mod_tcp_chaff.c

/*
 * mod_tcp_chaff.c
 *
 * Copyright (c) 2001 Dug Song <dugsong@monkey.org>
 *
 * $Id: mod_tcp_chaff.c,v 1.11 2002/04/07 22:55:20 dugsong Exp $
 */

#include "config.h"

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include "pkt.h"
#include "mod.h"
#include "randutil.h"

#define CHAFF_TYPE_CKSUM	1
#define CHAFF_TYPE_NULL		2
#define CHAFF_TYPE_PAWS		3
#define CHAFF_TYPE_REXMIT	4
#define CHAFF_TYPE_SEQ		5
#define CHAFF_TYPE_SYN		6
#define CHAFF_TYPE_TTL		7

struct tcp_chaff_data {
	rand_t		*rnd;
	int		 type;
	int		 ttl;
};

void *
tcp_chaff_close(void *d)
{
	struct tcp_chaff_data *data = (struct tcp_chaff_data *)d;
	
	if (data != NULL) {
		rand_close(data->rnd);
		free(data);
	}
	return (NULL);
}

void *
tcp_chaff_open(int argc, char *argv[])
{
	struct tcp_chaff_data *data;
	
	if (argc < 2)
		return (NULL);
	
	if ((data = calloc(1, sizeof(*data))) == NULL)
		return (NULL);

	data->rnd = rand_open();
	
	if (strcasecmp(argv[1], "cksum") == 0)
		data->type = CHAFF_TYPE_CKSUM;
	else if (strcasecmp(argv[1], "null") == 0)
		data->type = CHAFF_TYPE_NULL;
	else if (strcasecmp(argv[1], "paws") == 0)
		data->type = CHAFF_TYPE_PAWS;
	else if (strcasecmp(argv[1], "rexmit") == 0)
		data->type = CHAFF_TYPE_REXMIT;
	else if (strcasecmp(argv[1], "seq") == 0)
		data->type = CHAFF_TYPE_SEQ;
	else if (strcasecmp(argv[1], "syn") == 0)
		data->type = CHAFF_TYPE_SYN;
	else if ((data->ttl = atoi(argv[1])) > 0 && data->ttl < 256)
		data->type = CHAFF_TYPE_TTL;
	else
		return (tcp_chaff_close(data));
	
	return (data);
}

int
tcp_chaff_apply(void *d, struct pktq *pktq)
{
	struct tcp_chaff_data *data = (struct tcp_chaff_data *)d;
	struct pkt *pkt, *new, *next;
	struct tcp_opt opt;
	int i;
	
	for (pkt = TAILQ_FIRST(pktq); pkt != TAILQ_END(pktq); pkt = next) {
		next = TAILQ_NEXT(pkt, pkt_next);
		
		if (pkt->pkt_ip == NULL || pkt->pkt_ip->ip_p != IP_PROTO_TCP ||
		    pkt->pkt_tcp == NULL || pkt->pkt_tcp_data == NULL ||
		    (pkt->pkt_tcp->th_flags & TH_ACK) == 0)
			continue;
		
		new = pkt_dup(pkt);
		rand_strset(data->rnd, new->pkt_tcp_data, new->pkt_end -
		    new->pkt_tcp_data + 1);
	
		switch (data->type) {
		case CHAFF_TYPE_CKSUM:
			ip_checksum(new->pkt_ip,
			    new->pkt_ip_data - new->pkt_eth_data);
			new->pkt_tcp->th_sum = rand_uint16(data->rnd);
			break;
		case CHAFF_TYPE_NULL:
			new->pkt_tcp->th_flags = 0;
			ip_checksum(new->pkt_ip, new->pkt_end -
			    new->pkt_eth_data);
			break;
		case CHAFF_TYPE_PAWS:
			/* Delete any existing TCP options. */
			i = (new->pkt_tcp->th_off << 2) - TCP_HDR_LEN;
			new->pkt_tcp->th_off = 5;
			new->pkt_end -= i;
			new->pkt_ip->ip_len = htons(new->pkt_end -
			    new->pkt_eth_data);
			
			/* Insert initial timestamp, for PAWS elimination. */
			opt.opt_type = TCP_OPT_TIMESTAMP;
			opt.opt_len = TCP_OPT_LEN + 8;
			opt.opt_data.timestamp[0] = 0;
			opt.opt_data.timestamp[1] = 0;
			if ((i = ip_add_option(new->pkt_ip,
			    PKT_BUF_LEN - ETH_HDR_LEN,
			    IP_PROTO_TCP, &opt, opt.opt_len)) < 0) {
				pkt_free(new);
				continue;
			}
			new->pkt_end += i;
			ip_checksum(new->pkt_ip, new->pkt_end -
			    new->pkt_eth_data);
			pkt_decorate(new);
			break;
		case CHAFF_TYPE_REXMIT:
			new->pkt_ts.tv_usec = 1;
			ip_checksum(new->pkt_ip, new->pkt_end -
			    new->pkt_eth_data);
			break;
		case CHAFF_TYPE_SEQ:
			/* XXX - dunno recv window? */
			new->pkt_tcp->th_seq = htonl(666);
			new->pkt_tcp->th_ack = htonl(666);
			ip_checksum(new->pkt_ip, new->pkt_end -
			    new->pkt_eth_data);
			break;
		case CHAFF_TYPE_SYN:
			new->pkt_tcp->th_flags = TH_SYN;
			new->pkt_tcp->th_seq = rand_uint32(data->rnd);
			new->pkt_tcp->th_ack = 0;
			new->pkt_end = new->pkt_tcp_data;
			new->pkt_tcp_data = NULL;
			new->pkt_ip->ip_len = htons(new->pkt_end -
			    new->pkt_eth_data);
			ip_checksum(new->pkt_ip, new->pkt_end -
			    new->pkt_eth_data);
			break;
		case CHAFF_TYPE_TTL:
			new->pkt_ip->ip_ttl = data->ttl;
			ip_checksum(new->pkt_ip, new->pkt_end -
			    new->pkt_eth_data);
			break;
		}
		/* Minimal random reordering. */
		if ((new->pkt_tcp->th_sum & 1) == 0)
			TAILQ_INSERT_BEFORE(pkt, new, pkt_next);
		else
			TAILQ_INSERT_AFTER(pktq, pkt, new, pkt_next);
	}
	return (0);
}

struct mod mod_tcp_chaff = {
	"tcp_chaff",						/* name */
	"tcp_chaff cksum|null|paws|rexmit|seq|syn|<ttl>",	/* usage */
	tcp_chaff_open,						/* open */
	tcp_chaff_apply,					/* apply */
	tcp_chaff_close						/* close */
};