Do broken CRC32

ethertype-dump.c

@@ -2,6 +2,7 @@
 #include <assert.h>
 #include <err.h>
 #include <errno.h>
+#include <inttypes.h>
 #include <linux/if_packet.h>
 #include <net/ethernet.h>
 #include <net/if.h>
@@ -14,6 +15,34 @@
 #include <sys/types.h>
 #include <unistd.h>
 
+uint32_t crc32_table[256];
+
+void precompute_crc32_table(void) {
+	for (size_t index = 0; index < 256; index++) {
+		uint32_t value = index;
+
+		for (size_t i = 0; i < 8; i++) {
+			uint32_t low_bit = value & 1;
+			value = value >> 1;
+			if (low_bit) {
+				value ^= 0xEDB88320;
+			}
+		}
+
+		crc32_table[index] = value;
+	}
+}
+
+uint32_t crc32(unsigned char *buf, size_t len) {
+	uint32_t value = 0xffffffff;
+
+	for (size_t index = 0; index < len; index++) {
+		value = crc32_table[(value & 0xff) ^ buf[index]] ^ (value >> 8);
+	}
+
+	return value ^ 0xffffffff;
+}
+
 char hexify(int nybble) {
 	assert(0 <= nybble && nybble <= 16);
 	return "0123456789abcdef"[nybble];
@@ -35,6 +64,9 @@ int main(int argc, char **argv) {
 		exit(1);
 	}
 
+	// Set up CRC32
+	precompute_crc32_table();
+
 	// Create a packet socket
 	errno = 0;
 	int packet_socket = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
@@ -63,6 +95,10 @@ int main(int argc, char **argv) {
 	for(;;) {
 		unsigned char frame[1522]; // 1522 is the largest a 802.3 frame can be
 
+		if (fflush(stdout) == -1) {
+			err(1, "fflush");
+		}
+
 		errno = 0;
 		ssize_t frame_length = recv(packet_socket, frame, sizeof(frame), MSG_TRUNC);
 		if (frame_length == -1) {
@@ -77,6 +113,17 @@ int main(int argc, char **argv) {
 			continue;
 		}
 
+		// Compute the CRC32 of the frame
+		uint32_t crc32_residue = crc32(frame, frame_length);
+
+		if (crc32_residue != 0xc704dd7b) {
+			errno = 0;
+			if (printf("CRC failed (%04"PRIx32")\n", crc32_residue) == -1) {
+				err(1, "printf");
+			}
+			continue;
+		}
+
 		// Extract the MACs
 		// 012345 012345
 		// dest   source
@@ -104,7 +151,7 @@ int main(int argc, char **argv) {
 		}
 
 		errno = 0;
-		if(printf("%s -> %s ethertype: %02x%02x", source_mac, destination_mac, ethertype >> 8, ethertype & 0xff) == -1) {
+		if(printf("%s -> %s ethertype: %04" PRIx16, source_mac, destination_mac, ethertype) == -1) {
 			err(1, "printf");
 		}
 
@@ -118,10 +165,6 @@ int main(int argc, char **argv) {
 				err(1, "printf");
 			}
 		}
-
-		if (fflush(stdout) == -1) {
-			err(1, "fflush");
-		}
 	}
 
 	return 0;