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

     /*      $NetBSD: ip_id.c,v 1.11 2006/08/30 18:54:19 christos Exp $      */
     /*      $OpenBSD: ip_id.c,v 1.6 2002/03/15 18:19:52 millert Exp $       */
     
     /*
      * Copyright 1998 Niels Provos <provos@citi.umich.edu>
      * All rights reserved.
      *
      * Theo de Raadt <deraadt@openbsd.org> came up with the idea of using
      * such a mathematical system to generate more random (yet non-repeating)
     * ids to solve the resolver/named problem.  But Niels designed the
     * actual system based on the constraints.
     *
     * Redistribution and use in source and binary forms, with or without
     * modification, are permitted provided that the following conditions
     * are met:
     * 1. Redistributions of source code must retain the above copyright
     *    notice, this list of conditions and the following disclaimer.
     * 2. Redistributions in binary form must reproduce the above copyright
     *    notice, this list of conditions and the following disclaimer in the
     *    documentation and/or other materials provided with the distribution.
     *
     * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
     * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
     * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
     * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
     * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
     * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
     * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
     * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
     * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
     * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
     */
    
    /*
     * seed = random 15bit
     * n = prime, g0 = generator to n,
     * j = random so that gcd(j,n-1) == 1
     * g = g0^j mod n will be a generator again.
     *
     * X[0] = random seed.
     * X[n] = a*X[n-1]+b mod m is a Linear Congruential Generator
     * with a = 7^(even random) mod m,
     *      b = random with gcd(b,m) == 1
     *      m = 31104 and a maximal period of m-1.
     *
     * The transaction id is determined by:
     * id[n] = seed xor (g^X[n] mod n)
     *
     * Effectively the id is restricted to the lower 15 bits, thus
     * yielding two different cycles by toggling the msb on and off.
     * This avoids reuse issues caused by reseeding.
     */
    
    #include <sys/cdefs.h>
    __KERNEL_RCSID(0, "$NetBSD: ip_id.c,v 1.11 2006/08/30 18:54:19 christos Exp $");
    
    #include "opt_inet.h"
    
    #include <sys/param.h>
    #include <lib/libkern/libkern.h>
    
    #include <net/if.h>
    #include <netinet/in.h>
    #include <netinet/ip_var.h>
    
    #define RU_OUT  180             /* Time after wich will be reseeded */
    #define RU_MAX  30000           /* Uniq cycle, avoid blackjack prediction */
    #define RU_GEN  2               /* Starting generator */
    #define RU_N    32749           /* RU_N-1 = 2*2*3*2729 */
    #define RU_AGEN 7               /* determine ru_a as RU_AGEN^(2*rand) */
    #define RU_M    31104           /* RU_M = 2^7*3^5 - don't change */
    
    #define PFAC_N 3
    static const u_int16_t pfacts[PFAC_N] = {
            2,
            3,
            2729
    };
    
    static u_int16_t ru_x;
    static u_int16_t ru_seed, ru_seed2;
    static u_int16_t ru_a, ru_b;
    static u_int16_t ru_g;
    static u_int16_t ru_counter = 0;
    static u_int16_t ru_msb = 0;
    static long ru_reseed;
    static u_int32_t tmp;           /* Storage for unused random */
    
    static u_int16_t pmod(u_int16_t, u_int16_t, u_int16_t);
    static void ip_initid(void);
    
    /*
     * Do a fast modular exponation, returned value will be in the range
     * of 0 - (mod-1)
     */
    
    static u_int16_t
    pmod(u_int16_t gen, u_int16_t expo, u_int16_t mod)
    {
           u_int16_t s, t, u;
   
           s = 1;
           t = gen;
           u = expo;
   
           while (u) {
                   if (u & 1)
                           s = (s * t) % mod;
                   u >>= 1;
                   t = (t * t) % mod;
           }
           return (s);
   }
   
   /*
    * Initalizes the seed and chooses a suitable generator. Also toggles
    * the msb flag. The msb flag is used to generate two distinct
    * cycles of random numbers and thus avoiding reuse of ids.
    *
    * This function is called from id_randomid() when needed, an
    * application does not have to worry about it.
    */
   static void
   ip_initid(void)
   {
           u_int16_t j, i;
           int noprime = 1;
   
           ru_x = ((tmp = arc4random()) & 0xFFFF) % RU_M;
   
           /* 15 bits of random seed */
           ru_seed = (tmp >> 16) & 0x7FFF;
           ru_seed2 = arc4random() & 0x7FFF;
   
           /* Determine the LCG we use */
           ru_b = ((tmp = arc4random()) & 0xfffe) | 1;
           ru_a = pmod(RU_AGEN, (tmp >> 16) & 0xfffe, RU_M);
           while (ru_b % 3 == 0)
                   ru_b += 2;
   
           j = (tmp = arc4random()) % RU_N;
           tmp = tmp >> 16;
   
           /*
            * Do a fast gcd(j,RU_N-1), so we can find a j with
            * gcd(j, RU_N-1) == 1, giving a new generator for
            * RU_GEN^j mod RU_N
            */
   
           while (noprime) {
                   for (i = 0; i < PFAC_N; i++)
                           if (j % pfacts[i] == 0)
                                   break;
   
                   if (i >= PFAC_N)
                           noprime = 0;
                   else
                           j = (j + 1) % RU_N;
           }
   
           ru_g = pmod(RU_GEN, j, RU_N);
           ru_counter = 0;
   
           ru_reseed = time_second + RU_OUT;
           ru_msb = ru_msb == 0x8000 ? 0 : 0x8000;
   }
   
   u_int16_t
   ip_randomid(void)
   {
           int i, n;
   
           if (ru_counter >= RU_MAX || time_second > ru_reseed)
                   ip_initid();
   
   #if 0
           if (!tmp)
                   tmp = arc4random();
   
           /* Skip a random number of ids */
           n = tmp & 0x3; tmp = tmp >> 2;
           if (ru_counter + n >= RU_MAX)
                   ip_initid();
   #else
           n = 0;
   #endif
   
           for (i = 0; i <= n; i++)
                   /* Linear Congruential Generator */
                   ru_x = (ru_a * ru_x + ru_b) % RU_M;
   
           ru_counter += i;
   
           return (ru_seed ^ pmod(ru_g, ru_seed2 + ru_x, RU_N)) | ru_msb;
   }

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