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