How to get notified about network interface change

问题:

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.

回答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);
    struct ifaddrmsg *ifa=NLMSG_DATA(msg);
    char ifname[1024];
    switch (msg->nlmsg_type)
    {
        case RTM_NEWADDR:
            if_indextoname(ifi->ifi_index,ifname);
            printf("msg_handler: RTM_NEWADDR : %s\n",ifname);
            break;
        case RTM_DELADDR:
            if_indextoname(ifi->ifi_index,ifname);
            printf("msg_handler: RTM_DELADDR : %s\n",ifname);
            break;
        case RTM_NEWLINK:
            if_indextoname(ifa->ifa_index,ifname);
            printf("msg_handler: RTM_NEWLINK\n");
            netlink_link_state(nl, msg);
            break;
        case RTM_DELLINK:
            if_indextoname(ifa->ifa_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;
}

回答2:

#include<stdio.h>
#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>
static void network_loop(void *data)
{
    struct sockaddr_nl addr;
    int nls, len, rtl;
    char buffer[4096];
    struct nlmsghdr *nlh;
    struct ifaddrmsg *ifa;
    struct rtattr *rth;

    if ((nls = socket(PF_NETLINK, SOCK_RAW, NETLINK_ROUTE)) == -1)
        printf("MY_PROG: socket failure\n");

    memset (&addr,0,sizeof(addr));
    addr.nl_family = AF_NETLINK;
    addr.nl_groups = RTMGRP_IPV4_IFADDR;

    if (bind(nls, (struct sockaddr *)&addr, sizeof(addr)) == -1)
        printf("MY_PROG:network_loop bind failure\n");

    nlh = (struct nlmsghdr *)buffer;
    while ((len = recv(nls,nlh,4096,0)) > 0)
    {
        for (; (NLMSG_OK (nlh, len)) && (nlh->nlmsg_type != NLMSG_DONE); nlh = NLMSG_NEXT(nlh, len))
        {
            if (nlh->nlmsg_type == RTM_NEWADDR)
                printf("MY_PROG:network_loop RTM_NEWADDR\n");
            else if (nlh->nlmsg_type == RTM_DELADDR)
                printf("MY_PROG:network_loop RTM_DELADDR\n");
            else
                printf("MY_PROG:network_loop nlmsg_type=%d\n", nlh->nlmsg_type);

            if ((nlh->nlmsg_type != RTM_NEWADDR) && (nlh->nlmsg_type != RTM_DELADDR)) continue; /* some other kind of announcement */

            ifa = (struct ifaddrmsg *) NLMSG_DATA (nlh);

            rth = IFA_RTA (ifa);
            rtl = IFA_PAYLOAD (nlh);
            for (; rtl && RTA_OK (rth, rtl); rth = RTA_NEXT (rth,rtl))
            {
                char name[IFNAMSIZ];
                uint32_t ipaddr;

                if (rth->rta_type != IFA_LOCAL) continue;

                ipaddr = * ((uint32_t *)RTA_DATA(rth));
                ipaddr = htonl(ipaddr);

                printf("MY_PROG:network_loop %s is now %X\n",if_indextoname(ifa->ifa_index,name),ipaddr);
            }
        }
    }

}

main()
{
    network_loop("MYPROG");
    printf("EXIT main()\n");
}

Simple program to know network change notification in Linux system and tested.