I'm working on a small firewall, i had to retrieve the datas from each tcp packet from port 80 (http) for parsing them. This code works well on a debian 32 bits virtual machine, i'm able to print the headers of each web page, but when i try to load my kernel module and to transfer some datas through the http port, it prints no datas.
When i compile, it shows those warnings only on my 64bits computer :
/home/dev3/C/FIREWALL/firewall.c: In function ‘hook_func’:
/home/dev3/C/FIREWALL/firewall.c:179: warning: cast from pointer to integer of different size
/home/dev3/C/FIREWALL/firewall.c:179: warning: cast to pointer from integer of different size
Anyone got any ideas, please ?
Thanks.
Code :
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/netfilter.h>
#undef __KERNEL__
#include <linux/netfilter_ipv4.h>
#define __KERNEL__
#include <linux/ip.h>
#include <linux/tcp.h>
static struct nf_hook_ops nfho;
unsigned int hook_func( 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 tcphdr * tcph;
unsigned char * http_port = "\x00\x50";
char * data;
if (skb)
{
iph = ip_hdr(skb);
if (iph && iph->protocol && (iph->protocol == IPPROTO_TCP))
{
tcph = (struct tcphdr *)((__u32 *)iph + iph->ihl);
if ((tcph->source) == *(unsigned short *)http_port)
{
data = (char *)((int)tcph + (int)(tcph->doff * 4));
printk(KERN_DEBUG "TCP source : %hu, TCP dest : %hu\n", ntohs(tcph->source), ntohs(tcph->dest));
printk(KERN_DEBUG "TCP seq : %u, TCP ack_seq : %u\n", ntohl(tcph->seq), ntohl(tcph->ack_seq));
printk(KERN_DEBUG "TCP doff : %hu, TCP window : %hu\n", ntohs(tcph->doff), ntohs(tcph->window));
printk(KERN_DEBUG "TCP check : 0x%hx, TCP urg_ptr : %hu\n", ntohs(tcph->check), ntohs(tcph->urg_ptr));
printk(KERN_DEBUG "FLAGS=%c%c%c%c%c%c\n\n",
tcph->urg ? 'U' : '-',
tcph->ack ? 'A' : '-',
tcph->psh ? 'P' : '-',
tcph->rst ? 'R' : '-',
tcph->syn ? 'S' : '-',
tcph->fin ? 'F' : '-');
//printk(KERN_DEBUG "sending packet to : %pI4\n", &iph->daddr);
printk(KERN_DEBUG "data len : %d\ndata : \n", (int) strlen(data));
printk(KERN_DEBUG "%s\n", data);
}
}
}
return NF_ACCEPT;
}
int init_module()
{
int result;
nfho.hook = (nf_hookfn *) hook_func;
nfho.hooknum = NF_IP_POST_ROUTING;
nfho.pf = PF_INET;
nfho.priority = NF_IP_PRI_FIRST;
result = nf_register_hook(&nfho);
if(result)
{
printk(KERN_DEBUG "firewall : erreur nf_register_hook !\n");
return 1;
}
printk(KERN_DEBUG "firewall : module charge.\n");
return 0;
}
void cleanup_module()
{
nf_unregister_hook(&nfho);
printk(KERN_DEBUG "firewall : module decharge.\n");
}
(char *)((int)tcph + (int)(tcph->doff * 4));
is wrong, it should be
(char *)((unsigned char *)tcph + (tcph->doff * 4));
Note that printk(KERN_DEBUG "data len : %d\ndata : \n", (int) strlen(data));
is not at all a safe thing to do. You don't know if the data contains text, and if it does, it might not contain a 0 terminated string which strlen(data) would need to work as you expect.
You probably need to care about endianess as well, when comparing the ports and possibly other fields.