1 | /* $NetBSD: ip_id.c,v 1.15 2011/11/19 22:51:25 tls Exp $ */ |
2 | |
3 | /*- |
4 | * Copyright (c) 2008 The NetBSD Foundation, Inc. |
5 | * All rights reserved. |
6 | * |
7 | * This code is derived from software contributed to The NetBSD Foundation |
8 | * by the 3am Software Foundry ("3am"). It was developed by Matt Thomas. |
9 | * |
10 | * Redistribution and use in source and binary forms, with or without |
11 | * modification, are permitted provided that the following conditions |
12 | * are met: |
13 | * 1. Redistributions of source code must retain the above copyright |
14 | * notice, this list of conditions and the following disclaimer. |
15 | * 2. Redistributions in binary form must reproduce the above copyright |
16 | * notice, this list of conditions and the following disclaimer in the |
17 | * documentation and/or other materials provided with the distribution. |
18 | * |
19 | * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS |
20 | * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED |
21 | * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR |
22 | * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS |
23 | * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR |
24 | * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF |
25 | * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS |
26 | * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN |
27 | * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) |
28 | * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE |
29 | * POSSIBILITY OF SUCH DAMAGE. |
30 | */ |
31 | |
32 | #include <sys/cdefs.h> |
33 | __KERNEL_RCSID(0, "$NetBSD: ip_id.c,v 1.15 2011/11/19 22:51:25 tls Exp $" ); |
34 | |
35 | #include <sys/param.h> |
36 | #include <sys/kmem.h> |
37 | #include <sys/mutex.h> |
38 | #include <sys/cprng.h> |
39 | |
40 | #include <net/if.h> |
41 | #include <netinet/in.h> |
42 | #include <netinet/in_var.h> |
43 | |
44 | #include <lib/libkern/libkern.h> |
45 | |
46 | #define IPID_MAXID 65535 |
47 | #define IPID_NUMIDS 32768 |
48 | |
49 | struct ipid_state { |
50 | kmutex_t ids_lock; |
51 | uint16_t ids_start_slot; |
52 | uint16_t ids_slots[IPID_MAXID]; |
53 | }; |
54 | |
55 | static inline uint32_t |
56 | ipid_random(void) |
57 | { |
58 | return cprng_fast32(); |
59 | } |
60 | |
61 | /* |
62 | * Initalizes the |
63 | * the msb flag. The msb flag is used to generate two distinct |
64 | * cycles of random numbers and thus avoiding reuse of ids. |
65 | * |
66 | * This function is called from id_randomid() when needed, an |
67 | * application does not have to worry about it. |
68 | */ |
69 | ipid_state_t * |
70 | ip_id_init(void) |
71 | { |
72 | ipid_state_t *ids; |
73 | size_t i; |
74 | |
75 | ids = kmem_alloc(sizeof(ipid_state_t), KM_SLEEP); |
76 | mutex_init(&ids->ids_lock, MUTEX_DEFAULT, IPL_SOFTNET); |
77 | |
78 | ids->ids_start_slot = ipid_random(); |
79 | for (i = 0; i < __arraycount(ids->ids_slots); i++) { |
80 | ids->ids_slots[i] = i; |
81 | } |
82 | |
83 | /* |
84 | * Shuffle the array. |
85 | */ |
86 | for (i = __arraycount(ids->ids_slots); --i > 0;) { |
87 | size_t k = ipid_random() % (i + 1); |
88 | uint16_t t = ids->ids_slots[i]; |
89 | ids->ids_slots[i] = ids->ids_slots[k]; |
90 | ids->ids_slots[k] = t; |
91 | } |
92 | return ids; |
93 | } |
94 | |
95 | void |
96 | ip_id_fini(ipid_state_t *ids) |
97 | { |
98 | |
99 | mutex_destroy(&ids->ids_lock); |
100 | kmem_free(ids, sizeof(ipid_state_t)); |
101 | } |
102 | |
103 | uint16_t |
104 | ip_randomid(ipid_state_t *ids, uint16_t salt) |
105 | { |
106 | uint32_t r, k, id; |
107 | |
108 | /* A random number. */ |
109 | r = ipid_random(); |
110 | |
111 | /* |
112 | * We do a modified Fisher-Yates shuffle but only one position at a |
113 | * time. Instead of the last entry, we swap with the first entry and |
114 | * then advance the start of the window by 1. The next time that |
115 | * swapped-out entry can be used is at least 32768 iterations in the |
116 | * future. |
117 | * |
118 | * The easiest way to visual this is to imagine a card deck with 52 |
119 | * cards. First thing we do is split that into two sets, each with |
120 | * half of the cards; call them deck A and deck B. Pick a card |
121 | * randomly from deck A and remember it, then place it at the |
122 | * bottom of deck B. Then take the top card from deck B and add it |
123 | * to deck A. Pick another card randomly from deck A and ... |
124 | */ |
125 | mutex_enter(&ids->ids_lock); |
126 | k = (r & (IPID_NUMIDS - 1)) + ids->ids_start_slot; |
127 | if (k >= IPID_MAXID) { |
128 | k -= IPID_MAXID; |
129 | } |
130 | id = ids->ids_slots[k]; |
131 | if (k != ids->ids_start_slot) { |
132 | ids->ids_slots[k] = ids->ids_slots[ids->ids_start_slot]; |
133 | ids->ids_slots[ids->ids_start_slot] = id; |
134 | } |
135 | if (++ids->ids_start_slot == IPID_MAXID) { |
136 | ids->ids_start_slot = 0; |
137 | } |
138 | mutex_exit(&ids->ids_lock); |
139 | |
140 | /* |
141 | * Add an optional salt to the id to further obscure it. |
142 | */ |
143 | id += salt; |
144 | if (id >= IPID_MAXID) { |
145 | id -= IPID_MAXID; |
146 | } |
147 | return (uint16_t)htons(id + 1); |
148 | } |
149 | |