Receive raw Ethernet frames in Linux

recvRawEth.c

/*
 *  This program is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 */

#include <arpa/inet.h>
#include <linux/if_packet.h>
#include <linux/ip.h>
#include <linux/udp.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <net/if.h>
#include <netinet/ether.h>

#define DEST_MAC0	0x00
#define DEST_MAC1	0x00
#define DEST_MAC2	0x00
#define DEST_MAC3	0x00
#define DEST_MAC4	0x00
#define DEST_MAC5	0x00

#define ETHER_TYPE	0x0800

#define DEFAULT_IF	"eth0"
#define BUF_SIZ		1024

int main(int argc, char *argv[])
{
	char sender[INET6_ADDRSTRLEN];
	int sockfd, ret, i;
	int sockopt;
	ssize_t numbytes;
	struct ifreq ifopts;	/* set promiscuous mode */
	struct ifreq if_ip;	/* get ip addr */
	struct sockaddr_storage their_addr;
	uint8_t buf[BUF_SIZ];
	char ifName[IFNAMSIZ];
	
	/* Get interface name */
	if (argc > 1)
		strcpy(ifName, argv[1]);
	else
		strcpy(ifName, DEFAULT_IF);

	/* Header structures */
	struct ether_header *eh = (struct ether_header *) buf;
	struct iphdr *iph = (struct iphdr *) (buf + sizeof(struct ether_header));
	struct udphdr *udph = (struct udphdr *) (buf + sizeof(struct iphdr) + sizeof(struct ether_header));

	memset(&if_ip, 0, sizeof(struct ifreq));

	/* Open PF_PACKET socket, listening for EtherType ETHER_TYPE */
	if ((sockfd = socket(PF_PACKET, SOCK_RAW, htons(ETHER_TYPE))) == -1) {
		perror("listener: socket");	
		return -1;
	}

	/* Set interface to promiscuous mode - do we need to do this every time? */
	strncpy(ifopts.ifr_name, ifName, IFNAMSIZ-1);
	ioctl(sockfd, SIOCGIFFLAGS, &ifopts);
	ifopts.ifr_flags |= IFF_PROMISC;
	ioctl(sockfd, SIOCSIFFLAGS, &ifopts);
	/* Allow the socket to be reused - incase connection is closed prematurely */
	if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &sockopt, sizeof sockopt) == -1) {
		perror("setsockopt");
		close(sockfd);
		exit(EXIT_FAILURE);
	}
	/* Bind to device */
	if (setsockopt(sockfd, SOL_SOCKET, SO_BINDTODEVICE, ifName, IFNAMSIZ-1) == -1)	{
		perror("SO_BINDTODEVICE");
		close(sockfd);
		exit(EXIT_FAILURE);
	}

repeat:	printf("listener: Waiting to recvfrom...\n");
	numbytes = recvfrom(sockfd, buf, BUF_SIZ, 0, NULL, NULL);
	printf("listener: got packet %lu bytes\n", numbytes);

	/* Check the packet is for me */
	if (eh->ether_dhost[0] == DEST_MAC0 &&
			eh->ether_dhost[1] == DEST_MAC1 &&
			eh->ether_dhost[2] == DEST_MAC2 &&
			eh->ether_dhost[3] == DEST_MAC3 &&
			eh->ether_dhost[4] == DEST_MAC4 &&
			eh->ether_dhost[5] == DEST_MAC5) {
		printf("Correct destination MAC address\n");
	} else {
		printf("Wrong destination MAC: %x:%x:%x:%x:%x:%x\n",
						eh->ether_dhost[0],
						eh->ether_dhost[1],
						eh->ether_dhost[2],
						eh->ether_dhost[3],
						eh->ether_dhost[4],
						eh->ether_dhost[5]);
		ret = -1;
		goto done;
	}

	/* Get source IP */
	((struct sockaddr_in *)&their_addr)->sin_addr.s_addr = iph->saddr;
	inet_ntop(AF_INET, &((struct sockaddr_in*)&their_addr)->sin_addr, sender, sizeof sender);

	/* Look up my device IP addr if possible */
	strncpy(if_ip.ifr_name, ifName, IFNAMSIZ-1);
	if (ioctl(sockfd, SIOCGIFADDR, &if_ip) >= 0) { /* if we can't check then don't */
		printf("Source IP: %s\n My IP: %s\n", sender, 
				inet_ntoa(((struct sockaddr_in *)&if_ip.ifr_addr)->sin_addr));
		/* ignore if I sent it */
		if (strcmp(sender, inet_ntoa(((struct sockaddr_in *)&if_ip.ifr_addr)->sin_addr)) == 0)	{
			printf("but I sent it :(\n");
			ret = -1;
			goto done;
		}
	}

	/* UDP payload length */
	ret = ntohs(udph->len) - sizeof(struct udphdr);

	/* Print packet */
	printf("\tData:");
	for (i=0; i<numbytes; i++) printf("%02x:", buf[i]);
	printf("\n");

done:	goto repeat;

	close(sockfd);
	return ret;
}

thanks for this, great starting point!

Thanks for sharing.

Thanks for the software. This really helped to get input from the eth0 to my c program quick.

perhaps #include <unistd.h> is missing as a compiler warning for implicit close definition occurs.

i've expanded this code significantly and joined it with the send example to relay packets. i'm adding thread support next for two-way relaying or a handy bridge.

one bug i've run into is that the kernel seems to re-assmble the TCP segments into large packets (>1500 bytes) requiring jumbo frames on the tx side. any comments on addressing this is appreciated.

Label "done" should be on a next line.

Nice example.
I updated it a little-bit with addition of ability to set DST MAC in command line like ./sendRawEth enp0s29u1u1u2 2E:58:2F:73:73:40
If of any interest please find it here https://gist.github.com/abrodkin/8854874f3b34e92f71653a7fa35c9900

Suggesting to use
#define ETHER_TYPE ETHERTYPE_IP
instead of
#define ETHER_TYPE 0x0800
ETHERTYPE_IP is already defined when including netinet/ether.h

You should set sockopt to 1 because at this moment it is undefined if SO_REUSEADDR is set or not.

SO_BINDTODEVICE works only with AF_INET

HI. I am getting an error of "SO_BINDTODEVICE : NO SUCH DEVICE". Would deeply appreciate if I can get help from anybody here. I had tried googling the error but i still cant find a prominent solution.

Thanks.
PS. I think your "done:" label is on the wrong line ;)

Can you please explain how to run these codes on both sender and receiver end using wlan0?

Thanks a lot for sharing. It does not show 802.1q tags similar to another program that I got from here.
I can capture the complete ethernet frame including the VLAN tags using tcpdump. For some reason, these programs drop/exclude 802.1q tags.
Does anybody know the reason?

Warning: sockopt is undefined in this code. It probably should be initialized to 1.

I modify it a little bit, so that it only prints the packet with specified source or dest mac address.

#include <arpa/inet.h>
#include <linux/if_packet.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <net/if.h>
#include <netinet/ether.h>
#include <unistd.h>

// #define DEST_MAC0 0x00
// #define DEST_MAC1 0x00
// #define DEST_MAC2 0x00
// #define DEST_MAC3 0x00
// #define DEST_MAC4 0x00
// #define DEST_MAC5 0x00

#define DEST_MAC0 0x52
#define DEST_MAC1 0x54
#define DEST_MAC2 0x00
#define DEST_MAC3 0xcb
#define DEST_MAC4 0x4b
#define DEST_MAC5 0xca

#define ETHER_TYPE 0x0800

#define DEFAULT_IF "eth0"
#define BUF_SIZ 1024

int main(int argc, char *argv[])
{
	char sender[INET6_ADDRSTRLEN];
	int sockfd, ret, i;
	int sockopt;
	ssize_t numbytes;
	struct ifreq ifopts; /* set promiscuous mode */
	struct sockaddr_storage their_addr;
	uint8_t buf[BUF_SIZ];
	char ifName[IFNAMSIZ];

	/* Get interface name */
	if (argc > 1)
		strcpy(ifName, argv[1]);
	else
		strcpy(ifName, DEFAULT_IF);

	/* Header structures */
	struct ether_header *eh = (struct ether_header *)buf;

	/* Open PF_PACKET socket, listening for EtherType ETHER_TYPE */
	if ((sockfd = socket(PF_PACKET, SOCK_RAW, htons(ETHER_TYPE))) == -1)
	{
		perror("listener: socket");
		return -1;
	}

	/* Set interface to promiscuous mode - do we need to do this every time? */
	strncpy(ifopts.ifr_name, ifName, IFNAMSIZ - 1);
	ioctl(sockfd, SIOCGIFFLAGS, &ifopts);
	ifopts.ifr_flags |= IFF_PROMISC;
	ioctl(sockfd, SIOCSIFFLAGS, &ifopts);
	/* Allow the socket to be reused - incase connection is closed prematurely */
	if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &sockopt, sizeof sockopt) == -1)
	{
		perror("setsockopt");
		close(sockfd);
		exit(EXIT_FAILURE);
	}
	/* Bind to device */
	if (setsockopt(sockfd, SOL_SOCKET, SO_BINDTODEVICE, ifName, IFNAMSIZ - 1) == -1)
	{
		perror("SO_BINDTODEVICE");
		close(sockfd);
		exit(EXIT_FAILURE);
	}

	while (1)
	{
		numbytes = recvfrom(sockfd, buf, BUF_SIZ, 0, NULL, NULL);
		if (
			// eh->ether_dhost[0] == DEST_MAC0 &&
			// eh->ether_dhost[1] == DEST_MAC1 &&
			// eh->ether_dhost[2] == DEST_MAC2 &&
			// eh->ether_dhost[3] == DEST_MAC3 &&
			// eh->ether_dhost[4] == DEST_MAC4 &&
			// eh->ether_dhost[5] == DEST_MAC5 &&
			eh->ether_shost[0] == 0x52 &&
			eh->ether_shost[1] == 0x54 &&
			eh->ether_shost[2] == 0x00 &&
			eh->ether_shost[3] == 0xb7 &&
			eh->ether_shost[4] == 0xd4 &&
			eh->ether_shost[5] == 0x39)
		{
			printf("source mac: %x:%x:%x:%x:%x:%x\n",
				   eh->ether_shost[0],
				   eh->ether_shost[1],
				   eh->ether_shost[2],
				   eh->ether_shost[3],
				   eh->ether_shost[4],
				   eh->ether_shost[5]);
			printf("destination mac: %x:%x:%x:%x:%x:%x\n",
				   eh->ether_dhost[0],
				   eh->ether_dhost[1],
				   eh->ether_dhost[2],
				   eh->ether_dhost[3],
				   eh->ether_dhost[4],
				   eh->ether_dhost[5]);

			printf("Data: ");
			for (i = 0; i < numbytes; i++)
				printf("%02x ", buf[i]);
			printf("\n\n");
		}
	}
}