How to get notified about network interface changes with Netlist and RTMGRP_LINK signal?

Question

I write a program which needs to get notified if the network interfaces have changed, in particular new one appeared or existed one gone. My research brought to the netlink and its RTMGRP_LINK signal. This manpage gives an example which is not clear for me yet.

It has this code:

memset(&sa, 0, sizeof(sa));
snl.nl_family = AF_NETLINK;
snl.nl_groups = RTMGRP_LINK | RTMGRP_IPV4_IFADDR;

fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
bind(fd, (struct sockaddr*)&sa, sizeof(sa));

My understading is this is an initialization part to enable some signals. I guess there must be a handler subroutine which processes the event.

My questions are:

• Is this a correct code snippet for enabling the event?

• How to process the event: do I need to have some handler?

• What are the data structures that contains a relevant information (about network interfaces and their changes)?

Thanks for any help.

Answer 1

The following code has been taken from here and I have added the logic to get the name of the concerned interface.

    #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>

    #define MYPROTO NETLINK_ROUTE
    #define MYMGRP RTMGRP_IPV4_ROUTE

    int open_netlink()
    {
        int sock = socket(AF_NETLINK,SOCK_RAW,MYPROTO);
        struct sockaddr_nl addr;

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

        if (sock<0)
            return sock;
        addr.nl_family = AF_NETLINK;
        addr.nl_pid = getpid();
        addr.nl_groups = RTMGRP_LINK|RTMGRP_IPV4_IFADDR|RTMGRP_IPV6_IFADDR;
        if (bind(sock,(struct sockaddr *)&addr,sizeof(addr))<0)
            return -1;
        return sock;
    }


    int read_event(int sockint, int (*msg_handler)(struct sockaddr_nl *,
                                                   struct nlmsghdr *))
    {
        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;

        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
            }

            /* Call message handler */
            if(msg_handler)
            {
                ret = (*msg_handler)(&snl, h);
                if(ret < 0)
                {
                    printf("read_netlink: Message hander error %d\n", ret);
                    return ret;
                }
            }
            else
            {
                printf("read_netlink: Error NULL message handler\n");
                return -1;
            }
        }



        return ret;
    }

    static int netlink_link_state(struct sockaddr_nl *nl, struct nlmsghdr *msg)
    {
        int len;
        struct ifinfomsg *ifi;

        nl = nl;

        ifi = NLMSG_DATA(msg);
        char ifname[1024];if_indextoname(ifi->ifi_index,ifname);

        printf("netlink_link_state: Link %s %s\n", 
                /*(ifi->ifi_flags & IFF_RUNNING)?"Up":"Down");*/
            ifname,(ifi->ifi_flags & IFF_UP)?"Up":"Down");
        return 0;
    }

    static int msg_handler(struct sockaddr_nl *nl, struct nlmsghdr *msg)
    {
        struct ifinfomsg *ifi=NLMSG_DATA(msg); /* for RTM_{NEW,DEL}ADDR */
        struct ifaddrmsg *ifa=NLMSG_DATA(msg); /* for RTM_{NEW,DEL}LINK */
        char ifname[1024];
        switch (msg->nlmsg_type)
        {
            case RTM_NEWADDR:
                if_indextoname(ifa->ifa_index,ifname);
                printf("msg_handler: RTM_NEWADDR : %s\n",ifname);
                break;
            case RTM_DELADDR:
                if_indextoname(ifa->ifa_index,ifname);
                printf("msg_handler: RTM_DELADDR : %s\n",ifname);
                break;
            case RTM_NEWLINK:
                if_indextoname(ifi->ifi_index,ifname);
                printf("msg_handler: RTM_NEWLINK\n");
                netlink_link_state(nl, msg);
                break;
            case RTM_DELLINK:
                if_indextoname(ifi->ifi_index,ifname);
                printf("msg_handler: RTM_DELLINK : %s\n",ifname);
                break;
            default:
                printf("msg_handler: Unknown netlink nlmsg_type %d\n",
                        msg->nlmsg_type);
                break;
        }
        return 0;
    }
    int main(int argc, char *argv[])
    {
        int nls = open_netlink();
        printf("Started watching:\n");
        if (nls<0) {
            printf("Open Error!");
        }
        while (1)
            read_event(nls, msg_handler);
        return 0;
    }