How to print data from TCP packets
Below is an example which does exactly what you need: hook received TCP packets and print their payloads. If you want to print some other information from received packet (like binary data), you just need to modify a bit the section under this comment:
/* ----- Print all needed information from received TCP packet ------ */
If you need to trace transmitted packets instead of received ones, you can replace this line:
nfho.hooknum = NF_INET_PRE_ROUTING;
with this one:
nfho.hooknum = NF_INET_POST_ROUTING;
Save next files and issue make command to build kernel module. Then do sudo insmod print_tcp.ko to load it. After that you will be able to see sniffed information using dmesg command. If you want to unload your module, run sudo rmmod print_tcp command.
print_tcp.c:
#include <linux/module.h>
#include <linux/netfilter.h>
#include <linux/netfilter_ipv4.h>
#include <linux/ip.h>
#include <linux/tcp.h>
#define PTCP_WATCH_PORT 80 /* HTTP port */
static struct nf_hook_ops nfho;
static unsigned int ptcp_hook_func(const struct nf_hook_ops *ops,
struct sk_buff *skb,
const struct net_device *in,
const struct net_device *out,
int (*okfn)(struct sk_buff *))
{
struct iphdr *iph; /* IPv4 header */
struct tcphdr *tcph; /* TCP header */
u16 sport, dport; /* Source and destination ports */
u32 saddr, daddr; /* Source and destination addresses */
unsigned char *user_data; /* TCP data begin pointer */
unsigned char *tail; /* TCP data end pointer */
unsigned char *it; /* TCP data iterator */
/* Network packet is empty, seems like some problem occurred. Skip it */
if (!skb)
return NF_ACCEPT;
iph = ip_hdr(skb); /* get IP header */
/* Skip if it's not TCP packet */
if (iph->protocol != IPPROTO_TCP)
return NF_ACCEPT;
tcph = tcp_hdr(skb); /* get TCP header */
/* Convert network endianness to host endiannes */
saddr = ntohl(iph->saddr);
daddr = ntohl(iph->daddr);
sport = ntohs(tcph->source);
dport = ntohs(tcph->dest);
/* Watch only port of interest */
if (sport != PTCP_WATCH_PORT)
return NF_ACCEPT;
/* Calculate pointers for begin and end of TCP packet data */
user_data = (unsigned char *)((unsigned char *)tcph + (tcph->doff * 4));
tail = skb_tail_pointer(skb);
/* ----- Print all needed information from received TCP packet ------ */
/* Show only HTTP packets */
if (user_data[0] != 'H' || user_data[1] != 'T' || user_data[2] != 'T' ||
user_data[3] != 'P') {
return NF_ACCEPT;
}
/* Print packet route */
pr_debug("print_tcp: %pI4h:%d -> %pI4h:%d
", &saddr, sport,
&daddr, dport);
/* Print TCP packet data (payload) */
pr_debug("print_tcp: data:
");
for (it = user_data; it != tail; ++it) {
char c = *(char *)it;
if (c == '')
break;
printk("%c", c);
}
printk("
");
return NF_ACCEPT;
}
static int __init ptcp_init(void)
{
int res;
nfho.hook = (nf_hookfn *)ptcp_hook_func; /* hook function */
nfho.hooknum = NF_INET_PRE_ROUTING; /* received packets */
nfho.pf = PF_INET; /* IPv4 */
nfho.priority = NF_IP_PRI_FIRST; /* max hook priority */
res = nf_register_hook(&nfho);
if (res < 0) {
pr_err("print_tcp: error in nf_register_hook()
");
return res;
}
pr_debug("print_tcp: loaded
");
return 0;
}
static void __exit ptcp_exit(void)
{
nf_unregister_hook(&nfho);
pr_debug("print_tcp: unloaded
");
}
module_init(ptcp_init);
module_exit(ptcp_exit);
MODULE_AUTHOR("Sam Protsenko");
MODULE_DESCRIPTION("Module for printing TCP packet data");
MODULE_LICENSE("GPL");
Makefile:
ifeq ($(KERNELRELEASE),)
KERNELDIR ?= /lib/modules/$(shell uname -r)/build
module:
$(MAKE) -C $(KERNELDIR) M=$(PWD) C=1 modules
clean:
$(MAKE) -C $(KERNELDIR) M=$(PWD) C=1 clean
.PHONY: module clean
else
MODULE = print_tcp.o
CFLAGS_$(MODULE) := -DDEBUG
obj-m := $(MODULE)
endif
Explanation
I would recommend you to read this book: [4]. Particularly you are interested in next chapters:
- chapter 11: Layer 4 Protocols
- TCP (Transmission Control Protocol)
- Receiving Packets from the Network Layer (L3) with TCP
- Sending Packets with TCP
- chapter 9: Netfilter
How to obtain Linux kernel source code
You can obtain kernel source code using one of ways you prefer:
Vanilla kernel from kernel.org (more specifically from kernel/git/torvalds/linux.git), using Git. E.g. if you need k3.13, it can be done next way:
$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
$ cd linux/
$ git checkout v3.13
Kernel sources from your distro. E.g. in Debian you can just install linux-source package (sources will be installed to /usr/src). For Ubuntu see these instructions.
Details:
[1] How to get TCP header from sk_buff
[2] Network flow control in Linux kernel
[3] Writing Loadable Kernel Modules using netfilter hooks
[4] "Linux Kernel Networking: Implementation and Theory" by Rami Rosen
[5] How to access data/payload from tcphdr
UPDATE
where the hook captures packets for this example? In other words, is it upon TCP stack so that I don't need to take care of packet losing, reordering, etc.?
Netfilter hook is called in ip_rcv() function (here), so you are basically working in IPv4 layer (which is Network layer in OSI). So I believe packet loss handling, packet reordering etc. is not handled yet in that netfilter hook.
See next links for insights:
If you want a hook packets upon Transport layer (TCP) -- netfilter is not sufficient for this task, as it works exclusively in Network layer (IPv4).
