make ip_id random but ensure that ids dont repeat for some period.

1 files changed, 199 insertions, 0 deletions

diff --git a/sys/netinet/ip_id.c b/sys/netinet/ip_id.c
new file mode 100644
index 00000000000..54ab20fbf42
--- /dev/null
+++ b/sys/netinet/ip_id.c
@@ -0,0 +1,199 @@
+/* $OpenBSD: ip_id.c,v 1.1 1998/12/26 12:35:12 provos Exp $ */
+
+/*
+ * Copyright 1998 Niels Provos <provos@citi.umich.edu>
+ * All rights reserved.
+ *
+ * Theo de Raadt <deraadt@openbsd.org> came up with the idea of using
+ * such a mathematical system to generate more random (yet non-repeating)
+ * ids to solve the resolver/named problem.  But Niels designed the
+ * actual system based on the constraints.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. All advertising materials mentioning features or use of this software
+ *    must display the following acknowledgement:
+ *      This product includes software developed by Niels Provos.
+ * 4. The name of the author may not be used to endorse or promote products
+ *    derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+ * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+ * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+/* 
+ * seed = random 15bit
+ * n = prime, g0 = generator to n,
+ * j = random so that gcd(j,n-1) == 1
+ * g = g0^j mod n will be a generator again.
+ *
+ * X[0] = random seed.
+ * X[n] = a*X[n-1]+b mod m is a Linear Congruential Generator
+ * with a = 7^(even random) mod m, 
+ *      b = random with gcd(b,m) == 1
+ *      m = 31104 and a maximal period of m-1.
+ *
+ * The transaction id is determined by:
+ * id[n] = seed xor (g^X[n] mod n)
+ *
+ * Effectivly the id is restricted to the lower 15 bits, thus
+ * yielding two different cycles by toggling the msb on and off.
+ * This avoids reuse issues caused by reseeding.
+ */
+
+#include <sys/param.h>
+#include <sys/time.h>
+#include <sys/kernel.h>
+
+#include <dev/rndvar.h>
+
+#define RU_OUT  180             /* Time after wich will be reseeded */
+#define RU_MAX	30000		/* Uniq cycle, avoid blackjack prediction */
+#define RU_GEN	2		/* Starting generator */
+#define RU_N	32749		/* RU_N-1 = 2*2*3*2729 */
+#define RU_AGEN	7               /* determine ru_a as RU_AGEN^(2*rand) */
+#define RU_M	31104           /* RU_M = 2^7*3^5 - don't change */
+
+#define PFAC_N 3
+const static u_int16_t pfacts[PFAC_N] = {
+	2, 
+	3,
+	2729
+};
+
+static u_int16_t ru_x;
+static u_int16_t ru_seed;
+static u_int16_t ru_a, ru_b;
+static u_int16_t ru_g;
+static u_int16_t ru_counter = 0;
+static u_int16_t ru_msb = 0;
+static long ru_reseed;
+static u_int32_t tmp;                /* Storage for unused random */
+
+static u_int16_t pmod __P((u_int16_t, u_int16_t, u_int16_t));
+static void ip_initid __P((void));
+u_int16_t ip_randomid __P((void));
+
+/*
+ * Do a fast modular exponation, returned value will be in the range
+ * of 0 - (mod-1)
+ */
+
+#ifdef __STDC__
+static u_int16_t
+pmod(u_int16_t gen, u_int16_t exp, u_int16_t mod)
+#else
+static u_int16_t
+pmod(gen, exp, mod)
+	u_int16_t gen, exp, mod;
+#endif
+{
+	u_int16_t s, t, u;
+
+	s = 1;
+	t = gen;
+	u = exp;
+
+	while (u) {
+		if (u & 1)
+			s = (s*t) % mod;
+		u >>= 1;
+		t = (t*t) % mod;
+	}
+	return (s);
+}
+
+/* 
+ * Initalizes the seed and chooses a suitable generator. Also toggles 
+ * the msb flag. The msb flag is used to generate two distinct
+ * cycles of random numbers and thus avoiding reuse of ids.
+ *
+ * This function is called from id_randomid() when needed, an 
+ * application does not have to worry about it.
+ */
+static void 
+ip_initid(void)
+{
+	u_int16_t j, i;
+	int noprime = 1;
+
+	get_random_bytes((void *) &tmp, sizeof(tmp));
+	ru_x = (tmp & 0xFFFF) % RU_M;
+
+	/* 15 bits of random seed */
+	ru_seed = (tmp >> 16) & 0x7FFF;
+
+	get_random_bytes((void *) &tmp, sizeof(tmp));
+
+	/* Determine the LCG we use */
+	ru_b = (tmp & 0xfffe) | 1;
+	ru_a = pmod(RU_AGEN, (tmp >> 16) & 0xfffe, RU_M);
+	while (ru_b % 3 == 0)
+	  ru_b += 2;
+	
+	get_random_bytes((void *) &tmp, sizeof(tmp));
+	j = tmp % RU_N;
+	tmp = tmp >> 16;
+
+	/* 
+	 * Do a fast gcd(j,RU_N-1), so we can find a j with
+	 * gcd(j, RU_N-1) == 1, giving a new generator for
+	 * RU_GEN^j mod RU_N
+	 */
+
+	while (noprime) {
+		for (i=0; i<PFAC_N; i++)
+			if (j%pfacts[i] == 0)
+				break;
+
+		if (i>=PFAC_N)
+			noprime = 0;
+		else 
+			j = (j+1) % RU_N;
+	}
+
+	ru_g = pmod(RU_GEN,j,RU_N);
+	ru_counter = 0;
+
+	ru_reseed = time.tv_sec + RU_OUT;
+	ru_msb = ru_msb == 0x8000 ? 0 : 0x8000; 
+}
+
+u_int16_t
+ip_randomid(void)
+{
+        int i, n;
+
+	if (ru_counter >= RU_MAX || time.tv_sec > ru_reseed)
+		ip_initid();
+
+	if (!tmp)
+		get_random_bytes((void *) &tmp, sizeof(tmp));
+
+	/* Skip a random number of ids */
+	n = tmp & 0x3; tmp = tmp >> 2;
+	if (ru_counter + n >= RU_MAX)
+                ip_initid();
+
+	for (i = 0; i <= n; i++)
+	        /* Linear Congruential Generator */
+	        ru_x = (ru_a*ru_x + ru_b) % RU_M;
+
+	ru_counter += i;
+
+	return (ru_seed ^ pmod(ru_g,ru_x,RU_N)) | ru_msb;
+}