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Replace the very predictable 'random' IP sequence number generator with
something far less predictable.  Use DragonFly's built-in PRNG and add a
shuffle algorithm.  We eat 128KB of ram for the shuffle array but I've
pretty much had it with roll-your-own PRNGs.  At least this way any PRNG
issues can be corrected in one place -- our system PRNG.

Turn net.inet.ip.random_id on by default.  Eat the minor loss in performance.
On the bright side, our PRNG is very fast, so this should not represent
a burden.

References: Analysis of BSD ip randomizer algorithm by "Amit Klein"
	    <amit.klein@trusteer.com> showing serious vulnerabilities in
	    the algorithm.

/*
 * Copyright (c) 2007 The DragonFly Project.  All rights reserved.
 * 
 * This code is derived from software contributed to The DragonFly Project
 * by Matthew Dillon <dillon@backplane.com>
 * 
 * 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.
 * 3. Neither the name of The DragonFly Project nor the names of its
 *    contributors may be used to endorse or promote products derived
 *    from this software without specific, prior written permission.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * ``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
 * COPYRIGHT HOLDERS OR CONTRIBUTORS 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.
 * 
 * $DragonFly: src/sys/netinet/ip_id.c,v 1.7 2007/11/22 19:57:14 dillon Exp $
 */

/*
 * Random ip sequence number generator.  Use the system PRNG to shuffle the
 * 65536 entry ID space.  We reshuffle the front-side of the array as we
 * index through it, guarenteeing an id will not be reused for at least
 * 32768 calls.
 */
#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/random.h>
#include <sys/spinlock.h>
#include <sys/globaldata.h>
#include <netinet/ip_var.h>

#include <sys/spinlock2.h>

static struct spinlock ip_shuffle_spin = SPINLOCK_INITIALIZER(ip_shuffle_spin);
static u_int16_t ip_shuffle[65536];

/*
 * Initialize the shuffle.  We assume that the system PRNG won't be all that
 * good this early in the boot sequence but use it anyway.  The ids will be
 * reshuffled as they are popped and the PRNG should be better then.
 */
static void
ip_initshuffle(void *dummy __unused)
{
	int i;

	kprintf("INITSHUFFLE1\n");
	for (i = 0; i < 65536; ++i)
		ip_shuffle[i] = i;
	for (i = 0; i < 65536; ++i)
		ip_randomid();
	kprintf("INITSHUFFLE2\n");
}

SYSINIT(ipshuffle, SI_SUB_PSEUDO, SI_ORDER_ANY, ip_initshuffle, NULL);

/*
 * Return a random IP id.  Use a forward shuffle over half the index
 * space to avoid duplicates occuring too quickly.  Since the initial
 * shuffle may not have had a good random basis we returned the element
 * at the shuffle target instead of the current element.
 *
 * XXX make per-cpu so the spinlock can be removed?
 */
u_int16_t
ip_randomid(void)
{
	static int isindex;
	u_int16_t si, r;
	int i1, i2;

	read_random_unlimited(&si, sizeof(si));
	spin_lock_wr(&ip_shuffle_spin);
	i1 = isindex & 0xFFFF;
	i2 = (isindex + (si & 0x7FFF)) & 0xFFFF;
	r = ip_shuffle[i2];
	ip_shuffle[i2] = ip_shuffle[i1];
	ip_shuffle[i1] = r;
	++isindex;
	spin_unlock_wr(&ip_shuffle_spin);

	return(r);
}