netlink sockets examples in linux

Kernel side code:

//kernel module: netlink-exam-kern.c

#include <linux/module.h>

#include <linux/netlink.h>

#include <linux/sched.h>

#include <net/sock.h>

#include <linux/proc_fs.h>

#include <linux/version.h>

#define BUF_SIZE 16384

static struct sock *netlink_exam_sock;

static unsigned char buffer[BUF_SIZE];

static unsigned int buffer_tail = 0;

#define NETLINK_TEST 21

#if LINUX_VERSION_CODE < KERNEL_VERSION(2, 6, 35)

#define sk_sleep(sk) sk->sk_sleep

#endif

#define RCV_SKB_FAIL(err) do { netlink_ack(skb, nlh, (err)); return; } while (0)

static void recv_handler(struct sk_buff *skb)

{

struct nlmsghdr *nlhdr = (struct nlmsghdr *)skb->data;

size_t len = 0;

if (nlhdr->nlmsg_len < sizeof(struct nlmsghdr)) {

printk("Corrupt netlink message.\n");

}

len = nlhdr->nlmsg_len - NLMSG_LENGTH(0);

if (len + buffer_tail > BUF_SIZE) {

printk("netlink buffer is full.\n");

}

else {

memcpy(buffer + buffer_tail, NLMSG_DATA(nlhdr), len);

buffer_tail += len;

}

}

static int netlink_exam_readproc(char *page, char **start, off_t off,

int count, int *eof, void *data)

{

int len;

if (off >= buffer_tail) {

* eof = 1;

return 0;

}

else {

len = count;

if (count > PAGE_SIZE) {

len = PAGE_SIZE;

}

if (len > buffer_tail - off) {

len = buffer_tail - off;

}

memcpy(page, buffer + off, len);

*start = page;

return len;

}

}

static int __init netlink_exam_init(void)

{

netlink_exam_sock = netlink_kernel_create(&init_net, NETLINK_TEST, 0, recv_handler, NULL, THIS_MODULE);

if (!netlink_exam_sock) {

printk("Fail to create netlink socket.\n");

return -1;

}

create_proc_read_entry("netlink_exam_buffer", 0444, NULL, netlink_exam_readproc, 0);

return 0;

}

static void __exit netlink_exam_exit(void)

{

wake_up(sk_sleep(netlink_exam_sock));

sock_release(netlink_exam_sock->sk_socket);

remove_proc_entry("netlink_exam_buffer", NULL);

}

module_init(netlink_exam_init);

module_exit(netlink_exam_exit);

MODULE_LICENSE("GPL");

Make File:

mname :=file name

$(mname)-objs := netlink-exam-kern.o

obj-m := $(mname).o

KERNELDIR := /lib/modules/`uname -r`/build

all:

$(MAKE) -C $(KERNELDIR) M=`pwd` modules

clean:

$(MAKE) -C $(KERNELDIR) M=`pwd` clean

User side Code:

Send :

//application sender: netlink-exam-user-send.c

#include <stdio.h>

#include <string.h>

#include <stdlib.h>

#include <sys/types.h>

#include <sys/socket.h>

#include <linux/netlink.h>

#define MAX_MSGSIZE 1024

#define NETLINK_TEST 21

int main(int argc, char * argv[])

{

FILE * fp;

struct sockaddr_nl saddr, daddr;

struct nlmsghdr *nlhdr = NULL;

struct msghdr msg;

struct iovec iov;

int sd;

char text_line[MAX_MSGSIZE];

int ret = -1;

if (argc < 2) {

printf("Usage: %s atextfilename\n", argv[0]);

exit(1);

}

if ((fp = fopen(argv[1], "r")) == NULL) {

printf("File %s dosen't exist.\n", argv[1]);

exit(1);

}

sd = socket(AF_NETLINK, SOCK_RAW, NETLINK_TEST);

if (sd == -1) {

printf("Cannot create netlink\n");

exit(1);

}

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

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

saddr.nl_family = AF_NETLINK;

saddr.nl_pid = getpid();

saddr.nl_groups = 11;

bind(sd, (struct sockaddr*)&saddr, sizeof(saddr));

daddr.nl_family = AF_NETLINK;

daddr.nl_pid = 0;

daddr.nl_groups = 1;

nlhdr = (struct nlmsghdr *)malloc(NLMSG_SPACE(MAX_MSGSIZE));

while (fgets(text_line, MAX_MSGSIZE, fp)) {

memcpy(NLMSG_DATA(nlhdr), text_line, strlen(text_line));

memset(&msg, 0 ,sizeof(struct msghdr));

nlhdr->nlmsg_len = NLMSG_LENGTH(strlen(text_line));

nlhdr->nlmsg_pid = getpid(); /* self pid */

nlhdr->nlmsg_flags = 0;

iov.iov_base = (void *)nlhdr;

iov.iov_len = nlhdr->nlmsg_len;

msg.msg_name = (void *)&daddr;

msg.msg_namelen = sizeof(daddr);

msg.msg_iov = &iov;

msg.msg_iovlen = 1;

ret = sendmsg(sd, &msg, 0);

if (ret == -1) {

perror("sendmsg error:");

}

}

close(sd);

}

Receive :

//application receiver: netlink-exam-user-recv.c

#include <stdio.h>

#include <sys/types.h>

#include <sys/socket.h>

#include <linux/netlink.h>

#include <string.h>

#include <stdlib.h>

#define MAX_MSGSIZE 1024

#define NETLINK_TEST 21

int main(void)

{

struct sockaddr_nl saddr, daddr;

struct nlmsghdr *nlhdr = NULL;

struct msghdr msg;

struct iovec iov;

int sd;

int ret = 1;

sd = socket(AF_NETLINK, SOCK_RAW, NETLINK_TEST);

if (sd == -1) {

printf("cannot create netlink\n");

exit(-1);

}

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

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

saddr.nl_family = AF_NETLINK;

saddr.nl_pid = getpid();

saddr.nl_groups = 11;

bind(sd, (struct sockaddr*)&saddr, sizeof(saddr));

nlhdr = (struct nlmsghdr *)malloc(NLMSG_SPACE(MAX_MSGSIZE));

while (1) {

memset(nlhdr, 0, NLMSG_SPACE(MAX_MSGSIZE));

iov.iov_base = (void *)nlhdr;

iov.iov_len = NLMSG_SPACE(MAX_MSGSIZE);

msg.msg_name = (void *)&daddr;

msg.msg_namelen = sizeof(daddr);

msg.msg_iov = &iov;

msg.msg_iovlen = 1;

ret = recvmsg(sd, &msg, 0);

if (ret == 0) {

printf("Exit.\n");

exit(0);

}

else if (ret == -1) {

perror("recvmsg:");

exit(1);

}

printf("%s", (char *)NLMSG_DATA(nlhdr));

}

close(sd);

}
Make file:

.PHONY: clean

all:

gcc netlink-exam-user-recv.c -o netlink-exam-user-recv

gcc netlink-exam-user-send.c -o netlink-exam-user-send

clean:

rm netlink-exam-user-recv netlink-exam-user-send