1/*	$OpenBSD: ip_id.c,v 1.25 2021/03/10 10:21:48 jsg Exp $ */
2
3/*
4 * Copyright (c) 2008 Theo de Raadt, Ryan McBride
5 *
6 * Slightly different algorithm from the one designed by
7 * Matthew Dillon <dillon@backplane.com> for The DragonFly Project
8 *
9 * Permission to use, copy, modify, and distribute this software for any
10 * purpose with or without fee is hereby granted, provided that the above
11 * copyright notice and this permission notice appear in all copies.
12 *
13 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
14 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
15 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
16 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
17 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
18 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
19 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
20 */
21
22/*
23 * Random ip sequence number generator.  Use the system PRNG to shuffle
24 * the 65536 entry ID space.  We reshuffle the ID we pick out of the array
25 * into the previous 32767 cells, providing an guarantee that an ID will not
26 * be reused for at least 32768 calls.
27 */
28#include <sys/param.h>
29#include <sys/systm.h>
30
31static u_int16_t ip_shuffle[65536];
32static int isindex = 0;
33
34u_int16_t ip_randomid(void);
35
36/*
37 * Return a random IP id.  Shuffle the new value we get into the previous half
38 * of the ip_shuffle ring (-32767 or swap with ourself), to avoid duplicates
39 * occurring too quickly but also still be random.
40 *
41 * 0 is a special IP ID -- don't return it.
42 */
43u_int16_t
44ip_randomid(void)
45{
46	static int ipid_initialized;
47	u_int16_t si, r;
48	int i, i2;
49
50	if (!ipid_initialized) {
51		ipid_initialized = 1;
52
53		/*
54		 * Initialize with a random permutation. Do so using Knuth
55		 * which avoids the exchange in the Durstenfeld shuffle.
56		 * (See "The Art of Computer Programming, Vol 2" 3rd ed, pg. 145).
57		 *
58		 * Even if our PRNG is imperfect at boot time, we have deferred
59		 * doing this until the first packet being sent and now must
60		 * generate an ID.
61		 */
62		for (i = 0; i < nitems(ip_shuffle); ++i) {
63			i2 = arc4random_uniform(i + 1);
64			ip_shuffle[i] = ip_shuffle[i2];
65			ip_shuffle[i2] = i;
66		}
67	}
68
69	do {
70		arc4random_buf(&si, sizeof(si));
71		i = isindex & 0xFFFF;
72		i2 = (isindex - (si & 0x7FFF)) & 0xFFFF;
73		r = ip_shuffle[i];
74		ip_shuffle[i] = ip_shuffle[i2];
75		ip_shuffle[i2] = r;
76		isindex++;
77	} while (r == 0);
78
79	return (r);
80}
81