Netfilter and Scapy

Question

I play with netfilter hooks in the kernel module. And I want to be able to capture packages created by scapy.

Both hooks, as well as packet generation via scapy, work on the same physical node. It seems that none of the available line filter hooks can capture the packet.

I also tried sending the same packet from a virtual machine, but this also does not work.

I suspect the problem is that it is working on a loopback iterface, since all this is in one field.

I could, of course, go with two physical hosts, but unfortunately this is not possible right now :(

static unsigned int out_hook(unsigned int hooknum,
                struct sk_buff *skb,
                const struct net_device *in,
                const struct net_device *out,
                int (*okfn)(struct sk_buff *))
{
    sock_buff = skb; 

    if (!sock_buff) {
        return NF_ACCEPT;
    } else {
            ip_header = (struct iphdr *)skb_network_header(sock_buff);
        if (!ip_header) {
            return NF_ACCEPT;
        } else {
            if (ip_header->protocol == IPPROTO_TCP) {
                th = (struct tcphdr *)(skb_transport_header(sock_buff)+sizeof(struct iphdr));

                printk(KERN_INFO "[LOCAL_OUT] %d.%d.%d.%d:%d -> %d.%d.%d.%d:%d\n", ip_header->saddr & 0x000000FF, (ip_header->saddr & 0x0000FF00) >> 8,(ip_header->saddr & 0x00FF0000) >> 16,(ip_header->saddr & 0xFF000000) >> 24, th->source, ip_header->daddr & 0x000000FF, (ip_header->daddr & 0x0000FF00) >> 8,(ip_header->daddr & 0x00FF0000) >> 16,(ip_header->daddr & 0xFF000000) >> 24, th->dest);

                unsigned int len = sock_buff->len - sizeof(struct tcphdr) - sizeof(struct iphdr);
                printk(KERN_INFO "\t [skbuf->len]=%d", sock_buff->len);
                printk(KERN_INFO "\t [skbuf->data_len]=%d", sock_buff->data_len); 


                return NF_ACCEPT;

            } else {
                return NF_ACCEPT;
            }
        }
    }
}

The above is a hook.

#!/usr/bin/env python

import sys
sys.path.append('/usr/local/bin')

import time
from threading import Thread
from scapy.all import *
from hashlib import sha1, md5
import random
import crypt

conf.iface='wlan0'


packet = IP(dst="192.168.0.104") / TCP(sport=1234, dport=2222) / Raw("testtest")
send(packet)

Above - send.py

Answer 1

Scapy Sniffing

[mpenning@Bucksnort ~]$ sudo python
Python 2.5.2 (r252:60911, Jan 24 2010, 14:53:14)
[GCC 4.3.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from scapy.all import sniff
>>> from scapy.all import wrpcap
>>> foo = sniff(filter="icmp", count=3)
>>> wrpcap("icmppaks.pcap", foo)
>>> quit()
[mpenning@Bucksnort ~]$ tshark -r icmppaks.pcap
  1   0.000000 192.0.2.178 -> 192.0.2.6 ICMP Echo (ping) request
  2   0.000065 192.0.2.6 -> 192.0.2.178 ICMP Echo (ping) reply
  3   1.004224 192.0.2.178 -> 192.0.2.6 ICMP Echo (ping) request
[mpenning@Bucksnort ~]$