added fallback random source ("/dev/urandom") for systems with low entropy available and "/dev/random" activated

Signed-off-by: Toni Uhlig <matzeton@googlemail.com>

3 files changed, 31 insertions, 5 deletions

diff --git a/configure.ac b/configure.ac
index 239a78b..80050d3 100644
--- a/configure.ac
+++ b/configure.ac
@@ -160,13 +160,14 @@ esac
 
 dnl `--with-randomdev`: Default value /dev/random
 use_customrng=no
+use_rngfallback=no
 AC_MSG_CHECKING([for random device])
 AC_ARG_WITH([rngdev],
 	[AS_HELP_STRING([--with-rngdev], [Set an alternative random device. (default: /dev/random)])],
 	[use_customrng=yes], [with_rngdev="/dev/random"])
 case ${with_rngdev} in
 	yes) with_rngdev="/dev/random" ;;
-	/dev/random) ;;
+	/dev/random) use_rngfallback=yes ;;
 	/dev/urandom) ;;
 	*) AC_MSG_ERROR([Unknown random device \`${with_rngdev}\` for --with-rngdev: Only \`/dev/random\` xor \`/dev/urandom\` allowed. This option is unused on Windows targets.]) ;;
 esac
@@ -236,6 +237,7 @@ AM_CONDITIONAL([IS_WINDOWS], [test x"${use_msw}" = xyes])
 AM_CONDITIONAL([HAVE_ICMPFILTER], [test x"${with_icmp_filter}" = xyes])
 AM_CONDITIONAL([HAVE_ARC4RANDOM], [test x"${arc4random_enabled}" = xyes])
 AM_CONDITIONAL([USE_CUSTOMRNG], [test x"${use_customrng}" = xyes])
+AM_CONDITIONAL([USE_RNGFALLBACK], [test x"${use_rngfallback}" = xyes])
 
 dnl output config headers
 AC_CONFIG_HEADERS([src/config.h:src/config.h.in])
diff --git a/src/Makefile.am b/src/Makefile.am
index 6f9ca23..97b4545 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -30,6 +30,10 @@ if USE_CUSTOMRNG
 ptunnel_ng_CFLAGS += -DUSE_CUSTOMRNG=1
 endif
 
+if USE_RNGFALLBACK
+ptunnel_ng_CFLAGS += -DUSE_RNGFALLBACK=1
+endif
+
 ptunnel_ng_SOURCES = \
 	md5.c \
 	challenge.c \
diff --git a/src/utils.c b/src/utils.c
index 64f6f88..6752890 100644
--- a/src/utils.c
+++ b/src/utils.c
@@ -174,12 +174,32 @@ int pt_random(void) {
 	}
 	bytes_read = read(rng_fd, &rnd_val, sizeof rnd_val);
 	if (bytes_read != sizeof rnd_val) {
-		if (bytes_read < 0)
-			pt_log(kLog_error, "FATAL: Read from random device failed: %s\n",
+		if (bytes_read < 0) {
+			pt_log(kLog_error, "Read from random device failed: %s\n",
 			       strerror(errno));
-		else
-			pt_log(kLog_error, "FATAL: Read only %zd bytes (wanted %zd bytes)\n",
+        } else {
+			pt_log(kLog_info, "Read only %zd random bytes (wanted %zd bytes)\n",
 			       bytes_read, sizeof rnd_val);
+		}
+#ifdef USE_RNGFALLBACK
+		/* use /dev/urandom if previous random device failed */
+		static int fallback_rng_fd = -1;
+		if (fallback_rng_fd < 0) {
+			fallback_rng_fd = open("/dev/urandom", O_RDONLY);
+			if (fallback_rng_fd < 0) {
+				pt_log(kLog_error, "FATAL: Could not open fallback random device '%s': %s\n",
+				       "/dev/urandom", strerror(errno));
+				exit(EXIT_FAILURE);
+			}
+		}
+		if (bytes_read < 0) {
+			bytes_read = 0;
+		}
+		if (read(fallback_rng_fd, &rnd_val + bytes_read, sizeof rnd_val - bytes_read) == sizeof rnd_val - bytes_read) {
+			return rnd_val;
+		}
+#endif
+		pt_log(kLog_error, "FATAL: No more RNG sources available\n");
 		exit(EXIT_FAILURE);
 	}
 	return rnd_val;