *ifa; // structure for network interface data
				struct rtattr *tba[IFA_MAX+1];

				ifa = (struct ifaddrmsg*)NLMSG_DATA(h); // get data from the network interface

				parseRtattr(tba, IFA_MAX, IFA_RTA(ifa), h->nlmsg_len);

				if (tba[IFA_LOCAL])
				{
					inet_ntop(AF_INET, RTA_DATA(tba[IFA_LOCAL]), ifAddress, sizeof(ifAddress)); // get IP addr
				}

				switch (h->nlmsg_type)   // what is actually happenned?
				{
					case RTM_DELADDR:
						printf("Interface %s: address was removed\n", ifName);
						break;

					case RTM_DELLINK:
						printf("Network interface %s was removed\n", ifName);
						break;

					case RTM_NEWLINK:
						printf("New network interface %s, state: %s %s\n", ifName, ifUpp, ifRunn);
						break;

					case RTM_NEWADDR:
						printf("Interface %s: new address was assigned: %s\n", ifName, ifAddress);
						break;
				}
			}

			status -= NLMSG_ALIGN(len); // align offsets by the message length, this is important

			h = (struct nlmsghdr*)((char*)h + NLMSG_ALIGN(len));    // get next message
		}

		usleep(250000); // sleep for a while
	}

	close(fd);  // close socket

	return 0;
}