149 lines
4.3 KiB
C
149 lines
4.3 KiB
C
/* $NetBSD: ip_id.c,v 1.15 2011/11/19 22:51:25 tls Exp $ */
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/*-
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* Copyright (c) 2008 The NetBSD Foundation, Inc.
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* All rights reserved.
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*
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* This code is derived from software contributed to The NetBSD Foundation
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* by the 3am Software Foundry ("3am"). It was developed by Matt Thomas.
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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 NETBSD FOUNDATION, INC. AND CONTRIBUTORS
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* ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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* TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
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* BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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* POSSIBILITY OF SUCH DAMAGE.
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*/
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#include <sys/cdefs.h>
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__KERNEL_RCSID(0, "$NetBSD: ip_id.c,v 1.15 2011/11/19 22:51:25 tls Exp $");
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#include <sys/param.h>
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#include <sys/kmem.h>
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#include <sys/mutex.h>
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#include <sys/cprng.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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#include <lib/libkern/libkern.h>
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#define IPID_MAXID 65535
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#define IPID_NUMIDS 32768
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struct ipid_state {
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kmutex_t ids_lock;
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uint16_t ids_start_slot;
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uint16_t ids_slots[IPID_MAXID];
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};
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static inline uint32_t
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ipid_random(void)
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{
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return cprng_fast32();
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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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ipid_state_t *
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ip_id_init(void)
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{
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ipid_state_t *ids;
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size_t i;
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ids = kmem_alloc(sizeof(ipid_state_t), KM_SLEEP);
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mutex_init(&ids->ids_lock, MUTEX_DEFAULT, IPL_SOFTNET);
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ids->ids_start_slot = ipid_random();
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for (i = 0; i < __arraycount(ids->ids_slots); i++) {
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ids->ids_slots[i] = i;
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}
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/*
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* Shuffle the array.
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*/
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for (i = __arraycount(ids->ids_slots); --i > 0;) {
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size_t k = ipid_random() % (i + 1);
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uint16_t t = ids->ids_slots[i];
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ids->ids_slots[i] = ids->ids_slots[k];
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ids->ids_slots[k] = t;
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}
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return ids;
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}
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void
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ip_id_fini(ipid_state_t *ids)
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{
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mutex_destroy(&ids->ids_lock);
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kmem_free(ids, sizeof(ipid_state_t));
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}
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uint16_t
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ip_randomid(ipid_state_t *ids, uint16_t salt)
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{
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uint32_t r, k, id;
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/* A random number. */
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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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mutex_enter(&ids->ids_lock);
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k = (r & (IPID_NUMIDS - 1)) + ids->ids_start_slot;
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if (k >= IPID_MAXID) {
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k -= IPID_MAXID;
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}
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id = ids->ids_slots[k];
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if (k != ids->ids_start_slot) {
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ids->ids_slots[k] = ids->ids_slots[ids->ids_start_slot];
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ids->ids_slots[ids->ids_start_slot] = id;
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}
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if (++ids->ids_start_slot == IPID_MAXID) {
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ids->ids_start_slot = 0;
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}
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mutex_exit(&ids->ids_lock);
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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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}
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return (uint16_t)htons(id + 1);
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}
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