path: root/src/dht.c

#include <time.h>
#include <sys/random.h>
#include <errno.h>
#include <sys/socket.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <arpa/inet.h>
#include <netinet/ip.h>
#include <arpa/nameser.h>
#include <resolv.h>
#include <limits.h>
#include <assert.h>
#include <sha1.h> // http://github.com/clibs/sha1 and in major distributions
#define ECB 1
#define AES128 1
#include <aes.c>
#include <bencoding.c>

time_t seconds (void) {
	struct timespec tp;
	clock_gettime(CLOCK_MONOTONIC, &tp);
	return tp.tv_sec;
}

int family (const unsigned char * addr) {
	return memcmp("\0\0\0\0\0\0\0\0\0\0\xFF\xFF", addr, 12) ? AF_INET6 : AF_INET;
}

#define K 8

/**
 * writes a hexadecimal representation of a binary string. does not NULL terminate destination.
 *
 * h and b may not overlap. period.
 * 
 * @param h	[out]	destination (requires 2l space)
 * @param b	[in]	source
 * @param l	[in]	length of source
 */

void bin2hex (char * h, const unsigned char * b, size_t l) {
	for (size_t i = 0; i < l; i++) {
		int ms = b[i] >> 4;
		int ls = b[i] & 0xF;
		assert(ms < 16);
		assert(ls < 16);
		*h++ = ms < 10 ? '0'+ms : 'a'+ms-10;
		*h++ = ls < 10 ? '0'+ls : 'a'+ls-10;
	}
}

/**
 * node representation
 */

struct node {
	unsigned char id[20];
	struct sockaddr_in6 addr;
	int unanswered; /**< number of packets I've sent since last_received */
	time_t last_received; /**< time when I received the last packet from it */
	time_t last_sent; /**< time when I sent the last query to it */
	struct node * next;
};

/**
 * creates a new node
 */

struct node * node_init (void) {
	struct node * n = calloc(1, sizeof *n);
	if (!n)
		return NULL;
	n->last_received = seconds();
	n->addr.sin6_family = AF_INET6;
	return n;
}

/**
 * frees a node
 *
 * @param n	[in]	the node to be freed
 */

void node_free (struct node * n) {
	free(n);
}

/**
 * print node to stream, for debugging
 *
 * @param s	[out]	the stream to which to print to
 * @param n	[in]	the node to be printed
 */

void node_print (FILE * s, const struct node * n) {
	char buf[41];
	buf[40] = '\0';
	bin2hex(buf, n->id, 20);
	char remote[INET6_ADDRSTRLEN + 64];
	if (!inet_ntop(n->addr.sin6_family, n->addr.sin6_addr.s6_addr, remote, INET6_ADDRSTRLEN+7))
		snprintf(remote, sizeof remote,  "(inet_ntop: %s)", strerror(errno));
	fprintf(s, "%s %s/%d unans=%d", buf, remote, ntohs(n->addr.sin6_port), n->unanswered);
}

/**
 * returns which 20 byte bytestring is closer with XOR metric to target 20 byte bytestring
 *
 * at most 20 bytes are read from each pointer
 *{
 * @param a	[in]	20 byte to consider
 * @param b	[in]	20 byte string to consider
 * @param t	[in]	target
 * @return 0 if a is closer or 1 if b is closer, -1 if both are the same
 */

int closer (const unsigned char * a, const unsigned char * b, const unsigned char * t) {
	for (int i = 0; i < 20; i++) {
		if ((a[i] ^ t[i]) < (b[i] ^ t[i]))
			return 0;
		if ((a[i] ^ t[i]) > (b[i] ^ t[i]))
			return 1;
	}
	return -1;
}

/**
 * bucket representation
 */

struct bucket {
	unsigned char id[20]; /**< bucket spans from id inclusive to next->id exclusive */
	struct node * nodes;
	struct bucket * next;
};

/**
 * creates a new bucket
 */

struct bucket * bucket_init (void) {
	struct bucket * b = calloc(1, sizeof *b);
	if (!b)
		return NULL;
	return b;
}

/**
 * frees a bucket
 *
 * @param b	[in]	the bucket to be freed
 */

void bucket_free (struct bucket * b) {
	struct node * node = b->nodes;
	while (node) {
		struct node * old = node;
		node = node->next;
		node_free(old);
	}
	free(b);
}

/**
 * peer of a torrent
 */

struct peer {
	int socket; /**< tcp socket for bep-0009 */
	struct sockaddr_in6 addr; /**< peer ip address and port */
	struct peer * next;
};

/**
 * init a torrent peer
 */

struct peer * peer_init () {
	struct peer * p = calloc(1, sizeof *p);
	p->addr.sin6_family = AF_INET6;
	return p;
}

/**
 * free a torrent peer
 */

void peer_free (struct peer * p) {
	free(p);
}

/**
 * print a peer, for debugging purposes
 *
 * @param s	[out]	stream
 * @param p	[in]	peer
 */

void peer_print (FILE * s, const struct peer * p) {
	char remote[INET6_ADDRSTRLEN + 64];
	if (!inet_ntop(p->addr.sin6_family, p->addr.sin6_addr.s6_addr, remote, INET6_ADDRSTRLEN+7))
		snprintf(remote, sizeof remote,  "(inet_ntop: %s)", strerror(errno));
	fprintf(s, "%s:%d", remote, ntohs(p->addr.sin6_port));
}

/**
 * reason why a torrent is in our database. 0 just means it is stored as a result of an announce
 */

enum interested {
	announce = 1 << 0, /**< will announce myself on every work() call with no packet */
	peers = 1 << 1, /**< will get peers on every work() call with no packet */
	info = 1 << 2 /**< download metadata into `dht->dl`/$hash.info */
};

/**
 * torrent we are interested in
 */

struct torrent {
	enum interested type; /**< is truthy only for manually added torrents */
	unsigned char hash[20]; /**< infohash */
	struct peer * peers;
	struct node * nodes; /**< closest K DHT nodes to this hash, used only for announce, peers and info torrents */
	struct torrent * next;
	struct torrent * prev; /**< prev is here so that we can easily pop the oldest torrent. dht->last_torrent is useful here */
};

/**
 * free a torrent object and it's nodes and peers
 */

void torrent_free (struct torrent * t) {
	struct node * n = t->nodes;
	while (n) {
		struct node * old = n;
		n = n->next;
		node_free(old);
	}
	struct peer * p = t->peers;
	while (p) {
		struct peer * old = p;
		p = p->next;
		peer_free(old);
	}
	free(t);
}

/**
 * compares two torrent objects based on their hash
 */

int torrent_compare (const void * a, const void * b) {
	return memcmp(((const struct torrent *) a)->hash, ((const struct torrent *) b)->hash, 20);
}

/**
 * prints a torrent, for debugging purposes
 *
 * @param s	[out]	stream to which to write to
 * @param t	[in]	the torrent to print
 */

void torrent_print (FILE * s, const struct torrent * t) {
	char buf[41];
	buf[40] = '\0';
	bin2hex(buf, t->hash, 20);
	printf("magnet:?xt=urn:btih:%s\n\t**** PEERS ****\n", buf);
	struct peer * p = t->peers;
	while (p) {
		fprintf(s, "\t");
		peer_print(s, p);
		fprintf(s, "\n");
		p = p->next;
	}
	printf("\t**** NODES ****\n");
	struct node * n = t->nodes;
	while (n) {
		fprintf(s, "\t");
		node_print(s, n);
		fprintf(s, "\n");
		n = n->next;
	}
}

/**
 * handle for the library
 */

struct dht {
	unsigned char id[20]; /**< own id */
	int socket; /**< v4&v6 UDP socket that is bound on UDP and sends to nodes */
	unsigned char secret[16]; /**< for calculating opaque write token, random */
	FILE * log; /**< FILE to log to, defaults to stderr */
	struct bucket * buckets;
	struct bucket * buckets6; /**< IPv6 routing table */
	struct torrent * torrents; /**< linked list of torrents for which we want to know peers */
	int dl; /**< dirfd storage directory for info torrents */
	void (* possible_torrent)(struct dht *, const unsigned char *); /**< a user callback function that is called whenever we come across a torrent hash from a network */
	void * userdata; /**< unused, but left for the library user to set so he can refer back to his structures from callback code, such as dht->possible_torrent(d, h) */
	unsigned torrents_num; /**< number of torrents. this number can rise indefinitely, so it can, and should be capped by the caller, depending on available memory */
	unsigned peers_num; /**< number of peers. same notice regarding memory applies here as for torrents */
	unsigned torrents_max; /**< max number of torrents that we are allowed to store */
	unsigned peers_max; /**< max number of peers that we are allowed to store */
	struct torrent * last_torrent; /**< to quickly go to the end of the doubly linked list of torrents is helpful when reaching torrents_max and needing to pop oldest */
	unsigned peers_per_torrent_max; /**< max number of peers to store per torrent - applies to ->type and !->type torrents */
	unsigned time; /**< statistics: time of initialization, for amount of seconds since initialization, check seconds() */
	unsigned rxp; /**< statistics: total number of received packets */
	unsigned txp; /**< statistics: total number of sent packets */
	unsigned rxb; /**< statistics: total number of bytes in received UDP bodies */
	unsigned txb; /**< statistics: total number of bytes in transmitted UDP bodies */
};

/**
 * prints a dht, for debugging
 *
 * @param s	[out]	stream
 * @param d	[in]	dht
 */

void dht_print (FILE * s, const struct dht * d) {
	char buf[41];
	buf[40] = '\0';
	bin2hex(buf, d->id, 20);
	char secret[17*2];
	secret[17*2+1] = '\0';
	bin2hex(secret, d->secret, 16);
	fprintf(s, "id=%s socket=%d dl=%d t=%u p=%u tmax=%u pmax=%u p/t-max=%u runsec=%ld rxp=%u txp=%u rxb=%u txb=%u secret=%s\n", buf, d->socket, d->dl, d->torrents_num, d->peers_num, d->torrents_max, d->peers_max, d->peers_per_torrent_max, seconds()-d->time, d->rxp, d->txp, d->rxb, d->txb, secret);
	printf("**** TORRENTS ****\n");
	struct torrent * t = d->torrents;
	while (t) {
		torrent_print(s, t);
		t = t->next;
	}
	fprintf(s, "**** NODES ****\n");
	int nodes = 0;
	for (int i = 0; i <= 1; i++) {
		struct bucket * b = d->buckets;
		if (i)
			b = d->buckets6;
		int buckets = 0;
		while (b) {
			buckets++;
			bin2hex(buf, b->id, 20);
			fprintf(s, "\tBUCKET id=%s\n", buf);
			struct node * n = b->nodes;
			while (n) {
				nodes++;
				fprintf(s, "\t");
				node_print(s, n);
				fprintf(s, "\n");
				n = n->next;
			}
			b = b->next;
		}
		fprintf(s, "\t**** COUNT OF %s BUCKETS: %d\n", i ? "IPv4" : "IPV6", buckets);
	}
	fprintf(s, "**** COUNT OF NODES: %d\n", nodes);
}

/**
 * a dummy function that does nothing that is set as the default for possible_torrent in struct dht
 *
 * @param d	[in]	the dht library handler
 * @param h	[in]	the infohash of the found torrent
 */

void possible_torrent (struct dht * d __attribute__((unused)), const unsigned char * h __attribute__((unused))) {
	return;
}

/**
 * macro for printing logs
 *
 * @param o	[in]	FILE * to which to write to
 * @param f	[in]	printf style format string
 * @param ...	[in]	variable arguments to go with f
 */

#define L(Lo, Lf, ...) do {char Lt[512]; time_t Ln = time(NULL); strftime(Lt, 512, "%c", localtime(&Ln)); fprintf(Lo, "[%s] %s()%s:%d: " Lf "\n", Lt, __func__, __FILE__, __LINE__ __VA_OPT__(,) __VA_ARGS__);} while (0)

/**
 * sends a bencoding object to the remote node. does not free the input bencoding. inserts a v key to the input bencoding.
 *
 * @param d	[in]	the dht library handle
 * @param b	[in]	the bencoding to send serialized, m. ownership NOT transfered
 * @param a	[in]	destination address
 */

void sendb (struct dht * d, struct bencoding * b, const struct sockaddr_in6 * a) {
	char remote[INET6_ADDRSTRLEN + 64];
	if (!inet_ntop(a->sin6_family, &a->sin6_addr, remote, INET6_ADDRSTRLEN+7))
		snprintf(remote, sizeof remote,  "(inet_ntop: %s)", strerror(errno));
	sprintf(remote+strlen(remote), "/%d", ntohs(((struct sockaddr_in6 *) a)->sin6_port));
	struct bencoding * v = bstr(strdup("TK00"));
	v->key = bstr(strdup("v"));
	binsert(b, v);
	struct bencoding * ip = calloc(1, sizeof *ip);
	ip->type = string;
	ip->key = bstr(strdup("key"));
	memcpy((b->value = malloc(18)), a->sin6_addr.s6_addr + (family(a->sin6_addr.s6_addr) == AF_INET ? 12 : 0), (b->valuelen = family(a->sin6_addr.s6_addr) == AF_INET ? 6 : 18));
	memcpy(b->value + (family(a->sin6_addr.s6_addr) == AF_INET ? 4 : 16), &a->sin6_port, 2);
	binsert(b, ip);
	int len = b2json_length(b);
	char json[len+1];
	b2json(json, b);
	json[len] = '\0';
	L(d->log, "sending to %s: %s", remote, json);
	len = bencode_length(b);
	char text[len];
	bencode(text, b);
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wincompatible-pointer-types"
	if (sendto(d->socket, text, len, MSG_DONTWAIT | MSG_NOSIGNAL, a, sizeof *a) == -1)
		L(d->log, "sendto(%s): %s", remote, strerror(errno));
	else {
		d->txp++;
		d->txb += len;
	}
#pragma GCC diagnostic pop
}

#define GENERIC_T "a@4" "a.si" /**< generic token that we send if we have no statekeeping to do - developer's email address */

/**
 * sends a find_node query to a "raw node"
 * 
 * @param d	[in]	handle
 * @param a	[in]	address of the remote node
 * @param query	[in]	20 byte id we are querying
 */

void find_node (struct dht * d, const struct sockaddr_in6 * addr, const unsigned char * query) {
	struct bencoding * b = calloc(1, sizeof *b);
	b->type = dict;
	struct bencoding * t = bstr(strdup(GENERIC_T));
	t->key = bstr(strdup("t"));
	t->type = string;
	binsert(b, t);
	struct bencoding * y = bstr(strdup("q"));
	y->key = bstr(strdup("y"));
	y->type = string;
	binsert(b, y);
	struct bencoding * q = bstr(strdup("find_node"));
	q->key = bstr(strdup("q"));
	q->type = string;
	binsert(b, q);
	struct bencoding * a = calloc(1, sizeof *a);
	a->type = dict;
	a->key = bstr(strdup("a"));
	struct bencoding * id = calloc(1, sizeof *id);
	id->type = string;
	id->valuelen = 20;
	id->key = bstr(strdup("id"));
	memcpy((id->value = malloc(20)), d->id, 20);
	binsert(a, id);
	struct bencoding * want = calloc(1, sizeof *want); // BEP-0032
	want->key = bstr(strdup("want"));
	want->type = list;
	binsert(want, bstr(strdup("n4")));
	binsert(want, bstr(strdup("n6")));
	binsert(a, want);
	struct bencoding * target = calloc(1, sizeof *target);
	target->key = bstr(strdup("target"));
	target->type = string;
	memcpy((target->value = malloc(20)), query, (target->valuelen = 20));
	binsert(a, target);
	binsert(b, a);
	sendb(d, b, addr);
	free_bencoding(b);
}

/**
 * ping a raw node by sending a find_node
 *
 * instead of sending a ping query, we send a find_node query. this gets us useful information of peers around our ID instead of just a blank ping reply. infolgedessen we don't have to actively search for our neighbour nodes, since we'll get them through pings anyways
 *
 * DEV THOUGHT: instead of sending a find_node for an ID close to ours, we could send a find_node for a random ID far from us. though those buckets will probably quickly be filled by torrent searches.
 *
 * @param d	[in]	library handle
 * @param a	[in]	address of node
 */

void ping_node (struct dht * d, const struct sockaddr_in6 * a) {
	unsigned char target[20];
	memcpy(target, d->id, 20);
	if (target[19] & 1) // flip the last bit, so the other node doesn't just return
		target[19] &= 0xFE;	// our ID but K ids around it
	else
		target[19] |= 1;
	find_node(d, a, target);
}

/**
 * creates a handle. you can override log in the result struct.
 *
 * this function does not log, as log fd is not known yet
 *
 * socket must be close()d before being overriden, if the caller wants to use custom binding.
 *
 * binds UDP to all ifaces
 *
 * @param c	[in]	bencoding object containing the persistent config file, generated from a previous run with persistent(). memory ownership is NOT transfered, it's the caller's responsibility to free the object. can be NULL.
 */

struct dht * dht_init (const struct bencoding * c) {
	struct dht * d = calloc(1, sizeof *d);
	d->time = seconds();
	d->log = stderr;
	d->dl = -1;
	d->buckets = bucket_init();
	d->buckets6 = bucket_init();
	d->possible_torrent = &possible_torrent;
	d->torrents_max = UINT_MAX; // this is hardcore - so many torrents makes LL traversal too slow
	d->peers_max = UINT_MAX; // there's no way there even are this many peers on the entire network at a time xDDDDDDDDDDD
	d->peers_per_torrent_max = UINT_MAX;
	errno = 0;
	if (!d)
		return NULL;
	if (getrandom(d->id, 20, GRND_NONBLOCK) == -1)
		goto e;
	if (getrandom(d->secret, 16, GRND_NONBLOCK) == -1)
		goto e;
	d->socket = socket(AF_INET6, SOCK_DGRAM | SOCK_NONBLOCK | SOCK_CLOEXEC, 0);
	if (d->socket == -1)
		goto e;
	struct sockaddr_in6 a = {
		.sin6_family = AF_INET6,
		.sin6_addr = in6addr_any
	};
	const struct bencoding * port = NULL;
	if (c && (port = bpath(c, "port")) && port->type & num)
		a.sin6_port = htons(port->intvalue);
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wincompatible-pointer-types"
	if (bind(d->socket, &a, sizeof a) == -1)
		if (errno == EADDRINUSE) {
			a.sin6_port = 0;
			if (bind(d->socket, &a, sizeof a) == -1)
#pragma GCC diagnostic pop
				goto e;
		}
	if (c) {
		const struct bencoding * id = bpath(c, "id");
		if (id && id->type & string && id->valuelen == 20)
			memcpy(d->id, id->value, 20);
		bforeach (bpath(c, "nodes"), str) {
			struct sockaddr_in6 addr = {
				.sin6_family = AF_INET6
			};
			char remote[INET6_ADDRSTRLEN + 7];
			strncpy(remote, str->value, str->valuelen);
			char * port = strchr(remote, '/');
			port[0] = '\0';
			if (port) {
				if (inet_pton(AF_INET6, remote, addr.sin6_addr.s6_addr) == 1) {
					addr.sin6_port = htons(atoi(++port));
					ping_node(d, &addr);
				}
			}
		}
	}
	return d;
	e:
	free(d);
	return NULL;
}

/**
 * frees a handle. does nothing if handle is NULL. does not fclose log. closes socket. please set socket to -1 before calling if you don't want to close it.
 */

void dht_free (struct dht * d) {
	if (d->socket != -1)
		if (close(d->socket) == -1)
			L(d->log, "close(d->socket) == -1");
	struct bucket * bucket = d->buckets;
	while (bucket) {
		struct bucket * old = bucket;
		bucket = bucket->next;
		bucket_free(old);
	}
	struct bucket * bucket6 = d->buckets6;
	while (bucket6) {
		struct bucket * old = bucket6;
		bucket6 = bucket6->next;
		bucket_free(old);
	}
	struct torrent * torrent = d->torrents;
	while (torrent) {
		struct torrent * old = torrent;
		torrent = torrent->next;
		torrent_free(old);
	}
	free(d);
}

/**
 * generate a bencoding struct containing the persistent storage config
 *
 * this config contains:
 * 	- nodes from the routing table that can be used to bootstrap into the dht net
 * 	- this node's id
 *
 * please make sure that you do not start two nodes from the same config
 *
 * @param d	[in]	library handle
 * @return bencoding object, whose memory ownership is transfered to the caller, which must call bencoding_free() on it.
 */

struct bencoding * persistent (const struct dht * d) {
	struct bencoding * b = calloc(1, sizeof *b);
	b->type = dict;
	struct bencoding * id = calloc(1, sizeof *id);
	id->type = string;
	memcpy((id->value = malloc(20)), d->id, (id->valuelen = 20));
	id->key = bstr(strdup("id"));
	binsert(b, id);
	struct sockaddr_in6 bound;
	socklen_t size = sizeof bound;
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wincompatible-pointer-types"
	if (getsockname(d->socket, &bound, &size) == -1)
		L(d->log, "getsockname: %s", strerror(errno));
	else {
		struct bencoding * port = bnum(ntohs(bound.sin6_port));
		port->key = bstr(strdup("port"));
		binsert(b, port);
	}
#pragma GCC diagnostic pop
	struct bencoding * nodes = calloc(1, sizeof *nodes);
	nodes->type = list;
	nodes->key = bstr(strdup("nodes"));
	for (int i = 0; i <= 1; i++) {
		struct bucket * bucket = d->buckets;
		if (i)
			bucket = d->buckets6;
		while (bucket) {
			struct node * node = bucket->nodes;
			while (node) {
				char remote[INET6_ADDRSTRLEN + 7];
				if (inet_ntop(AF_INET6, &node->addr.sin6_addr, remote, INET6_ADDRSTRLEN+7)) {
					sprintf(remote+strlen(remote), "/%u", ntohs(node->addr.sin6_port));
					binsert(nodes, bstr(strdup(remote)));
				}
				node = node->next;
			}
			bucket = bucket->next;
		}
	}
	binsert(b, nodes);
	return b;
}

/**
 * generates a 16 byte token for allowing a node to store it's IP addres in our node. verify with valid()
 *
 * @param d	[in]	used to obtain the secret key
 * @param t	[out]	the destination to which to write the 16 bytes
 * @param a	[in]	16 bytes of the node's IP address, from which get_peers was received - addr.sin6_addr.s6_addr
 * @param l	[in]	the length of socket address struct a
 */

void token (const struct dht * d, unsigned char * t, const unsigned char * addr) {
	struct AES_ctx aes;
	memcpy(t, addr, 16);
	AES_init_ctx(&aes, d->secret);
	AES_ECB_encrypt(&aes, t);
}

/**
 * verifies a 16 byte token, if it was really generated with token(), to prevent unsolicited adding of IPs to storage with src ip addr spoofing.
 *
 * @param d	[in]	used to obtain the secret key
 * @param t	[in]	the address from which to obtain the 16 byte token
 * @param a	[in]	16 bytes of the node's IP address, from which announce was received - addr.sin6_addr.s6_addr
 * @param l	[in]	the length of socket address struct a
 * @return 1 if the token is valid for this node, 0 otherwise
 */

int valid (const struct dht * d, const unsigned char * t, const unsigned char * addr) {
	unsigned char try[16];
	memcpy(try, t, 16);
	token(d, try, addr);
	return !memcmp(try, t, 16);
}

/**
 * sends an error rpc packet to a node. make sure that this is always similar size to the received packet, otherwise we could get amplification attacks.
 *
 * @param d	[in]	the dht library handle, for logging and for socket fd
 * @param b	[in]	the incoming packet that caused the error, to get key t
 * @param a	[in]	address of the incoming packet to which an error is sent
 * @param num	[in]	error number, as specified by BEP-0005
 * @param text	[in]	error text. memory ownership is transfered and string is freed, so make sure it's allocated on heap and no longer used by the caller - for static strings use strdup()
 */

void send_error (struct dht * d, const struct bencoding * b, struct sockaddr_in6 * a, int errnum, char * text) {
	struct bencoding * e = calloc(1, sizeof *e);
	e->type = list;
	e->key = bstr(strdup("e"));
	binsert(e, bstr(text));
	binsert(e, bnum(errnum));
	struct bencoding * y = bstr(strdup("e"));
	y->key = bstr(strdup("y"));
	struct bencoding * response = calloc(1, sizeof *response);
	binsert(response, y);
	binsert(response, e);
	binsert(response, bpath(b, "t"));
	sendb(d, response, a);
	free_bencoding(response);
}

/**
 * decides if a node id belongs to a bucket or not. this does not actually check the bucket for it's contents, just the ranges.
 *
 * @param id	[in]	the node id
 * @param b	[in]	the bucket
 * @return 1 if belongs to a bucket, 0 otherwise
 */

int in_bucket (const unsigned char * id, const struct bucket * b) {
	return memcmp(id, b->id, 20) >= 0 && (!b->next || memcmp(id, b->next->id, 20) < 0);
}

/**
 * searches for a stored node based on id
 *
 * @param id	[in]	the node id
 * @param b		pointer to a variable containing a pointer to the first bucket in ll. after the call the value at this pointer is overwritten to the bucket that should contain this node. since it's overwritten, do NOT just pass &dht->buckets. do struct bucket * b = dht->buckets and pass &b instead.
 * @param n	[out]	the node directly before the searched for node. NULL is written if this node would be placed at the start of the bucket. NULL may be passed without consequences.
 * @return the pointer to the node or NULL if not found
 */

struct node * find (const unsigned char * id, struct bucket ** b, struct node ** n) {
	while (!in_bucket(id, *b))
		*b = (*b)->next;
	struct node * node = (*b)->nodes;
	struct node * prev = NULL;
	while (node && memcmp(node->id, id, 20) < 0) {
		prev = node;
		node = node->next;
	}
	if (n)
		*n = prev;
	if (node && !memcmp((node)->id, id, 20))
		return node;
	else
		return NULL;
}

/**
 * add two 20 byte numbers
 *
 * @param a	[io]	result and first operand
 * @param b	[in]	second operand
 */

void add (unsigned char * a, const unsigned char * b) {
	unsigned carry = 0;
	for (int i = 19; i >= 0; i--) {
		unsigned cursum = (unsigned) a[i] + (unsigned) b[i] + carry;
		carry = 0;
		if (cursum > 255) {
			carry = cursum / 256;
			a[i] = cursum % 256;
		} else
			a[i] = cursum;
	}
}

/**
 * subtract two 20 byte numbers
 *
 * @param a	[io]	result and first operand
 * @param b	[in]	second operand
 */

void subtract (unsigned char * a, const unsigned char * b) {
	int carry = 0;
	for (int i = 19; i >= 0; i--) {
		int cur = (int) a[i] - ((int) b[i] + carry);
		carry = 0;
		while (cur < 0) {
			cur += 256;
			carry++;
		}
		a[i] = cur;
	}
}

/**
 * divide a 20 byte number with 2
 *
 * @param a	[io]	result and operand
 */

void divide (unsigned char * a) {
	for (int i = 19; i >= 0; i--) {
		a[i] >>= 1;
		if (i && a[i-1] & 1)
			a[i] |= 1 << 7;
	}
}

/**
 * find the middle point of a bucket/or two 20 byte bytestrings
 *
 * arguments must not overlap
 *
 * @param r	[out]	midpoint. caller must provide 20 bytes here
 * @param a	[in]	lower boundary
 * @param b	[in]	upper boundary. if NULL, 0xffffffffffffffffffffffffffffffffffffffff is assumed
 */

void midpoint (unsigned char * r, const unsigned char * a, const unsigned char * b) {
	for (int i = 0; i < 20; i++)
		r[i] = 0xFF;
	if (b)
		memcpy(r, b, 20);
	subtract(r, a);
	divide(r);
	add(r, a);
}

/**
 * splits a bucket
 *
 * @param b	[in]	the bucket to split
 */

void split (struct bucket * b) {
	struct bucket * new = bucket_init();
	midpoint(new->id, b->id, b->next ? b->next->id : NULL);
	new->next = b->next;
	struct node ** n = &b->nodes;
	while (*n && !in_bucket((*n)->id, new))
		n = &(*n)->next;
	new->nodes = *n;
	*n = NULL;
	b->next = new;
}

/**
 * returns a count of nodes in a linked list
 *
 * @param n	[in]	first node in ll
 */

int node_count (const struct node * n) {
	int c = 0;
	while (n) {
		c++;
		n = n->next;
	}
	return c;
}

/**
 * returns the distance between two ids. if it cannot be represented with an unsigned int, UINT_MAX is returned.
 *
 * TODO: test util
 */

unsigned int distance (const unsigned char * a, const unsigned char * b) {
	unsigned char xor[20];
	memcpy(xor, a, 20);
	for (long unsigned int i = 0; i < 20; i++) {
		xor[i] ^= b[i];
		if (i < 20 - sizeof (unsigned int) && xor[i])
			return UINT_MAX;
	}
	unsigned r = 0;
	for (long unsigned int i = 0; i < sizeof (unsigned); i++)
		r |= xor[i] << (sizeof (unsigned) - 1 - i) * 8;
	return r;
}

enum node_grade {
	good,
	bad,
	questionable
};

/**
 * determines if node is considered good, bad or questionable
 *
 * @param n	[in]	node
 * @return enum node_grade
 */

enum node_grade node_grade (const struct node * n) {
	if (n->last_received + 15*60 < seconds()) {
		if (n->last_sent + 14*60 < seconds() && n->unanswered > 1)
			return bad;
		return questionable;
	}
	return good;
}

/**
 * returns 1 if bucket is perfect, meaning it is fresh, has K nodes, and all nodes are good. bucket that contains id is almost never perfect, as it can usually be split into smaller buckets, that's why param d is required to get own id
 *
 * if d is NULL, it's not checked whether we fall into the bucket and whether it could be split
 *
 * @param d	[in]	library handle to get own id
 * @param b	[in]	the bucket
 */

int bucket_good (const struct dht * d, const struct bucket * b) {
	if (d) {
		if (!bucket_good(NULL, b))
			return 0;
		if (in_bucket(d->id, b)) {
			struct node * n = b->nodes;
			while (n->next) {
				if (distance(n->id, n->next->id) > 1)	// we allow holes of 1, since we don't actually store our ID in the bucket
					return 0;			// it's really impossible that a non-malicious node would by chance get so close
				n = n->next;
			}
			// L(d->log, "our bucket filled");
			return 1;
		}
		// L(d->log, "not our bucket and filled");
		return 1;
	} else {
		if (node_count(b->nodes) < K)
			return 0;
		struct node * n = b->nodes;
		if (n) {
			if (node_grade(n) != good)
				return 0;
			n = n->next;
		}
		return 1;
	}
}

/**
 * informs the library of a successfully received response packet from a node, knowing it's id and ip:port. do not call if the node queried us, if that's the case, use potential_node().
 *
 * if the node is new, it's added in a bucket.
 *
 * if the node is found in a bucket, it's last received time and unanswered are updated
 *
 * TODO: this could be refactored so that it reuses logic from potential_node, like done in replied_torrent_node() below
 *
 * @param d	[in]	handle
 * @param id	[in]	node id that was received
 * @param addr	[in]	address from which the id was received
 */

void replied (const struct dht * d, const unsigned char * id, const struct sockaddr_in6 * addr) {
	if (!memcmp(d->id, id, 20)) // WE COULDN'T'VE POSSIBLY REPLIED TO OURSELVES!
		return;
	char buf[41];
	buf[40] = '\0';
	bin2hex(buf, id, 20);
	// L(d->log, "%s replied", buf);
	struct bucket * b = d->buckets;
	if (family(addr->sin6_addr.s6_addr) == AF_INET6)
		b = d->buckets6;
	struct node * n;
	struct node * found = find(id, &b, &n);
	if (found) {
		found->last_received = seconds();
		found->unanswered = 0;
		// L(d->log, "found");
		return;
	}
	if (bucket_good(d, b)) {
		// L(d->log, "bucket good");
		return;
	}
	struct node * node = node_init();
	memcpy(&node->addr, addr, sizeof *addr);
	memcpy(node->id, id, 20);
	if (!n) {
		b->nodes = node;
		return;
	}
	if (node_count(b->nodes) < K) {
		struct node * index = b->nodes;
		while (index->next && memcmp(node->id, index->next->id, 20) > 1)
			index = index->next;
		node->next = index->next;
		index->next = node;
		return;
	}
	if (in_bucket(d->id, b)) {
		node_free(node);
		split(b);
		replied(d, id, addr); // find bucket again
	}
}

/**
 * when we are sure that a node exists on a specific ip:port and we know it's id, but we are unsure if the node is already in the routing table, we call this function, which makes a query to this node if it's a candidate for filling the routing table. this doesn't yet add it to the routing table, because we are unsure if it's a good node / can respond to queries. replied() is called if a node replied to our query.
 *
 * see NOTE02
 * 
 * @param d	[in]	library handle
 * @param a	[in]	pointer to sockaddr of the node
 * @param id	[in]	id of the node, 20 bytes is read from this address
 */

void potential_node (struct dht * d, const struct sockaddr_in6 * a, const unsigned char * id) {
	if (!memcmp(d->id, id, 20)) // we are not a potential node of ourselves
		return;
	struct bucket * bucket = d->buckets;
	if (family(a->sin6_addr.s6_addr) == AF_INET6)
		bucket = d->buckets6;
	if (find(id, &bucket, NULL))
		return;
	if (!bucket_good(d, bucket))
		ping_node(d, a);
}

#define MAXT 15 /**< see NOTE04 */

/**
 * find a torrent based on hash
 *
 * NOTE04: full length 20 byte binary hash can't be stored in "t" because some nodes' software discards t if it's longer than MAXT bytes
 * 	
 * that's why this function accepts shorter lengths to compare.
 *
 * @param d	[in]	the library handle
 * @param h	[in]	an l-byte infohash
 * @param l	[in]	number of bytes of infohash to compare - for truncated hashes. full hash is 20 bytes long.
 * @return pointer to torrent or NULL
 */

struct torrent * find_torrent (struct dht * d, const unsigned char * h, int l) {
	struct torrent * t = d->torrents;
	while (t) {
		if (!memcmp(t->hash, h, l))
			return t;
		t = t->next;
	}
	return NULL;
}

/**
 * deletes a torrent from storage. if you downloaded a torrent and set peers/announce flags, do not remove_torrent once you're done with it, but instead just clear peers/announce bits. this will remove the torrent when necessary.
 *
 * @param d	[in]	the library handle
 * @param t	[in]	the pointer to the torrent to be deleted. do not craft torrent yourself, it must be stored in dht and that specific instance must be passed
 */

void remove_torrent (struct dht * d, struct torrent * t) {
	if (!t)
		return;
	if (!t->next)
		d->last_torrent = t->prev;
	if (t->prev)
		t->prev->next = t->next;
	if (t->next)
		t->next->prev = t->prev;
	struct peer * p = t->peers;
	while (p) {
		d->peers_num--;
		p = p->next;
	}
	d->torrents_num--;
	torrent_free(t);
}

/**
 * what to do when there are too many torrents and their peers stored, used in add_peer and add_torrent
 *
 * removes last added torrent that wasn't manually added
 *
 * @param d	[in]	libhandle
 */

void oom (struct dht * d) {
	struct torrent * drop = d->last_torrent;
	while (drop && drop->type)
		drop = drop->prev;
	remove_torrent(d, drop);
}

/**
 * adds a torrent to a list of torrents
 * 
 * if the torrent already exists in the database flags of this one will be anded with the flags of the old one, meaning this function can be used to set peers, announce, info and dl flags. see @return for important details.
 *
 * @param d	[in]	dht library handler, for counting torrents in storage
 * @param t	[in]	torrent object, whose memory ownership is transfered to the library and must be heap allocated
 * @return the new pointer to the torrent. if the torrent is already in the storage, the passed torrent will be freed, so the return value must be checked if you intend to use the torrent weiter
 */

struct torrent * add_torrent (struct dht * d, struct torrent * t) {
	struct torrent * found = find_torrent(d, t->hash, 20);
	if (found) {
		found->type |= t->type;
		torrent_free(t);
		return found;
	}
	if (d->torrents)
		d->torrents->prev = t;
	else
		d->last_torrent = t;
	t->prev = NULL;
	t->next = d->torrents;
	d->torrents = t;
	d->torrents_num++;
	if (d->torrents_num >= d->torrents_max)
		oom(d);
	return t;
}

/**
 * adds a peer to a torrent, memory ownership is transfered, so make sure it's allocated on heap. if this peer already exists, the input peer is freed and the old peer is returned.
 *
 * @param d	[in]	library handle, for counting peers
 * @param t	[in]	torrent
 * @param p	[in]	peer to add
 * @param this peer in storage. could be different if old one was freed, so discard value that you passed in and replace it with this value. memory ownership is NOT transfered from storage to caller
 */

struct peer * add_peer (struct dht * d, struct torrent * t, struct peer * p) {
	struct peer * peer = t->peers;
	while (peer) {
		if (!memcmp(&peer->addr, &p->addr, sizeof p->addr)) {
			peer_free(p);
			return peer;
		}
		peer = peer->next;
	}
	p->next = t->peers;
	t->peers = p;
	d->peers_num++;
	if (d->peers_num >= d->peers_max)
		oom(d);
	return peer;
}

/**
 * sends a get_peers query to a raw node
 *
 * @param d	[in]	handle
 * @param a	[in]	address
 * @param q	[in]	20 byte hash we query for
 */

void get_peers (struct dht * d, const struct sockaddr_in6 * addr, const unsigned char * q) {
	struct bencoding * b = calloc(1, sizeof *b);
	b->type = dict;
	struct bencoding * t = calloc(1, sizeof *t);
	memcpy((t->value = malloc(MAXT)), q, (t->valuelen = MAXT));
	t->key = bstr(strdup("t"));
	t->type = string;
	binsert(b, t);
	struct bencoding * y = bstr(strdup("q"));
	y->key = bstr(strdup("y"));
	y->type = string;
	binsert(b, y);
	struct bencoding * q_elem = bstr(strdup("get_peers"));
	q_elem->key = bstr(strdup("q"));
	q_elem->type = string;
	binsert(b, q_elem);
	struct bencoding * a = calloc(1, sizeof *a);
	a->key = bstr(strdup("a"));
	a->type = dict;
	struct bencoding * id = calloc(1, sizeof *id);
	id->type = string;
	id->valuelen = 20;
	id->key = bstr(strdup("id"));
	memcpy((id->value = malloc(20)), d->id, 20);
	binsert(a, id);
	struct bencoding * want = calloc(1, sizeof *want); // BEP-0032
	want->key = bstr(strdup("want"));
	want->type = list;
	binsert(want, bstr(strdup("n4")));
	binsert(want, bstr(strdup("n6")));
	binsert(a, want);
	struct bencoding * info_hash = calloc(1, sizeof *info_hash);
	info_hash->key = bstr(strdup("info_hash"));
	info_hash->type = string;
	memcpy((info_hash->value = malloc(20)), q, (info_hash->valuelen = 20));
	binsert(a, info_hash);
	binsert(b, a);
	sendb(d, b, addr);
	free_bencoding(b);
}

/**
 * called from compact() on response to get_peers, this function sends a get_peers to a node if it would improve the node pool of a torrent
 *
 * @param d	[in]	library handle. if NULL, get_peers is not sent and it's expected that caller will insert a new node into torrent (because one good will be removed if the potential node is closer and there's no bad node and nodelist is full)
 * @param t	[in]	torrent
 * @param a	[in]	address of node
 * @param i	[in]	20 byte id of node
 * @return the position to which the new node should be written. the caller, if overwrites the pointed memory with this address, must update ->next of inserted node to the value that this address pointed to before
 */

struct node ** potential_torrent_node (struct dht * d, struct torrent * t, const struct sockaddr_in6 * a, const unsigned char * i) {
	if (!t->nodes) {
		if (d)
			get_peers(d, a, t->hash);
		return &t->nodes;
	}
	int l = 0;
	struct node * n = t->nodes;
	struct node ** rs = NULL; // return if enough space
	struct node ** farthest = &t->nodes;
	struct node * prev = NULL;
	while (n) {
		l++;
		if (node_grade(n) == bad) {
			if (prev)
				prev->next = n->next;
			else
				t->nodes = n->next;
			node_free(n);
			l--;
			if (prev)
				n = prev->next;
			else
				n = t->nodes;
			continue;
		}
		int cp = memcmp(n->id, i, 20);
		if (!cp)
			return NULL;
		if (!n->next)
			rs = &n->next;
		else {
			int cn = memcmp(i, n->next->id, 20);
			if (!cn)
				return NULL;
			if (cp == 1 && cn == -1)
				rs = &n->next;
		}
		if (!closer(n->id, (*farthest)->id, t->hash)) {
			if (prev)
				farthest = &t->nodes;
			else
				farthest = &prev->next;
		}
		prev = n;
		n = n->next;
	}
	if (l < K) {
		if (d)
			get_peers(d, a, t->hash);
		return rs;
	}
	if (!closer(i, (*farthest)->id, t->hash)) {
		if (!d) {
			node_free(*farthest);
			*farthest = prev;
			if (d)
				get_peers(d, a, t->hash);
			return farthest;
		}
	}
	return NULL;
}

/**
 * called on a response to get_peers, the node that responded is added to a list of good nodes of a torrent if it's a potential torrent node, if it's already in the list, it's stats are updated.
 *
 * note that torrent nodelists have no housekeeping like the routing table has with refresh(), so bad nodes are removed only in potential_torrent_node.
 *
 * @param t	[in]	torrent
 * @param a	[in]	address of node
 * @param i	[in]	node id
 */

void replied_torrent_node (struct torrent * t, const struct sockaddr_in6 * a, const unsigned char * i) {
	struct node * existing = t->nodes;
	int r;
	while (existing && (r = memcmp(existing->id, i, 20)) < 0)
		existing = existing->next;
	if (!r) {
		existing->last_received = seconds();
		existing->unanswered = 0;
		return;
	}
	struct node ** dst = potential_torrent_node(NULL, t, a, i);
	if (!dst)
		return;
	struct node * n = node_init();
	memcpy(n->id, i, 20);
	memcpy(&n->addr, a, sizeof n->addr);
	n->next = *dst;
	*dst = n;
}

/**
 * parses a compact node description in either ipv4 or ipv6 from nodes or nodes6 and invoke actions. call this for every string in nodes and nodes6 array in incoming response packets.
 *
 * if the newly found node is useful for filling the K closest nodes ll of the torrent, it will be added in the LL. it may also replace an existing node if the existing node is bad or furthest away from torrent hash and this one is closer. upon insertion, the node is queried for find_node.
 *
 * @param d	[in]	libh
 * @param value	[in]	compact node info buffer pointer
 * @param len	[in]	length of buffer, can be either 20+4+2 for ipv4 or 20+16+2 for ipv6
 * @param t
 */

void compact (struct dht * d, const char * value, int len, struct torrent * t) {
	if (len != 4+2+20 && len != 16+2+20) {
		L(d->log, "received packet contained an invalid compact node");
		return;
	}
	struct sockaddr_in6 addr = {
		.sin6_family = AF_INET6
	};
	memcpy(addr.sin6_addr.s6_addr, "\0\0\0\0\0\0\0\0\0\0\xFF\xFF", 12);
	addr.sin6_port = *((uint16_t *) (value + len-2));
	memcpy(addr.sin6_addr.s6_addr+(len == 4+2+20 ? 12 : 0), value + 20, len == 4+2+20 ? 4 : 16);
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wpointer-sign"
	potential_node(d, &addr, value); // NOTE02 at the beginning, a lot of packets will be sent, since every reply of potential_node will generate K replies. naively this would generate an exponentially increasing number of packets, in increasing powers of 8 (8**n). to prevent an absolute resource hog, this is only done when node would be useful and would contribute to the routing table
	if (t)
		potential_torrent_node(d, t, &addr, value);
#pragma GCC diagnostic pop
	/*
		if (t) {
			int i = 0;
			struct node ** replaceable = NULL;
			struct node ** farthest = &t->nodes;
			struct node ** index = &t->nodes;
			while (*index) {
				i++;
				if (node_grade(*index) == bad)
					replaceable = index;
				if (!closer((*index)->id, (*farthest)->id, t->hash))
					farthest = index;
				index = &(*index)->next;
			}
			if (i <= K) {
				node->next = t->nodes;
				t->nodes = node->next;
			} else if (replaceable) {
				node->next = (*replaceable)->next;
				node_free(*replaceable);
				*replaceable = node;
				find_node(d, &node->addr, t->hash);
			} else if (!closer(node->id, (*farthest)->id, t->hash)) {
				node->next = (*farthest)->next;
				node_free(*farthest);
				*farthest = node;
				find_node(d, &node->addr, t->hash);
			} else
				node_free(node);
		} else
			node_free(node);
	*/
}

/**
 * returns a dict containing nodes or nodes6 bencoding list (with a key) with compact nodes in the bucket. if exact node is found, only that one is in the list.
 *
 * @param d	[in]	library handle
 * @param id	[in]	target node id, 20 bytes is read from this location
 * @param f	[in]	address family, AF_INET or AF_INET6
 * @return bencoding object whose memory ownership and free responsibility is transfered to the caller
 */

struct bencoding * nodes (const struct dht * d, const unsigned char * id, sa_family_t f) {
	struct bencoding * nodes = calloc(1, sizeof *nodes);
	nodes->type = list;
	nodes->key = bstr(strdup(f == AF_INET ? "nodes" : "nodes6"));
	struct bucket * bucket = (f == AF_INET ? d->buckets : d->buckets6);
	struct node * found = find(id, &bucket, NULL);
#define ADDRLEN(f) (f == AF_INET ? 4 : 16)
	if (found) {
		struct bencoding * compact = calloc(1, sizeof *compact);
		compact->type = string;
		compact->value = malloc((compact->valuelen = 20+ADDRLEN(f)+2));
		memcpy(compact->value, found->id, 20);
		memcpy(compact->value+20, found->addr.sin6_addr.s6_addr+(16-ADDRLEN(f)), ADDRLEN(f));
		memcpy(compact->value+20+ADDRLEN(f), &found->addr.sin6_port, 2);
		binsert(nodes, compact);
	} else {
		struct node * node = bucket->nodes;
		while (node) {
			struct bencoding * compact = calloc(1, sizeof *compact);
			compact->type = string;
			compact->value = malloc((compact->valuelen = 20+ADDRLEN(f)+2));
			memcpy(compact->value, node->id, 20);
			memcpy(compact->value+20, node->addr.sin6_addr.s6_addr+(16-ADDRLEN(f)), ADDRLEN(f));
			memcpy(compact->value+20+ADDRLEN(f), &node->addr.sin6_port, 2);
			binsert(nodes, compact);
			node = node->next;
		}
	}
	return nodes;
}

/**
 * sends an announce_peer query to a raw node
 *
 * @param d	[in]	lh
 * @param a	[in]	node addr
 * @param t	[in]	bencoding object from r/token to be inserted into a/token when sending. memory ownership and responsibility is transfered and this object is freed after call to this function, so use something along the lines of bclone(bpath(b, "r/token")) as argument to call
 * @param h	[in]	torrent hash to announce from which 20 bytes are read
 */

void announce_peer (struct dht * d, const struct sockaddr_in6 * addr, struct bencoding * t, const unsigned char * h) {
	struct bencoding * b = calloc(1, sizeof *b);
	b->type = dict;
	struct bencoding * t_elem = bstr(strdup(GENERIC_T));
	t_elem->key = bstr(strdup("t"));
	binsert(b, t_elem);
	struct bencoding * y = bstr(strdup("q"));
	y->key = bstr(strdup("y"));
	y->type = string;
	binsert(b, y);
	struct bencoding * q = bstr(strdup("announce_peer"));
	q->key = bstr(strdup("q"));
	q->type = string;
	binsert(b, q);
	struct bencoding * a = calloc(1, sizeof *a);
	a->type = dict;
	struct bencoding * id = calloc(1, sizeof *id);
	id->type = string;
	id->valuelen = 20;
	id->key = bstr(strdup("id"));
	memcpy((id->value = malloc(20)), d->id, 20);
	binsert(a, id);
	struct bencoding * want = calloc(1, sizeof *want); // BEP-0032
	want->key = bstr(strdup("want"));
	want->type = list;
	binsert(want, bstr(strdup("n4")));
	binsert(want, bstr(strdup("n6")));
	binsert(a, want);
	struct bencoding * info_hash = calloc(1, sizeof *info_hash);
	info_hash->key = bstr(strdup("info_hash"));
	info_hash->type = string;
	info_hash->valuelen = 20;
	memcpy((info_hash->value = malloc(20)), h, 20);
	binsert(a, info_hash);
	struct bencoding * implied_port = bnum(1);
	implied_port->key = bstr(strdup("implied_port"));
	binsert(b, implied_port);
	binsert(b, t);
	struct sockaddr_in6 bound;
	socklen_t size = sizeof bound;
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wincompatible-pointer-types"
	if (getsockname(d->socket, &bound, &size) == -1)
		L(d->log, "getsockname: %s", strerror(errno));
	else {
		struct bencoding * port = bnum(ntohs(bound.sin6_port));
		b->key = bstr(strdup("port"));
		binsert(b, port);
	}
#pragma GCC diagnostic pop
	binsert(b, a);
	sendb(d, b, addr);
	free_bencoding(b);
}

/**
 * handles an incoming packet, be it: (polyglot packets are technically possible)
 * 	- bencoded packet from another DHT node
 * 	- DNS server response for bootstrapping queries
 * 	- uTP packet from a peer (TODO)
 *
 * @param d	[in]	library handle
 * @param pkt	[in]	incoming UDP packet content
 * @param len	[in]	length of the packet
 * @param addr	[in]	IP address and port of sender
 */

void handle (struct dht * d, char * pkt, int len, struct sockaddr_in6 addr) {
	struct bencoding * b = bdecode(pkt, len, replace);
	struct bencoding * v = bpath(b, "v");
	char * node_ver = "";
	char remote[INET_ADDRSTRLEN + INET6_ADDRSTRLEN + 64 + (v && v->type & string ? v->valuelen : 0)];
	if (!inet_ntop(addr.sin6_family, &addr.sin6_addr, remote, INET6_ADDRSTRLEN+7+INET_ADDRSTRLEN)) {
		snprintf(remote, sizeof remote,  "(inet_ntop: %s)", strerror(errno));
		sprintf(remote+strlen(remote), "/%d", ntohs(addr.sin6_port));
	}
	if (v && v->type & string) {
		node_ver = v->value;
		sprintf(remote+strlen(remote), "-%s", node_ver);
	}
	if (b) {
		int len = b2json_length(b);
		char out[len+1];
		char * end = b2json(out, b);
		*end = '\0';
		assert(out+len == end);
		L(d->log, "handle(%s): %s", remote, out);
	}
	struct bencoding * y = bpath(b, "y");
	char * msg_type = "";
	if (y && y->type & string)
		msg_type = y->value;
	switch (msg_type[0]) {
		case 'Q':
		case 'q':
			;
			struct bencoding * q = bpath(b, "q");
			char * qtype = "";
			if (q && q->type & string)
				qtype = q->value;
			struct bencoding * ro = bpath(b, "ro");
			struct bencoding * rid = bpath(b, "a/id");
			if (rid && rid->type & string && rid->valuelen == 20 && (!ro || !(ro->type & num) || ro->intvalue != 1 /* BEP-0043 */)) {
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wpointer-sign"
				potential_node(d, &addr, rid->value);
#pragma GCC diagnostic pop
			}
			else { // see NOTE01
				int len = b2json_length(b);
				char j[len+1];
				b2json(j, b);
				j[len] = '\0';
				L(d->log, "%s did not send a valid id in %s", remote, j);
			}
			switch (qtype[0]) {
				case 'P': // ping
				case 'p':
					;
					struct bencoding * id = calloc(1, sizeof *id);
					id->type = string;
					id->key = bstr(strdup("id"));
					memcpy((id->value = malloc((id->valuelen = 20))), d->id, 20);
					struct bencoding * r = calloc(1, sizeof *r);
					r->type = dict;
					r->key = bstr(strdup("r"));
					binsert(r, id);
					struct bencoding * y = bstr(strdup("r"));
					y->key = bstr(strdup("y"));
					struct bencoding * response = calloc(1, sizeof *response);
					response->type = dict;
					binsert(response, y);
					binsert(response, r);
					binsert(response, bclone(bpath(b, "t")));
					sendb(d, response, &addr);
					free_bencoding(response);
					break;
				case 'F': // find_node
				case 'f':
					;
					struct bencoding * target = bpath(b, "a/target");
					if (!target || !(target->type & string) || target->valuelen != 20)
						break; // see NOTE01
					response = calloc(1, sizeof *response);
					response->type = dict;
					y = bstr(strdup("r"));
					y->key = bstr(strdup("y"));
					binsert(response, y);
					binsert(response, bclone(bpath(b, "t")));
					r = calloc(1, sizeof *r);
					r->key = bstr(strdup("r"));
					r->type = dict;
					id = calloc(1, sizeof *id);
					id->type = string;
					id->key = bstr(strdup("id"));
					memcpy((id->value = malloc((id->valuelen = 20))), d->id, 20);
					binsert(r, id);
					binsert(response, r);
					if (family(addr.sin6_addr.s6_addr) == AF_INET || bval(bpath(b, "a/want"), bstrs("v4"))) {
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wpointer-sign"
						binsert(response, nodes(d, target->value, AF_INET));
#pragma GCC diagnostic pop
					}
					if (family(addr.sin6_addr.s6_addr) == AF_INET6 || bval(bpath(b, "a/want"), bstrs("v6"))) {
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wpointer-sign"
						binsert(response, nodes(d, target->value, AF_INET6));
#pragma GCC diagnostic pop
					}
					sendb(d, response, &addr);
					free_bencoding(response);
					break;
				case 'G': // get_peers
				case 'g':
					;
					struct bencoding * hash = bpath(b, "a/info_hash");
					if (!hash || !(hash->type & string) || hash->valuelen != 20)
						break; // see NOTE01
					else {
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wpointer-sign"
						d->possible_torrent(d, hash->value);
#pragma GCC diagnostic pop
					}
					response = calloc(1, sizeof *response);
					response->type = dict;
					y = bstr(strdup("r"));
					y->key = bstr(strdup("y"));
					binsert(response, y);
					binsert(response, bclone(bpath(b, "t")));
					r = calloc(1, sizeof *r);
					r->key = bstr(strdup("r"));
					r->type = dict;
					id = calloc(1, sizeof *id);
					id->type = string;
					id->key = bstr(strdup("id"));
					memcpy((id->value = malloc((id->valuelen = 20))), d->id, 20);
					binsert(r, id);
					binsert(response, r);
					if (family(addr.sin6_addr.s6_addr) == AF_INET || bval(bpath(b, "a/want"), bstrs("v4"))) {
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wpointer-sign"
						binsert(response, nodes(d, hash->value, AF_INET));
#pragma GCC diagnostic pop
					}
					if (family(addr.sin6_addr.s6_addr) == AF_INET6 || bval(bpath(b, "a/want"), bstrs("v6"))) {
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wpointer-sign"
						binsert(response, nodes(d, hash->value, AF_INET6));
#pragma GCC diagnostic pop
					}
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wpointer-sign"
					struct torrent * torrent = find_torrent(d, hash->value, 20);
#pragma GCC diagnostic pop
					if (torrent) {
						struct peer * peer = torrent->peers;
						struct bencoding * values = calloc(1, sizeof *values);
						values->type = list;
						while (peer) { // TODO implement peer preference: prefer sending peers that responded to us
							if (family(peer->addr.sin6_addr.s6_addr) == family(addr.sin6_addr.s6_addr)) { // possible
								struct bencoding * value = calloc(1, sizeof *value);
								memcpy((value->value = malloc((value->valuelen = ADDRLEN(family(peer->addr.sin6_addr.s6_addr))+2))), peer->addr.sin6_addr.s6_addr, ADDRLEN(family(peer->addr.sin6_addr.s6_addr)));
								memcpy(value->value+ADDRLEN(family(peer->addr.sin6_addr.s6_addr)), &peer->addr.sin6_port, 2);
								binsert(values, value); // possible stack overflow if there are a lot of peers, see limit in bdecode() wrapper
							} // TODO add a random IP address for plausible deniability
							peer = peer->next;
						}
						binsert(r, values);
					}
					struct bencoding * tok = calloc(1, sizeof *tok);
					tok->type = string;
					tok->key = bstr(strdup("token"));
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wpointer-sign"
					token(d, (tok->value = malloc((tok->valuelen = 16))), addr.sin6_addr.s6_addr);
#pragma GCC diagnostic pop
					binsert(r, tok);
					sendb(d, response, &addr);
					free_bencoding(response);
					break;
				case 'A': // announce
				case 'a':
					;
					tok = bpath(b, "a/token");
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wpointer-sign"
					if (!tok || !(tok->type & string) || tok->valuelen != 16 || !valid(d, tok->value, addr.sin6_addr.s6_addr))
						break; // see NOTE01
#pragma GCC diagnostic pop
					hash = bpath(b, "a/info_hash");
					if (!hash || !(hash->type & string) || hash->valuelen != 20)
						break; // see NOTE01
					response = calloc(1, sizeof *response);
					response->type = dict;
					binsert(response, bclone(bpath(b, "t")));
					r = calloc(1, sizeof *r);
					r->type = dict;
					r->key = bstr(strdup("r"));
					id = calloc(1, sizeof *id);
					id->key = bstr(strdup("id"));
					id->type = string;
					memcpy((id->value = malloc((id->valuelen = 20))), d->id, 20);
					binsert(r, id);
					binsert(response, r);
					y = bstr(strdup("r"));
					y->key = bstr(strdup("y"));
					binsert(response, y);
					torrent = calloc(1, sizeof *torrent);
					memcpy(torrent->hash, hash->value, 20);
					torrent = add_torrent(d, torrent);
					struct peer * peer = calloc(1, sizeof *peer);
					memcpy(&peer->addr, &addr, sizeof addr);
					if (bpath(b, "a/port") && (!bpath(b, "a/implied_port") || !bpath(b, "a/implied_port")->intvalue))
						peer->addr.sin6_port = htons(bpath(b, "a/port")->intvalue);
					add_peer(d, torrent, peer);
					sendb(d, response, &addr);
					free_bencoding(response);
					break;
				default: // see NOTE01
					;
					int len = b2json_length(b);
					char json[len+1];
					b2json(json, b);
					json[len] = '\0';
					L(d->log, "%s sent an unknown query type: %s", remote, json);
					break;
			}
			break;
		case 'R': // we only ever query and expect responses to get_peers and find_node, so it's egal
		case 'r': 
			;
			rid = bpath(b, "r/id");
			if (rid && rid->type & string && rid->valuelen == 20) {
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wpointer-sign"
				replied(d, rid->value, &addr); // since here I'm only really interested about nodes and values or
#pragma GCC diagnostic pop
			}
			struct bencoding * t = bpath(b, "t");
			struct torrent * torrent = NULL;
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wpointer-sign"
			if (t && t->type & string && t->valuelen == MAXT) {
				if ((torrent = find_torrent(d, t->value, MAXT)) && torrent->type) {
#pragma GCC diagnostic pop
						if (rid && rid->type & string && rid->valuelen == 20) {
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wpointer-sign"
							replied_torrent_node(torrent, &addr, rid->value);
#pragma GCC diagnostic pop
						}
						bforeach (bpath(b, "r/values"), p) {
							if (!(p->type & string) || (p->valuelen != 6 && p->valuelen != 18))
								break;
							struct peer * peer = peer_init();
							memcpy(peer->addr.sin6_addr.s6_addr, "\0\0\0\0\0\0\0\0\0\0\xFF\xFF", 12);
							peer->addr.sin6_port = *((uint16_t *) (p->value + p->valuelen-2));
							memcpy(peer->addr.sin6_addr.s6_addr+(p->valuelen == 6 ? 12 : 0), p->value, p->valuelen == 6 ? 4 : 16);
							add_peer(d, torrent, peer);
						}
						if (torrent->type & announce)
							announce_peer(d, &addr, bclone(bpath(b, "r/token")), torrent->hash);
				}
			}
			struct bencoding * nodes = bpath(b, "r/nodes");
			struct bencoding * nodes6 = bpath(b, "r/nodes6");
			// L(d->log, "r/nodes length: %zu, r/nodes6 length: %zu", nodes ? nodes->valuelen : 0, nodes6 ? nodes6->valuelen : 0);
			if (nodes && nodes->type & string && !(nodes->valuelen % 26))
				for (unsigned i = 0; i < nodes->valuelen; i += 26)
					compact(d, nodes->value+i, 26, torrent);
			if (nodes6 && nodes6->type & string && !(nodes6->valuelen % 38))
				for (unsigned i = 0; i < nodes6->valuelen; i += 38)
					compact(d, nodes6->value+i, 38, torrent);
			break;
		case 'E':
		case 'e':
			;
			struct bencoding * e = bpath(b, "e");
			char * errtype = "Unspecified Error";
			if (e && e->child)
				switch (e->child->intvalue) {
					case 201:
						errtype = "Generic Error";
						break;
					case 202:
						errtype = "Server Error";
						break;
					case 203:
						errtype = "Protocol Error, such as a malformed packet, invalid arguments, or bad token";
						break;
					case 204:
						errtype = "Method Unknown";
						break;
					default:
						errtype = "Unknown Error";
						break;
				}
			char * msg = NULL;
			if (e && e->child && e->child->next && e->child->next->type & string)
				msg = e->child->next->value;
			L(d->log, "%s sent %s%s%s", remote, errtype, msg ? ": " : "", msg ? msg : "");
			break;
		case '\0': // y is not present, this may just be a DNS response,
			break; // do not log
		default: // NOTE01 sending an error is unfortunately bad in this case, since clever hackers can force two servers speaking entirely different UDP based protcols into sending error messages to each other, telling one another that they don't understand each other's messages.
			; // we don't report, since it may be DNS packet
			int len = b2json_length(b);
			char json[len+1];
			b2json(json, b);
			json[len] = '\0';
			L(d->log, "%s sent an unknown type: %s", remote, json);
			// send_error(d, b, &addr, addrlen, 203, "unknown type");
			break;
	}
	free_bencoding(b);
	ns_msg handle; // parsing incoming DNS packet as utils/dns.c
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wpointer-sign"
	if (ns_initparse(pkt, len, &handle) != -1) { // queries are sent by periodic
#pragma GCC diagnostic pop
		for (int i = 0; i < ns_msg_count(handle, ns_s_an); i++) {
			struct __ns_rr rr;
			if (ns_parserr(&handle, ns_s_an, i, &rr) == -1) {
				L(d->log, "ns_parserr(%s) == -1", remote);
				break;
			}
			if (rr.type != ns_t_srv && rr.type != ns_t_a && rr.type != ns_t_aaaa) {
				L(d->log, "%s unknown RR type %d", remote, rr.type);
				continue;
			}
			char address[INET_ADDRSTRLEN+INET6_ADDRSTRLEN+7];
			switch (rr.rdlength) {
				case 4:
					if (!inet_ntop(AF_INET, rr.rdata, address, INET6_ADDRSTRLEN+INET_ADDRSTRLEN+7)) {
						L(d->log, "%s !inet_ntop(AF_INET)", remote);
						break; // this can't fail!?
					}
					sprintf(address+strlen(address), ":%u", ntohs(*((uint16_t *) pkt)));
					L(d->log, "%s: A %s", remote, address);
					struct sockaddr_in6 a = {
						.sin6_family = AF_INET6,
						.sin6_port = *((uint16_t *) pkt)
					};
					memcpy(a.sin6_addr.s6_addr, "\0\0\0\0\0\0\0\0\0\0\xFF\xFF", 12);
					memcpy(a.sin6_addr.s6_addr+12, rr.rdata, 4);
					ping_node(d, &a);
					break;
				case 16:
					if (!inet_ntop(AF_INET6, rr.rdata, address, INET6_ADDRSTRLEN+INET_ADDRSTRLEN+7))
						L(d->log, "%s !inet_ntop(AF_INET6)", remote);
					sprintf(address+strlen(address), ":%u", ntohs(*((uint16_t *) pkt)));
					L(d->log, "%s: AAAA %s", remote, address);
					struct sockaddr_in6 aaaa = {
						.sin6_family = AF_INET6,
						.sin6_port = *((uint16_t *) pkt)
					};
					memcpy(aaaa.sin6_addr.s6_addr, rr.rdata, 16);
					ping_node(d, &aaaa);
					break;
				default: // SRV
					if (rr.rdlength < 3*2+3) // . indicates
						break; // disabled service - useless
					char target[NS_MAXDNAME];
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wpointer-sign"
					if (ns_name_uncompress(pkt, pkt+len, rr.rdata+3*2, target, NS_MAXDNAME) == -1) {
#pragma GCC diagnostic pop
						break;
						L(d->log, "ns_name_uncompress(%s) == -1", remote);
					}
					for (int j = 0; j <= 1; j++) {
						struct __res_state state;
						if (res_ninit(&state) == -1) {
							L(d->log, "res_ninit(%s, %s) == -1", remote, target);
							continue;
						}
						unsigned char packet[65536];
						int size = res_nmkquery(&state, QUERY, target, ns_c_in, j ? ns_t_a : ns_t_aaaa, NULL, 0, NULL, packet, 65536);
						if (size == -1) {
							L(d->log, "res_nmkquery(%s) == -1", target);
							goto d;
						}
						memcpy(packet, rr.rdata+4, 2);
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wincompatible-pointer-types"
						if (sendto(d->socket, packet, size, MSG_DONTWAIT | MSG_NOSIGNAL, &addr, sizeof addr) == -1)
							L(d->log, "sendto(%s, %s)", remote, target);
						d->txp++;
						d->txb += size;
#pragma GCC diagnostic pop
d:
						res_nclose(&state);
					}
					break;
					
			}
		}
	} // do not log, it may have been a bencoded reply
}

/**
 * do periodic housekeeping on the routing table LL, making sure no nodes are bad. removes bad nodes and does not ping questionable nodes. see NOTE03
 *
 * @param b	[in]	first bucket in LL - basically either d->buckets or d->buckets6
 * @return number of good nodes
 */

int refresh (struct bucket * b) {
	int nrgood = 0;
	while (b) {
		struct node ** n = &b->nodes;
		while (*n) {
			switch (node_grade(*n)) {
				case bad:
					;
					struct node * old = *n;
					*n = (*n)->next;
					node_free(old);
					break;
				case questionable:
					// ping_node(d, *n); // NOTE03 about not pinging questionable nodes: this ensures a constant regeneration of the routing table. this is just an idea, if the client frequently gets in a situation without any nodes in the routing table, remove the comment before ping_node call.
					break;
				case good:
					nrgood++;
					break;
			}
			n = &(*n)->next;
		}
		b = b->next;
	}
	return nrgood;
}

/**
 * does periodic work for the library, called every 13 minutes
 *
 * namely, it sends UDP packets:
 * 	- searching deeper DHT storage nodes for torrents with peers and announce
 * 	- get_peers on torrents with peers or announce. when a response is received for a torrent with announce, an announce will be sent as well
 *
 * for bootstrapping, an **IPv4** nameserver is required in /etc/resolv.conf. res_*() functions only provide IPv4 nameservers. example script with this algorithm is available in utils/dns.c
 *
 * this can be a lot of packets, so please keep number of torrents with peers and announce low
 */

void periodic (struct dht * d) {
	int dns = 0;
	if (!refresh(d->buckets))
		dns++;
	if (!refresh(d->buckets6))
		dns++;
	if (dns) {
		char packet[512];
		struct __res_state state;
		if (res_ninit(&state) == -1) {
			L(d->log, "res_ninit(&state) == -1");
			goto t;
		}
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wpointer-sign"
		int size = res_nmkquery(&state, QUERY, "_dht._udp.travnik.sijanec.eu", ns_c_in, ns_t_srv, NULL, 0, NULL, packet, 512) != -1; // for some reason always returns 1
#pragma GCC diagnostic pop
		if (size == -1) {
			L(d->log, "res_nmkquery(SRV) == -1");
			goto d;
		}
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wincompatible-pointer-types"
		for (int i = 0; i < state.nscount; i++)
			if (state.nsaddr_list[i].sin_family == AF_INET) { // leider
				if (sendto(d->socket, packet, 512, MSG_DONTWAIT | MSG_NOSIGNAL, &state.nsaddr_list[i], sizeof state.nsaddr_list[i]) == -1)
					L(d->log, "sendto: %s", strerror(errno));
				d->txp++;
				d->txb += size;
			}
#pragma GCC diagnostic pop
		d:
		res_nclose(&state); // receiving and resolving SRV->{A,AAAA} in handle()
	}
	t:
	;
	struct torrent * t = d->torrents;
	while (t) {
		if (t->type & (peers | announce)) {
			struct node * n = t->nodes;
			int sent = 0;
			while (n) {
				sent++;
				n->unanswered++;
				n->last_sent = seconds();
				get_peers(d, &n->addr, t->hash);
				n = n->next;
			}
			if (sent < K) {
#define RTGP(buckets)		{struct bucket * b = d->buckets; \
				find(t->hash, &b, NULL); \
				struct node * n = b->nodes; \
				while (sent < K && n) { \
					sent++; \
					n->unanswered++; \
					n->last_sent = seconds(); \
					get_peers(d, &n->addr, t->hash); \
					n = n->next; \
				}}
				RTGP(buckets);
				RTGP(buckets6);
			}
			if (sent < K) {
				struct bucket * b = d->buckets;
				while (sent < K && b) {
					n = b->nodes;
					while (sent < K && n) {
						sent++;
						n->unanswered++;
						n->last_sent = seconds();
						get_peers(d, &n->addr, t->hash);
						n = n->next;
					}
					b = b->next;
				}
			}
		}
		t = t->next;
	}
	return;
}

/**
 * does work; syncs with the network, handles incoming queries.
 *
 * call this:
 * 	- whenever socket can be read from (via poll/epoll/select/...)
 * 	- every 13 minutes, even if it was called for incoming packets in this time period
 * 		+ stale buckets are refreshed
 *		+ peers are refetched for joined torrents and announcements are sent
 *		+ calling it more often is discouraged, since every periodic call sends out UDP packets for PEX and DHT searches/announces of torrents
 *
 * @param d	[in]	dht library handle
 */

void work (struct dht * d) {
	char packet[65536];
	struct sockaddr_in6 addr;
	socklen_t addrlen = sizeof addr;
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wincompatible-pointer-types"
	int ret = recvfrom(d->socket, packet, 65536, MSG_DONTWAIT | MSG_TRUNC, &addr, &addrlen);
#pragma GCC diagnostic pop
	if (addrlen != sizeof addr)
		L(d->log, "addrlen changed, not parsing packet");
	else if (ret > 65536)
		L(d->log, "recvfrom()d larger packet than 65536, not parsing packet");
	else if (ret < 0) {
		if (errno != EAGAIN)
			L(d->log, "recvfrom(): %s (%d)", strerror(errno), errno);
		else
			periodic(d);
	} else {
		d->rxp++;
		d->rxb += ret;
		handle(d, packet, ret, addr);
	}
}