152 lines
4.7 KiB
C
152 lines
4.7 KiB
C
/* $NetBSD: ip_id.c,v 1.12 2008/02/06 03:20:51 matt Exp $ */
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/* $OpenBSD: ip_id.c,v 1.6 2002/03/15 18:19:52 millert Exp $ */
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/*
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* Copyright 1998 Niels Provos <provos@citi.umich.edu>
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* All rights reserved.
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*
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* Theo de Raadt <deraadt@openbsd.org> came up with the idea of using
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* such a mathematical system to generate more random (yet non-repeating)
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* ids to solve the resolver/named problem. But Niels designed the
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* actual system based on the constraints.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* 1. Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution.
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*
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* THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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* IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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* OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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* IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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* NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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* THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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*/
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/*
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* seed = random 15bit
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* n = prime, g0 = generator to n,
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* j = random so that gcd(j,n-1) == 1
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* g = g0^j mod n will be a generator again.
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*
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* X[0] = random seed.
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* X[n] = a*X[n-1]+b mod m is a Linear Congruential Generator
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* with a = 7^(even random) mod m,
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* b = random with gcd(b,m) == 1
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* m = 31104 and a maximal period of m-1.
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*
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* The transaction id is determined by:
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* id[n] = seed xor (g^X[n] mod n)
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*
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* Effectively the id is restricted to the lower 15 bits, thus
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* yielding two different cycles by toggling the msb on and off.
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* This avoids reuse issues caused by reseeding.
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*/
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#include <sys/cdefs.h>
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__KERNEL_RCSID(0, "$NetBSD: ip_id.c,v 1.12 2008/02/06 03:20:51 matt Exp $");
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#include "opt_inet.h"
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#include <sys/param.h>
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#include <lib/libkern/libkern.h>
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#include <net/if.h>
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#include <netinet/in.h>
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#include <netinet/in_var.h>
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#define IPID_MAXID 65535
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#define IPID_NUMIDS 32768
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static struct ipid_state {
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uint16_t ids_start_slot;
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uint16_t ids_slots[IPID_MAXID];
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} idstate;
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static inline uint32_t
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ipid_random(void)
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{
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return arc4random();
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}
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/*
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* Initalizes the
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* the msb flag. The msb flag is used to generate two distinct
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* cycles of random numbers and thus avoiding reuse of ids.
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*
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* This function is called from id_randomid() when needed, an
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* application does not have to worry about it.
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*/
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void
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ip_initid(void)
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{
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size_t i;
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idstate.ids_start_slot = ipid_random();
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for (i = 0; i < __arraycount(idstate.ids_slots); i++)
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idstate.ids_slots[i] = i;
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/*
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* Shuffle the array.
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*/
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for (i = __arraycount(idstate.ids_slots); --i > 0;) {
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size_t k = ipid_random() % (i + 1);
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uint16_t t = idstate.ids_slots[i];
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idstate.ids_slots[i] = idstate.ids_slots[k];
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idstate.ids_slots[k] = t;
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}
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}
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uint16_t
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ip_randomid(uint16_t salt)
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{
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uint32_t r, k, id;
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/*
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* We need a random number
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*/
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r = ipid_random();
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/*
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* We do a modified Fisher-Yates shuffle but only one position at a
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* time. Instead of the last entry, we swap with the first entry and
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* then advance the start of the window by 1. The next time that
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* swapped-out entry can be used is at least 32768 iterations in the
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* future.
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*
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* The easiest way to visual this is to imagine a card deck with 52
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* cards. First thing we do is split that into two sets, each with
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* half of the cards; call them deck A and deck B. Pick a card
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* randomly from deck A and remember it, then place it at the
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* bottom of deck B. Then take the top card from deck B and add it
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* to deck A. Pick another card randomly from deck A and ...
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*/
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k = (r & (IPID_NUMIDS-1)) + idstate.ids_start_slot;
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if (k >= IPID_MAXID)
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k -= IPID_MAXID;
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id = idstate.ids_slots[k];
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if (k != idstate.ids_start_slot) {
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idstate.ids_slots[k] = idstate.ids_slots[idstate.ids_start_slot];
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idstate.ids_slots[idstate.ids_start_slot] = id;
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}
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if (++idstate.ids_start_slot == IPID_MAXID)
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idstate.ids_start_slot = 0;
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/*
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* Add an optional salt to the id to further obscure it.
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*/
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id += salt;
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if (id >= IPID_MAXID)
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id -= IPID_MAXID;
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return (uint16_t) htons(id + 1);
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}
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