Netlink example in C

Instructions

make
gcc -o netlinkUser netlinkUser.c
sudo insmod netlinkKernel.ko
./netlinkUser
sudo rmmod netlinkKernel
make lean

Makefile

KDIR := /lib/modules/$(shell uname -r)/build

obj-m += netlinkKernel.o

all:
	$(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules

clean:
	rm -rf *.o *.ko *.mod.* *.cmd .module* modules* Module* .*.cmd .tmp*
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

netlinkKernel.c

#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__);
	//This is for 3.6 kernels and above.
	struct netlink_kernel_cfg cfg = {
	    .input = hello_nl_recv_msg,
	};
	nl_sk = netlink_kernel_create(&init_net, NETLINK_USER, &cfg);
	//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");

netlinkUser.c

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

int 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 */
	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", (char *)NLMSG_DATA(nlh));
	close(sock_fd);
}