lwip_tcp_isn.c

00001 /**
00002  * @file
00003  *
00004  * Reference implementation of the TCP ISN algorithm standardized in RFC 6528.
00005  * Produce TCP Initial Sequence Numbers by combining an MD5-generated hash
00006  * based on the new TCP connection's identity and a stable secret, with the
00007  * current time at 4-microsecond granularity.
00008  *
00009  * Specifically, the implementation uses MD5 to compute a hash of the input
00010  * buffer, which contains both the four-tuple of the new TCP connection (local
00011  * and remote IP address and port), as well as a 16-byte secret to make the
00012  * results unpredictable to external parties.  The secret must be given at
00013  * initialization time and should ideally remain the same across system
00014  * reboots.  To be sure: the spoofing-resistance of the resulting ISN depends
00015  * mainly on the strength of the supplied secret!
00016  *
00017  * The implementation takes 32 bits from the computed hash, and adds to it the
00018  * current time, in 4-microsecond units.  The current time is computed from a
00019  * boot time given at initialization, and the current uptime as provided by
00020  * sys_now().  Thus, it assumes that sys_now() returns a time value that is
00021  * relative to the boot time, i.e., that it starts at 0 at system boot, and
00022  * only ever increases monotonically.
00023  *
00024  * For efficiency reasons, a single MD5 input buffer is used, and partially
00025  * filled in at initialization time.  Specifically, of this 64-byte buffer, the
00026  * first 36 bytes are used for the four-way TCP tuple data, followed by the
00027  * 16-byte secret, followed by 12-byte zero padding.  The 64-byte size of the
00028  * buffer should achieve the best performance for the actual MD5 computation.
00029  *
00030  * Basic usage:
00031  *
00032  * 1. in your lwipopts.h, add the following lines:
00033  *
00034  *    #include <lwip/arch.h>
00035  *    struct ip_addr;
00036  *    u32_t lwip_hook_tcp_isn(const struct ip_addr *local_ip, u16_t local_port,
00037  *      const struct ip_addr *remote_ip, u16_t remote_port);
00038  *   "#define LWIP_HOOK_TCP_ISN lwip_hook_tcp_isn";
00039  *
00040  * 2. from your own code, call lwip_init_tcp_isn() at initialization time, with
00041  *    appropriate parameters.
00042  */
00043 
00044 /*
00045  * Copyright (c) 2016 The MINIX 3 Project.
00046  * All rights reserved.
00047  *
00048  * Redistribution and use in source and binary forms, with or without modification,
00049  * are permitted provided that the following conditions are met:
00050  *
00051  * 1. Redistributions of source code must retain the above copyright notice,
00052  *    this list of conditions and the following disclaimer.
00053  * 2. Redistributions in binary form must reproduce the above copyright notice,
00054  *    this list of conditions and the following disclaimer in the documentation
00055  *    and/or other materials provided with the distribution.
00056  * 3. The name of the author may not be used to endorse or promote products
00057  *    derived from this software without specific prior written permission.
00058  *
00059  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED
00060  * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
00061  * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT
00062  * SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
00063  * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT
00064  * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
00065  * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
00066  * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
00067  * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
00068  * OF SUCH DAMAGE.
00069  *
00070  * Author: David van Moolenbroek <david@minix3.org>
00071  */
00072 
00073 #include "lwip/opt.h"
00074 #include "lwip/ip_addr.h"
00075 #include "lwip/sys.h"
00076 #include <string.h>
00077 
00078 /* pull in md5 of ppp? */
00079 #if !PPP_SUPPORT
00080 #undef   PPP_SUPPORT
00081 #define  PPP_SUPPORT  1
00082 #define  PPP_FAKED_ON 1
00083 #endif
00084 
00085 #include "netif/ppp/ppp_opts.h"
00086 #include "netif/ppp/ppp.h"
00087 #include "netif/ppp/pppcrypt.h"
00088 
00089 #if PPP_FAKED_ON && !LWIP_USE_EXTERNAL_POLARSSL && !LWIP_USE_EXTERNAL_MBEDTLS
00090 #undef  LWIP_INCLUDED_POLARSSL_MD5
00091 #define LWIP_INCLUDED_POLARSSL_MD5 1
00092 #include "netif/ppp/polarssl/lwip_md5.c"
00093 #endif
00094 
00095 static u8_t input[64];
00096 static u32_t base_time;
00097 
00098 /**
00099  * Initialize the TCP ISN module, with the boot time and a secret.
00100  *
00101  * @param boot_time Wall clock boot time of the system, in seconds.
00102  * @param secret_16_bytes A 16-byte secret used to randomize the TCP ISNs.
00103  */
00104 void
00105 lwip_init_tcp_isn(u32_t boot_time, const u8_t *secret_16_bytes)
00106 {
00107   /* Initialize the input buffer with the secret and trailing zeroes. */
00108   memset(input, 0, sizeof(input));
00109 
00110   MEMCPY(&input[36], secret_16_bytes, 16);
00111 
00112   /* Save the boot time in 4-us units. Overflow is no problem here. */
00113   base_time = boot_time * 250000;
00114 }
00115 
00116 /**
00117  * Hook to generate an Initial Sequence Number (ISN) for a new TCP connection.
00118  *
00119  * @param local_ip The local IP address.
00120  * @param local_port The local port number, in host-byte order.
00121  * @param remote_ip The remote IP address.
00122  * @param remote_port The remote port number, in host-byte order.
00123  * @return The ISN to use for the new TCP connection.
00124  */
00125 
00126 u32_t
00127 lwip_hook_tcp_isn(const void *local_ip_ptr, u16_t local_port,
00128         const void *remote_ip_ptr, u16_t remote_port)
00129 {
00130   lwip_md5_context ctx;
00131   u8_t output[16];
00132   u32_t isn;
00133   const ip_addr_t *local_ip = local_ip_ptr;
00134   const ip_addr_t *remote_ip = remote_ip_ptr;
00135 
00136 #if LWIP_IPV4 && LWIP_IPV6
00137   if (IP_IS_V6(local_ip))
00138 #endif /* LWIP_IPV4 && LWIP_IPV6 */
00139 #if LWIP_IPV6
00140   {
00141     const ip6_addr_t *local_ip6, *remote_ip6;
00142 
00143     local_ip6  = ip_2_ip6(local_ip);
00144     remote_ip6 = ip_2_ip6(remote_ip);
00145 
00146     SMEMCPY(&input[0],  &local_ip6->addr,  16);
00147     SMEMCPY(&input[16], &remote_ip6->addr, 16);
00148   }
00149 #endif /* LWIP_IPV6 */
00150 #if LWIP_IPV4 && LWIP_IPV6
00151   else
00152 #endif /* LWIP_IPV4 && LWIP_IPV6 */
00153 #if LWIP_IPV4
00154   {
00155     const ip4_addr_t *local_ip4, *remote_ip4;
00156 
00157     local_ip4  = ip_2_ip4(local_ip);
00158     remote_ip4 = ip_2_ip4(remote_ip);
00159 
00160     /* Represent IPv4 addresses as IPv4-mapped IPv6 addresses, to ensure that
00161      * the IPv4 and IPv6 address spaces are completely disjoint. */
00162     memset(&input[0], 0, 10);
00163     input[10] = 0xff;
00164     input[11] = 0xff;
00165     SMEMCPY(&input[12], &local_ip4->addr, 4);
00166     memset(&input[16], 0, 10);
00167     input[26] = 0xff;
00168     input[27] = 0xff;
00169     SMEMCPY(&input[28], &remote_ip4->addr, 4);
00170   }
00171 #endif /* LWIP_IPV4 */
00172 
00173   input[32] = local_port >> 8;
00174   input[33] = local_port & 0xff;
00175   input[34] = remote_port >> 8;
00176   input[35] = remote_port & 0xff;
00177 
00178   /* The secret and padding are already filled in. */
00179 
00180   /* Generate the hash, using MD5. */
00181   lwip_md5_starts(&ctx);
00182   lwip_md5_update(&ctx, input, sizeof(input));
00183   lwip_md5_finish(&ctx, output);
00184 
00185   /* Arbitrarily take the first 32 bits from the generated hash. */
00186   MEMCPY(&isn, output, sizeof(isn));
00187 
00188   /* Add the current time in 4-microsecond units. */
00189   return isn + base_time + sys_now() * 250;
00190 }