Start working on the network-facing side of Ethermess

.gitignore

@@ -2,3 +2,4 @@
 *.o
 ethertype-dump
 arp-request
+ethermess

Makefile

@@ -7,7 +7,7 @@ CFLAGS += -Os -g -Wall -Wextra -pedantic
 CPPFLAGS +=
 LDFLAGS +=
 
-BINS := ethertype-dump arp-request
+BINS := ethertype-dump arp-request ethermess
 
 .SUFFIXES:
 .SUFFIXES: .c .o
@@ -22,6 +22,9 @@ ethertype-dump: ethertype-dump.o
 arp-request: arp-request.o
 	$(CC) -o $@ $< $(LDFLAGS)
 
+ethermess: ethermess.o
+	$(CC) -o $@ $< $(LDFLAGS)
+
 .c.o:
 	$(CC) -c $(CFLAGS) $(CPPFLAGS) -o $@ $<
 

ethermess.c

@@ -0,0 +1,200 @@
+#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 <poll.h>
+#include <stdbool.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>
+
+bool running = true;
+
+int packet_socket;
+
+unsigned char own_mac[6];
+
+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");
+	}
+}
+
+void read_command(void) {
+	int cmd = getchar();
+	if (cmd == EOF) {
+		err(1, "getchar");
+	}
+
+	if (cmd == 'q') {
+		running = false;
+	} else if (cmd == '\n') {
+		// Ignore
+	} else {
+		fprintf(stderr, "?"); //debg
+	}
+}
+
+void process_frame(void) {
+	unsigned char frame[1518]; // Largest a 802.3 frame can be without FCS
+
+	errno = 0;
+	if (recv(packet_socket, frame, sizeof(frame), 0) == -1) {
+		errx(1, "recv");
+	}
+
+	fprintf(stderr, "."); // debg
+}
+
+void eventloop(void) {
+	// Listen on both stdin for commands and network interface for packets
+	struct pollfd pollfds[2];
+
+	// stdin
+	pollfds[0].fd = 0;
+	pollfds[0].events = POLLIN;
+
+	// Network interface
+	pollfds[1].fd = packet_socket;
+	pollfds[1].events = POLLIN;
+
+	while (running) {
+		errno = 0;
+		int ready = poll(pollfds, sizeof(pollfds) / sizeof(*pollfds), -1);
+		if (ready == -1) {
+			err(1, "poll");
+		}
+
+		// stdin
+		if (ready > 0 && pollfds[0].revents != 0) {
+			ready--;
+
+			if (pollfds[0].revents & POLLIN) {
+				// Read a command
+				read_command();
+			} else {
+				errx(1, "Got poll event %hd on stdin\n", pollfds[0].revents);
+			}
+		}
+
+		// packet_socket
+		if (ready > 0 && pollfds[1].revents != 0) {
+			ready--;
+
+			if (pollfds[1].revents & POLLIN) {
+				// Process a frame
+				process_frame();
+			} else {
+				errx(1, "Got poll event %hd on packet socket\n", pollfds[1].revents);
+			}
+		}
+
+		if (ready > 0) {
+			errx(1, "poll(1) says we have ready fds, but neither was ready");
+		}
+	}
+}
+
+int main(int argc, char **argv) {
+	if (argc != 2) {
+		fprintf(stderr, "Usage: %s interface\n", argv[0]);
+		exit(1);
+	}
+
+	const char *interface_name = argv[1];
+
+	// Create a packet socket
+	errno = 0;
+	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");
+	}
+
+	// Get our own 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(own_mac, ifr.ifr_hwaddr.sa_data, sizeof(own_mac));
+
+	// Print it out
+	char own_mac_str[18];
+	format_mac(own_mac, own_mac_str);
+	fprintf(stderr, "%s\n", own_mac_str);
+
+	// Start the event loop
+	eventloop();
+
+	// Close the socket (tho I'm not 100% sure it's needed)
+	errno = 0;
+	if (close(packet_socket) == -1) {
+		err(1, "close");
+	}
+
+	// Flush stdout
+	errno = 0;
+	if (fflush(stdout) == EOF) {
+		err(1, "fflush");
+	}
+
+	return 0;
+}