The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


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

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    1 /* $OpenBSD: ip_id.c,v 1.2 1999/08/26 13:37:01 provos Exp $ */
    2 
    3 /*-
    4  * Copyright 1998 Niels Provos <provos@citi.umich.edu>
    5  * All rights reserved.
    6  *
    7  * Theo de Raadt <deraadt@openbsd.org> came up with the idea of using
    8  * such a mathematical system to generate more random (yet non-repeating)
    9  * ids to solve the resolver/named problem.  But Niels designed the
   10  * actual system based on the constraints.
   11  *
   12  * Redistribution and use in source and binary forms, with or without
   13  * modification, are permitted provided that the following conditions
   14  * are met:
   15  * 1. Redistributions of source code must retain the above copyright
   16  *    notice, this list of conditions and the following disclaimer.
   17  * 2. Redistributions in binary form must reproduce the above copyright
   18  *    notice, this list of conditions and the following disclaimer in the
   19  *    documentation and/or other materials provided with the distribution.
   20  * 3. All advertising materials mentioning features or use of this software
   21  *    must display the following acknowledgement:
   22  *      This product includes software developed by Niels Provos.
   23  * 4. The name of the author may not be used to endorse or promote products
   24  *    derived from this software without specific prior written permission.
   25  *
   26  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
   27  * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
   28  * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
   29  * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
   30  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
   31  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
   32  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
   33  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
   34  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
   35  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
   36  *
   37  * $FreeBSD: releng/6.2/sys/netinet/ip_id.c 139823 2005-01-07 01:45:51Z imp $
   38  */
   39 
   40 /*
   41  * seed = random 15bit
   42  * n = prime, g0 = generator to n,
   43  * j = random so that gcd(j,n-1) == 1
   44  * g = g0^j mod n will be a generator again.
   45  *
   46  * X[0] = random seed.
   47  * X[n] = a*X[n-1]+b mod m is a Linear Congruential Generator
   48  * with a = 7^(even random) mod m,
   49  *      b = random with gcd(b,m) == 1
   50  *      m = 31104 and a maximal period of m-1.
   51  *
   52  * The transaction id is determined by:
   53  * id[n] = seed xor (g^X[n] mod n)
   54  *
   55  * Effectivly the id is restricted to the lower 15 bits, thus
   56  * yielding two different cycles by toggling the msb on and off.
   57  * This avoids reuse issues caused by reseeding.
   58  */
   59 
   60 #include "opt_pf.h"
   61 #include <sys/param.h>
   62 #include <sys/time.h>
   63 #include <sys/kernel.h>
   64 #include <sys/random.h>
   65 
   66 #define RU_OUT  180             /* Time after wich will be reseeded */
   67 #define RU_MAX  30000           /* Uniq cycle, avoid blackjack prediction */
   68 #define RU_GEN  2               /* Starting generator */
   69 #define RU_N    32749           /* RU_N-1 = 2*2*3*2729 */
   70 #define RU_AGEN 7               /* determine ru_a as RU_AGEN^(2*rand) */
   71 #define RU_M    31104           /* RU_M = 2^7*3^5 - don't change */
   72 
   73 #define PFAC_N 3
   74 const static u_int16_t pfacts[PFAC_N] = {
   75         2,
   76         3,
   77         2729
   78 };
   79 
   80 static u_int16_t ru_x;
   81 static u_int16_t ru_seed, ru_seed2;
   82 static u_int16_t ru_a, ru_b;
   83 static u_int16_t ru_g;
   84 static u_int16_t ru_counter = 0;
   85 static u_int16_t ru_msb = 0;
   86 static long ru_reseed;
   87 static u_int32_t tmp;           /* Storage for unused random */
   88 
   89 static u_int16_t pmod(u_int16_t, u_int16_t, u_int16_t);
   90 static void ip_initid(void);
   91 u_int16_t ip_randomid(void);
   92 
   93 /*
   94  * Do a fast modular exponation, returned value will be in the range
   95  * of 0 - (mod-1)
   96  */
   97 
   98 #ifdef __STDC__
   99 static u_int16_t
  100 pmod(u_int16_t gen, u_int16_t exp, u_int16_t mod)
  101 #else
  102 static u_int16_t
  103 pmod(gen, exp, mod)
  104         u_int16_t gen, exp, mod;
  105 #endif
  106 {
  107         u_int16_t s, t, u;
  108 
  109         s = 1;
  110         t = gen;
  111         u = exp;
  112 
  113         while (u) {
  114                 if (u & 1)
  115                         s = (s*t) % mod;
  116                 u >>= 1;
  117                 t = (t*t) % mod;
  118         }
  119         return (s);
  120 }
  121 
  122 /*
  123  * Initalizes the seed and chooses a suitable generator. Also toggles
  124  * the msb flag. The msb flag is used to generate two distinct
  125  * cycles of random numbers and thus avoiding reuse of ids.
  126  *
  127  * This function is called from id_randomid() when needed, an
  128  * application does not have to worry about it.
  129  */
  130 static void
  131 ip_initid(void)
  132 {
  133         u_int16_t j, i;
  134         int noprime = 1;
  135         struct timeval time;
  136 
  137         getmicrotime(&time);
  138         read_random((void *) &tmp, sizeof(tmp));
  139         ru_x = (tmp & 0xFFFF) % RU_M;
  140 
  141         /* 15 bits of random seed */
  142         ru_seed = (tmp >> 16) & 0x7FFF;
  143         read_random((void *) &tmp, sizeof(tmp));
  144         ru_seed2 = tmp & 0x7FFF;
  145 
  146         read_random((void *) &tmp, sizeof(tmp));
  147 
  148         /* Determine the LCG we use */
  149         ru_b = (tmp & 0xfffe) | 1;
  150         ru_a = pmod(RU_AGEN, (tmp >> 16) & 0xfffe, RU_M);
  151         while (ru_b % 3 == 0)
  152           ru_b += 2;
  153 
  154         read_random((void *) &tmp, sizeof(tmp));
  155         j = tmp % RU_N;
  156         tmp = tmp >> 16;
  157 
  158         /*
  159          * Do a fast gcd(j,RU_N-1), so we can find a j with
  160          * gcd(j, RU_N-1) == 1, giving a new generator for
  161          * RU_GEN^j mod RU_N
  162          */
  163 
  164         while (noprime) {
  165                 for (i=0; i<PFAC_N; i++)
  166                         if (j%pfacts[i] == 0)
  167                                 break;
  168 
  169                 if (i>=PFAC_N)
  170                         noprime = 0;
  171                 else
  172                         j = (j+1) % RU_N;
  173         }
  174 
  175         ru_g = pmod(RU_GEN,j,RU_N);
  176         ru_counter = 0;
  177 
  178         ru_reseed = time.tv_sec + RU_OUT;
  179         ru_msb = ru_msb == 0x8000 ? 0 : 0x8000;
  180 }
  181 
  182 u_int16_t
  183 ip_randomid(void)
  184 {
  185         int i, n;
  186         struct timeval time;
  187 
  188         /* XXX not reentrant */
  189         getmicrotime(&time);
  190         if (ru_counter >= RU_MAX || time.tv_sec > ru_reseed)
  191                 ip_initid();
  192 
  193         if (!tmp)
  194                 read_random((void *) &tmp, sizeof(tmp));
  195 
  196         /* Skip a random number of ids */
  197         n = tmp & 0x3; tmp = tmp >> 2;
  198         if (ru_counter + n >= RU_MAX)
  199                 ip_initid();
  200 
  201         for (i = 0; i <= n; i++)
  202                 /* Linear Congruential Generator */
  203                 ru_x = (ru_a*ru_x + ru_b) % RU_M;
  204 
  205         ru_counter += i;
  206 
  207         return (ru_seed ^ pmod(ru_g,ru_seed2 ^ ru_x,RU_N)) | ru_msb;
  208 }
  209 

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