bonjour/src/mdns.h

/* mdns.h  -  mDNS/DNS-SD library  -  Public Domain  -  2017 Mattias Jansson
 *
 * This library provides a cross-platform mDNS and DNS-SD library in C.
 * The implementation is based on RFC 6762 and RFC 6763.
 *
 * The latest source code is always available at
 *
 * https://github.com/mjansson/mdns
 *
 * This library is put in the public domain; you can redistribute it and/or modify it without any restrictions.
 *
 */

#pragma once
#include <stdint.h>
#include <stddef.h>
#include <stdlib.h>
#include <string.h>

#include <fcntl.h>
#ifdef _WIN32
#include <Winsock2.h>
#include <Ws2tcpip.h>
#define strncasecmp _strnicmp
#else
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#endif

#ifdef __cplusplus
extern "C" {
#endif

#define MDNS_INVALID_POS ((size_t)-1)

#define MDNS_STRING_CONST(s) (s), (sizeof((s))-1)
#define MDNS_STRING_FORMAT(s) (int)((s).length), s.str

#define MDNS_POINTER_OFFSET(p, ofs) ((void*)((char*)(p) + (ptrdiff_t)(ofs)))
#define MDNS_POINTER_OFFSET_CONST(p, ofs) ((const void*)((const char*)(p) + (ptrdiff_t)(ofs)))
#define MDNS_POINTER_DIFF(a, b) ((size_t)((const char*)(a) - (const char*)(b)))

#define MDNS_PORT 5353

enum mdns_record_type {
	MDNS_RECORDTYPE_IGNORE = 0,
	//Address
	MDNS_RECORDTYPE_A = 1,
	//Domain Name pointer
	MDNS_RECORDTYPE_PTR = 12,
	//Arbitrary text string
	MDNS_RECORDTYPE_TXT = 16,
	//IP6 Address [Thomson]
	MDNS_RECORDTYPE_AAAA = 28,
	//Server Selection [RFC2782]
	MDNS_RECORDTYPE_SRV = 33
};

enum mdns_entry_type {
	MDNS_ENTRYTYPE_QUESTION = 0,
	MDNS_ENTRYTYPE_ANSWER = 1,
	MDNS_ENTRYTYPE_AUTHORITY = 2,
	MDNS_ENTRYTYPE_ADDITIONAL = 3
};

enum mdns_class {
	MDNS_CLASS_IN = 1
};

typedef enum mdns_record_type  mdns_record_type_t;
typedef enum mdns_entry_type   mdns_entry_type_t;
typedef enum mdns_class        mdns_class_t;

typedef int (* mdns_record_callback_fn)(int sock, const struct sockaddr* from, size_t addrlen,
                                        mdns_entry_type_t entry, uint16_t transaction_id,
                                        uint16_t rtype, uint16_t rclass, uint32_t ttl,
                                        const void* data, size_t size, size_t offset, size_t length,
                                        void* user_data);

typedef struct mdns_string_t       mdns_string_t;
typedef struct mdns_string_pair_t  mdns_string_pair_t;
typedef struct mdns_record_srv_t   mdns_record_srv_t;
typedef struct mdns_record_txt_t   mdns_record_txt_t;

#ifdef _WIN32
typedef int mdns_size_t;
#else
typedef size_t mdns_size_t;
#endif

struct mdns_string_t {
	const char* str;
	size_t length;
};

struct mdns_string_pair_t {
	size_t  offset;
	size_t  length;
	int     ref;
};

struct mdns_record_srv_t {
	uint16_t priority;
	uint16_t weight;
	uint16_t port;
	mdns_string_t name;
};

struct mdns_record_txt_t {
	mdns_string_t key;
	mdns_string_t value;
};

struct mdns_header_t {
	uint16_t transaction_id;
	uint16_t flags;
	uint16_t questions;
	uint16_t answer_rrs;
	uint16_t authority_rrs;
	uint16_t additional_rrs;
};

//! Open and setup a IPv4 socket for mDNS/DNS-SD. To send discovery requests and queries pass
//  0 as port to assign a random user level port. To run discovery service listening for incoming
//  discoveries and queries, pass MDNS_PORT as port.
static int
mdns_socket_open_ipv4(int port);

//! Setup an already opened IPv4 socket for mDNS/DNS-SD. To send discovery requests and queries pass
//  0 as port to assign a random user level port. To run discovery service listening for incoming
//  discoveries and queries, pass MDNS_PORT as port.
static int
mdns_socket_setup_ipv4(int sock, int port);

//! Open and setup a IPv6 socket for mDNS/DNS-SD. To send discovery requests and queries pass
//  0 as port to assign a random user level port. To run discovery service listening for incoming
//  discoveries and queries, pass MDNS_PORT as port.
static int
mdns_socket_open_ipv6(int port);

//! Setup an already opened IPv6 socket for mDNS/DNS-SD. To send discovery requests and queries pass
//  0 as port to assign a random user level port. To run discovery service listening for incoming
//  discoveries and queries, pass MDNS_PORT as port.
static int
mdns_socket_setup_ipv6(int sock, int port);

//! Close a socket opened with mdns_socket_open_ipv4 and mdns_socket_open_ipv6.
static void
mdns_socket_close(int sock);

//! Listen for incoming multicast DNS-SD and mDNS query requests. The socket should have been
//  opened on port MDNS_PORT using one of the mdns open or setup socket functions.
static size_t
mdns_socket_listen(int sock, void* buffer, size_t capacity,
                   mdns_record_callback_fn callback, void* user_data);

//! Send a multicast DNS-SD reqeuest on the given socket to discover available services.
static int
mdns_discovery_send(int sock);

//! Recieve unicast responses to a DNS-SD sent with mdns_discovery_send. Any data will be piped to the
//  given callback for parsing.
static size_t
mdns_discovery_recv(int sock, void* buffer, size_t capacity,
                    mdns_record_callback_fn callback, void *user_data);

//! Send a unicast DNS-SD answer with a single record to the given address.
static int
mdns_discovery_answer(int sock, const void* address, size_t address_size, void* buffer,
                      size_t capacity, const char* record, size_t length);

//! Send a multicast mDNS query on the given socket for the given service name. The supplied buffer
//  will be used to build the query packet.
static int
mdns_query_send(int sock, mdns_record_type_t type, const char* name, size_t length,
                void* buffer, size_t capacity);

//! Receive unicast responses to a mDNS query sent with mdns_discovery_recv. Any data will be piped
//  to the given callback for parsing.
static size_t
mdns_query_recv(int sock, void* buffer, size_t capacity,
                mdns_record_callback_fn callback, void* user_data,
                int only_last_query);

//! Send a unicast mDNS query answer with a single record to the given address.
static int
mdns_query_answer(int sock, const void* address, size_t address_size, void* buffer, size_t capacity,
                  uint16_t transaction_id, const char* service, size_t service_length,
                  const char* hostname, size_t hostname_length, uint32_t ipv4, const uint8_t* ipv6,
                  uint16_t port, const char* txt, size_t txt_length);

static mdns_string_t
mdns_string_extract(const void* buffer, size_t size, size_t* offset,
                    char* str, size_t capacity);

static int
mdns_string_skip(const void* buffer, size_t size, size_t* offset);

static int
mdns_string_equal(const void* buffer_lhs, size_t size_lhs, size_t* ofs_lhs,
                  const void* buffer_rhs, size_t size_rhs, size_t* ofs_rhs);

static void*
mdns_string_make(void* data, size_t capacity, const char* name, size_t length);

static void*
mdns_string_make_ref(void* data, size_t capacity, size_t ref_offset);

static void*
mdns_string_make_with_ref(void* data, size_t capacity, const char* name, size_t length,
                          size_t ref_offset);

static mdns_string_t
mdns_record_parse_ptr(const void* buffer, size_t size, size_t offset, size_t length,
                      char* strbuffer, size_t capacity);

static mdns_record_srv_t
mdns_record_parse_srv(const void* buffer, size_t size, size_t offset, size_t length,
                      char* strbuffer, size_t capacity);

static struct sockaddr_in*
mdns_record_parse_a(const void* buffer, size_t size, size_t offset, size_t length,
                    struct sockaddr_in* addr);

static struct sockaddr_in6*
mdns_record_parse_aaaa(const void* buffer, size_t size, size_t offset, size_t length,
                       struct sockaddr_in6* addr);

static size_t
mdns_record_parse_txt(const void* buffer, size_t size, size_t offset, size_t length,
                      mdns_record_txt_t* records, size_t capacity);

// Implementations

static int
mdns_socket_open_ipv4(int port) {
	int sock = (int)socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
	if (sock < 0)
		return -1;
	if (mdns_socket_setup_ipv4(sock, port)) {
		mdns_socket_close(sock);
		return -1;
	}
	return sock;
}

static int
mdns_socket_setup_ipv4(int sock, int port) {
	unsigned char ttl = 1;
	unsigned char loopback = 1;
	unsigned int reuseaddr = 1;
	struct ip_mreq req;

	setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, (const char*)&reuseaddr, sizeof(reuseaddr));
#ifdef SO_REUSEPORT
	setsockopt(sock, SOL_SOCKET, SO_REUSEPORT, (const char*)&reuseaddr, sizeof(reuseaddr));
#endif
	setsockopt(sock, IPPROTO_IP, IP_MULTICAST_TTL, (const char*)&ttl, sizeof(ttl));
	setsockopt(sock, IPPROTO_IP, IP_MULTICAST_LOOP, (const char*)&loopback, sizeof(loopback));

	memset(&req, 0, sizeof(req));
	req.imr_multiaddr.s_addr = htonl((((uint32_t)224U) << 24U) | ((uint32_t)251U));
	req.imr_interface.s_addr = INADDR_ANY;
	if (setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP, (char*)&req, sizeof(req)))
		return -1;

	struct sockaddr_in saddr;
	memset(&saddr, 0, sizeof(saddr));
	saddr.sin_family = AF_INET;
	saddr.sin_addr.s_addr = INADDR_ANY;
	if (port)
		saddr.sin_port = htons((unsigned short)port);
#ifdef __APPLE__
	saddr.sin_len = sizeof(saddr);
#endif

	if (bind(sock, (struct sockaddr*)&saddr, sizeof(saddr)))
		return -1;

#ifdef _WIN32
	unsigned long param = 1;
	ioctlsocket(sock, FIONBIO, &param);
#else
	const int flags = fcntl(sock, F_GETFL, 0);
	fcntl(sock, F_SETFL, flags | O_NONBLOCK);
#endif

	return 0;
}

static int
mdns_socket_open_ipv6(int port) {
	int sock = (int)socket(AF_INET6, SOCK_DGRAM, IPPROTO_UDP);
	if (sock < 0)
		return -1;
	if (mdns_socket_setup_ipv6(sock, port)) {
		mdns_socket_close(sock);
		return -1;
	}
	return sock;
}

static int
mdns_socket_setup_ipv6(int sock, int port) {
	int hops = 1;
	unsigned int loopback = 1;
	unsigned int reuseaddr = 1;
	struct ipv6_mreq req;

	setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, (const char*)&reuseaddr, sizeof(reuseaddr));
#ifdef SO_REUSEPORT
	setsockopt(sock, SOL_SOCKET, SO_REUSEPORT, (const char*)&reuseaddr, sizeof(reuseaddr));
#endif
	setsockopt(sock, IPPROTO_IPV6, IPV6_MULTICAST_HOPS, (const char*)&hops, sizeof(hops));
	setsockopt(sock, IPPROTO_IPV6, IPV6_MULTICAST_LOOP, (const char*)&loopback, sizeof(loopback));

	memset(&req, 0, sizeof(req));
	req.ipv6mr_multiaddr.s6_addr[0] = 0xFF;
	req.ipv6mr_multiaddr.s6_addr[1] = 0x02;
	req.ipv6mr_multiaddr.s6_addr[15] = 0xFB;
	if (setsockopt(sock, IPPROTO_IPV6, IPV6_JOIN_GROUP, (char*)&req, sizeof(req)))
		return -1;

	struct sockaddr_in6 saddr;
	memset(&saddr, 0, sizeof(saddr));
	saddr.sin6_family = AF_INET6;
	saddr.sin6_addr = in6addr_any;
	if (port)
		saddr.sin6_port = htons((unsigned short)port);
#ifdef __APPLE__
	saddr.sin6_len = sizeof(saddr);
#endif

	if (bind(sock, (struct sockaddr*)&saddr, sizeof(saddr)))
		return -1;

#ifdef _WIN32
	unsigned long param = 1;
	ioctlsocket(sock, FIONBIO, &param);
#else
	const int flags = fcntl(sock, F_GETFL, 0);
	fcntl(sock, F_SETFL, flags | O_NONBLOCK);
#endif

	return 0;
}

static void
mdns_socket_close(int sock) {
#ifdef _WIN32
	closesocket(sock);
#else
	close(sock);
#endif
}

static int
mdns_is_string_ref(uint8_t val) {
	return (0xC0 == (val & 0xC0));
}

static mdns_string_pair_t
mdns_get_next_substring(const void* rawdata, size_t size, size_t offset) {
	const uint8_t* buffer = (const uint8_t*)rawdata;
	mdns_string_pair_t pair = {MDNS_INVALID_POS, 0, 0};
	if (!buffer[offset]) {
		pair.offset = offset;
		return pair;
	}
	if (mdns_is_string_ref(buffer[offset])) {
		if (size < offset + 2)
			return pair;

		offset = 0x3fff & ntohs(*(uint16_t*)MDNS_POINTER_OFFSET(buffer, offset));
		if (offset >= size)
			return pair;

		pair.ref = 1;
	}

	size_t length = (size_t)buffer[offset++];
	if (size < offset + length)
		return pair;

	pair.offset = offset;
	pair.length = length;

	return pair;
}

static int
mdns_string_skip(const void* buffer, size_t size, size_t* offset) {
	size_t cur = *offset;
	mdns_string_pair_t substr;
	do {
		substr = mdns_get_next_substring(buffer, size, cur);
		if (substr.offset == MDNS_INVALID_POS)
			return 0;
		if (substr.ref) {
			*offset = cur + 2;
			return 1;
		}
		cur = substr.offset + substr.length;
	}
	while (substr.length);

	*offset = cur + 1;
	return 1;
}

static int
mdns_string_equal(const void* buffer_lhs, size_t size_lhs, size_t* ofs_lhs,
                  const void* buffer_rhs, size_t size_rhs, size_t* ofs_rhs) {
	size_t lhs_cur = *ofs_lhs;
	size_t rhs_cur = *ofs_rhs;
	size_t lhs_end = MDNS_INVALID_POS;
	size_t rhs_end = MDNS_INVALID_POS;
	mdns_string_pair_t lhs_substr;
	mdns_string_pair_t rhs_substr;
	do {
		lhs_substr = mdns_get_next_substring(buffer_lhs, size_lhs, lhs_cur);
		rhs_substr = mdns_get_next_substring(buffer_rhs, size_rhs, rhs_cur);
		if ((lhs_substr.offset == MDNS_INVALID_POS) || (rhs_substr.offset == MDNS_INVALID_POS))
			return 0;
		if (lhs_substr.length != rhs_substr.length)
			return 0;
		if (strncasecmp((const char*)buffer_rhs + rhs_substr.offset,
		                (const char*)buffer_lhs + lhs_substr.offset, rhs_substr.length))
			return 0;
		if (lhs_substr.ref && (lhs_end == MDNS_INVALID_POS))
			lhs_end = lhs_cur + 2;
		if (rhs_substr.ref && (rhs_end == MDNS_INVALID_POS))
			rhs_end = rhs_cur + 2;
		lhs_cur = lhs_substr.offset + lhs_substr.length;
		rhs_cur = rhs_substr.offset + rhs_substr.length;
	}
	while (lhs_substr.length);

	if (lhs_end == MDNS_INVALID_POS)
		lhs_end = lhs_cur + 1;
	*ofs_lhs = lhs_end;

	if (rhs_end == MDNS_INVALID_POS)
		rhs_end = rhs_cur + 1;
	*ofs_rhs = rhs_end;

	return 1;
}

static mdns_string_t
mdns_string_extract(const void* buffer, size_t size, size_t* offset,
                    char* str, size_t capacity) {
	size_t cur = *offset;
	size_t end = MDNS_INVALID_POS;
	mdns_string_pair_t substr;
	mdns_string_t result = {str, 0};
	char* dst = str;
	size_t remain = capacity;
	do {
		substr = mdns_get_next_substring(buffer, size, cur);
		if (substr.offset == MDNS_INVALID_POS)
			return result;
		if (substr.ref && (end == MDNS_INVALID_POS))
			end = cur + 2;
		if (substr.length) {
			size_t to_copy = (substr.length < remain) ? substr.length : remain;
			memcpy(dst, (const char*)buffer + substr.offset, to_copy);
			dst += to_copy;
			remain -= to_copy;
			if (remain) {
				*dst++ = '.';
				--remain;
			}
		}
		cur = substr.offset + substr.length;
	}
	while (substr.length);

	if (end == MDNS_INVALID_POS)
		end = cur + 1;
	*offset = end;

	result.length = capacity - remain;
	return result;
}

static size_t
mdns_string_find(const char* str, size_t length, char c, size_t offset) {
	const void* found;
	if (offset >= length)
		return MDNS_INVALID_POS;
	found = memchr(str + offset, c, length - offset);
	if (found)
		return (size_t)((const char*)found - str);
	return MDNS_INVALID_POS;
}

static void*
mdns_string_make(void* data, size_t capacity, const char* name, size_t length) {
	size_t pos = 0;
	size_t last_pos = 0;
	size_t remain = capacity;
	unsigned char* dest = (unsigned char*)data;
	while ((last_pos < length) && ((pos = mdns_string_find(name, length, '.', last_pos)) != MDNS_INVALID_POS)) {
		size_t sublength = pos - last_pos;
		if (sublength < remain) {
			*dest = (unsigned char)sublength;
			memcpy(dest + 1, name + last_pos, sublength);
			dest += sublength + 1;
			remain -= sublength + 1;
		}
		else {
			return 0;
		}
		last_pos = pos + 1;
	}
	if (last_pos < length) {
		size_t sublength = length - last_pos;
		if (sublength < remain) {
			*dest = (unsigned char)sublength;
			memcpy(dest + 1, name + last_pos, sublength);
			dest += sublength + 1;
			remain -= sublength + 1;
		}
		else {
			return 0;
		}
	}
	if (!remain)
		return 0;
	*dest++ = 0;
	return dest;
}

static void*
mdns_string_make_ref(void* data, size_t capacity, size_t ref_offset) {
	if (capacity < 2)
		return 0;
	uint16_t* udata = (uint16_t*)data;
	*udata++ = htons(0xC000 | (uint16_t)ref_offset);
	return udata;
}

static void*
mdns_string_make_with_ref(void* data, size_t capacity, const char* name, size_t length, size_t ref_offset) {
	void* remaindata = mdns_string_make(data, capacity, name, length);
	capacity -= MDNS_POINTER_DIFF(remaindata, data);
	if (!data || !capacity)
		return 0;
	return mdns_string_make_ref(MDNS_POINTER_OFFSET(remaindata, -1), capacity + 1, ref_offset);
}

static size_t
mdns_records_parse(int sock, const struct sockaddr* from, size_t addrlen, const void* buffer, size_t size,
                   size_t* offset, mdns_entry_type_t type, uint16_t transaction_id, size_t records,
                   mdns_record_callback_fn callback, void* user_data) {
	size_t parsed = 0;
	int do_callback = (callback ? 1 : 0);
	for (size_t i = 0; i < records; ++i) {
		mdns_string_skip(buffer, size, offset);
		const uint16_t* data = (const uint16_t*)((const char*)buffer + (*offset));

		uint16_t rtype = ntohs(*data++);
		uint16_t rclass = ntohs(*data++);
		uint32_t ttl = ntohl(*(const uint32_t*)(const void*)data); data += 2;
		uint16_t length = ntohs(*data++);

		*offset += 10;

		if (do_callback) {
			++parsed;
			if (callback(sock, from, addrlen, type, transaction_id, rtype, rclass, ttl,
			             buffer, size, *offset, length, user_data))
				do_callback = 0;
		}

		*offset += length;
	}
	return parsed;
}

static int
mdns_unicast_send(int sock, const void* address, size_t address_size, const void* buffer, size_t size) {
	if (sendto(sock, (const char*)buffer, (mdns_size_t)size, 0, (const struct sockaddr*)address, (socklen_t)address_size) < 0)
		return -1;
	return 0;
}

static int
mdns_multicast_send(int sock, const void* buffer, size_t size) {
	struct sockaddr_storage addr_storage;
	struct sockaddr_in addr;
	struct sockaddr_in6 addr6;
	struct sockaddr* saddr = (struct sockaddr*)&addr_storage;
	socklen_t saddrlen = sizeof(struct sockaddr_storage);
	if (getsockname(sock, saddr, &saddrlen))
		return -1;
	if (saddr->sa_family == AF_INET6) {
		memset(&addr6, 0, sizeof(struct sockaddr_in6));
		addr6.sin6_family = AF_INET6;
#ifdef __APPLE__
		addr6.sin6_len = sizeof(struct sockaddr_in6);
#endif
		addr6.sin6_addr.s6_addr[0] = 0xFF;
		addr6.sin6_addr.s6_addr[1] = 0x02;
		addr6.sin6_addr.s6_addr[15] = 0xFB;
		addr6.sin6_port = htons((unsigned short)MDNS_PORT);
		saddr = (struct sockaddr*)&addr6;
		saddrlen = sizeof(struct sockaddr_in6);
	}
	else {
		memset(&addr, 0, sizeof(struct sockaddr_in));
		addr.sin_family = AF_INET;
#ifdef __APPLE__
		addr.sin_len = sizeof(struct sockaddr_in);
#endif
		addr.sin_addr.s_addr = htonl((((uint32_t)224U) << 24U) | ((uint32_t)251U));
		addr.sin_port = htons((unsigned short)MDNS_PORT);
		saddr = (struct sockaddr*)&addr;
		saddrlen = sizeof(struct sockaddr_in);
	}

	if (sendto(sock, (const char*)buffer, (mdns_size_t)size, 0, saddr, saddrlen) < 0)
		return -1;
	return 0;
}

static const uint8_t mdns_services_query[] = {
	// Transaction ID
	0x00, 0x00,
	// Flags
	0x00, 0x00,
	// 1 question
	0x00, 0x01,
	// No answer, authority or additional RRs
	0x00, 0x00,
	0x00, 0x00,
	0x00, 0x00,
	// _services._dns-sd._udp.local.
	0x09, '_', 's', 'e', 'r', 'v', 'i', 'c', 'e', 's',
	0x07, '_', 'd', 'n', 's', '-', 's', 'd',
	0x04, '_', 'u', 'd', 'p',
	0x05, 'l', 'o', 'c', 'a', 'l',
	0x00,
	// PTR record
	0x00, MDNS_RECORDTYPE_PTR,
	// QU (unicast response) and class IN
	0x80, MDNS_CLASS_IN
};

static int
mdns_discovery_send(int sock) {
	return mdns_multicast_send(sock, mdns_services_query, sizeof(mdns_services_query));
}

static size_t
mdns_discovery_recv(int sock, void* buffer, size_t capacity,
                    mdns_record_callback_fn callback, void* user_data) {
	struct sockaddr_in6 addr;
	struct sockaddr* saddr = (struct sockaddr*)&addr;
	memset(&addr, 0, sizeof(addr));
	saddr->sa_family = AF_INET;
#ifdef __APPLE__
	saddr->sa_len = sizeof(addr);
#endif
	socklen_t addrlen = sizeof(addr);
	int ret = recvfrom(sock, (char*)buffer, (mdns_size_t)capacity, 0, saddr, &addrlen);
	if (ret <= 0)
		return 0;

	size_t data_size = (size_t)ret;
	size_t records = 0;
	uint16_t* data = (uint16_t*)buffer;

	uint16_t transaction_id = ntohs(*data++);
	uint16_t flags          = ntohs(*data++);
	uint16_t questions      = ntohs(*data++);
	uint16_t answer_rrs     = ntohs(*data++);
	uint16_t authority_rrs  = ntohs(*data++);
	uint16_t additional_rrs = ntohs(*data++);

	if (transaction_id || (flags != 0x8400))
		return 0; //Not a reply to our question

	if (questions != 1)
		return 0;

	int i;
	for (i = 0; i < questions; ++i) {
		size_t ofs = (size_t)((char*)data - (char*)buffer);
		size_t verify_ofs = 12;
		//Verify it's our question, _services._dns-sd._udp.local.
		if (!mdns_string_equal(buffer, data_size, &ofs,
		                       mdns_services_query, sizeof(mdns_services_query), &verify_ofs))
			return 0;
		data = (uint16_t*)((char*)buffer + ofs);

		uint16_t rtype = ntohs(*data++);
		uint16_t rclass = ntohs(*data++);

		//Make sure we get a reply based on our PTR question for class IN
		if ((rtype != MDNS_RECORDTYPE_PTR) || ((rclass & 0x7FFF) != MDNS_CLASS_IN))
			return 0;
	}

	int do_callback = 1;
	for (i = 0; i < answer_rrs; ++i) {
		size_t ofs = (size_t)((char*)data - (char*)buffer);
		size_t verify_ofs = 12;
		//Verify it's an answer to our question, _services._dns-sd._udp.local.
		int is_answer = mdns_string_equal(buffer, data_size, &ofs,
		                                  mdns_services_query, sizeof(mdns_services_query), &verify_ofs);
		data = (uint16_t*)((char*)buffer + ofs);

		uint16_t rtype = ntohs(*data++);
		uint16_t rclass = ntohs(*data++);
		uint32_t ttl = ntohl(*(uint32_t*)(void*)data); data += 2;
		uint16_t length = ntohs(*data++);
		if (length >= (data_size - ofs))
			return 0;

		if (is_answer && do_callback) {
			++records;
			if (callback(sock, saddr, addrlen, MDNS_ENTRYTYPE_ANSWER, transaction_id, rtype, rclass, ttl,
			             buffer, data_size, (size_t)((char*)data - (char*)buffer), length, user_data))
				do_callback = 0;
		}
		data = (uint16_t*)((char*)data + length);
	}

	size_t offset = (size_t)((char*)data - (char*)buffer);
	records += mdns_records_parse(sock, saddr, addrlen, buffer, data_size, &offset,
	                              MDNS_ENTRYTYPE_AUTHORITY, transaction_id, authority_rrs,
	                              callback, user_data);
	records += mdns_records_parse(sock, saddr, addrlen, buffer, data_size, &offset,
	                              MDNS_ENTRYTYPE_ADDITIONAL, transaction_id, additional_rrs,
	                              callback, user_data);

	return records;
}

static size_t
mdns_socket_listen(int sock, void* buffer, size_t capacity,
                   mdns_record_callback_fn callback, void* user_data) {
	struct sockaddr_in6 addr;
	struct sockaddr* saddr = (struct sockaddr*)&addr;
	memset(&addr, 0, sizeof(addr));
	saddr->sa_family = AF_INET;
#ifdef __APPLE__
	saddr->sa_len = sizeof(addr);
#endif
	socklen_t addrlen = sizeof(addr);
	int ret = recvfrom(sock, (char*)buffer, (mdns_size_t)capacity, 0, saddr, &addrlen);
	if (ret <= 0)
		return 0;

	size_t data_size = (size_t)ret;
	size_t records = 0;
	uint16_t* data = (uint16_t*)buffer;

	uint16_t transaction_id = ntohs(*data++);
	uint16_t flags          = ntohs(*data++);
	uint16_t questions      = ntohs(*data++);
	uint16_t answer_rrs     = ntohs(*data++);
	uint16_t authority_rrs  = ntohs(*data++);
	uint16_t additional_rrs = ntohs(*data++);

	size_t parsed = 0;
	for (int iquestion = 0; iquestion < questions; ++iquestion) {
		size_t question_offset = (size_t)((char*)data - (char*)buffer);
		size_t offset = question_offset;
		size_t verify_ofs = 12;
		if (mdns_string_equal(buffer, data_size, &offset,
		                      mdns_services_query, sizeof(mdns_services_query), &verify_ofs)) {
			if (transaction_id || flags || (questions != 1))
				return 0;
		}
		else {
			offset = question_offset;
			if (!mdns_string_skip(buffer, data_size, &offset))
				break;
		}
		size_t length = offset - question_offset;
		data = (uint16_t*)((char*)buffer + offset);

		uint16_t rtype = ntohs(*data++);
		uint16_t rclass = ntohs(*data++);

		//Make sure we get a PTR question of class IN
		if ((rtype != MDNS_RECORDTYPE_PTR) || ((rclass & 0x7FFF) != MDNS_CLASS_IN))
			return 0;

		if (callback)
			callback(sock, saddr, addrlen, MDNS_ENTRYTYPE_QUESTION, transaction_id, rtype, rclass, 0,
			         buffer, data_size, question_offset, length, user_data);

		++parsed;
	}

	return parsed;
}

static int
mdns_discovery_answer(int sock, const void* address, size_t address_size, void* buffer,
                      size_t capacity, const char* record, size_t length) {
	if (capacity < (sizeof(mdns_services_query) + 32 + length))
		return -1;

	uint16_t* data = (uint16_t*)buffer;
	//Basic reply structure
	memcpy(data, mdns_services_query, sizeof(mdns_services_query));
	//Flags
	uint16_t* flags = data + 1;
	*flags = htons(0x8400);
	//One answer
	uint16_t* answers = data + 3;
	*answers = htons(1);

	//Fill in answer PTR record
	data = (uint16_t*)((char*)buffer + sizeof(mdns_services_query));
	//Reference _services._dns-sd._udp.local. string in question
	*data++ = htons(0xC000 | 12);
	//Type
	*data++ = htons(MDNS_RECORDTYPE_PTR);
	//Rclass
	*data++ = htons(MDNS_CLASS_IN);
	//TTL
	*(uint32_t*)data = htonl(10); data += 2;
	//Record string length
	uint16_t* record_length = data++;
	uint8_t* record_data = (uint8_t*)data;
	size_t remain = capacity - (sizeof(mdns_services_query) + 10);
	record_data = (uint8_t*)mdns_string_make(record_data, remain, record, length);
	*record_length = htons((uint16_t)(record_data - (uint8_t*)data));
	*record_data++ = 0;

	ptrdiff_t tosend = (char*)record_data - (char*)buffer;
	return mdns_unicast_send(sock, address, address_size, buffer, (size_t)tosend);
}

static uint16_t mdns_transaction_id = 0;

static int
mdns_query_send(int sock, mdns_record_type_t type, const char* name, size_t length,
                void* buffer, size_t capacity) {
	if (capacity < (17 + length))
		return -1;

	uint16_t* data = (uint16_t*)buffer;
	//Transaction ID
	*data++ = htons(++mdns_transaction_id);
	//Flags
	*data++ = 0;
	//Questions
	*data++ = htons(1);
	//No answer, authority or additional RRs
	*data++ = 0;
	*data++ = 0;
	*data++ = 0;
	//Name string
	data = (uint16_t*)mdns_string_make(data, capacity - 17, name, length);
	if (!data)
		return -1;
	//Record type
	*data++ = htons(type);
	//! Unicast response, class IN
	*data++ = htons(0x8000U | MDNS_CLASS_IN);

	ptrdiff_t tosend = (char*)data - (char*)buffer;
	return mdns_multicast_send(sock, buffer, (size_t)tosend);
}

static size_t
mdns_query_recv(int sock, void* buffer, size_t capacity,
                mdns_record_callback_fn callback, void* user_data,
                int only_last_query) {
	struct sockaddr_in6 addr;
	struct sockaddr* saddr = (struct sockaddr*)&addr;
	memset(&addr, 0, sizeof(addr));
	saddr->sa_family = AF_INET;
#ifdef __APPLE__
	saddr->sa_len = sizeof(addr);
#endif
	socklen_t addrlen = sizeof(addr);
	int ret = recvfrom(sock, (char*)buffer, (mdns_size_t)capacity, 0, saddr, &addrlen);
	if (ret <= 0)
		return 0;

	size_t data_size = (size_t)ret;
	uint16_t* data = (uint16_t*)buffer;

	uint16_t transaction_id = ntohs(*data++);
	++data;// uint16_t flags = ntohs(*data++);
	uint16_t questions      = ntohs(*data++);
	uint16_t answer_rrs     = ntohs(*data++);
	uint16_t authority_rrs  = ntohs(*data++);
	uint16_t additional_rrs = ntohs(*data++);

	if (only_last_query && transaction_id != mdns_transaction_id)// || (flags != 0x8400))
		return 0; //Not a reply to our last question

	if (questions > 1)
		return 0;

	//Skip questions part
	int i;
	for (i = 0; i < questions; ++i) {
		size_t ofs = (size_t)((char*)data - (char*)buffer);
		if (!mdns_string_skip(buffer, data_size, &ofs))
			return 0;
		data = (uint16_t*)((char*)buffer + ofs);
		++data;
		++data;
	}

	size_t records = 0;
	size_t offset = MDNS_POINTER_DIFF(data, buffer);
	records += mdns_records_parse(sock, saddr, addrlen, buffer, data_size, &offset,
	                              MDNS_ENTRYTYPE_ANSWER, transaction_id, answer_rrs,
	                              callback, user_data);
	records += mdns_records_parse(sock, saddr, addrlen, buffer, data_size, &offset,
	                              MDNS_ENTRYTYPE_AUTHORITY, transaction_id, authority_rrs,
	                              callback, user_data);
	records += mdns_records_parse(sock, saddr, addrlen, buffer, data_size, &offset,
	                              MDNS_ENTRYTYPE_ADDITIONAL, transaction_id, additional_rrs,
	                              callback, user_data);
	return records;
}

static int
mdns_query_answer(int sock, const void* address, size_t address_size, void* buffer, size_t capacity,
                  uint16_t transaction_id, const char* service, size_t service_length,
                  const char* hostname, size_t hostname_length, uint32_t ipv4, const uint8_t* ipv6,
                  uint16_t port, const char* txt, size_t txt_length) {
	if (capacity < (sizeof(struct mdns_header_t) + 32 + service_length + hostname_length))
		return -1;

	int use_ipv4 = (ipv4 != 0);
	int use_ipv6 = (ipv6 != 0);
	int use_txt = (txt && txt_length && (txt_length <= 255));

	//Basic answer structure
	struct mdns_header_t* header = (struct mdns_header_t*)buffer;
	header->transaction_id = htons(transaction_id);
	header->flags = htons(0x8400);
	header->questions = htons(1);
	header->answer_rrs = htons(2 + use_ipv4 + use_ipv6 + use_txt);
	header->authority_rrs = 0;
	header->additional_rrs = 0;

	//Fill in question
	void* data = MDNS_POINTER_OFFSET(buffer, sizeof(struct mdns_header_t));
	size_t service_offset = MDNS_POINTER_DIFF(data, buffer);
	size_t remain = capacity - service_offset;
	data = mdns_string_make(data, remain, service, service_length);
	size_t local_offset = MDNS_POINTER_DIFF(data, buffer) - 7;
	remain = capacity - MDNS_POINTER_DIFF(data, buffer);
	if (!data || (remain <= 4))
		return -1;

	uint16_t* udata = (uint16_t*)data;
	*udata++ = htons(MDNS_RECORDTYPE_PTR);
	*udata++ = htons(MDNS_CLASS_IN);
	data = udata;
	remain = capacity - MDNS_POINTER_DIFF(data, buffer);

	//Fill in answers
	//PTR record for service
	data = mdns_string_make_ref(data, remain, service_offset);
	remain = capacity - MDNS_POINTER_DIFF(data, buffer);
	if (!data || (remain <= 10))
		return -1;
	udata = (uint16_t*)data;
	*udata++ = htons(MDNS_RECORDTYPE_PTR); //type
	*udata++ = htons(MDNS_CLASS_IN); //rclass
	*(uint32_t*)udata = htonl(10); udata += 2; //ttl
	uint16_t* record_length = udata++; //length
	data = udata;
	remain = capacity - MDNS_POINTER_DIFF(data, buffer);
	data = mdns_string_make_with_ref(data, remain, hostname, hostname_length, service_offset);
	remain = capacity - MDNS_POINTER_DIFF(data, buffer);
	if (!data || (remain <= 10))
		return -1;
	*record_length = htons((uint16_t)MDNS_POINTER_DIFF(data, record_length + 1));

	//SRV record
	data = mdns_string_make_ref(data, remain, service_offset);
	remain = capacity - MDNS_POINTER_DIFF(data, buffer);
	if (!data || (remain <= 10))
		return -1;
	udata = (uint16_t*)data;
	*udata++ = htons(MDNS_RECORDTYPE_SRV); //type
	*udata++ = htons(MDNS_CLASS_IN); //rclass
	*(uint32_t*)udata = htonl(10); udata += 2; //ttl
	record_length = udata++; //length
	*udata++ = htons(0); //priority
	*udata++ = htons(0); //weight
	*udata++ = htons(port); //port
	//Make a string <hostname>.local.
	data = udata;
	remain = capacity - MDNS_POINTER_DIFF(data, buffer);
	data = mdns_string_make_with_ref(data, remain, hostname, hostname_length, local_offset);
	remain = capacity - MDNS_POINTER_DIFF(data, buffer);
	if (!data || (remain <= 10))
		return -1;
	*record_length = htons((uint16_t)MDNS_POINTER_DIFF(data, record_length + 1));

	//A record
	if (use_ipv4) {
		data = mdns_string_make_ref(data, remain, service_offset);
		remain = capacity - MDNS_POINTER_DIFF(data, buffer);
		if (!data || (remain <= 14))
			return -1;
		udata = (uint16_t*)data;
		*udata++ = htons(MDNS_RECORDTYPE_A); //type
		*udata++ = htons(MDNS_CLASS_IN); //rclass
		*(uint32_t*)udata = htonl(10); udata += 2; //ttl
		*udata++ = htons(4); //length
		*(uint32_t*)udata = ipv4; udata += 2; //ipv4
		data = udata;
		remain = capacity - MDNS_POINTER_DIFF(data, buffer);
	}

	//AAAA record
	if (use_ipv6) {
		data = mdns_string_make_ref(data, remain, service_offset);
		remain = capacity - MDNS_POINTER_DIFF(data, buffer);
		if (!data || (remain <= 26))
			return -1;
		udata = (uint16_t*)data;
		*udata++ = htons(MDNS_RECORDTYPE_AAAA); //type
		*udata++ = htons(MDNS_CLASS_IN); //rclass
		*(uint32_t*)udata = htonl(10); udata += 2; //ttl
		*udata++ = htons(16); //length
		memcpy(udata, ipv6, 16);
		data = MDNS_POINTER_OFFSET(udata, 16);
		remain = capacity - MDNS_POINTER_DIFF(data, buffer);
	}

	//TXT record
	if (use_txt) {
		data = mdns_string_make_ref(data, remain, service_offset);
		remain = capacity - MDNS_POINTER_DIFF(data, buffer);
		if (!data || (remain <= (11 + txt_length)))
			return -1;
		udata = (uint16_t*)data;
		*udata++ = htons(MDNS_RECORDTYPE_TXT); //type
		*udata++ = htons(MDNS_CLASS_IN); //rclass
		*(uint32_t*)udata = htonl(10); udata += 2; //ttl
		*udata++ = htons((unsigned short)(txt_length + 1)); //length
		char* txt_record = (char*)udata;
		*txt_record++ = (char)txt_length;
		memcpy(txt_record, txt, txt_length);
		data = MDNS_POINTER_OFFSET(txt_record, txt_length);
		remain = capacity - MDNS_POINTER_DIFF(data, buffer);
	}

	size_t tosend = MDNS_POINTER_DIFF(data, buffer);
	return mdns_unicast_send(sock, address, address_size, buffer, tosend);
}

static mdns_string_t
mdns_record_parse_ptr(const void* buffer, size_t size, size_t offset, size_t length,
                      char* strbuffer, size_t capacity) {
	//PTR record is just a string
	if ((size >= offset + length) && (length >= 2))
		return mdns_string_extract(buffer, size, &offset, strbuffer, capacity);
	mdns_string_t empty = {0, 0};
	return empty;
}

static mdns_record_srv_t
mdns_record_parse_srv(const void* buffer, size_t size, size_t offset, size_t length,
                      char* strbuffer, size_t capacity) {
	mdns_record_srv_t srv;
	memset(&srv, 0, sizeof(mdns_record_srv_t));
	// Read the priority, weight, port number and the discovery name
	// SRV record format (http://www.ietf.org/rfc/rfc2782.txt):
	// 2 bytes network-order unsigned priority
	// 2 bytes network-order unsigned weight
	// 2 bytes network-order unsigned port
	// string: discovery (domain) name, minimum 2 bytes when compressed
	if ((size >= offset + length) && (length >= 8)) {
		const uint16_t* recorddata = (const uint16_t*)((const char*)buffer + offset);
		srv.priority = ntohs(*recorddata++);
		srv.weight = ntohs(*recorddata++);
		srv.port = ntohs(*recorddata++);
		offset += 6;
		srv.name = mdns_string_extract(buffer, size, &offset, strbuffer, capacity);
	}
	return srv;
}

static struct sockaddr_in*
mdns_record_parse_a(const void* buffer, size_t size, size_t offset, size_t length,
                    struct sockaddr_in* addr) {
	memset(addr, 0, sizeof(struct sockaddr_in));
	addr->sin_family = AF_INET;
#ifdef __APPLE__
	addr->sin_len = sizeof(struct sockaddr_in);
#endif
	if ((size >= offset + length) && (length == 4))
		addr->sin_addr.s_addr = *(const uint32_t*)((const char*)buffer + offset);
	return addr;
}

static struct sockaddr_in6*
mdns_record_parse_aaaa(const void* buffer, size_t size, size_t offset, size_t length,
                       struct sockaddr_in6* addr) {
	memset(addr, 0, sizeof(struct sockaddr_in6));
	addr->sin6_family = AF_INET6;
#ifdef __APPLE__
	addr->sin6_len = sizeof(struct sockaddr_in6);
#endif
	if ((size >= offset + length) && (length == 16))
		addr->sin6_addr = *(const struct in6_addr*)((const char*)buffer + offset);
	return addr;
}

static size_t
mdns_record_parse_txt(const void* buffer, size_t size, size_t offset, size_t length,
                      mdns_record_txt_t* records, size_t capacity) {
	size_t parsed = 0;
	const char* strdata;
	size_t separator, sublength;
	size_t end = offset + length;

	if (size < end)
		end = size;

	while ((offset < end) && (parsed < capacity)) {
		strdata = (const char*)buffer + offset;
		sublength = *(const unsigned char*)strdata;

		++strdata;
		offset += sublength + 1;

		separator = 0;
		for (size_t c = 0; c < sublength; ++c) {
			//DNS-SD TXT record keys MUST be printable US-ASCII, [0x20, 0x7E]
			if ((strdata[c] < 0x20) || (strdata[c] > 0x7E))
				break;
			if (strdata[c] == '=') {
				separator = c;
				break;
			}
		}

		if (!separator)
			continue;

		if (separator < sublength) {
			records[parsed].key.str = strdata;
			records[parsed].key.length = separator;
			records[parsed].value.str = strdata + separator + 1;
			records[parsed].value.length = sublength - (separator + 1);
		}
		else {
			records[parsed].key.str = strdata;
			records[parsed].key.length = sublength;
		}

		++parsed;
	}

	return parsed;
}

#ifdef _WIN32
#undef strncasecmp
#endif

#ifdef __cplusplus
}
#endif