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Display MAC addresses

This commit is contained in:
Juhani Krekelä 2019-07-03 23:05:29 +03:00
parent 17b8c26404
commit 4ce7f9c309
1 changed files with 38 additions and 5 deletions
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43
ethertype-dump.c
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@ -1,4 +1,5 @@
#include <arpa/inet.h>
#include <assert.h>
#include <err.h>
#include <errno.h>
#include <linux/if_packet.h>
@ -12,6 +13,21 @@
#include <sys/types.h>
#include <unistd.h>
char hexify(int nybble) {
assert(0 <= nybble && nybble <= 16);
return "0123456789abcdef"[nybble];
}
void format_mac(const unsigned char binary_address[6], char formatted[18]) {
for (size_t i = 0; i < 6; i++) {
unsigned char byte = binary_address[i];
formatted[3*i] = hexify(byte >> 4);
formatted[3*i + 1] = hexify(byte & 0xf);
formatted[3*i + 2] = ':';
}
formatted[17] = '\0';
}
int main(void) {
// Create a packet socket
errno = 0;
@ -47,13 +63,25 @@ int main(void) {
err(1, "recv");
}
if (frame_length < 64) {
errno = 0;
if (printf("Runt frame (%zdB)\n", frame_length) == -1) {
err(1, "printf");
}
continue;
}
// Extract the MACs
// 012345 012345
// dest source
char destination_mac[18], source_mac[18];
format_mac(&frame[0], destination_mac);
format_mac(&frame[6], source_mac);
// Extract EtherType / length field
// 012345 012345 01
// dest source ^^
// It is stored in the network byte order, that is, high byte 1st
if (frame_length < 14) {
errx(1, "Very weird frame of only %zdB???", frame_length);
}
uint16_t ethertype = (frame[12] << 8) | frame[13];
const char *ethertype_meaning = NULL;
@ -69,13 +97,18 @@ int main(void) {
ethertype_meaning = "IPv6";
}
errno = 0;
if(printf("%s -> %s ethertype: %02x%02x", source_mac, destination_mac, ethertype >> 8, ethertype & 0xff) == -1) {
err(1, "printf");
}
errno = 0;
if (ethertype_meaning == NULL) {
if (printf("ethertype: %02x%02x, length: %zd\n", ethertype >> 8, ethertype & 0xff, frame_length) == -1) {
if (printf(", length: %zd\n", frame_length) == -1) {
err(1, "printf");
}
} else {
if (printf("ethertype: %02x%02x (%s), length: %zd\n", ethertype >> 8, ethertype & 0xff, ethertype_meaning, frame_length) == -1) {
if (printf("(%s), length: %zd\n", ethertype_meaning, frame_length) == -1) {
err(1, "printf");
}
}