Linux Netlink 编程

Netlink 是内核与用户空间进程通信的一种机制，采用与 UDP socket 非常类似的编程风格。（关于 UDP socket 编程，请看这里）

与 UDP 类似，Netlink 通信也有服务器端和客户端。例如，Linux 内核作为服务器端，已经运行在那里。我们则编写 Netlink 客户端，给内核发消息，并获得我们想要的信息。

例子 1

通过 Netlink，从内核中得到所有网口的名字。

link-list.c：

/*
 *  Display all network interface names
 */
#include             //printf, perror
#include            //memset, strlen
#include            //exit
#include            //close
#include        //msghdr
#include         //inet_ntop
#include     //sockaddr_nl
#include   //rtgenmsg,ifinfomsg

#define BUFSIZE 8192

struct nl_req_s {
  struct nlmsghdr hdr;
  struct rtgenmsg gen;
};

void die(char *s)
{
    perror(s);
    exit(1);
}

void rtnl_print_link(struct nlmsghdr * h)
{
    struct ifinfomsg * iface;
    struct rtattr * attr;
    int len;

    iface = NLMSG_DATA(h);
    len = RTM_PAYLOAD(h);

    /* loop over all attributes for the NEWLINK message */
    for (attr = IFLA_RTA(iface); RTA_OK(attr, len); attr = RTA_NEXT(attr, len))
    {
        switch (attr->rta_type)
        {
        case IFLA_IFNAME:
            printf("Interface %d : %s\n", iface->ifi_index, (char *)RTA_DATA(attr));
            break;
        default:
            break;
        }
    }
}

int main(void)
{
    struct sockaddr_nl kernel;
    int s, end=0, len;
    struct msghdr msg;
    struct nl_req_s req;
    struct iovec io;
    char buf[BUFSIZE];

    //build kernel netlink address
    memset(&kernel, 0, sizeof(kernel));
    kernel.nl_family = AF_NETLINK;
    kernel.nl_groups = 0;

    //create a Netlink socket
    if ((s=socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE)) < 0)
    {
        die("socket");
    }

    //build netlink message
    memset(&req, 0, sizeof(req));
    req.hdr.nlmsg_len = NLMSG_LENGTH(sizeof(struct rtgenmsg));
    req.hdr.nlmsg_type = RTM_GETLINK;
    req.hdr.nlmsg_flags = NLM_F_REQUEST | NLM_F_DUMP;
    req.hdr.nlmsg_seq = 1;
    req.hdr.nlmsg_pid = getpid();
    req.gen.rtgen_family = AF_INET;

    memset(&io, 0, sizeof(io));
    io.iov_base = &req;
    io.iov_len = req.hdr.nlmsg_len;

    memset(&msg, 0, sizeof(msg));
    msg.msg_iov = &io;
    msg.msg_iovlen = 1;
    msg.msg_name = &kernel;
    msg.msg_namelen = sizeof(kernel);

    //send the message
    if (sendmsg(s, &msg, 0) < 0)
    {
        die("sendmsg");
    }

    //parse reply
    while (!end)
    {
        memset(buf, 0, BUFSIZE);
        msg.msg_iov->iov_base = buf;
        msg.msg_iov->iov_len = BUFSIZE;
        if ((len=recvmsg(s, &msg, 0)) < 0)
        {
            die("recvmsg");
        }
        for (struct nlmsghdr * msg_ptr = (struct nlmsghdr *)buf;
             NLMSG_OK(msg_ptr, len); msg_ptr = NLMSG_NEXT(msg_ptr, len))
        {
            switch (msg_ptr->nlmsg_type)
            {
            case NLMSG_DONE:
                end++;
                break;
            case RTM_NEWLINK:
                rtnl_print_link(msg_ptr);
                break;
            default:
                printf("Ignored msg: type=%d, len=%d\n", msg_ptr->nlmsg_type, msg_ptr->nlmsg_len);
                break;
            }
        }
    }

    close(s);
    return 0;
}

# gcc link-list.c -o link-list && ./link-list
Interface 1 : lo
Interface 2 : eth0

例子 ２

通过 Netlink，从内核中得到所有网口的 IP 地址。

/*
 *  Display all IPv4 addresses
 */
#include             //printf, perror
#include            //memset, strlen
#include            //exit
#include            //close
#include        //msghdr
#include         //inet_ntop
#include     //sockaddr_nl
#include   //rtgenmsg,ifinfomsg

#define BUFSIZE 8192

struct nl_req_s {
  struct nlmsghdr hdr;
  struct rtgenmsg gen;
};

void die(char *s)
{
    perror(s);
    exit(1);
}

void rtnl_print_addr(struct nlmsghdr * h)
{
    struct ifaddrmsg * addr;
    struct rtattr * attr;
    int len;

    addr = NLMSG_DATA(h);
    len = RTM_PAYLOAD(h);

    /* loop over all attributes for the NEWLINK message */
    for (attr = IFLA_RTA(addr); RTA_OK(attr, len); attr = RTA_NEXT(attr, len))
    {
        switch (attr->rta_type)
        {
        case IFA_LABEL:
            printf("Interface  : %s\n", (char *)RTA_DATA(attr));
            break;
        case IFA_LOCAL:
        {
            int ip = *(int*)RTA_DATA(attr);
            unsigned char bytes[4];
            bytes[0] = ip & 0xFF;
            bytes[1] = (ip >> 8) & 0xFF;
            bytes[2] = (ip >> 16) & 0xFF;
            bytes[3] = (ip >> 24) & 0xFF;
            printf("IP Address : %d.%d.%d.%d\n", bytes[0], bytes[1], bytes[2], bytes[3]);
            break;
        }
        default:
            break;
        }
    }
}

int main(void)
{
    struct sockaddr_nl kernel;
    int s, end=0, len;
    struct msghdr msg;
    struct nl_req_s req;
    struct iovec io;
    char buf[BUFSIZE];

    //build kernel netlink address
    memset(&kernel, 0, sizeof(kernel));
    kernel.nl_family = AF_NETLINK;
    kernel.nl_groups = 0;

    //create a Netlink socket
    if ((s=socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE)) < 0)
    {
        die("socket");
    }

    //build netlink message
    memset(&req, 0, sizeof(req));
    req.hdr.nlmsg_len = NLMSG_LENGTH(sizeof(struct rtgenmsg));
    req.hdr.nlmsg_type = RTM_GETADDR;
    req.hdr.nlmsg_flags = NLM_F_REQUEST | NLM_F_DUMP;
    req.hdr.nlmsg_seq = 1;
    req.hdr.nlmsg_pid = getpid();
    req.gen.rtgen_family = AF_INET;

    memset(&io, 0, sizeof(io));
    io.iov_base = &req;
    io.iov_len = req.hdr.nlmsg_len;

    memset(&msg, 0, sizeof(msg));
    msg.msg_iov = &io;
    msg.msg_iovlen = 1;
    msg.msg_name = &kernel;
    msg.msg_namelen = sizeof(kernel);

    //send the message
    if (sendmsg(s, &msg, 0) < 0)
    {
        die("sendmsg");
    }

    //parse reply
    while (!end)
    {
        memset(buf, 0, BUFSIZE);
        msg.msg_iov->iov_base = buf;
        msg.msg_iov->iov_len = BUFSIZE;
        if ((len=recvmsg(s, &msg, 0)) < 0)
        {
            die("recvmsg");
        }
        for (struct nlmsghdr * msg_ptr = (struct nlmsghdr *)buf;
             NLMSG_OK(msg_ptr, len); msg_ptr = NLMSG_NEXT(msg_ptr, len))
        {
            switch (msg_ptr->nlmsg_type)
            {
            case NLMSG_DONE:
                end++;
                break;
            case RTM_NEWADDR:
                rtnl_print_addr(msg_ptr);
                break;
            default:
                printf("Ignored msg: type=%d, len=%d\n", msg_ptr->nlmsg_type, msg_ptr->nlmsg_len);
                break;
            }
        }
    }

    close(s);
    return 0;
}

# gcc addr-list.c -o addr-list && ./addr-list
IP Address : 127.0.0.1
Interface  : lo
IP Address : 10.254.115.149
Interface  : eth0

例子 ３

通过 Netlink 列出当前系统中主路由表中的所有路由。

/*
 *  Display all routes
 */
#include             //printf, perror
#include            //memset, strlen
#include            //exit
#include            //close
#include        //msghdr
#include         //inet_ntop
#include     //sockaddr_nl,NLMSG_DATA
#include   //rtgenmsg,ifinfomsg

#define BUFSIZE 8192

struct nl_req_s {
  struct nlmsghdr hdr;
  struct rtgenmsg gen;
};

void die(char *s)
{
    perror(s);
    exit(1);
}

void rtnl_print_route(struct nlmsghdr * h)
{
    struct rtmsg * rte = NLMSG_DATA(h);
    struct rtattr * attr = RTM_RTA(rte);
    int len = RTM_PAYLOAD(h);
    char dest[32] = {0};
    char gway[32] = {"unspecified"};

    if (rte->rtm_table != RT_TABLE_MAIN) {
        return;
    }

    for (; RTA_OK(attr, len); attr = RTA_NEXT(attr, len))
    {
        switch (attr->rta_type)
        {
        case RTA_DST:
            inet_ntop(AF_INET, RTA_DATA(attr), dest, sizeof(dest));
            break;
        case RTA_GATEWAY:
            inet_ntop(AF_INET, RTA_DATA(attr), gway, sizeof(gway));
            break;
        default:
            break;
        }
    }

    printf("%s/%d gateway %s\n", dest, rte->rtm_dst_len, gway);
}

int main(void)
{
    struct sockaddr_nl kernel;
    int s, end=0, len;
    struct msghdr msg;
    struct nl_req_s req;
    struct iovec io;
    char buf[BUFSIZE];

    //build kernel netlink address
    memset(&kernel, 0, sizeof(kernel));
    kernel.nl_family = AF_NETLINK;
    kernel.nl_groups = 0;

    //create a Netlink socket
    if ((s=socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE)) < 0)
    {
        die("socket");
    }

    //build netlink message
    memset(&req, 0, sizeof(req));
    req.hdr.nlmsg_len = NLMSG_LENGTH(sizeof(struct rtgenmsg));
    req.hdr.nlmsg_type = RTM_GETROUTE;
    req.hdr.nlmsg_flags = NLM_F_REQUEST | NLM_F_DUMP;
    req.hdr.nlmsg_seq = 1;
    req.hdr.nlmsg_pid = getpid();
    req.gen.rtgen_family = AF_INET;

    memset(&io, 0, sizeof(io));
    io.iov_base = &req;
    io.iov_len = req.hdr.nlmsg_len;

    memset(&msg, 0, sizeof(msg));
    msg.msg_iov = &io;
    msg.msg_iovlen = 1;
    msg.msg_name = &kernel;
    msg.msg_namelen = sizeof(kernel);

    //send the message
    if (sendmsg(s, &msg, 0) < 0)
    {
        die("sendmsg");
    }

    // parse reply
    while (!end)
    {
        memset(buf, 0, BUFSIZE);
        msg.msg_iov->iov_base = buf;
        msg.msg_iov->iov_len = BUFSIZE;

        if ((len=recvmsg(s, &msg, 0)) < 0)
        {
            die("recvmsg");
        }

        for (struct nlmsghdr * msg_ptr = (struct nlmsghdr *)buf;
             NLMSG_OK(msg_ptr, len); msg_ptr = NLMSG_NEXT(msg_ptr, len))
        {
            switch (msg_ptr->nlmsg_type)
            {
            case NLMSG_DONE:
                end++;
                break;
            case RTM_NEWROUTE:
                rtnl_print_route(msg_ptr);
                break;
            default:
                printf("Ignored msg: type=%d, len=%d\n", msg_ptr->nlmsg_type, msg_ptr->nlmsg_len);
                break;
            }
        }
    }

    close(s);
    return 0;
}

# gcc route-list.c -o route-list && ./route-list
/0 gateway 10.254.96.136
10.254.96.0/19 gateway unspecified

例子 4

通过 Netlink 监测主路由表中路由的变化。

/*
 *  Monitor route change
 */
#include             //printf, perror
#include            //memset, strlen
#include            //exit
#include            //close
#include        //msghdr
#include         //inet_ntop
#include     //sockaddr_nl,NLMSG_DATA
#include   //rtgenmsg,ifinfomsg

#define BUFSIZE 4096

void die(char *s)
{
    perror(s);
    exit(1);
}

void print_route(struct nlmsghdr * h)
{
    struct rtmsg * rte = NLMSG_DATA(h);
    struct rtattr * attr = RTM_RTA(rte);
    int len = RTM_PAYLOAD(h);
    char dest[32] = {0};
    char gway[32] = {"unspecified"};

    if (rte->rtm_table != RT_TABLE_MAIN) {
        return;
    }

    for (; RTA_OK(attr, len); attr = RTA_NEXT(attr, len))
    {
        switch (attr->rta_type)
        {
        case RTA_DST:
            inet_ntop(AF_INET, RTA_DATA(attr), dest, sizeof(dest));
            break;
        case RTA_GATEWAY:
            inet_ntop(AF_INET, RTA_DATA(attr), gway, sizeof(gway));
            break;
        default:
            break;
        }
    }

    if (h->nlmsg_type == RTM_NEWROUTE) {
        printf("add ");
    }
    else if (h->nlmsg_type == RTM_DELROUTE) {
        printf("del ");
    }
    else {
        printf("nlmsg_type=%d ", h->nlmsg_type);
    }
    printf("%s/%d gateway %s\n", dest, rte->rtm_dst_len, gway);
}

int main(void)
{
    int s, len;
    struct sockaddr_nl me;
    char buf[BUFSIZE];

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

    if ((s = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE)) < 0)
    {
        die("socket");
    }

    me.nl_family = AF_NETLINK;
    me.nl_groups = RTMGRP_IPV4_ROUTE;

    if (bind(s,(struct sockaddr *)&me, sizeof(me)) < 0)
    {
        die("bind");
    }

    while (1)
    {
        memset(buf, 0, BUFSIZE);
        if ((len=recv(s, buf, BUFSIZE, 0)) < 0)
        {
            die("recv");
        }

        for (struct nlmsghdr * msg_ptr = (struct nlmsghdr *)buf;
             NLMSG_OK(msg_ptr, len); msg_ptr = NLMSG_NEXT(msg_ptr, len))
        {
            switch (msg_ptr->nlmsg_type)
            {
            case RTM_NEWROUTE:
            case RTM_DELROUTE:
                print_route(msg_ptr);
                break;
            default:
                printf("Ignored msg: type=%d\n", msg_ptr->nlmsg_type);
                break;
            }
        }
    }

    close(s);
    return 0;
}

Terminal 1:
# gcc route-chg.c -o route-chg && ./route-chg

Terminal 2:
# ip route add 199.1.1.0/24 via 10.254.96.136
# ip route del 199.1.1.0/24 via 10.254.96.136

Terminal 1:
# gcc route-chg.c -o route-chg && ./route-chg
add 199.1.1.0/24 gateway 10.254.96.136
del 199.1.1.0/24 gateway 10.254.96.136

例子 5

通过 Netlink 监测网口的变化。

/*
 *  Monitor network interface change
 */
#include             //printf, perror
#include            //memset, strlen
#include            //exit
#include            //close
#include        //msghdr
#include         //inet_ntop
#include     //sockaddr_nl,NLMSG_DATA
#include   //rtgenmsg,ifinfomsg

#define BUFSIZE 4096

void die(char *s)
{
    perror(s);
    exit(1);
}

void print_link(struct nlmsghdr * h)
{
    struct ifinfomsg * iface;
    struct rtattr * attr;
    int len;

    iface = NLMSG_DATA(h); //point to payload
    len = RTM_PAYLOAD(h);

    /* loop over all attributes for the NEWLINK message */
    for (attr = IFLA_RTA(iface); RTA_OK(attr, len); attr = RTA_NEXT(attr, len))
    {
        switch (attr->rta_type)
        {
        case IFLA_IFNAME:
            if (h->nlmsg_type == RTM_NEWLINK) {
                printf("add ");
            }
            else if (h->nlmsg_type == RTM_DELLINK) {
                printf("del ");
            }
            else {
                printf("nlmsg_type=%d ", h->nlmsg_type);
            }

            printf("Interface %d : %s\n", iface->ifi_index, (char *)RTA_DATA(attr));
            break;
        default:
            break;
        }
    }
}

int main(void)
{
    int s, len;
    struct sockaddr_nl me;
    char buf[BUFSIZE];

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

    if ((s = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE)) < 0)
    {
        die("socket");
    }

    int on = 1;
    if (setsockopt(s, SOL_NETLINK, NETLINK_LISTEN_ALL_NSID, &on, sizeof(on)) < 0)
    {
        die("setsockopt");
    }

    me.nl_family = AF_NETLINK;
    me.nl_groups = RTMGRP_LINK;

    if (bind(s,(struct sockaddr *)&me, sizeof(me)) < 0)
    {
        die("bind");
    }

    while (1)
    {
        memset(buf, 0, BUFSIZE);
        if ((len=recv(s, buf, BUFSIZE, 0)) < 0)
        {
            die("recv");
        }

        for (struct nlmsghdr * msg_ptr = (struct nlmsghdr *)buf;
             NLMSG_OK(msg_ptr, len); msg_ptr = NLMSG_NEXT(msg_ptr, len))
        {
            switch (msg_ptr->nlmsg_type)
            {
            case RTM_NEWLINK:
            case RTM_DELLINK:
                print_link(msg_ptr);
                break;
            default:
                printf("Ignored msg: type=%d\n", msg_ptr->nlmsg_type);
                break;
            }
        }
    }

    close(s);
    return 0;
}

Terminal 1:
# gcc link-chg.c -o link-chg && ./link-chg

Terminal 2:
# ip link add veth1 type veth
# ip link del veth1 type veth

Terminal 1:
# gcc link-chg.c -o link-chg && ./link-chg
add Interface 8 : veth0
add Interface 9 : veth1
del Interface 9 : veth1
del Interface 8 : veth0

例子 6

通过 Netlink 监测 IP 地址的变化。

/*
 *  Monitor IP address change
 */
#include             //printf, perror
#include            //memset, strlen
#include            //exit
#include            //close
#include        //msghdr
#include         //inet_ntop
#include     //sockaddr_nl,NLMSG_DATA
#include   //rtgenmsg,ifinfomsg

#define BUFSIZE 4096

void die(char *s)
{
    perror(s);
    exit(1);
}

void print_addr(struct nlmsghdr * h)
{
    struct ifaddrmsg * addr;
    struct rtattr * attr;
    int len;

    addr = NLMSG_DATA(h);
    len = RTM_PAYLOAD(h);

    /* loop over all attributes for the NEWLINK message */
    for (attr = IFLA_RTA(addr); RTA_OK(attr, len); attr = RTA_NEXT(attr, len))
    {
        switch (attr->rta_type)
        {
        case IFA_LABEL:
            printf("Interface  : %s\n", (char *)RTA_DATA(attr));
            break;
        case IFA_LOCAL:
        {
            if (h->nlmsg_type == RTM_NEWADDR) {
                printf("add ");
            }
            else if (h->nlmsg_type == RTM_DELADDR) {
                printf("del ");
            }
            else {
                printf("nlmsg_type=%d ", h->nlmsg_type);
            }

            int ip = *(int*)RTA_DATA(attr);
            unsigned char bytes[4];
            bytes[0] = ip & 0xFF;
            bytes[1] = (ip >> 8) & 0xFF;
            bytes[2] = (ip >> 16) & 0xFF;
            bytes[3] = (ip >> 24) & 0xFF;
            printf("IP Address : %d.%d.%d.%d\n", bytes[0], bytes[1], bytes[2], bytes[3]);
            break;
        }
        default:
            break;
        }
    }
}

int main(void)
{
    int s, len;
    struct sockaddr_nl me;
    char buf[BUFSIZE];

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

    if ((s = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE)) < 0)
    {
        die("socket");
    }

    int on = 1;
    if (setsockopt(s, SOL_NETLINK, NETLINK_LISTEN_ALL_NSID, &on, sizeof(on)) < 0)
    {
        die("setsockopt");
    }

    me.nl_family = AF_NETLINK;
    me.nl_groups = RTMGRP_IPV4_IFADDR;

    if (bind(s,(struct sockaddr *)&me, sizeof(me)) < 0)
    {
        die("bind");
    }

    while (1)
    {
        memset(buf, 0, BUFSIZE);
        if ((len=recv(s, buf, BUFSIZE, 0)) < 0)
        {
            die("recv");
        }

        for (struct nlmsghdr * msg_ptr = (struct nlmsghdr *)buf;
             NLMSG_OK(msg_ptr, len); msg_ptr = NLMSG_NEXT(msg_ptr, len))
        {
            switch (msg_ptr->nlmsg_type)
            {
            case RTM_NEWADDR:
            case RTM_DELADDR:
                print_addr(msg_ptr);
                break;
            default:
                printf("Ignored msg: type=%d\n", msg_ptr->nlmsg_type);
                break;
            }
        }
    }

    close(s);
    return 0;
}

Terminal 1:
# gcc addr-chg.c -o addr-chg && ./addr-chg

Terminal 2:
# ip address add 20.20.20.2/24 dev eth0
# ip address del 20.20.20.2/24 dev eth0

Terminal 1:
# gcc addr-chg.c -o addr-chg && ./addr-chg
add IP Address : 20.20.20.2
Interface  : eth0
del IP Address : 20.20.20.2
Interface  : eth0

例子 7

通过 Netlink 监测所有 namespace 中的 IP 地址的变化。

/*
 *  Monitor IP address change
 */
#include             //printf, perror
#include            //memset, strlen
#include            //exit
#include            //close
#include        //msghdr
#include         //inet_ntop
#include     //sockaddr_nl,NLMSG_DATA
#include   //rtgenmsg,ifinfomsg

#define BUFSIZE 4096

void die(char *s)
{
    perror(s);
    exit(1);
}

void print_addr(struct nlmsghdr * h)
{
    struct ifaddrmsg * addr;
    struct rtattr * attr;
    int len;

    addr = NLMSG_DATA(h);
    len = RTM_PAYLOAD(h);

    /* loop over all attributes for the NEWLINK message */
    for (attr = IFLA_RTA(addr); RTA_OK(attr, len); attr = RTA_NEXT(attr, l)
    {
        switch (attr->rta_type)
        {
        case IFA_LABEL:
            printf("Interface  : %s\n", (char *)RTA_DATA(attr));
            break;
        case IFA_LOCAL:
        {
            if (h->nlmsg_type == RTM_NEWADDR) {
                printf("ADD ");
            }
            else if (h->nlmsg_type == RTM_DELADDR) {
                printf("DEL ");
            }
            else {
                printf("nlmsg_type=%d ", h->nlmsg_type);
            }

            int ip = *(int*)RTA_DATA(attr);
            unsigned char bytes[4];
            bytes[0] = ip & 0xFF;
            bytes[1] = (ip >> 8) & 0xFF;
            bytes[2] = (ip >> 16) & 0xFF;
            bytes[3] = (ip >> 24) & 0xFF;
            printf("IP Address : %d.%d.%d.%d\n", bytes[0], bytes[1], bytes, bytes[3]);
            break;
        }
        default:
            break;
        }
    }
    printf("\n");
}

int main(void)
{
    int s, len;
    struct sockaddr_nl me;
    struct msghdr msg;
    struct iovec io;
    char buf[BUFSIZE];
    char cmsgbuf[BUFSIZE];

    if ((s = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE)) < 0)
    {
        die("socket");
    }

    int on = 1;
    if (setsockopt(s, SOL_NETLINK, NETLINK_LISTEN_ALL_NSID, &on, sizeof(on< 0)
    {
        die("setsockopt");
    }

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

    if (bind(s,(struct sockaddr *)&me, sizeof(me)) < 0)
    {
        die("bind");
    }

    memset(&msg, 0, sizeof(msg));
    msg.msg_iov = &io;
    msg.msg_iovlen = 1;
    msg.msg_name = &me;
    msg.msg_namelen = sizeof(me);
    msg.msg_control = &cmsgbuf;
    msg.msg_controllen = BUFSIZE;

    memset(&io, 0, sizeof(io));
    msg.msg_iov->iov_base = buf;
    msg.msg_iov->iov_len = BUFSIZE;

    while (1)
    {
        memset(buf, 0, BUFSIZE);
        memset(cmsgbuf, 0, BUFSIZE);
        msg.msg_iov->iov_len = BUFSIZE;
        msg.msg_controllen = BUFSIZE;

        if ((len=recvmsg(s, &msg, 0)) < 0)
        {
            die("recvmsg");
        }

        for (struct cmsghdr * cmsg = CMSG_FIRSTHDR(&msg); cmsg; cmsg = CMSXTHDR(&msg, cmsg))
        {
            if (cmsg->cmsg_level == SOL_NETLINK &&
                cmsg->cmsg_type == NETLINK_LISTEN_ALL_NSID &&
                cmsg->cmsg_len == CMSG_LEN(sizeof(int)))
            {
                int * data = (int *)CMSG_DATA(cmsg);
                printf("Namespace ID : %d\n", *data);
            }
        }

        for (struct nlmsghdr * msg_ptr = (struct nlmsghdr *)buf;
             NLMSG_OK(msg_ptr, len); msg_ptr = NLMSG_NEXT(msg_ptr, len))
        {
            switch (msg_ptr->nlmsg_type)
            {
            case RTM_NEWADDR:
            case RTM_DELADDR:
                print_addr(msg_ptr);
                break;
            default:
                printf("Ignored msg: type=%d\n", msg_ptr->nlmsg_type);
                break;
            }
        }
    }

    close(s);
    return 0;
}

Terminal 1:
# gcc addr-chg.c -o addr-chg && ./addr-chg

Terminal 2:
# ip netns add red
# ip netns set red 123
# ip netns
red (id: 123)
# ip address add 20.20.20.2/24 dev lo
# ip address del 20.20.20.2/24 dev lo
# ip netns exec red ip address add 20.20.20.3/24 dev lo
# ip netns exec red ip address del 20.20.20.3/24 dev lo

Terminal 1:
# gcc addr-chg.c -o addr-chg && ./addr-chg
ADD IP Address : 20.20.20.2
Interface  : lo

DEL IP Address : 20.20.20.2
Interface  : lo

Namespace ID : 123
ADD IP Address : 20.20.20.3
Interface  : lo

Namespace ID : 123
DEL IP Address : 20.20.20.3
Interface  : lo

参考文献

1 http://man7.org/linux/man-pages/man7/rtnetlink.7.html
2 http://man7.org/linux/man-pages/man7/netlink.7.html
3 https://gist.github.com/cl4u2/5204374