ip_id.c

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    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 
   31 static u_int16_t ip_shuffle[65536];
   32 static int isindex = 0;
   33 
   34 u_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  */
   43 u_int16_t
   44 ip_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 }

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FreeBSD/Linux Kernel Cross Reference sys/netinet/ip_id.c

FreeBSD/Linux Kernel Cross Reference
sys/netinet/ip_id.c