Linux Kernel /proc Interface – sample kernel module /proc access to control netfilter hook

Here is my sample source-code discussed in the video:

/* proc_example.c - /proc access to drop pkts
 * Author: Kiran Kankipati
 * Updated: 25-feb-2017
 */
#include <linux/kernel.h>
#include <linux/types.h>
#include <linux/module.h> 
#include <linux/errno.h>
#include <linux/slab.h>
#include <linux/netfilter.h>
#include <linux/netfilter_ipv4.h>
#include <linux/skbuff.h>  
#include <linux/udp.h>
#include <linux/ip.h>
#include <linux/in.h>
#include <linux/string.h>
#include <linux/init.h>
#include <linux/net.h>
#include <linux/netdevice.h>
#include <linux/socket.h>
#include <linux/sockios.h>
#include <linux/inet.h>
#include <linux/inetdevice.h>
#include <linux/netdevice.h>
#include <linux/etherdevice.h>
#include <linux/if_arp.h>
#include <linux/icmp.h>
#include <linux/proc_fs.h>
#include <linux/netlink.h>
#include <linux/mroute.h>
#include <net/checksum.h>
#include <net/inet_ecn.h>
#include <net/xfrm.h>
#include <net/route.h>
#include <net/sock.h>
#include <net/ip.h>
#include <net/tcp.h>
#include <net/arp.h>
#include <net/udp.h>
#include <net/icmp.h>
#include <net/inetpeer.h>
#include <net/protocol.h>
#include <net/flow.h>
#include <asm/types.h>

struct proc_dir_entry *tlc_proc_a;

int drop_pkts=0;

#define PROCFS_MAX_SIZE 30
char proc_buf[PROCFS_MAX_SIZE];

static ssize_t tlc_proc_a_write(struct file *fp, const char *buf, size_t len, loff_t * off)
{	int val=0;
	if(len > PROCFS_MAX_SIZE) { return -EFAULT; }
   if(copy_from_user(proc_buf, buf, len)) { return -EFAULT; }

	val=simple_strtoul(proc_buf,NULL,10); if(val==0 || val==1) { drop_pkts=val; }
	return len;
}


static ssize_t tlc_proc_a_read(struct file *fp, char *buf, size_t len, loff_t * off)
{	static int finished=0; if(finished) {finished=0;return 0;} finished=1;
	
	if(drop_pkts) { sprintf(buf, "drop_pkts: true\n"); } else { sprintf(buf, "drop_pkts: false\n"); }
	return strlen(buf);
}

static struct file_operations tlc_proc_a_fops = { .owner=THIS_MODULE, .read=tlc_proc_a_read, .write=tlc_proc_a_write, };

static struct nf_hook_ops nfho_pre_routing;

unsigned int pre_routing_hook_func(void *priv, struct sk_buff *skb, const struct nf_hook_state *state)
{
	if(drop_pkts) { return NF_DROP; }
	return NF_ACCEPT;
}

static int hello_init(void)
{
 	tlc_proc_a = proc_create( "tlc_proc_a", 0666, NULL, &tlc_proc_a_fops); 
 	if(tlc_proc_a==NULL) {	printk(KERN_ALERT "Error: Could not initialize %s\n", "tlc_proc_a"); goto hello_init_end; }
 	
 	//Packet RX
	nfho_pre_routing.hook = pre_routing_hook_func;
	nfho_pre_routing.hooknum = NF_INET_PRE_ROUTING;
	nfho_pre_routing.pf = PF_INET;
	nfho_pre_routing.priority = NF_IP_PRI_FIRST;
	nf_register_hook(&nfho_pre_routing);

hello_init_end:	
	return 0;
}

static void hello_exit(void) 
{  remove_proc_entry("tlc_proc_a", NULL);
	nf_unregister_hook(&nfho_pre_routing);
}

module_init(hello_init);
module_exit(hello_exit);

Its makefile:

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

I also conduct sessions/classes on Systems and Network Software Programming, Linux Kernel Programming and Architecture. If you are interested, click HERE for more details.

If you have any queries or anything to discuss further on Linux Kernel Programming and writing Kernel modules kindly feel free to contact me.