So here’s what I am trying to do(trivial, I know; I am doing this to learn something for a project):
I’ve built this module to catch all outgoing traffic, check if it’s ICMP echo message traffic.
If it is, it simply re-calculates the checksum of the ICMP packet and then let it go on its way.
Every time I insmod this module, all PING traffic fails >.<
Can you please tell me what I am doing wrong here?
/*
Coder: Adel *. *******
Creation Date: April/7th/2012
Last Modification Date: April/9th/2012
Purpose: This module is merely a prototype on how to change the IP/ICMP pakcet information and still let it go without problems
Testing: This module is being tested on a machine running the Linux kernel 2.6.32-33 on a 64bits Intel Processor
Notes: N/A
*/
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/inet.h>
#include <linux/ip.h>
#include <linux/icmp.h>
#include <linux/tcp.h>
#include <linux/in.h>
#include <linux/netfilter.h>
#include <linux/netfilter_ipv4.h>
static struct nf_hook_ops nfho;
static void printICMPHeader(struct icmphdr *icmph);
/*
* in_cksum --
* Checksum routine for Internet Protocol
* family headers (C Version)
*/
unsigned short in_cksum(unsigned short *addr, int len)
{
register int sum = 0;
u_short answer = 0;
register u_short *w = addr;
register int nleft = len;
/*
* Our algorithm is simple, using a 32 bit accumulator (sum), we add
* sequential 16 bit words to it, and at the end, fold back all the
* carry bits from the top 16 bits into the lower 16 bits.
*/
while (nleft > 1)
{
sum += *w++;
nleft -= 2;
}
/* mop up an odd byte, if necessary */
if (nleft == 1)
{
*(u_char *) (&answer) = *(u_char *) w;
sum += answer;
}
/* add back carry outs from top 16 bits to low 16 bits */
sum = (sum >> 16) + (sum & 0xffff); /* add hi 16 to low 16 */
sum += (sum >> 16); /* add carry */
answer = ~sum; /* truncate to 16 bits */
return (answer);
}
static unsigned int icmp_check(unsigned int hooknum,
struct sk_buff *skb,
const struct net_device *in,
const struct net_device *out,
int (*okfn)(struct sk_buff *))
{
struct iphdr *iph;
struct icmphdr *icmph;
struct tcphdr *tcph;
if(skb == NULL)
return -1;
iph = ip_hdr(skb);
if(iph->protocol == IPPROTO_ICMP){
printk(KERN_DEBUG"ICMP traffic!\n");
icmph = icmp_hdr(skb);
if(icmph->type == ICMP_ECHO){
printICMPHeader(icmph);
icmph->checksum = in_cksum((unsigned short *)icmph, sizeof(struct icmphdr));
printICMPHeader(icmph);
}
}/* If IPPROTO_ICMP */
return NF_ACCEPT;
}
static void printICMPHeader(struct icmphdr *icmph)
{
printk(KERN_DEBUG "ICMP print function begin \n");
printk(KERN_DEBUG "ICMP type = %d\n", icmph->type);
printk(KERN_DEBUG "ICMP code = %d\n", icmph->code);
printk(KERN_DEBUG "ICMP checksum = %d\n", icmph->checksum);
printk(KERN_DEBUG "ICMP id = %d\n", icmph->un.echo.id);
printk(KERN_DEBUG "ICMP sequence = %d\n", icmph->un.echo.sequence);
printk(KERN_DEBUG "ICMP print function exit \n");
}
static int __init startup(void)
{
printk(KERN_INFO "Loading Test module...\n");
printk(KERN_ALERT "Hello world\n");
/* Fill in our hook structure */
nfho.hook = icmp_check; /* Handler function */
nfho.hooknum = NF_INET_POST_ROUTING; /* Just before it hits the wire */
nfho.pf = PF_INET;
nfho.priority = NF_IP_PRI_FILTER;
nf_register_hook(&nfho);
//pinger();
return 0;
}
static void __exit cleanup(void)
{
nf_unregister_hook(&nfho);
printk(KERN_ALERT "Goodbye Mr.\n");
}
module_init(startup);
module_exit(cleanup);
EDIT:
To debug the code a little bit, I’ve made my own user-space ping utility and I’ve filled all of its IP and ICMP headers using RAW_SOCKETS
icmp->type = ICMP_ECHO;
icmp->code = 0;
icmp->un.echo.id = 0;
icmp->un.echo.sequence = 0;
icmp-> checksum = in_cksum((unsigned short *)icmp, sizeof(struct icmphdr));
This utility works perfectly fine as long as my Module is not loaded.
Weirdly enough, when I load my module and check the Kernel Debug file, look at what I get:
Apr 9 10:42:10 DHS-1022CYB kernel: [ 2521.862356] ICMP traffic!
Apr 9 10:42:58 DHS-1022CYB kernel: [ 2569.572346] ICMP traffic!
Apr 9 10:43:22 DHS-1022CYB kernel: [ 2593.317201] ICMP traffic!
Apr 9 10:43:56 DHS-1022CYB kernel: [ 2627.331320] ICMP traffic!
Apr 9 10:44:05 DHS-1022CYB kernel: [ 2636.802236] ICMP traffic!
Apr 9 10:44:08 DHS-1022CYB kernel: [ 2639.876490] ICMP traffic!
Apr 9 10:45:27 DHS-1022CYB kernel: [ 2718.422229] ICMP traffic!
This basically means that I, for some odd reason, am not even able to catch the ECHO traffic in my module!
(When I cannot catch it, it simply goes out and works perfectly fine)
P.S I tried to change the hook to LOCAL_OUT and got the same result
EDIT2: results of the DEBUG file changed is this
Apr 9 10:57:24 DHS-1022CYB kernel: [ 3435.916336] ICMP print function exit
Apr 9 10:57:25 DHS-1022CYB kernel: [ 3436.922656] ICMP traffic!
Apr 9 10:57:25 DHS-1022CYB kernel: [ 3436.922665] ICMP print function begin
Apr 9 10:57:25 DHS-1022CYB kernel: [ 3436.922670] ICMP type = 8
Apr 9 10:57:25 DHS-1022CYB kernel: [ 3436.922674] ICMP code = 0
Apr 9 10:57:25 DHS-1022CYB kernel: [ 3436.922677] ICMP checksum = 50252
Apr 9 10:57:25 DHS-1022CYB kernel: [ 3436.922681] ICMP id = 3673
Apr 9 10:57:25 DHS-1022CYB kernel: [ 3436.922685] ICMP sequence = 512
Apr 9 10:57:25 DHS-1022CYB kernel: [ 3436.922688] ICMP print function exit
Apr 9 10:57:25 DHS-1022CYB kernel: [ 3436.922691] ICMP print function begin
Apr 9 10:57:25 DHS-1022CYB kernel: [ 3436.922695] ICMP type = 8
Apr 9 10:57:25 DHS-1022CYB kernel: [ 3436.922698] ICMP code = 0
Apr 9 10:57:25 DHS-1022CYB kernel: [ 3436.922702] ICMP checksum = 11090
Apr 9 10:57:25 DHS-1022CYB kernel: [ 3436.922705] ICMP id = 3673
Apr 9 10:57:25 DHS-1022CYB kernel: [ 3436.922709] ICMP sequence = 512
Apr 9 10:57:25 DHS-1022CYB kernel: [ 3436.922712] ICMP print function exit
Note however that this is the result of the Linux utility ping, not my hand-written PING(Which I still for some reason cannot intercept).
And the Linux ping is not working as long as my Module is loaded.
You’re not calculating the checksum correctly… as you can see by your logs. The ICMP checksum is calculated over the whole message, not just the header. So in your case:
Should be:
also, don’t forget to initialize the field to 0.