用户态监控网络接口up、down事件

网上搜索（https://blog.csdn.net/qq123386926/article/details/50695725）可以直接使用netlink现成的接口实现：

#include <sys/types.h>
#include <sys/socket.h>
#include <asm/types.h>
#include <linux/netlink.h>
#include <linux/rtnetlink.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/ioctl.h>
#include <linux/if.h>
#include <string.h>  

#define BUFLEN 20480  

int main(int argc, char *argv[])
{
    int fd, retval;
    char buf[BUFLEN] = {};
    int len = BUFLEN;
    struct sockaddr_nl addr;
    struct nlmsghdr *nh;
    struct ifinfomsg *ifinfo;
    struct rtattr *attr;  

    fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
    setsockopt(fd, SOL_SOCKET, SO_RCVBUF, &len, sizeof(len));
    memset(&addr, , sizeof(addr));
    addr.nl_family = AF_NETLINK;
    addr.nl_groups = RTNLGRP_LINK;
    bind(fd, (struct sockaddr*)&addr, sizeof(addr));
    while ((retval = read(fd, buf, BUFLEN)) > )
    {
        for (nh = (struct nlmsghdr *)buf; NLMSG_OK(nh, retval); nh = NLMSG_NEXT(nh, retval))
        {
            if (nh->nlmsg_type == NLMSG_DONE)
                break;
            else if (nh->nlmsg_type == NLMSG_ERROR)
                return -;
            else if (nh->nlmsg_type != RTM_NEWLINK)
                continue;
            ifinfo = NLMSG_DATA(nh);
            printf("%u: %s", ifinfo->ifi_index,
                    (ifinfo->ifi_flags & IFF_LOWER_UP) ? "up" : "down" );
            attr = (struct rtattr*)(((char*)nh) + NLMSG_SPACE(sizeof(*ifinfo)));
            len = nh->nlmsg_len - NLMSG_SPACE(sizeof(*ifinfo));
            for (; RTA_OK(attr, len); attr = RTA_NEXT(attr, len))
            {
                if (attr->rta_type == IFLA_IFNAME)
                {
                    printf(" %s", (char*)RTA_DATA(attr));
                    break;
                }
            }
            printf("\n");
        }
    }  

    return ;
}  

对应内核代码位于net/core/rtnetlink.c:rtnetlink_event

static int rtnetlink_event(struct notifier_block *this, unsigned long event, void *ptr)
{
        struct net_device *dev = netdev_notifier_info_to_dev(ptr);

        switch (event) {
        case NETDEV_UP:
        case NETDEV_DOWN:
        case NETDEV_PRE_UP:
        case NETDEV_POST_INIT:
        case NETDEV_REGISTER:
        case NETDEV_CHANGE:
        case NETDEV_PRE_TYPE_CHANGE:
        case NETDEV_GOING_DOWN:
        case NETDEV_UNREGISTER:
        case NETDEV_UNREGISTER_FINAL:
        case NETDEV_RELEASE:
        case NETDEV_JOIN:
                break;
        default:
                rtmsg_ifinfo(RTM_NEWLINK, dev, , GFP_KERNEL);
                break;
        }
        return NOTIFY_DONE;
}

可见netlink没有对所有事件都进行上报。

如果想监控所有事件，可以自行实现内核模块，注册netdevice钩子（调用register_netdevice_notifier），在回调中实现netlink事件推送。