How to use netlink socket to communicate with a kernel module?

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 31struct 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);    struct netlink_kernel_cfg cfg = {        .input = hello_nl_recv_msg,    };    nl_sk = netlink_kernel_create(&init_net, NETLINK_USER, &cfg);    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 <linux/netlink.h>#include <stdio.h>#include <stdlib.h>#include <string.h>#include <sys/socket.h>#include <unistd.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));    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);}

Related thread about the magic constant NETLINK_USER 31: Can I have more than 32 netlink sockets in kernelspace?

Just in case anybody doesn't know how to compile, google "how to compile and load kernel module"

refer to http://www.cyberciti.biz/tips/compiling-linux-kernel-module.html

Grab kernel source code to which you'll compile module against http://kernel.org

Or simply update your headers if you are running intended kernel

# apt-get install kernel-headers-$(uname -r)

Create a makefile, for example

obj-m = hello.oKVERSION = $(shell uname -r)all:        make -C /lib/modules/$(KVERSION)/build M=$(PWD) modulesclean:        make -C /lib/modules/$(KVERSION)/build M=$(PWD) clean

Make and you'll get bunch of files. *.ko is the one you'll load into your kernel, run

# insmod hello.ko

if you us lsmod to check all loaded modules, you'll find yours, most likely you will see:

hello       12575  0 

In our case, compile and run user code:

gcc hello.c -o hello.o./hello.o

If everything is OK, you'll get following message using binW's code:

Sending message to kernelWaiting for message from kernel Received message payload: Hello from kernel

Finally, remove the module using:

# rmmod hello

It works for me with kernel 3.2. For kernel 3.6 & above, it needs a bit of a change at the netlink_kernel_create function.

 struct netlink_kernel_cfg cfg = {                .groups = 1,                .input = hello_nl_recv_msg,        };        printk("Entering: %s\n", __FUNCTION__);        nl_sk = netlink_kernel_create(&init_net, NETLINK_USER, &cfg);