Add arp-request.c

.gitignore

@@ -1,3 +1,4 @@
 *.swp
 *.o
 ethertype-dump
+arp-request

Makefile

@@ -7,24 +7,29 @@ CFLAGS += -Os -g -Wall -Wextra -pedantic
 CPPFLAGS +=
 LDFLAGS +=
 
+BINS := ethertype-dump arp-request
+
 .SUFFIXES:
 .SUFFIXES: .c .o
 
 .PHONY: all install uninstall clean distclean
 
-all: ethertype-dump
+all: $(BINS)
 
 ethertype-dump: ethertype-dump.o
 	$(CC) -o $@ $< $(LDFLAGS)
 
+arp-request: arp-request.o
+	$(CC) -o $@ $< $(LDFLAGS)
+
 .c.o:
 	$(CC) -c $(CFLAGS) $(CPPFLAGS) -o $@ $<
 
-install: all
-	install ethertype-dump $(BINDIR)
+install: $(BINS)
+	install $(BINS) $(BINDIR)
 
 uninstall:
-	rm $(BINDIR)/ethertype-dump
+	cd $(BINDIR) && rm $(BINS)
 
 clean:
 	rm -f ethertype-dump *.o

arp-request.c

@@ -0,0 +1,187 @@
+#define _GNU_SOURCE
+#include <arpa/inet.h>
+#include <assert.h>
+#include <err.h>
+#include <errno.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>
+
+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';
+}
+
+void drop_privileges(void) {
+	uid_t uid = getuid();
+	gid_t gid = getgid();
+
+	errno = 0;
+	if (setresgid(gid, gid, gid) == -1) {
+		err(1, "setresgid");
+	}
+	errno = 0;
+	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
+	errno = 0;
+	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);
+	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");
+	}
+
+	// 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
+	errno = 0;
+	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
+	errno = 0;
+	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));
+	errno = 0;
+	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
+	errno = 0;
+	if (write(packet_socket, frame, sizeof(frame)) != sizeof(frame)) {
+		err(1, "write");
+	}
+
+	// Close the socket (tho I'm not 100% sure it's needed)
+	errno = 0;
+	if (close(packet_socket) == -1) {
+		err(1, "close");
+	}
+
+	return 0;
+}