ethermess/arp-request.c

#define _GNU_SOURCE
#include <arpa/inet.h>
#include <assert.h>
#include <err.h>
#include <inttypes.h>
#include <linux/if_packet.h>
#include <net/ethernet.h>
#include <net/if.h>
#include <net/if_arp.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>

void drop_privileges(void) {
	uid_t uid = getuid();
	gid_t gid = getgid();

	if (setresgid(gid, gid, gid) == -1) {
		err(1, "setresgid");
	}
	if (setresuid(uid, uid, uid) == -1) {
		err(1, "setresuid");
	}
}

int main(int argc, char **argv) {
	if (argc != 3) {
		fprintf(stderr, "Usage: %s interface ip\n", argv[0]);
		exit(1);
	}

	const char *interface_name = argv[1];
	const char *target_ip = argv[2];

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

	// Only creating the socket requires root privs
	drop_privileges();

	// Find the index of the network interface
	struct ifreq ifr;
	strncpy(ifr.ifr_name, interface_name, IFNAMSIZ);
	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;
	if (bind(packet_socket, (const struct sockaddr*)&sll, sizeof(sll)) == -1) {
		err(1, "bind");
	}

	// Construct the Ethernet frame with ARP packet inside it
	//
	// NOTE: This uses network byte order. That is, most significant
	// byte goes first, not last like with x86
	//
	// -- Ethernet --
	//  0 Destination MAC (6B) == FF:FF:FF:FF:FF:FF (broadcast address)
	//  6 Source MAC (6B)
	// 12 EtherType (2B) == 0x0806
	// -- ARP --
	// 14 Hardware type (2B) == 0x0001 (Ethernet)
	// 16 Protocol type (2B) == 0x0800 (IPv4 EtherType)
	// 18 Hardware address length (1B) == 6
	// 19 Protocol address length (1B) == 4
	// 20 Opcode (2B) == 0x0001 (request)
	// 22 Sender hardware address (6B for Ethernet)
	// 28 Sender protocol address (4B for IPv4)
	// 32 Target hardware address (6B for Ethernet) == FF:FF:FF:FF:FF:FF (the value is ignored)
	// 38 Target protocol address (4B for IPv4)
	//
	// = 42B
	unsigned char frame[42];
	memset(frame, 0, sizeof(frame));

	// Destination MAC
	memset(&frame[0], 0xff, 6);

	// Source MAC
	strncpy(ifr.ifr_name, interface_name, IFNAMSIZ);
	if (ioctl(packet_socket, SIOCGIFHWADDR, &ifr) == -1) {
		err(1, "ioctl");
	}
	if (ifr.ifr_hwaddr.sa_family != ARPHRD_ETHER) {
		errx(1, "Not an Ethernet interface");
	}
	memcpy(&frame[6], ifr.ifr_hwaddr.sa_data, 6);

	// EtherType
	frame[12] = 0x08;
	frame[13] = 0x06;

	// Hardware type
	frame[14] = 0x00;
	frame[15] = 0x01;

	// Protocol type
	frame[16] = 0x08;
	frame[17] = 0x00;

	// Hardware address length
	frame[18] = 6;

	// Protocol address length
	frame[19] = 4;

	// Opcode
	frame[20] = 0x00;
	frame[21] = 0x01;

	// Sender hardware address
	// This is the same as source MAC in the Ethernet header
	memcpy(&frame[22], &frame[6], 6);

	// Sender protocol address
	strncpy(ifr.ifr_name, interface_name, IFNAMSIZ);
	if (ioctl(packet_socket, SIOCGIFADDR, &ifr) == -1) {
		err(1, "ioctl");
	}
	if (ifr.ifr_addr.sa_family != AF_INET) {
		errx(1, "Somehow, we've gotten a non-IPv4 address from kernel");
	}
	struct sockaddr_in sin;
	memcpy(&sin, &ifr.ifr_addr, sizeof(sin));
	memcpy(&frame[28], &sin.sin_addr.s_addr, 4);

	// Target hardware address
	memset(&frame[32], 0xff, 6);

	// Target protocol address
	memset(&sin, 0, sizeof(sin));
	if (inet_pton(AF_INET, target_ip, &sin.sin_addr) == -1) {
		err(1, "inet_pton");
	}
	memcpy(&frame[38], &sin.sin_addr.s_addr, 4);

	// Send the frame
	if (write(packet_socket, frame, sizeof(frame)) != sizeof(frame)) {
		err(1, "write");
	}

	// Close the socket (tho I'm not 100% sure it's needed)
	if (close(packet_socket) == -1) {
		err(1, "close");
	}

	return 0;
}