Kick Start your Netlink--->Basic netlink kernel and user module....

nluser.c



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

#define NETLINK_USER 12

#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;

int main()
{
int i=0;
sock_fd=socket(PF_NETLINK, SOCK_RAW, NETLINK_USER);
if(sock_fd<0 p=""> 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 p=""> bind(sock_fd, (struct sockaddr*)&src_addr,
sizeof(src_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");
while(1)
{
sendmsg(sock_fd,&msg,0);
printf("Waiting for message from kernel\n");
sleep(1);
/* Read message from kernel */
recvmsg(sock_fd, &msg, 0);
printf(" Received message payload: %s\n",
NLMSG_DATA(nlh));
nlh->nlmsg_pid = getpid();

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;
i++;
}
close(sock_fd);
}



nlkern.c



#include linux/module.h
#include linux/netlink.h
#include linux/skbuff.h
#include net/sock.h

#define NETLINK_USER 31

struct sock *nl_sk = NULL;
char *mesg[2]={"Hello From Kernel-111111111111111",
"Bye From Kernel-2111111111111"};
static void hello_nl_recv_msg(struct sk_buff *skb)
{
    struct nlmsghdr *nlh;
    int pid;
    struct sk_buff *skb_out;
    int msg_size;
  static int i=0;
    int res;



    msg_size=strlen(mesg[i]);
 
    nlh=(struct nlmsghdr*)skb->data;
    printk(KERN_INFO "Netlink received msg payload: %s& i=%d\n",(char*)nlmsg_data(nlh),i);
    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),mesg[i],msg_size);
 
    res=nlmsg_unicast(nl_sk,skb_out,pid);
    if (i==0)
i++;
else i=0;

    if(res<0 p="">        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");




Makefile(kernel module compilation)


obj-m = nlkern.o
KVERSION=$(shell uname -r)
all:
        make -C /lib/modules/$(KVERSION)/build M=$(PWD) modules

clean:
        make -C /lib/modules/$(KVERSION)/build M=$(PWD) clean

for nluser.c

gcc nluser.c -o nlusr_out