How to use Netlink Sockets

 After reading kernel source I finally managed to make netlink sockets work for me. Below is an example of Netlink socket basics i.e opening a netlink socket, reading and writing to it and closing it.

Kernel Module

#include <linux/module.h>

#include <net/sock.h>

#include <linux/netlink.h>

#include <linux/skbuff.h>

#define NETLINK_USER 31

struct sock *nl_sk = NULL;

static void hello_nl_recv_msg(struct sk_buff *skb)

{

    struct nlmsghdr *nlh;

    int pid;

    struct sk_buff *skb_out;

    int msg_size;

    char *msg="Hello from kernel";

    int res;

    printk(KERN_INFO "Entering: %s\n", __FUNCTION__);

    

    msg_size=strlen(msg);

    

    nlh=(struct nlmsghdr*)skb->data;

    printk(KERN_INFO "Netlink received msg payload: %s\n",(char*)nlmsg_data(nlh));

    pid = nlh->nlmsg_pid; /*pid of sending process */

    skb_out = nlmsg_new(msg_size,0);

    if(!skb_out)

    {

        printk(KERN_ERR "Failed to allocate new skb\n");

        return;

    } 

    nlh=nlmsg_put(skb_out,0,0,NLMSG_DONE,msg_size,0);

    NETLINK_CB(skb_out).dst_group = 0; /* not in mcast group */

    strncpy(nlmsg_data(nlh),msg,msg_size);

    

    res=nlmsg_unicast(nl_sk,skb_out,pid);

    

    if(res<0)

        printk(KERN_INFO "Error while sending bak to user\n");

}

static int __init hello_init(void)

{

    printk("Entering: %s\n",__FUNCTION__);

    nl_sk=netlink_kernel_create(&init_net, NETLINK_USER, 0, hello_nl_recv_msg, NULL, THIS_MODULE);

    if(!nl_sk)

    {

        printk(KERN_ALERT "Error creating socket.\n");

        return -10;

    }

    return 0;

}

static void __exit hello_exit(void){

    printk(KERN_INFO "exiting hello module\n");

    netlink_kernel_release(nl_sk);

}

module_init(hello_init);

module_exit(hello_exit);

MODULE_LICENSE("GPL");

User Program

#include <sys/socket.h>

#include <linux/netlink.h>

#define NETLINK_USER 31

#define MAX_PAYLOAD 1024  /* maximum payload size*/

struct sockaddr_nl src_addr, dest_addr;

struct nlmsghdr *nlh = NULL;

struct iovec iov;

int sock_fd;

struct msghdr msg;

void main() {

    sock_fd=socket(PF_NETLINK, SOCK_RAW, NETLINK_USER);

    if(sock_fd<0)

return -1;

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

    src_addr.nl_family = AF_NETLINK;

    src_addr.nl_pid = getpid();  /* self pid */

    /* interested in group 1<<0 */

    bind(sock_fd, (struct sockaddr*)&src_addr,

        sizeof(src_addr));

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

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

    dest_addr.nl_family = AF_NETLINK;

    dest_addr.nl_pid = 0;   /* For Linux Kernel */

    dest_addr.nl_groups = 0; /* unicast */

    

    nlh = (struct nlmsghdr *)malloc(NLMSG_SPACE(MAX_PAYLOAD));

    memset(nlh, 0, NLMSG_SPACE(MAX_PAYLOAD));

    nlh->nlmsg_len = NLMSG_SPACE(MAX_PAYLOAD);

    nlh->nlmsg_pid = getpid();

    nlh->nlmsg_flags = 0;

    strcpy(NLMSG_DATA(nlh), "Hello");

    iov.iov_base = (void *)nlh;

    iov.iov_len = nlh->nlmsg_len;

    msg.msg_name = (void *)&dest_addr;

    msg.msg_namelen = sizeof(dest_addr);

    msg.msg_iov = &iov;

    msg.msg_iovlen = 1;

    printf("Sending message to kernel\n");

    sendmsg(sock_fd,&msg,0);

    printf("Waiting for message from kernel\n");

    /* Read message from kernel */

    recvmsg(sock_fd, &msg, 0);

    printf(" Received message payload: %s\n",

           NLMSG_DATA(nlh));

    close(sock_fd);

}