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/* $OpenBSD: ip_id.c,v 1.25 2021/03/10 10:21:48 jsg Exp $ */
/*
* Copyright (c) 2008 Theo de Raadt, Ryan McBride
*
* Slightly different algorithm from the one designed by
* Matthew Dillon <dillon@backplane.com> for The DragonFly Project
*
* Permission to use, copy, modify, and distribute this software for any
* purpose with or without fee is hereby granted, provided that the above
* copyright notice and this permission notice appear in all copies.
*
* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/
/*
* Random ip sequence number generator. Use the system PRNG to shuffle
* the 65536 entry ID space. We reshuffle the ID we pick out of the array
* into the previous 32767 cells, providing an guarantee that an ID will not
* be reused for at least 32768 calls.
*/
#include <sys/param.h>
#include <sys/systm.h>
static u_int16_t ip_shuffle[65536];
static int isindex = 0;
u_int16_t ip_randomid(void);
/*
* Return a random IP id. Shuffle the new value we get into the previous half
* of the ip_shuffle ring (-32767 or swap with ourself), to avoid duplicates
* occurring too quickly but also still be random.
*
* 0 is a special IP ID -- don't return it.
*/
u_int16_t
ip_randomid(void)
{
static int ipid_initialized;
u_int16_t si, r;
int i, i2;
if (!ipid_initialized) {
ipid_initialized = 1;
/*
* Initialize with a random permutation. Do so using Knuth
* which avoids the exchange in the Durstenfeld shuffle.
* (See "The Art of Computer Programming, Vol 2" 3rd ed, pg. 145).
*
* Even if our PRNG is imperfect at boot time, we have deferred
* doing this until the first packet being sent and now must
* generate an ID.
*/
for (i = 0; i < nitems(ip_shuffle); ++i) {
i2 = arc4random_uniform(i + 1);
ip_shuffle[i] = ip_shuffle[i2];
ip_shuffle[i2] = i;
}
}
do {
arc4random_buf(&si, sizeof(si));
i = isindex & 0xFFFF;
i2 = (isindex - (si & 0x7FFF)) & 0xFFFF;
r = ip_shuffle[i];
ip_shuffle[i] = ip_shuffle[i2];
ip_shuffle[i2] = r;
isindex++;
} while (r == 0);
return (r);
}
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