Randomize message ID start

ethermess.c

@@ -3,6 +3,7 @@
 #include <assert.h>
 #include <err.h>
 #include <errno.h>
+#include <fcntl.h>
 #include <inttypes.h>
 #include <linux/if_packet.h>
 #include <net/ethernet.h>
@@ -16,6 +17,7 @@
 #include <string.h>
 #include <sys/ioctl.h>
 #include <sys/socket.h>
+#include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>
 
@@ -33,6 +35,7 @@
 bool running = true;
 
 int packet_socket;
+int urandom;
 
 unsigned char own_mac[6];
 unsigned char broadcast_mac[6] = {0xff, 0xff, 0xff, 0xff, 0xff, 0xff};
@@ -75,8 +78,11 @@ ssize_t msgid_cache_add(const unsigned char mac[6]) {
 
 	// If we are adding into a new slot (instead of overwriting one),
 	// expand the fill pointer
-	if (msgid_cache_fill < index) {
-		msgid_cache_fill++;
+	//
+	// + 1 because msgid_cache_fill of N means that cache slots [0, N-1]
+	// are in use
+	if (msgid_cache_fill < index + 1) {
+		msgid_cache_fill = index + 1;
 	}
 
 	memcpy(msgid_cache[index].other_mac, mac, sizeof(msgid_cache[index].other_mac));
@@ -86,6 +92,15 @@ ssize_t msgid_cache_add(const unsigned char mac[6]) {
 	return index;
 }
 
+unsigned char random_byte(void) {
+	unsigned char randomness;
+	errno = 0;
+	if (read(urandom, &randomness, 1) != 1) {
+		err(1, "read");
+	}
+	return randomness;
+}
+
 char hexify(int nybble) {
 	assert(0 <= nybble && nybble <= 16);
 	return "0123456789abcdef"[nybble];
@@ -199,9 +214,8 @@ void send_msgid(const unsigned char destination[6]) {
 
 	if (!msgid_cache[cache_index].know_receive) {
 		// We don't have receive ID stored in the cache
-		// In that case, start from 0
-		// TODO: Make it random instead
-		msgid_cache[cache_index].next_receive = 0;
+		// In that case, start from a random index
+		msgid_cache[cache_index].next_receive = (random_byte() << 8) | random_byte();
 		msgid_cache[cache_index].know_receive = true;
 	}
 
@@ -215,6 +229,7 @@ void send_msgid(const unsigned char destination[6]) {
 }
 
 void read_command(void) {
+	errno = 0;
 	int cmd = getchar();
 	if (cmd == EOF) {
 		err(1, "getchar");
@@ -356,7 +371,7 @@ void process_frame(void) {
 	errno = 0;
 	ssize_t res = recv(packet_socket, frame, sizeof(frame), 0);
 	if (res == -1) {
-		errx(1, "recv");
+		err(1, "recv");
 	} else if (res < 16) {
 		// Frame too short to contain enough information
 		return;
@@ -480,6 +495,9 @@ int main(int argc, char **argv) {
 
 	const char *interface_name = argv[1];
 
+	// Open /dev/urandom for getting randomness
+	urandom = open("/dev/urandom", O_RDONLY);
+
 	// Create a packet socket
 	errno = 0;
 	packet_socket = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
@@ -545,5 +563,11 @@ int main(int argc, char **argv) {
 		err(1, "fflush");
 	}
 
+	// Close urandom
+	errno = 0;
+	if (close(urandom) == -1) {
+		err(1, "close");
+	}
+
 	return 0;
 }