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+/**
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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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+/*
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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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+
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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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+
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+#ifdef LWIP_HOOK_TCP_ISN
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+
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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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+
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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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+/**
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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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+
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+ MEMCPY(&input[36], secret_16_bytes, 16);
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+
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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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+/**
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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+ /* The secret and padding are already filled in. */
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+
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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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+
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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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+
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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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+
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+#endif /* LWIP_HOOK_TCP_ISN */
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Mahavir Jain