183 lines
6.5 KiB
C
183 lines
6.5 KiB
C
/**
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* @file
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*
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* Reference implementation of the TCP ISN algorithm standardized in RFC 6528.
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* Produce TCP Initial Sequence Numbers by combining an MD5-generated hash
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* based on the new TCP connection's identity and a stable secret, with the
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* current time at 4-microsecond granularity.
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*
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* Specifically, the implementation uses MD5 to compute a hash of the input
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* buffer, which contains both the four-tuple of the new TCP connection (local
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* and remote IP address and port), as well as a 16-byte secret to make the
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* results unpredictable to external parties. The secret must be given at
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* initialization time and should ideally remain the same across system
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* reboots. To be sure: the spoofing-resistance of the resulting ISN depends
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* mainly on the strength of the supplied secret!
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*
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* The implementation takes 32 bits from the computed hash, and adds to it the
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* current time, in 4-microsecond units. The current time is computed from a
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* boot time given at initialization, and the current uptime as provided by
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* sys_now(). Thus, it assumes that sys_now() returns a time value that is
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* relative to the boot time, i.e., that it starts at 0 at system boot, and
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* only ever increases monotonically.
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*
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* For efficiency reasons, a single MD5 input buffer is used, and partially
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* filled in at initialization time. Specifically, of this 64-byte buffer, the
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* first 36 bytes are used for the four-way TCP tuple data, followed by the
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* 16-byte secret, followed by 12-byte zero padding. The 64-byte size of the
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* buffer should achieve the best performance for the actual MD5 computation.
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*
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* Basic usage:
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*
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* 1. in your lwipopts.h, add the following lines:
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*
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* #include <lwip/arch.h>
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* struct ip_addr;
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* u32_t lwip_hook_tcp_isn(const struct ip_addr *local_ip, u16_t local_port,
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* const struct ip_addr *remote_ip, u16_t remote_port);
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* "#define LWIP_HOOK_TCP_ISN lwip_hook_tcp_isn";
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*
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* 2. from your own code, call lwip_init_tcp_isn() at initialization time, with
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* appropriate parameters.
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*/
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/*
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* Copyright (c) 2016 The MINIX 3 Project.
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* All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without modification,
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* are permitted provided that the following conditions are met:
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*
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* 1. Redistributions of source code must retain the above copyright notice,
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* this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright notice,
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* this list of conditions and the following disclaimer in the documentation
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* and/or other materials provided with the distribution.
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* 3. The name of the author may not be used to endorse or promote products
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* derived from this software without specific prior written permission.
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*
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* THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED
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* WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
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* MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT
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* SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
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* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT
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* OF 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) ARISING
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* IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
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* OF SUCH DAMAGE.
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*
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* Author: David van Moolenbroek <david@minix3.org>
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*/
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#include "tcp_isn.h"
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#include "lwip/ip_addr.h"
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#include "lwip/sys.h"
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#include <string.h>
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#ifdef LWIP_HOOK_TCP_ISN
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/* pull in md5 of ppp? */
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#include "netif/ppp/ppp_opts.h"
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#if !PPP_SUPPORT || (!LWIP_USE_EXTERNAL_POLARSSL && !LWIP_USE_EXTERNAL_MBEDTLS)
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#undef LWIP_INCLUDED_POLARSSL_MD5
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#define LWIP_INCLUDED_POLARSSL_MD5 1
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#include "netif/ppp/polarssl/md5.h"
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#endif
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static u8_t input[64];
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static u32_t base_time;
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/**
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* Initialize the TCP ISN module, with the boot time and a secret.
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*
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* @param boot_time Wall clock boot time of the system, in seconds.
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* @param secret_16_bytes A 16-byte secret used to randomize the TCP ISNs.
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*/
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void
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lwip_init_tcp_isn(u32_t boot_time, const u8_t *secret_16_bytes)
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{
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/* Initialize the input buffer with the secret and trailing zeroes. */
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memset(input, 0, sizeof(input));
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MEMCPY(&input[36], secret_16_bytes, 16);
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/* Save the boot time in 4-us units. Overflow is no problem here. */
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base_time = boot_time * 250000;
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}
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/**
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* Hook to generate an Initial Sequence Number (ISN) for a new TCP connection.
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*
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* @param local_ip The local IP address.
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* @param local_port The local port number, in host-byte order.
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* @param remote_ip The remote IP address.
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* @param remote_port The remote port number, in host-byte order.
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* @return The ISN to use for the new TCP connection.
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*/
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u32_t
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lwip_hook_tcp_isn(const ip_addr_t *local_ip, u16_t local_port,
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const ip_addr_t *remote_ip, u16_t remote_port)
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{
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md5_context ctx;
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u8_t output[16];
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u32_t isn;
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#if LWIP_IPV4 && LWIP_IPV6
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if (IP_IS_V6(local_ip))
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#endif /* LWIP_IPV4 && LWIP_IPV6 */
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#if LWIP_IPV6
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{
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const ip6_addr_t *local_ip6, *remote_ip6;
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local_ip6 = ip_2_ip6(local_ip);
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remote_ip6 = ip_2_ip6(remote_ip);
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SMEMCPY(&input[0], &local_ip6->addr, 16);
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SMEMCPY(&input[16], &remote_ip6->addr, 16);
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}
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#endif /* LWIP_IPV6 */
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#if LWIP_IPV4 && LWIP_IPV6
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else
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#endif /* LWIP_IPV4 && LWIP_IPV6 */
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#if LWIP_IPV4
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{
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const ip4_addr_t *local_ip4, *remote_ip4;
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local_ip4 = ip_2_ip4(local_ip);
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remote_ip4 = ip_2_ip4(remote_ip);
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/* Represent IPv4 addresses as IPv4-mapped IPv6 addresses, to ensure that
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* the IPv4 and IPv6 address spaces are completely disjoint. */
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memset(&input[0], 0, 10);
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input[10] = 0xff;
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input[11] = 0xff;
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SMEMCPY(&input[12], &local_ip4->addr, 4);
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memset(&input[16], 0, 10);
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input[26] = 0xff;
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input[27] = 0xff;
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SMEMCPY(&input[28], &remote_ip4->addr, 4);
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}
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#endif /* LWIP_IPV4 */
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input[32] = (u8_t)(local_port >> 8);
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input[33] = (u8_t)(local_port & 0xff);
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input[34] = (u8_t)(remote_port >> 8);
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input[35] = (u8_t)(remote_port & 0xff);
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/* The secret and padding are already filled in. */
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/* Generate the hash, using MD5. */
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md5_starts(&ctx);
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md5_update(&ctx, input, sizeof(input));
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md5_finish(&ctx, output);
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/* Arbitrarily take the first 32 bits from the generated hash. */
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MEMCPY(&isn, output, sizeof(isn));
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/* Add the current time in 4-microsecond units. */
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return isn + base_time + sys_now() * 250;
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}
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#endif /* LWIP_HOOK_TCP_ISN */
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