DnsLookup: Configurable dnsQueryId generation including random

Dns lookup logic has been updated to provide a configurable compile
time selection of dns query id generation logics, including random,
where possible, instead of the previous simple counter mode.

This should make dns poison attempts more difficult. The uclibc
developers wish to thank the white hat teams which alerted the
community about the possible weakness in the dns path, given the
increased resources with adversaries today.

Given that embedded systems may or may not have sources for trying
to generate random numbers, and also to try and keep the load on
the system low, by default it uses the standard random prng based
logic to indirectly generate the ids.

However if either urandom or else if realtime clock is available on
the target, then the same is used to reseed the prng periodically
in a slightly non deterministic manner. Also additional transform
(one way where possible) is used to avoid directly exposing the
internal random sequence.

The dns lookup logic maintains its own state wrt the random prng
functions, so that other users of the library's random prng are
not affected wrt their operations with the prng.

Note to Platform developers:

If you want to change from the default prngplus based logic, to one
of the other logics provided, then during compile/config time you can
switch to one of these additional choices wrt dns query id generation,
by using make config and companions.

If your platform doesnt support urandom nor a realtime clock backed
by a source with sufficient resolution, and or for some reason if you
want to revert to previous simple counter, rather than the transformed
random prng plus logic, you can force the same at compile time by
selecting SimpleCounter mode.

If you want to increase the randomness of the generated ids, and dont
mind the increased system load and latency then you could select the
Urandom mode during config. Do note that it will be dipping into the
entropy pool maintained by ur system.

If your target has a system realtime clock available and exposed to
user space, and inturn if you want to keep the underlying logic simple,
you could try using the clock option from the config. However do note
that the clock should have nanosecond resolution to help generate ids
which are plausibly random. Also improvements to processor and or io
performance can affect this.

Wrt the URandom and Clock modes, if there is a failure with generation
of the next random value, the logic tries to fallback to simple counter
mode.

If you want to change the underlying logic to make it more random
and or more simple, look at dnsrand_setup and dnsrand_next.

Signed-off-by: hanishkvc <hanishkvc@gmail.com>

extra/Configs/Config.in

@@ -1344,6 +1344,57 @@ config UCLIBC_HAS_RESOLVER_SUPPORT
 	  ns_name_pack, ns_name_compress, ns_name_skip, dn_skipname,
 	  ns_get16, ns_get32, ns_put16, ns_put32
 
+choice
+	prompt "DNS Query ID generation"
+	default UCLIBC_DNSRAND_MODE_PRNGPLUS
+	help
+	  Control how successive dns query ids' are generated during
+	  dns lookup.
+
+config UCLIBC_DNSRAND_MODE_URANDOM
+	bool "urandom"
+	help
+	  "urandom" uses /dev/urandom available under many unix flavours
+	  to generate dns query id. This can generate good random ids,
+	  by dipping into the entropy pool maintained by the system.
+	  However this is relatively slow compared to the other options,
+	  as it may involve cryptographic operations internally and
+	  kernel-userspace handshake.
+
+config UCLIBC_DNSRAND_MODE_CLOCK
+	bool "clock"
+	depends on UCLIBC_HAS_REALTIME
+	help
+	  "clock" uses CLOCK_REALTIME of the system to generate plausibly
+	  random dns query id. Systems require to have clock source with
+	  nanosec granularity mapped to this clock id for this to generate
+	  plausibly random values. However has processor and io performances
+	  improve in future, its effectiveness can get impacted.
+
+config UCLIBC_DNSRAND_MODE_PRNGPLUS
+	bool "prngplus"
+	help
+	  "prngplus" uses random prng available within uclibc, to indirectly
+	  generate the dns query id. This tries to provide a good balance
+	  between speed and randomness to an extent. It periodically reseeds
+	  the prng using random value generated from either the urandom or
+	  else the clock, if either of them is available. Additionally applies
+	  transform (one way, if possible) on internal generated random values.
+	  These make it difficult to infer internal state of prng from unbroken
+	  sequences of exposed random values.
+	  This is the default.
+
+config UCLIBC_DNSRAND_MODE_SIMPLECOUNTER
+	bool "simplecounter"
+	help
+	  "simplecounter" uses a simple counter to generate dns query id.
+	  This is a very simple logic and can be subjected to dns poison
+	  attack relatively easily.
+	  It is recommended to avoid this option.
+
+endchoice
+
+
 endif
 
 

libc/inet/resolv.c

@@ -255,6 +255,7 @@ Domain name in a message can be represented as either:
 #include <sys/stat.h>
 #include <sys/param.h>
 #include <bits/uClibc_mutex.h>
+#include <fcntl.h>
 #include "internal/parse_config.h"
 
 /* poll() is not supported in kernel <= 2.0, therefore if __NR_poll is
@@ -1045,6 +1046,236 @@ static int __decode_answer(const unsigned char *message, /* packet */
 	return i + RRFIXEDSZ + a->rdlength;
 }
 
+
+#if defined __UCLIBC_DNSRAND_MODE_URANDOM__ || defined __UCLIBC_DNSRAND_MODE_PRNGPLUS__
+
+/*
+ * Get a random int from urandom.
+ * Return 0 on success and -1 on failure.
+ *
+ * This will dip into the entropy pool maintaind by the system.
+ */
+int _dnsrand_getrandom_urandom(int *rand_value) {
+	static int urand_fd = -1;
+	static int errCnt = 0;
+	if (urand_fd == -1) {
+		urand_fd = open("/dev/urandom", O_RDONLY);
+		if (urand_fd == -1) {
+			if ((errCnt % 16) == 0) {
+				DPRINTF("uCLibC:WARN:DnsRandGetRand: urandom is unavailable...\n");
+			}
+			errCnt += 1;
+			return -1;
+		}
+	}
+	if (read(urand_fd, rand_value, sizeof(int)) == sizeof(int)) { /* small reads like few bytes here should be safe in general. */
+		DPRINTF("uCLibC:DBUG:DnsRandGetRand: URandom:0x%lx\n", *rand_value);
+		return 0;
+	}
+	return -1;
+}
+
+#endif
+
+#if defined __UCLIBC_DNSRAND_MODE_CLOCK__ || defined __UCLIBC_DNSRAND_MODE_PRNGPLUS__
+
+/*
+ * Try get a sort of random int by looking at current time in system realtime clock.
+ * Return 0 on success and -1 on failure.
+ *
+ * This requries the realtime related uclibc feature to be enabled and also
+ * the system should have a clock source with nanosec resolution to be mapped
+ * to CLOCK_REALTIME, for this to generate values that appear random plausibly.
+ */
+int _dnsrand_getrandom_clock(int *rand_value) {
+#if defined __USE_POSIX199309 && defined __UCLIBC_HAS_REALTIME__
+	struct timespec ts;
+	if (clock_gettime(CLOCK_REALTIME, &ts) == 0) {
+		*rand_value = (ts.tv_sec + ts.tv_nsec) % INT_MAX;
+		DPRINTF("uCLibC:DBUG:DnsRandGetRand: Clock:0x%lx\n", *rand_value);
+		return 0;
+	}
+#endif
+	return -1;
+}
+
+#endif
+
+#ifdef __UCLIBC_DNSRAND_MODE_PRNGPLUS__
+
+/*
+ * Try get a random int by first checking at urandom and then at realtime clock.
+ * Return 0 on success and -1 on failure.
+ *
+ * Chances are most embedded targets using linux/bsd/... could have urandom and
+ * also it can potentially give better random values, so try urandom first.
+ * However if there is failure wrt urandom, then try realtime clock based helper.
+ */
+int _dnsrand_getrandom_urcl(int *rand_value) {
+	if (_dnsrand_getrandom_urandom(rand_value) == 0) {
+		return 0;
+	}
+	if (_dnsrand_getrandom_clock(rand_value) == 0) {
+		return 0;
+	}
+	DPRINTF("uCLibC:DBUG:DnsRandGetRand: URCL:Nothing:0x%lx\n", *rand_value);
+	return -1;
+}
+
+#define DNSRAND_PRNGSTATE_INT32LEN 32
+#undef DNSRAND_PRNGRUN_SHORT
+#ifdef DNSRAND_PRNGRUN_SHORT
+#define DNSRAND_RESEED_OP1 (DNSRAND_PRNGSTATE_INT32LEN/2)
+#define DNSRAND_RESEED_OP2 (DNSRAND_PRNGSTATE_INT32LEN/4)
+#else
+#define DNSRAND_RESEED_OP1 (DNSRAND_PRNGSTATE_INT32LEN*6)
+#define DNSRAND_RESEED_OP2 DNSRAND_PRNGSTATE_INT32LEN
+#endif
+/*
+ * This logic uses uclibc's random PRNG to generate random int. This keeps the
+ * logic fast by not depending on a more involved CPRNG kind of logic nor on a
+ * kernel to user space handshake at the core.
+ *
+ * However to ensure that pseudo random sequences based on a given seeding of the
+ * PRNG logic, is not generated for too long so as to allow a advarsary to try guess
+ * the internal states of the prng logic and inturn the next number, srandom is
+ * used periodically to reseed PRNG logic, when and where possible.
+ *
+ * To help with this periodic reseeding, by default the logic will first try to
+ * see if it can get some relatively random number using /dev/urandom. If not it
+ * will try use the current time to generate plausibly random value as substitute.
+ * If neither of these sources are available, then the prng itself is used to seed
+ * a new state, so that the pseudo random sequence can continue, which is better
+ * than the fallback simple counter.
+ *
+ * Also to add bit more of variance wrt this periodic reseeding, the period interval
+ * at which this reseeding occurs keeps changing within a predefined window. The
+ * window is controlled based on how often this logic is called (which currently
+ * will depend on how often requests for dns query (and inturn dnsrand_next) occurs,
+ * as well as a self driven periodically changing request count boundry.
+ *
+ * The internally generated random values are not directly exposed, instead result
+ * of adjacent values large mult with mod is used to greatly reduce the possibility
+ * of trying to infer the internal values from externally exposed random values.
+ * This should also make longer run of prng ok to an extent.
+ *
+ * NOTE: The Random PRNG used here maintains its own internal state data, so that
+ * it doesnt impact any other users of random prng calls in the system/program
+ * compiled against uclibc.
+ *
+ * NOTE: If your target doesnt support int64_t, then the code uses XOR instead of
+ * mult with mod based transform on the internal random sequence, to generate the
+ * random number that is returned. However as XOR is not a one way transform, this
+ * is supported only in DNSRAND_PRNGRUN_SHORT mode by default, which needs to be
+ * explicitly enabled by the platform developer, by defining the same.
+ *
+ */
+int _dnsrand_getrandom_prng(int *rand_value) {
+	static int cnt = -1;
+	static int nextReSeedWindow = DNSRAND_RESEED_OP1;
+	static int32_t prngState[DNSRAND_PRNGSTATE_INT32LEN]; /* prng logic internally assumes int32_t wrt state array, so to help align if required */
+	static struct random_data prngData;
+	int32_t val, val2;
+	int calc;
+	int prngSeed = 0x19481869;
+
+	if (cnt == -1) {
+		_dnsrand_getrandom_urcl(&prngSeed);
+		memset(&prngData, 0, sizeof(prngData));
+		initstate_r(prngSeed, (char*)&prngState, DNSRAND_PRNGSTATE_INT32LEN*4, &prngData);
+	}
+	cnt += 1;
+	if ((cnt % nextReSeedWindow) == 0) {
+		if (_dnsrand_getrandom_urcl(&prngSeed) != 0) {
+			random_r(&prngData, &prngSeed);
+		}
+		srandom_r(prngSeed, &prngData);
+		random_r(&prngData, &val);
+		nextReSeedWindow = DNSRAND_RESEED_OP1 + (val % DNSRAND_RESEED_OP2);
+		DPRINTF("uCLibC:DBUG:DnsRandNext: PRNGWindow:%d\n", nextReSeedWindow);
+		cnt = 0;
+	}
+	random_r(&prngData, &val);
+	random_r(&prngData, &val2);
+#ifdef INT64_MAX
+	calc = ((int64_t)val * (int64_t)val2) % INT_MAX;
+#else
+# ifdef DNSRAND_PRNGRUN_SHORT
+	calc = val ^ val2;
+# warning "[No int64] using xor based random number transform logic in short prng run mode, bcas int64_t not supported on this target"
+# else
+# error "[No int64] using xor based random number transform logic only supported with short prng runs, you may want to define DNSRAND_PRNGRUN_SHORT"
+# endif
+#endif
+	*rand_value = calc;
+	DPRINTF("uCLibC:DBUG:DnsRandGetRand: PRNGPlus: %d, 0x%lx 0x%lx 0x%lx\n", cnt, val, val2, *rand_value);
+	return 0;
+}
+
+#endif
+
+/**
+ * If DNS query's id etal is generated using a simple counter, then it can be
+ * subjected to dns poisoning relatively easily, so adding some randomness can
+ * increase the difficulty wrt dns poisoning and is thus desirable.
+ *
+ * However given that embedded targets may or may not have different sources available
+ * with them to try generate random values, this logic tries to provides flexibility
+ * to the platform developer to decide, how they may want to handle this.
+ *
+ * If a given target doesnt support urandom nor realtime clock OR for some reason
+ * if the platform developer doesnt want to use random dns query id etal, then
+ * they can define __UCLIBC_DNSRAND_MODE_SIMPLECOUNTER__ so that a simple incrementing
+ * counter is used.
+ *
+ * However if the target has support for urandom or realtime clock, then the prngplus
+ * based random generation tries to give a good balance between randomness and performance.
+ * This is the default and is enabled when no other mode is defined. It is also indirectly
+ * enabled by defining __UCLIBC_DNSRAND_MODE_PRNGPLUS__ instead of the other modes.
+ *
+ * If urandom is available on the target and one wants to keep things simple and use
+ * it directly, then one can define __UCLIBC_DNSRAND_MODE_URANDOM__. Do note that this
+ * will be relatively slower compared to other options. But it can normally generate
+ * good random values/ids by dipping into the entropy pool available in the system.
+ *
+ * If system realtime clock is available on target and enabled, then if one wants to
+ * keep things simple and use it directly, then define __UCLIBC_DNSRAND_MODE_CLOCK__.
+ * Do note that this requires nanosecond resolution / granularity wrt the realtime
+ * clock source to generate plausibly random values/ids. As processor &/ io performance
+ * improves, the effectiveness of this strategy can be impacted in some cases.
+ *
+ * If either the URandom or Clock based get random fails, then the logic is setup to
+ * try fallback to the simple counter mode, with the help of the def_value, which is
+ * setup to be the next increment wrt the previously generated / used value, by the
+ * caller of dnsrand_next.
+ *
+ */
+int dnsrand_next(int def_value) {
+	int val = def_value;
+#if defined __UCLIBC_DNSRAND_MODE_SIMPLECOUNTER__
+	return val;
+#elif defined __UCLIBC_DNSRAND_MODE_URANDOM__
+	if (_dnsrand_getrandom_urandom(&val) == 0) {
+		return val;
+	}
+	return def_value;
+#elif defined __UCLIBC_DNSRAND_MODE_CLOCK__
+	if (_dnsrand_getrandom_clock(&val) == 0) {
+		return val;
+	}
+	return def_value;
+#else
+	if (_dnsrand_getrandom_prng(&val) == 0) {
+		return val;
+	}
+	return def_value;
+#endif
+}
+
+int dnsrand_setup(int def_value) {
+	return def_value;
+}
+
 /* On entry:
  *  a.buf(len) = auxiliary buffer for IP addresses after first one
  *  a.add_count = how many additional addresses are there already
@@ -1149,7 +1380,7 @@ int __dns_lookup(const char *name,
 		}
 		/* first time? pick starting server etc */
 		if (local_ns_num < 0) {
-			local_id = last_id;
+			local_id = dnsrand_setup(last_id);
 /*TODO: implement /etc/resolv.conf's "options rotate"
  (a.k.a. RES_ROTATE bit in _res.options)
 			local_ns_num = 0;
@@ -1159,7 +1390,7 @@ int __dns_lookup(const char *name,
 		}
 		if (local_ns_num >= __nameservers)
 			local_ns_num = 0;
-		local_id++;
+		local_id = dnsrand_next(++local_id);
 		local_id &= 0xffff;
 		/* write new values back while still under lock */
 		last_id = local_id;