ethermess/ethertype-dump.c

#include <arpa/inet.h>
#include <err.h>
#include <errno.h>
#include <linux/if_packet.h>
#include <net/ethernet.h>
#include <net/if.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>

int main(void) {
	// Create a packet socket
	errno = 0;
	int packet_socket = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
	if (packet_socket == -1) {
		err(1, "socket");
	}

	// Find the index of the network interface
	struct ifreq ifr;
	strncpy(ifr.ifr_name, "wlan0", IFNAMSIZ);
	errno = 0;
	if (ioctl(packet_socket, SIOCGIFINDEX, &ifr) == -1) {
		err(1, "ioctl");
	}

	// Bind to the network interface
	struct sockaddr_ll sll;
	sll.sll_family = AF_PACKET;
	sll.sll_protocol = htons(ETH_P_ALL);
	sll.sll_ifindex = ifr.ifr_ifindex;
	errno = 0;
	if (bind(packet_socket, (const struct sockaddr*)&sll, sizeof(sll)) == -1) {
		err(1, "bind");
	}

	for(;;) {
		unsigned char frame[1522]; // 1522 is the largest a 802.3 frame can be

		errno = 0;
		ssize_t frame_length = recv(packet_socket, frame, sizeof(frame), MSG_TRUNC);
		if (frame_length == -1) {
			err(1, "recv");
		}

		// 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;
		if (ethertype <= 1500) {
			ethertype_meaning = "packet length";
		} else if (ethertype < 0x0600) {
			ethertype_meaning = "undefined";
		} else if (ethertype == 0x0800) {
			ethertype_meaning = "IPv4";
		} else if (ethertype == 0x0806) {
			ethertype_meaning = "ARP";
		} else if (ethertype == 0x86dd) {
			ethertype_meaning = "IPv6";
		}

		errno = 0;
		if (ethertype_meaning == NULL) {
			if (printf("ethertype: %02x%02x, length: %zd\n", ethertype >> 8, ethertype & 0xff, 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) {
				err(1, "printf");
			}
		}
	}

	return 0;
}