ethermess/ethermess-backend.c

#define _GNU_SOURCE
#include <arpa/inet.h>
#include <assert.h>
#include <err.h>
#include <fcntl.h>
#include <inttypes.h>
#include <limits.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/stat.h>
#include <sys/types.h>
#include <time.h>
#include <unistd.h>

#define EM_PROTOCOL_VERSION 1
#define EM_MESSAGE_MAX_LENGTH (1500 - 2 - 2 - 2)
#define EM_STATUS_BROADCAST_TIME (60 * 1000 + 1000 * random_byte() / 64)
#define EM_RETRANSMIT_TIME (1000 + random_byte() * 2)
#define EM_MAX_RETRANSMIT 5

// Version 0
#define EMT_SPEAK_VERSION 0
#define EMT_STATUS_REQUEST 1
#define EMT_STATUS 2
#define EMT_MSGID_REQUEST 3
#define EMT_MSGID 4
#define EMT_MESSAGE 5
#define EMT_ACK 6

// Version 1
#define EMT_FILE_OFFER 7

#define EMS_AVAILABLE 0
#define EMS_UNAVAILABLE 1
#define EMS_OFFLINE 2

bool running = true;

int packet_socket;
int urandom;

unsigned char own_mac[6];
unsigned char broadcast_mac[6] = {0xff, 0xff, 0xff, 0xff, 0xff, 0xff};

unsigned char own_status = EMS_AVAILABLE;
unsigned char own_nick[256];
unsigned char own_nick_length = 0;
struct timespec next_status_broadcast;

enum message_send_states {IDLE, QUEUED, WAITING_MSGID, SENDING, WAITING_ACK};
enum message_send_states own_message_send_state = IDLE;
unsigned char own_message_destination_mac[6];
unsigned char own_message[EM_MESSAGE_MAX_LENGTH];
size_t own_message_length = 0;
uint16_t own_message_msgid = 0;
unsigned char retransmission_count = 0;
struct timespec next_retransmission;

struct msgid_cache_entry {
	unsigned char other_mac[6];
	bool know_send;
	bool know_receive;
	uint16_t next_send;
	uint16_t next_receive;
};

struct msgid_cache_entry msgid_cache[256];
ssize_t msgid_cache_fill = 0;
unsigned char next_slot = 0;

ssize_t msgid_cache_lookup(const unsigned char mac[6]) {
	for (ssize_t i = 0; i < msgid_cache_fill; i++) {
		if (memcmp(msgid_cache[i].other_mac, mac, 6) == 0) {
			// Found it
			return i;
		}
	}

	// Did not find it
	return -1;
}

ssize_t msgid_cache_add(const unsigned char mac[6]) {
	ssize_t index = next_slot++;

	// If we are adding into a new slot (instead of overwriting one),
	// expand the fill pointer
	//
	// + 1 because msgid_cache_fill of N means that cache slots [0, N-1]
	// are in use
	if (msgid_cache_fill < index + 1) {
		msgid_cache_fill = index + 1;
	}

	memcpy(msgid_cache[index].other_mac, mac, sizeof(msgid_cache[index].other_mac));
	msgid_cache[index].know_send = false;
	msgid_cache[index].know_receive = false;

	return index;
}

unsigned char random_byte(void) {
	unsigned char randomness;
	if (read(urandom, &randomness, 1) != 1) {
		err(1, "read");
	}
	return randomness;
}

struct timespec ms_in_future(intmax_t ms) {
	struct timespec ts;
	if (clock_gettime(CLOCK_MONOTONIC, &ts) == -1) {
		err(1, "clock_gettime");
	}

	// We can't really do anything to check for time_t overflow, because
	// there are no macros to test for its size, so let's just hope it
	// won't overflow
	ts.tv_sec += ms / 1000;
#if LONG_MAX < 1999999999L
#error This code does time arithmetic, and requires a long big enough to hold almost 2 seconds worth of nanoseconds
#endif
	ts.tv_nsec += (ms % 1000) * 1000 * 1000;
	if (ts.tv_nsec >= 1000 * 1000 * 1000) {
		ts.tv_sec += 1;
		ts.tv_nsec -= 1000 * 1000 * 1000;
	}

	return ts;
}

intmax_t saturating_add(intmax_t a, intmax_t b) {
	if (a < 0 && b < 0) {
		// a: [min, -1]
		// b: [min, -1]
		// min + min -> underflow
		// min + -1 -> underflow
		// -1 + -1 -> ok
		// a + b < INTMAX_MIN || - a
		// b < INTMAX_MIN - a
		if (b < INTMAX_MIN - a) {
			// Underflow
			return INTMAX_MIN;
		}
	} else if (a < 0 && b >= 0) {
		// a: [min, -1]
		// b: [0, max]
		// min + 0 -> ok
		// min + max -> ok
		// -1 + 0 -> ok
		// -1 + max -> ok
	} else if (a >= 0 && b < 0) {
		// See above but swap a and b
	} else if (a >= 0 && b >= 0) {
		// a: [0, max]
		// b: [0, max]
		// 0 + 0 -> ok
		// 0 + max -> ok
		// max + max -> overflow
		// a + b > INTMAX_MAX || -a
		// b > INTMAX_MAX - a
		if (b > INTMAX_MAX - a) {
			// Overflow
			return INTMAX_MAX;
		}
	}

	return a + b;
}

intmax_t saturating_sub(intmax_t a, intmax_t b) {
	// a - b = a + (-b)
	// Only case where -b is not safe is when b < -INTMAX_MAX (because
	// INTMAX_MIN can be smaller than -INTMAX_MAX, but INTMAX_MAX can't be
	// larger than -INTMAX_MIN)
	if (b < -INTMAX_MAX) {
		// a - b || + c - c
		// a - b + c - c
		// a - (b - c) - c
		// a + (c - b) - c
		intmax_t c = saturating_sub(b, -INTMAX_MAX);
		return saturating_sub(saturating_add(a, c - b), c);
	} else {
		return saturating_add(a, -b);
	}
}

intmax_t saturating_mul(intmax_t a, intmax_t b) {
	// Doesn't give 100% right results when one parameter is INTMAX_MIN,
	// but at least it won't ever overflow
	if (a == 0 || b == 0) {
		return 0;
	}
	if (a < 0) {
		return saturating_sub(0, saturating_mul(saturating_sub(0, a), b));
	}
	if (b < 0) {
		return saturating_sub(0, saturating_mul(a, saturating_sub(0, b)));
	}

	if (INTMAX_MAX / a < b) {
		// Overflow
		return INTMAX_MAX;
	}

	return a * b;
}

int wait_ms_until(struct timespec then) {
	// This function basically returns the difference in ms between the
	// current time and the given time, clamped to [0, INT_MAX]
	//
	// That format works for poll(2), which will immediately exit after
	// checking status if timeout is 0, and otherwise wait the given
	// number of ms

	struct timespec now;
	if (clock_gettime(CLOCK_MONOTONIC, &now) == -1) {
		err(1, "clock_gettime");
	}

	// Uses saturating arithmetic. I guess this might be wrong in some cases
	// but can't be bothered to deal with it any other way, especially as a
	// clamping the values to even something like [0, 1] would result in
	// mostly correct functioning
	intmax_t sec_diff = saturating_sub(then.tv_sec, now.tv_sec);
	intmax_t ns_diff = saturating_sub(then.tv_nsec, now.tv_nsec);
	intmax_t ms = saturating_add(saturating_mul(sec_diff, 1000), ns_diff / 1000 / 1000);

	// Clamp
	if (ms > INT_MAX) {
		ms = INT_MAX;
	} else if (ms < 0) {
		ms = 0;
	}

	return (int)ms;
}

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");
	}
}

void send_frame(const unsigned char *frame, size_t frame_length) {
	// 64B is the minimum frame side
	// We check for 60B, since kernel handles the FCS (4B)
	if (frame_length < 60) {
		unsigned char padded_frame[60];
		memset(padded_frame, 0, sizeof(padded_frame));
		memcpy(padded_frame, frame, frame_length);
		if (write(packet_socket, padded_frame, sizeof(padded_frame)) == -1) {
			err(1, "write");
		}
	} else {
		if (write(packet_socket, frame, frame_length) == -1) {
			err(1, "write");
		}
	}
}

void write_headers(unsigned char frame[14], const unsigned char destination_mac[6], unsigned char version, unsigned char packet_type) {
	// Destination MAC
	memcpy(&frame[0], destination_mac, 6);

	// Source MAC
	memcpy(&frame[6], own_mac, 6);

	// EtherType
	frame[12] = 0xda;
	frame[13] = 0x7a;

	// Ethermess version
	frame[14] = version;

	// Ethermess packet type
	frame[15] = packet_type;
}

void send_speak_version(const unsigned char destination[6]) {
	unsigned char frame[14 + 2 + 1];

	write_headers(frame, destination, 0, EMT_SPEAK_VERSION);

	// Version to speak
	frame[16] = EM_PROTOCOL_VERSION;

	send_frame(frame, sizeof(frame));
}

void send_status_request(const unsigned char destination[6]) {
	unsigned char frame[14 + 2];

	write_headers(frame, destination, 0, EMT_STATUS_REQUEST);

	send_frame(frame, sizeof(frame));
}

void send_status(const unsigned char destination[6]) {
	unsigned char frame[14 + 2 + 1 + 1 + own_nick_length];

	write_headers(frame, destination, 0, EMT_STATUS);

	// Status
	frame[16] = own_status;

	// Length of nick
	frame[17] = own_nick_length;

	// Nick
	memcpy(&frame[18], own_nick, own_nick_length);

	send_frame(frame, sizeof(frame));
}

void send_msgid_request(const unsigned char destination[6]) {
	unsigned char frame[14 + 2];

	write_headers(frame, destination, 0, EMT_MSGID_REQUEST);

	send_frame(frame, sizeof(frame));
}

void send_msgid(const unsigned char destination[6]) {
	unsigned char frame[14 + 2 + 2];

	write_headers(frame, destination, 0, EMT_MSGID);

	// Look up destination in the ID cache
	ssize_t cache_index = msgid_cache_lookup(destination);
	if (cache_index == -1) {
		// Not in the cache
		// Create a new entry
		cache_index = msgid_cache_add(destination);
	}

	if (!msgid_cache[cache_index].know_receive) {
		// We don't have receive ID stored in the cache
		// In that case, start from a random index
		msgid_cache[cache_index].next_receive = (random_byte() << 8) | random_byte();
		msgid_cache[cache_index].know_receive = true;
	}

	// Message ID of next message we're waiting to receive
	uint16_t msgid = msgid_cache[cache_index].next_receive;

	frame[16] = msgid >> 8;
	frame[17] = msgid & 0xff;

	send_frame(frame, sizeof(frame));
}

void send_message(void) {
	unsigned char frame[14 + 2 + 2 + 2 + own_message_length];

	write_headers(frame, own_message_destination_mac, 0, EMT_MESSAGE);

	// Message ID
	frame[16] = own_message_msgid >> 8;
	frame[17] = own_message_msgid & 0xff;

	// Message length
	frame[18] = own_message_length >> 8;
	frame[19] = own_message_length & 0xff;

	// Message
	memcpy(&frame[20], own_message, own_message_length);

	send_frame(frame, sizeof(frame));
}

void send_ack(const unsigned char destination[6], uint16_t msgid) {
	unsigned char frame[14 + 2 + 2];

	write_headers(frame, destination, 0, EMT_ACK);

	// Message ID
	frame[16] = msgid >> 8;
	frame[17] = msgid & 0xff;

	send_frame(frame, sizeof(frame));
}

void send_file_offer(const unsigned char destination[6]) {
	unsigned char frame[14 + 2];

	write_headers(frame, destination, 1, EMT_FILE_OFFER);

	send_frame(frame, sizeof(frame));
}

void readallx(int fd, unsigned char *buf, size_t length) {
	size_t completed = 0;
	while (completed < length) {
		ssize_t res = read(fd, &buf[completed], length - completed);
		if (res == -1) {
			err(1, "read");
		} else if (res == 0) {
			errx(1, "Unexpected EOF");
		}
		completed += res;
	}
}

void writeallx(int fd, const void *buf, size_t length) {
	const unsigned char *cbuf = buf;
	size_t completed = 0;
	while (completed < length) {
		ssize_t res = write(fd, &cbuf[completed], length - completed);
		if (res == -1) {
			err(1, "write");
		}
		completed += res;
	}
}

void writeallx_u16(int fd, uint16_t value) {
	unsigned char buf[2];
	buf[0] = value >> 8;
	buf[1] = value & 0xff;
	writeallx(fd, buf, sizeof(buf));
}

bool check_utf8(const unsigned char *data, size_t data_length, bool newline_tab_allowed);

void read_status(void) {
	unsigned char status;
	readallx(0, &status, 1);
	if (status != EMS_AVAILABLE && status != EMS_UNAVAILABLE && status != EMS_OFFLINE) {
		errx(1, "Frontend sent a status %u that we don't understand", status);
	}

	unsigned char length;
	readallx(0, &length, 1);

	unsigned char nick[length];
	readallx(0, nick, length);

	if (!check_utf8(nick, length, false)) {
		errx(1, "Frontend sent a nick with malformed utf-8 or control characters");
	}

	own_status = status;

	memcpy(own_nick, nick, length);
	own_nick_length = length;
}

void read_message(void) {
	if (own_message_send_state != IDLE) {
		errx(1, "Frontend sent a new message while we are still processing the old one");
	}

	unsigned char mac[6];
	readallx(0, mac, sizeof(mac));

	unsigned char length_raw[2];
	readallx(0, length_raw, 2);
	size_t length = (length_raw[0] << 8) | length_raw[1];

	if (length > EM_MESSAGE_MAX_LENGTH) {
		errx(1, "Frontend sent a message that is too long (%zuB, max is %uB)", length, EM_MESSAGE_MAX_LENGTH);
	}

	unsigned char message[length];
	readallx(0, message, length);

	if (!check_utf8(message, length, true)) {
		errx(1, "Frontend sent a nick with malformed utf-8 or control characters other than newline");
	}

	memcpy(own_message_destination_mac, mac, sizeof(own_message_destination_mac));
	memcpy(own_message, message, length);
	own_message_length = length;
	own_message_send_state = QUEUED;
}

void read_command(void) {
	unsigned char cmd;
	readallx(0, &cmd, 1);

	if (cmd == 'q') {
		running = false;
	} else if (cmd == 'r') {
		unsigned char mac[6];
		readallx(0, mac, sizeof(mac));
		send_status_request(mac);
	} else if (cmd == 's') {
		read_status();
		send_status(broadcast_mac);
	} else if (cmd == 'm') {
		read_message();
	} else if (cmd == 'f') {
		unsigned char mac[6];
		readallx(0, mac, sizeof(mac));
		send_file_offer(mac);
	} else {
		errx(1, "Frontend sent an unknown command %c", cmd);
	}
}

bool check_padding(const unsigned char *data, size_t index, size_t data_length) {
	// Valid padding is all zero bytes
	assert(index <= data_length);
	for (size_t i = index; i < data_length; i++) {
		if (data[i] != 0) {
			// Check failed
			return false;
		}
	}

	// Check succeeded
	return true;
}

bool check_utf8(const unsigned char *data, size_t data_length, bool newline_tab_allowed) {
	size_t remaining = 0;
	size_t length = 0;
	uint32_t codepoint = 0;

	for (size_t i = 0; i < data_length; i++) {
		unsigned char byte = data[i];

		if (byte <= 0x7f) {
			// 0xxxxxxx - single byte
			if (remaining != 0) {
				// Can't appear in the middle of a multibyte sequence
				return false;
			}
			remaining = 0;
			length = 1;
			codepoint = byte;
		} else if (byte <= 0xbf) {
			// 10xxxxxx - continuation byte
			if (remaining == 0) {
				// Can only appear in the middle of a multibyte sequence
				return false;
			}
			remaining--;
			length++;
			codepoint <<= 6;
			codepoint |= byte & 0x3f;
		} else if (byte <= 0xdf) {
			// 110xxxxx - first byte of double byte sequence
			if (remaining != 0) {
				// Can't appear in the middle of a multibyte sequence
				return false;
			}
			remaining = 1;
			length = 1;
			codepoint = byte & 0x1f;
		} else if (byte <= 0xef) {
			// 1110xxxx - first byte of triple byte sequence
			if (remaining != 0) {
				// Can't appear in the middle of a multibyte sequence
				return false;
			}
			remaining = 2;
			length = 1;
			codepoint = byte & 0x0f;
		} else if (byte <= 0xf7) {
			// 11110xxx - first byte of quadruple byte sequence
			if (remaining != 0) {
				// Can't appear in the middle of a multibyte sequence
				return false;
			}
			remaining = 3;
			length = 1;
			codepoint = byte & 0x07;
		}

		if (remaining == 0) {
			// Full codepoint constructed

			// Reject overlong encodings
			if (codepoint <= 0x007f && length > 1) {
				return false;
			} else if (codepoint <= 0x07ff && length > 2) {
				return false;
			} else if (codepoint <= 0xffff && length > 3) {
				return false;
			} else if (codepoint <= 0x10ffff && length > 4) {
				return false;
			}

			// Reject code points over U+10FFFF
			if (codepoint > 0x10ffff) {
				return false;
			}

			// Reject surrogate pairs
			if (0xd800 <= codepoint && codepoint <= 0xdfff) {
				return false;
			}

			// Reject non-characters
			if ((codepoint & 0xffff) == 0xfffe || (codepoint & 0xffff) == 0xffff) {
				// Plane end non-characters
				return false;
			} else if (0xfdd0 <= codepoint && codepoint <= 0xfdef) {
				// BMP non-character block
				return false;
			}

			// Reject control characters
			if (codepoint <= 0x1f) {
				// C0 control character
				if (!newline_tab_allowed || (codepoint != 0x0a && codepoint != 0x09)) {
					return false;
				}
			} else if (0x80 <= codepoint && codepoint <= 0x9f) {
				// C1 control character
				return false;
			} else if (codepoint == 0x2028) {
				// U+2028 LINE SEPARATOR
				return false;
			} else if (codepoint == 0x2029) {
				// U+2029 PARAGRAPH SEPARATOR
				return false;
			}
		}
	}

	if (remaining != 0) {
		// Can't end at the middle of a multibyte sequence
		return false;
	}

	return true;
}

void handle_speak_version(const unsigned char source_mac[6], const unsigned char *data, size_t data_length) {
	if (data_length < 1) {
		// Too short
		return;
	}

	unsigned char version = data[0];

	if (!check_padding(data, 1, data_length)) {
		// Malformed padding
		return;
	}

	// Type of event: Speak version
	writeallx(1, "v", 1);

	// MAC
	writeallx(1, source_mac, 6);

	// Version
	writeallx(1, &version, 1);
}

void handle_status(const unsigned char source_mac[6], const unsigned char *data, size_t data_length) {
	if (data_length < 2) {
		// Too short
		return;
	}

	unsigned char status = data[0];

	if (status != EMS_AVAILABLE && status != EMS_UNAVAILABLE && status != EMS_OFFLINE) {
		// Unknown status, throw away
		return;
	}

	unsigned char nick_length = data[1];

	if (nick_length > data_length - 2) {
		// Malformed length field
		return;
	}

	unsigned char nick[nick_length];
	memcpy(nick, &data[2], nick_length);

	if (!check_padding(data, 2 + nick_length, data_length)) {
		// Malformed padding
		return;
	}

	if (!check_utf8(nick, nick_length, false)) {
		// Malformed utf-8, or has control chars
		return;
	}

	// Type of event: Status
	writeallx(1, "s", 1);

	// MAC
	writeallx(1, source_mac, 6);

	// Status
	writeallx(1, &status, 1);

	// Nick
	writeallx(1, &nick_length, 1);
	writeallx(1, nick, nick_length);
}

void handle_msgid(const unsigned char source_mac[6], const unsigned char *data, size_t data_length) {
	if (data_length < 2) {
		// Too short
		return;
	}

	uint16_t msgid = (data[0] << 8) | data[1];

	if (!check_padding(data, 2, data_length)) {
		// Malformed padding
		return;
	}

	ssize_t cache_index = msgid_cache_lookup(source_mac);
	if (cache_index == -1) {
		// Not in the cache, so add it there
		cache_index = msgid_cache_add(source_mac);
	}

	if (msgid_cache[cache_index].know_send) {
		// There is sth in the cache, test whether we should update it
		uint16_t diff = msgid - msgid_cache[cache_index].next_send;
		if (diff < 0x8000) {
			// See the description in handle_message for what is going on
			msgid_cache[cache_index].next_send = msgid;
		}
	} else {
		// Nothing in the cache
		msgid_cache[cache_index].next_send = msgid;
		msgid_cache[cache_index].know_send = true;
	}
}

void handle_message(const unsigned char source_mac[6], const unsigned char *data, size_t data_length) {
	if (data_length < 4) {
		// Too short
		return;
	}

	uint16_t msgid = (data[0] << 8) | data[1];

	uint16_t message_length = (data[2] << 8) | data[3];

	if (message_length > data_length - 4 || message_length > EM_MESSAGE_MAX_LENGTH) {
		// Malformed length field
		return;
	}

	unsigned char message[message_length];
	memcpy(message, &data[4], message_length);

	if (!check_padding(data, 4 + message_length, data_length)) {
		// Malformed padding
		return;
	}

	if (!check_utf8(message, message_length, true)) {
		// Malformed utf-8, or has control chars other than newline
		return;
	}

	// See whether we've already received this message and update the next msgid if so
	ssize_t cache_index = msgid_cache_lookup(source_mac);
	if (cache_index == -1) {
		// No cache entry -> add an empty one
		cache_index = msgid_cache_add(source_mac);
	}

	if (!msgid_cache[cache_index].know_receive) {
		// We have a cache entry, but no idea what next message to receive
		// Assume this is not a repeat transmission, and accept the message
		// The easiest way to do this is to set this message's msgid as the
		// one we're looking to receive
		msgid_cache[cache_index].next_receive = msgid;
		msgid_cache[cache_index].know_receive = true;
	}

	uint16_t diff = msgid - msgid_cache[cache_index].next_receive;
	if (diff < 0x8000) {
		// The msgid counter can wrap around, so a simple larger than
		// comparison will not work. Instead, we look at the distance
		// between the message's msgid and the one we're expecting to
		// receive. If it is from 0 to 0x8000, we consider it to be
		// in the forwards direction, and if it is more, we consider it
		// to be in the backwards one. This leaves equally sized (2^15)
		// ranges for values in both cases.

		// In this case the msgid is considered to be of a message we
		// haven't yet seen. Therefore we'll continue processing, and
		// update the next msgid we're expecting to receive to this one
		// plus 1
		msgid_cache[cache_index].next_receive = msgid + 1;
	} else {
		// In this case we consider the message already received
		// We will send an ack, but not process it any further
		send_ack(source_mac, msgid);
		return;
	}

	// Type of event: Received a message
	writeallx(1, "m", 1);

	// MAC
	writeallx(1, source_mac, 6);

	// Message length
	writeallx_u16(1, message_length);

	// Message
	writeallx(1, message, message_length);

	send_ack(source_mac, msgid);
}

void handle_ack(const unsigned char source_mac[6], const unsigned char *data, size_t data_length) {
	if (data_length < 2) {
		// Too short
		return;
	}

	uint16_t msgid = (data[0] << 8) | data[1];

	if (!check_padding(data, 2, data_length)) {
		// Malformed padding
		return;
	}

	// Type of event: ACK received
	writeallx(1, "a", 1);

	// MAC
	writeallx(1, source_mac, 6);

	// Msgid
	writeallx_u16(1, msgid);

	if (own_message_send_state == WAITING_ACK && msgid == own_message_msgid) {
		own_message_send_state = IDLE;
	}
}

void process_frame(void) {
	unsigned char frame[1518]; // Largest a 802.3 frame can be without FCS

	ssize_t res = recv(packet_socket, frame, sizeof(frame), 0);
	if (res == -1) {
		err(1, "recv");
	} else if (res < 16) {
		// Frame too short to contain enough information
		return;
	}
	size_t packet_length = (size_t)res;

	// Check that the packet is Ethermess (EtherType DA7A)
	if (frame[12] != 0xda || frame[13] != 0x7a) {
		return;
	}

	bool broadcast;
	if (memcmp(frame, own_mac, 6) == 0) {
		// Targetted at us
		broadcast = false;
	} else if (memcmp(frame, broadcast_mac, 6) == 0) {
		// Broadcast
		broadcast = true;
	} else {
		// Does not concern us
		return;
	}

	// Extract source MAC
	unsigned char source_mac[6];
	memcpy(source_mac, &frame[6], sizeof(source_mac));

	// Extract version
	unsigned char version = frame[14];

	// If they speak a version we don't understand, tell them to speak ours
	// Exception: If it was a broadcast, just ingore it
	if (version > EM_PROTOCOL_VERSION) {
		if (!broadcast) {
			send_speak_version(source_mac);
		}
		return;
	}

	// Extract Ethermess packet type
	unsigned char packet_type = frame[15];

	// Process the packet based on the version and packet type
	if (version == 0) {
		switch (packet_type) {
			case EMT_SPEAK_VERSION:
				handle_speak_version(source_mac, &frame[16], packet_length - 16);
				break;

			case EMT_STATUS_REQUEST:
				if (check_padding(&frame[16], 0, packet_length - 16)) {
					send_status(source_mac);
				}
				break;

			case EMT_STATUS:
				handle_status(source_mac, &frame[16], packet_length - 16);
				break;

			case EMT_MSGID_REQUEST:
				if (check_padding(&frame[16], 0, packet_length - 16)) {
					send_msgid(source_mac);
				}
				break;

			case EMT_MSGID:
				handle_msgid(source_mac, &frame[16], packet_length - 16);
				break;

			case EMT_MESSAGE:
				handle_message(source_mac, &frame[16], packet_length - 16);
				break;

			case EMT_ACK:
				handle_ack(source_mac, &frame[16], packet_length - 16);
				break;
		}
	} else if (version == 1) {
		switch (packet_type) {
			case EMT_FILE_OFFER:
				fprintf(stderr, "File offered\n"); //debg
				break;
		}
	}
}

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) {
		int retransmit_wait_ms = INT_MAX;

		// (Attempt) to process a message send
		if (own_message_send_state == QUEUED) {
			// We need to have the correct msgid to be able to send
			ssize_t cache_index = msgid_cache_lookup(own_message_destination_mac);

			if (cache_index == -1 || !msgid_cache[cache_index].know_send) {
				// We don't know what the msgid should be
				// -> ask the other side
				send_msgid_request(own_message_destination_mac);
				// Wait around 1 to 1.5s before asking again
				next_retransmission = ms_in_future(EM_RETRANSMIT_TIME);
				retransmission_count = 0;
				own_message_send_state = WAITING_MSGID;
			} else {
				// It is in the cache
				own_message_msgid = msgid_cache[cache_index].next_send++;
				own_message_send_state = SENDING;
			}
		}

		if (own_message_send_state == WAITING_MSGID) {
			ssize_t cache_index = msgid_cache_lookup(own_message_destination_mac);

			if (cache_index == -1 || !msgid_cache[cache_index].know_send) {
				// Still no msgid
				retransmit_wait_ms = wait_ms_until(next_retransmission);

				if (retransmit_wait_ms == 0 && retransmission_count < EM_MAX_RETRANSMIT) {
					// Time to resend
					send_msgid_request(own_message_destination_mac);
					// Wait around 1 to 1.5s before asking again
					next_retransmission = ms_in_future(EM_RETRANSMIT_TIME);
					retransmission_count++;
				} else if (retransmit_wait_ms == 0 && retransmission_count >= EM_MAX_RETRANSMIT) {
					// Time to give up
					own_message_send_state = IDLE;

					// Type of event: Msgid failed
					writeallx(1, "I", 1);
				}
			} else {
				// Found msgid
				own_message_msgid = msgid_cache[cache_index].next_send++;
				own_message_send_state = SENDING;
			}
		}

		if (own_message_send_state == SENDING) {
			// Type of event: msgid for a message
			writeallx(1, "i", 1);

			// Msgid
			writeallx_u16(1, own_message_msgid);

			// Send message
			send_message();

			// Wait around 1 to 1.5 before sending again
			next_retransmission = ms_in_future(EM_RETRANSMIT_TIME);
			retransmission_count = 0;
			own_message_send_state = WAITING_ACK;
		}

		if (own_message_send_state == WAITING_ACK) {
			retransmit_wait_ms = wait_ms_until(next_retransmission);

			if (retransmit_wait_ms == 0 && retransmission_count < EM_MAX_RETRANSMIT) {
				// Time to resend
				send_message();
				// Wait around 1 to 1.5 before sending again
				next_retransmission = ms_in_future(EM_RETRANSMIT_TIME);
				retransmission_count++;
			} else if (retransmit_wait_ms == 0 && retransmission_count >= EM_MAX_RETRANSMIT) {
				// Time to give up
				own_message_send_state = IDLE;

				// Type of event: ACK not received
				writeallx(1, "A", 1);
			}
		}

		// Process status broadcasting
		int status_broadcast_wait_ms = wait_ms_until(next_status_broadcast);
		if (status_broadcast_wait_ms == 0) {
			// The time has come to send the status broadcast
			send_status(broadcast_mac);
			// Do next one in about a minute
			next_status_broadcast = ms_in_future(EM_STATUS_BROADCAST_TIME);
		}

		// Figure out how many ms to wait
		int wait_ms;
		if (retransmit_wait_ms < status_broadcast_wait_ms) {
			wait_ms = retransmit_wait_ms;
		} else {
			wait_ms = status_broadcast_wait_ms;
		}

		int ready = poll(pollfds, sizeof(pollfds) / sizeof(*pollfds), wait_ms);
		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 if (pollfds[0].revents & POLLHUP) {
				// Quit on frontend exiting
				exit(1);
			} 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 if (pollfds[1].revents & POLLERR) {
				// Lost connection
				errx(1, "Network went down");
			} 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];

	// Open /dev/urandom for getting randomness
	urandom = open("/dev/urandom", O_RDONLY);

	// Create a packet socket
	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");
	}

	// Get our own 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(own_mac, ifr.ifr_hwaddr.sa_data, sizeof(own_mac));

	// Print it out
	if (write(1, own_mac, 6) != 6) {
		err(1, "write");
	}

	// Initialize the message id cache
	memset(msgid_cache, 0, sizeof(msgid_cache));

	// Set our status and nick
	read_status();

	// Broadcast our status to the network to let them know we're here
	send_status(broadcast_mac);

	// Schedule next broadcast of our status about 1 min in the future
	next_status_broadcast = ms_in_future(EM_STATUS_BROADCAST_TIME);

	// Request status from everyone, so that we can get an idea of who is on the network
	send_status_request(broadcast_mac);

	// Start the event loop
	eventloop();

	// Set our status to going offline and broadcast that
	own_status = EMS_OFFLINE;
	send_status(broadcast_mac);

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

	// Flush stdout
	if (fflush(stdout) == EOF) {
		err(1, "fflush");
	}

	// Close urandom
	if (close(urandom) == -1) {
		err(1, "close");
	}

	return 0;
}