1 /**
2 * @file
3 *
4 * Reference implementation of the TCP ISN algorithm standardized in RFC 6528.
5 * Produce TCP Initial Sequence Numbers by combining an MD5-generated hash
6 * based on the new TCP connection's identity and a stable secret, with the
7 * current time at 4-microsecond granularity.
8 *
9 * Specifically, the implementation uses MD5 to compute a hash of the input
10 * buffer, which contains both the four-tuple of the new TCP connection (local
11 * and remote IP address and port), as well as a 16-byte secret to make the
12 * results unpredictable to external parties. The secret must be given at
13 * initialization time and should ideally remain the same across system
14 * reboots. To be sure: the spoofing-resistance of the resulting ISN depends
15 * mainly on the strength of the supplied secret!
16 *
17 * The implementation takes 32 bits from the computed hash, and adds to it the
18 * current time, in 4-microsecond units. The current time is computed from a
19 * boot time given at initialization, and the current uptime as provided by
20 * sys_now(). Thus, it assumes that sys_now() returns a time value that is
21 * relative to the boot time, i.e., that it starts at 0 at system boot, and
22 * only ever increases monotonically.
23 *
24 * For efficiency reasons, a single MD5 input buffer is used, and partially
25 * filled in at initialization time. Specifically, of this 64-byte buffer, the
26 * first 36 bytes are used for the four-way TCP tuple data, followed by the
27 * 16-byte secret, followed by 12-byte zero padding. The 64-byte size of the
28 * buffer should achieve the best performance for the actual MD5 computation.
29 *
30 * Basic usage:
31 *
32 * 1. in your lwipopts.h, add the following lines:
33 *
34 * #include <lwip/arch.h>
35 * struct ip_addr;
36 * u32_t lwip_hook_tcp_isn(const struct ip_addr *local_ip, u16_t local_port,
37 * const struct ip_addr *remote_ip, u16_t remote_port);
38 * "#define LWIP_HOOK_TCP_ISN lwip_hook_tcp_isn";
39 *
40 * 2. from your own code, call lwip_init_tcp_isn() at initialization time, with
41 * appropriate parameters.
42 */
43
44 /*
45 * Copyright (c) 2016 The MINIX 3 Project.
46 * All rights reserved.
47 *
48 * Redistribution and use in source and binary forms, with or without modification,
49 * are permitted provided that the following conditions are met:
50 *
51 * 1. Redistributions of source code must retain the above copyright notice,
52 * this list of conditions and the following disclaimer.
53 * 2. Redistributions in binary form must reproduce the above copyright notice,
54 * this list of conditions and the following disclaimer in the documentation
55 * and/or other materials provided with the distribution.
56 * 3. The name of the author may not be used to endorse or promote products
57 * derived from this software without specific prior written permission.
58 *
59 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED
60 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
61 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT
62 * SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
63 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT
64 * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
65 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
66 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
67 * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
68 * OF SUCH DAMAGE.
69 *
70 * Author: David van Moolenbroek <david@minix3.org>
71 */
72
73 #include "tcp_isn.h"
74 #include "lwip/ip_addr.h"
75 #include "lwip/sys.h"
76 #include <string.h>
77
78 #ifdef LWIP_HOOK_TCP_ISN
79
80 /* pull in md5 of ppp? */
81 #include "netif/ppp/ppp_opts.h"
82 #if !PPP_SUPPORT || (!LWIP_USE_EXTERNAL_POLARSSL && !LWIP_USE_EXTERNAL_MBEDTLS)
83 #undef LWIP_INCLUDED_POLARSSL_MD5
84 #define LWIP_INCLUDED_POLARSSL_MD5 1
85 #include "netif/ppp/polarssl/md5.h"
86 #endif
87
88 static u8_t input[64];
89 static u32_t base_time;
90
91 /**
92 * Initialize the TCP ISN module, with the boot time and a secret.
93 *
94 * @param boot_time Wall clock boot time of the system, in seconds.
95 * @param secret_16_bytes A 16-byte secret used to randomize the TCP ISNs.
96 */
97 void
lwip_init_tcp_isn(u32_t boot_time,const u8_t * secret_16_bytes)98 lwip_init_tcp_isn(u32_t boot_time, const u8_t *secret_16_bytes)
99 {
100 /* Initialize the input buffer with the secret and trailing zeroes. */
101 memset(input, 0, sizeof(input));
102
103 MEMCPY(&input[36], secret_16_bytes, 16);
104
105 /* Save the boot time in 4-us units. Overflow is no problem here. */
106 base_time = boot_time * 250000;
107 }
108
109 /**
110 * Hook to generate an Initial Sequence Number (ISN) for a new TCP connection.
111 *
112 * @param local_ip The local IP address.
113 * @param local_port The local port number, in host-byte order.
114 * @param remote_ip The remote IP address.
115 * @param remote_port The remote port number, in host-byte order.
116 * @return The ISN to use for the new TCP connection.
117 */
118 u32_t
lwip_hook_tcp_isn(const ip_addr_t * local_ip,u16_t local_port,const ip_addr_t * remote_ip,u16_t remote_port)119 lwip_hook_tcp_isn(const ip_addr_t *local_ip, u16_t local_port,
120 const ip_addr_t *remote_ip, u16_t remote_port)
121 {
122 md5_context ctx;
123 u8_t output[16];
124 u32_t isn;
125
126 #if LWIP_IPV4 && LWIP_IPV6
127 if (IP_IS_V6(local_ip))
128 #endif /* LWIP_IPV4 && LWIP_IPV6 */
129 #if LWIP_IPV6
130 {
131 const ip6_addr_t *local_ip6, *remote_ip6;
132
133 local_ip6 = ip_2_ip6(local_ip);
134 remote_ip6 = ip_2_ip6(remote_ip);
135
136 SMEMCPY(&input[0], &local_ip6->addr, 16);
137 SMEMCPY(&input[16], &remote_ip6->addr, 16);
138 }
139 #endif /* LWIP_IPV6 */
140 #if LWIP_IPV4 && LWIP_IPV6
141 else
142 #endif /* LWIP_IPV4 && LWIP_IPV6 */
143 #if LWIP_IPV4
144 {
145 const ip4_addr_t *local_ip4, *remote_ip4;
146
147 local_ip4 = ip_2_ip4(local_ip);
148 remote_ip4 = ip_2_ip4(remote_ip);
149
150 /* Represent IPv4 addresses as IPv4-mapped IPv6 addresses, to ensure that
151 * the IPv4 and IPv6 address spaces are completely disjoint. */
152 memset(&input[0], 0, 10);
153 input[10] = 0xff;
154 input[11] = 0xff;
155 SMEMCPY(&input[12], &local_ip4->addr, 4);
156 memset(&input[16], 0, 10);
157 input[26] = 0xff;
158 input[27] = 0xff;
159 SMEMCPY(&input[28], &remote_ip4->addr, 4);
160 }
161 #endif /* LWIP_IPV4 */
162
163 input[32] = (u8_t)(local_port >> 8);
164 input[33] = (u8_t)(local_port & 0xff);
165 input[34] = (u8_t)(remote_port >> 8);
166 input[35] = (u8_t)(remote_port & 0xff);
167
168 /* The secret and padding are already filled in. */
169
170 /* Generate the hash, using MD5. */
171 md5_starts(&ctx);
172 md5_update(&ctx, input, sizeof(input));
173 md5_finish(&ctx, output);
174
175 /* Arbitrarily take the first 32 bits from the generated hash. */
176 MEMCPY(&isn, output, sizeof(isn));
177
178 /* Add the current time in 4-microsecond units. */
179 return isn + base_time + sys_now() * 250;
180 }
181
182 #endif /* LWIP_HOOK_TCP_ISN */
183