Question

I’m creating a module that would capture the TCP/IP packets from TCP/IP stack and send this to Kernel which from ther sends to the userspace. My code right now has been completed for second half. It sends a message from Kernel to userspace. Can someone help me to capture packets. I dont wanna use libcap

#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";//here we have to send the packet data
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.c

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

int main()
{
printf("socket ");
sock_fd=socket(PF_NETLINK, SOCK_RAW, NETLINK_USER);

if(sock_fd<0)
{
printf("socket invalid\n");
    return -1;
}
printf("socket created\n");
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", NLMSG_DATA(nlh));
close(sock_fd);
return 0;
}

Answer 1

You can use the netfilter API to do this in kernel land. Here is a good article with a kernel module filtering some UDP packets.

May I know the reason for you not using libpcap?