通过C中的Netlink从内核到用户空间的多播

>协议族和组播组都需要在内核prog和用户prog中保持一致.您的协议系列是用户空间中的NETLINK_USERSOCK(2)和内核空间中的NETLINK_USER(31).
> addr.nl_groups = MYMGRP;由于某种原因不起作用.但这确实如下：setsockopt(sock,270,NETLINK_ADD_MEMBERSHIP,& group,sizeof(group)).

不致命：

>在这种情况下,模块不会侦听组消息,因此您不需要在netlink_kernel_create()参数中包含多播组.

此外,并非真正与netlink相关,但有用：

> strlen()不包含null chara.在消息分配期间,您应该添加一个字节来弥补这一点.
>在这种情况下,NLMSG_DATA(& nlh)是未定义的行为.这是因为你的标题和数据都在单独的内存块中,不能保证被粘合,并且所有宏都在nlh之后访问内存块：

#define NLMSG_DATA(nlh)((void *)(((char *)nlh)NLMSG_LENGTH(0)))

这是我的代码版本：

用户空间程序：

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <linux/netlink.h>
#include <unistd.h>

/* Protocol family,consistent in both kernel prog and user prog. */
#define MYPROTO NETLINK_USERSOCK
/* Multicast group,consistent in both kernel prog and user prog. */
#define MYMGRP 21

int open_netlink(void)
{
    int sock;
    struct sockaddr_nl addr;
    int group = MYMGRP;

    sock = socket(AF_NETLINK,MYPROTO);
    if (sock < 0) {
        printf("sock < 0.n");
        return sock;
    }

    memset((void *) &addr,sizeof(addr));
    addr.nl_family = AF_NETLINK;
    addr.nl_pid = getpid();
    /* This doesn't work for some reason. See the setsockopt() below. */
    /* addr.nl_groups = MYMGRP; */

    if (bind(sock,(struct sockaddr *) &addr,sizeof(addr)) < 0) {
        printf("bind < 0.n");
        return -1;
    }

    /*
     * 270 is SOL_NETLINK. See
     * http://lxr.free-electrons.com/source/include/linux/socket.h?v=4.1#L314
     * and
     * https://stackoverflow.com/questions/17732044/
     */
    if (setsockopt(sock,&group,sizeof(group)) < 0) {
        printf("setsockopt < 0n");
        return -1;
    }

    return sock;
}

void read_event(int sock)
{
    struct sockaddr_nl nladdr;
    struct msghdr msg;
    struct iovec iov;
    char buffer[65536];
    int ret;

    iov.iov_base = (void *) buffer;
    iov.iov_len = sizeof(buffer);
    msg.msg_name = (void *) &(nladdr);
    msg.msg_namelen = sizeof(nladdr);
    msg.msg_iov = &iov;
    msg.msg_iovlen = 1;

    printf("Ok,listening.n");
    ret = recvmsg(sock,0);
    if (ret < 0)
        printf("ret < 0.n");
    else
        printf("Received message payload: %sn",NLMSG_DATA((struct nlmsghdr *) &buffer));
}

int main(int argc,char *argv[])
{
    int nls;

    nls = open_netlink();
    if (nls < 0)
        return nls;

    while (1)
        read_event(nls);

    return 0;
}

这是内核模块：

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/netlink.h>
#include <net/netlink.h>
#include <net/net_namespace.h>

/* Protocol family,consistent in both kernel prog and user prog. */
#define MYGRP 21

static struct sock *nl_sk = NULL;

static void send_to_user(void)
{
    struct sk_buff *skb;
    struct nlmsghdr *nlh;
    char *msg = "Hello from kernel";
    int msg_size = strlen(msg) + 1;
    int res;

    pr_info("Creating skb.n");
    skb = nlmsg_new(NLMSG_ALIGN(msg_size + 1),GFP_KERNEL);
    if (!skb) {
        pr_err("Allocation failure.n");
        return;
    }

    nlh = nlmsg_put(skb,msg_size + 1,0);
    strcpy(nlmsg_data(nlh),msg);

    pr_info("Sending skb.n");
    res = nlmsg_multicast(nl_sk,skb,GFP_KERNEL);
    if (res < 0)
        pr_info("nlmsg_multicast() error: %dn",res);
    else
        pr_info("Success.n");
}

static int __init hello_init(void)
{
    pr_info("Inserting hello module.n");

    nl_sk = netlink_kernel_create(&init_net,MYPROTO,NULL);
    if (!nl_sk) {
        pr_err("Error creating socket.n");
        return -10;
    }

    send_to_user();

    netlink_kernel_release(nl_sk);
    return 0;
}

static void __exit hello_exit(void)
{
    pr_info("Exiting hello module.n");
}

module_init(hello_init);
module_exit(hello_exit);

MODULE_LICENSE("GPL");

在内核3.13中测试过.

(我可以建议人们使用libnl-3而不是原始套接字用于用户空间程序.它的多播Netlink文档是actually decent.)