mdns.h

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/* 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
#define MDNS_UNICAST_RESPONSE 0x8000U
#define MDNS_CACHE_FLUSH 0x8000U
 
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 query_id, uint16_t rtype,
                                       uint16_t rclass, uint32_t ttl, const void* data, size_t size,
                                       size_t name_offset, size_t name_length, size_t record_offset,
                                       size_t record_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 query_id;
        uint16_t flags;
        uint16_t questions;
        uint16_t answer_rrs;
        uint16_t authority_rrs;
        uint16_t additional_rrs;
};
 
// mDNS/DNS-SD public API
 
//  pass in the appropriate socket address in saddr, otherwise pass a null pointer for INADDR_ANY.
//  To send one-shot discovery requests and queries pass a null pointer or set 0 as port to assign
//  a random user level ephemeral port. To run discovery service listening for incoming
//  discoveries and queries, you must set MDNS_PORT as port.
static int
mdns_socket_open_ipv4(struct sockaddr_in* saddr);
 
//  pass in the appropriate socket address in saddr, otherwise pass a null pointer for INADDR_ANY.
//  To send one-shot discovery requests and queries pass a null pointer or set 0 as port to assign
//  a random user level ephemeral port. To run discovery service listening for incoming
//  discoveries and queries, you must set MDNS_PORT as port.
static int
mdns_socket_setup_ipv4(int sock, struct sockaddr_in* saddr);
 
//  pass in the appropriate socket address in saddr, otherwise pass a null pointer for in6addr_any.
//  To send one-shot discovery requests and queries pass a null pointer or set 0 as port to assign
//  a random user level ephemeral port. To run discovery service listening for incoming
//  discoveries and queries, you must set MDNS_PORT as port.
static int
mdns_socket_open_ipv6(struct sockaddr_in6* saddr);
 
//  pass in the appropriate socket address in saddr, otherwise pass a null pointer for in6addr_any.
//  To send one-shot discovery requests and queries pass a null pointer or set 0 as port to assign
//  a random user level ephemeral port. To run discovery service listening for incoming
//  discoveries and queries, you must set MDNS_PORT as port.
static int
mdns_socket_setup_ipv6(int sock, struct sockaddr_in6* saddr);
 
static void
mdns_socket_close(int sock);
 
//  opened on port MDNS_PORT using one of the mdns open or setup socket functions. Returns the
//  number of queries  parsed.
static size_t
mdns_socket_listen(int sock, void* buffer, size_t capacity, mdns_record_callback_fn callback,
                   void* user_data);
 
//  0 on success, or <0 if error.
static int
mdns_discovery_send(int sock);
 
//  the given callback for parsing. Returns the number of responses parsed.
static size_t
mdns_discovery_recv(int sock, void* buffer, size_t capacity, mdns_record_callback_fn callback,
                    void* user_data);
 
//  or <0 if error.
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);
 
//  will be used to build the query packet. The query ID can be set to non-zero to filter responses,
//  however the RFC states that the query ID SHOULD be set to 0 for multicast queries. The query
//  will request a unicast response if the socket is bound to an ephemeral port, or a multicast
//  response if the socket is bound to mDNS port 5353.
//  Returns the used query ID, or <0 if error.
static int
mdns_query_send(int sock, mdns_record_type_t type, const char* name, size_t length, void* buffer,
                size_t capacity, uint16_t query_id);
 
//  out any responses not matching the given query ID. Set the query ID to 0 to parse
//  all responses, even if it is not matching the query ID set in a specific query. Any data will
//  be piped to the given callback for parsing. Returns the number of responses parsed.
static size_t
mdns_query_recv(int sock, void* buffer, size_t capacity, mdns_record_callback_fn callback,
                void* user_data, int query_id);
 
//  answer will be sent multicast if address size is 0, otherwise it will be sent unicast to the
//  given address. Use the top bit of the query class field (MDNS_UNICAST_RESPONSE) to determine
//  if the answer should be sent unicast (bit set) or multicast (bit not set).
//  Returns 0 if success, or <0 if error.
static int
mdns_query_answer(int sock, const void* address, size_t address_size, void* buffer, size_t capacity,
                  uint16_t query_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);
 
// Internal functions
 
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(struct sockaddr_in* saddr) {
        int sock = (int)socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
        if (sock < 0)
                return -1;
        if (mdns_socket_setup_ipv4(sock, saddr)) {
                mdns_socket_close(sock);
                return -1;
        }
        return sock;
}
 
static int
mdns_socket_setup_ipv4(int sock, struct sockaddr_in* saddr) {
        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));
        if (saddr)
                req.imr_interface = saddr->sin_addr;
        if (setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP, (char*)&req, sizeof(req)))
                return -1;
 
        struct sockaddr_in sock_addr;
        if (!saddr) {
                saddr = &sock_addr;
                memset(saddr, 0, sizeof(struct sockaddr_in));
                saddr->sin_family = AF_INET;
                saddr->sin_addr.s_addr = INADDR_ANY;
#ifdef __APPLE__
                saddr->sin_len = sizeof(struct sockaddr_in);
#endif
        } else {
                setsockopt(sock, IPPROTO_IP, IP_MULTICAST_IF, (const char*)&saddr->sin_addr,
                           sizeof(saddr->sin_addr));
#ifndef _WIN32
                saddr->sin_addr.s_addr = INADDR_ANY;
#endif
        }
 
        if (bind(sock, (struct sockaddr*)saddr, sizeof(struct sockaddr_in)))
                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(struct sockaddr_in6* saddr) {
        int sock = (int)socket(AF_INET6, SOCK_DGRAM, IPPROTO_UDP);
        if (sock < 0)
                return -1;
        if (mdns_socket_setup_ipv6(sock, saddr)) {
                mdns_socket_close(sock);
                return -1;
        }
        return sock;
}
 
static int
mdns_socket_setup_ipv6(int sock, struct sockaddr_in6* saddr) {
        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 sock_addr;
        if (!saddr) {
                saddr = &sock_addr;
                memset(saddr, 0, sizeof(struct sockaddr_in6));
                saddr->sin6_family = AF_INET6;
                saddr->sin6_addr = in6addr_any;
#ifdef __APPLE__
                saddr->sin6_len = sizeof(struct sockaddr_in6);
#endif
        } else {
                unsigned int ifindex = 0;
                setsockopt(sock, IPPROTO_IPV6, IPV6_MULTICAST_IF, (const char*)&ifindex, sizeof(ifindex));
#ifndef _WIN32
                saddr->sin6_addr = in6addr_any;
#endif
        }
 
        if (bind(sock, (struct sockaddr*)saddr, sizeof(struct sockaddr_in6)))
                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;
        result.str = str;
        result.length = 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 query_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) {
                size_t name_offset = *offset;
                mdns_string_skip(buffer, size, offset);
                size_t name_length = (*offset) - name_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, query_id, rtype, rclass, ttl, buffer, size,
                                     name_offset, name_length, *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(addr6));
                addr6.sin6_family = AF_INET6;
#ifdef __APPLE__
                addr6.sin6_len = sizeof(addr6);
#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(addr6);
        } else {
                memset(&addr, 0, sizeof(addr));
                addr.sin_family = AF_INET;
#ifdef __APPLE__
                addr.sin_len = sizeof(addr);
#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(addr);
        }
 
        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[] = {
    // Query 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;
        socklen_t addrlen = sizeof(addr);
        memset(&addr, 0, sizeof(addr));
#ifdef __APPLE__
        saddr->sa_len = sizeof(addr);
#endif
        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 query_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++);
 
        // According to RFC 6762 the query ID MUST match the sent query ID (which is 0 in our case)
        if (query_id || (flags != 0x8400))
                return 0;  // Not a reply to our question
 
        // It seems some implementations do not fill the correct questions field,
        // so ignore this check for now and only validate answer string
        /*
        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.
                size_t name_offset = ofs;
                int is_answer = mdns_string_equal(buffer, data_size, &ofs, mdns_services_query,
                                                  sizeof(mdns_services_query), &verify_ofs);
                size_t name_length = ofs - name_offset;
                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;
                        ofs = (size_t)((char*)data - (char*)buffer);
                        if (callback(sock, saddr, addrlen, MDNS_ENTRYTYPE_ANSWER, query_id, rtype, rclass, ttl,
                                     buffer, data_size, name_offset, name_length, ofs, 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, query_id, authority_rrs, callback, user_data);
        records += mdns_records_parse(sock, saddr, addrlen, buffer, data_size, &offset,
                                      MDNS_ENTRYTYPE_ADDITIONAL, query_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;
        socklen_t addrlen = sizeof(addr);
        memset(&addr, 0, sizeof(addr));
#ifdef __APPLE__
        saddr->sa_len = sizeof(addr);
#endif
        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 query_id = ntohs(*data++);
        uint16_t flags = ntohs(*data++);
        uint16_t questions = ntohs(*data++);
        /*
        This data is unused at the moment, skip
        uint16_t answer_rrs = ntohs(*data++);
        uint16_t authority_rrs = ntohs(*data++);
        uint16_t additional_rrs = ntohs(*data++);
        */
        data += 3;
 
        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 (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 question of class IN
                if ((rclass & 0x7FFF) != MDNS_CLASS_IN)
                        return 0;
 
                if (callback)
                        callback(sock, saddr, addrlen, MDNS_ENTRYTYPE_QUESTION, query_id, rtype, rclass, 0,
                                 buffer, data_size, question_offset, length, 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(0x8400U);
        // 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(0xC000U | 12U);
        // 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 int
mdns_query_send(int sock, mdns_record_type_t type, const char* name, size_t length, void* buffer,
                size_t capacity, uint16_t query_id) {
        if (capacity < (17 + length))
                return -1;
 
        uint16_t rclass = MDNS_CLASS_IN | MDNS_UNICAST_RESPONSE;
 
        struct sockaddr_storage addr_storage;
        struct sockaddr* saddr = (struct sockaddr*)&addr_storage;
        socklen_t saddrlen = sizeof(addr_storage);
        if (getsockname(sock, saddr, &saddrlen) == 0) {
                if ((saddr->sa_family == AF_INET) &&
                    (ntohs(((struct sockaddr_in*)saddr)->sin_port) == MDNS_PORT))
                        rclass &= ~MDNS_UNICAST_RESPONSE;
                else if ((saddr->sa_family == AF_INET6) &&
                         (ntohs(((struct sockaddr_in6*)saddr)->sin6_port) == MDNS_PORT))
                        rclass &= ~MDNS_UNICAST_RESPONSE;
        }
 
        uint16_t* data = (uint16_t*)buffer;
        // Query ID
        *data++ = htons(query_id);
        // Flags
        *data++ = 0;
        // Questions
        *data++ = htons(1);
        // No answer, authority or additional RRs
        *data++ = 0;
        *data++ = 0;
        *data++ = 0;
        // Fill in question
        // Name string
        data = (uint16_t*)mdns_string_make(data, capacity - 17, name, length);
        if (!data)
                return -1;
        // Record type
        *data++ = htons(type);
        *data++ = htons(rclass);
 
        ptrdiff_t tosend = (char*)data - (char*)buffer;
        if (mdns_multicast_send(sock, buffer, (size_t)tosend))
                return -1;
        return query_id;
}
 
static size_t
mdns_query_recv(int sock, void* buffer, size_t capacity, mdns_record_callback_fn callback,
                void* user_data, int only_query_id) {
        struct sockaddr_in6 addr;
        struct sockaddr* saddr = (struct sockaddr*)&addr;
        socklen_t addrlen = sizeof(addr);
        memset(&addr, 0, sizeof(addr));
#ifdef __APPLE__
        saddr->sa_len = sizeof(addr);
#endif
        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 query_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++);
        (void)sizeof(flags);
 
        if ((only_query_id > 0) && (query_id != only_query_id))
                return 0;  // Not a reply to the wanted one-shot query
 
        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);
                uint16_t rtype = ntohs(*data++);
                uint16_t rclass = ntohs(*data++);
                (void)sizeof(rtype);
                (void)sizeof(rclass);
        }
 
        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, query_id, answer_rrs, callback, user_data);
        records +=
            mdns_records_parse(sock, saddr, addrlen, buffer, data_size, &offset,
                               MDNS_ENTRYTYPE_AUTHORITY, query_id, authority_rrs, callback, user_data);
        records += mdns_records_parse(sock, saddr, addrlen, buffer, data_size, &offset,
                                      MDNS_ENTRYTYPE_ADDITIONAL, query_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 query_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 unicast = (address_size ? 1 : 0);
        int use_ipv4 = (ipv4 != 0);
        int use_ipv6 = (ipv6 != 0);
        int use_txt = (txt && txt_length && (txt_length <= 255));
 
        uint16_t question_rclass = (unicast ? MDNS_UNICAST_RESPONSE : 0) | MDNS_CLASS_IN;
        uint16_t rclass = (unicast ? MDNS_CACHE_FLUSH : 0) | MDNS_CLASS_IN;
        uint32_t ttl = (unicast ? 10 : 60);
        uint32_t a_ttl = ttl;
 
        // Basic answer structure
        struct mdns_header_t* header = (struct mdns_header_t*)buffer;
        header->query_id = (address_size ? htons(query_id) : 0);
        header->flags = htons(0x8400);
        header->questions = htons(unicast ? 1 : 0);
        header->answer_rrs = htons(1);
        header->authority_rrs = 0;
        header->additional_rrs = htons((unsigned short)(1 + use_ipv4 + use_ipv6 + use_txt));
 
        void* data = MDNS_POINTER_OFFSET(buffer, sizeof(struct mdns_header_t));
        uint16_t* udata;
        size_t remain, service_offset = 0, local_offset = 0, full_offset, host_offset;
 
        // Fill in question if unicast
        if (unicast) {
                service_offset = MDNS_POINTER_DIFF(data, buffer);
                remain = capacity - service_offset;
                data = mdns_string_make(data, remain, service, service_length);
                local_offset = MDNS_POINTER_DIFF(data, buffer) - 7;
                remain = capacity - MDNS_POINTER_DIFF(data, buffer);
                if (!data || (remain <= 4))
                        return -1;
 
                udata = (uint16_t*)data;
                *udata++ = htons(MDNS_RECORDTYPE_PTR);
                *udata++ = htons(question_rclass);
                data = udata;
        }
        remain = capacity - MDNS_POINTER_DIFF(data, buffer);
 
        // Fill in answers
        // PTR record for service
        if (unicast) {
                data = mdns_string_make_ref(data, remain, service_offset);
        } else {
                service_offset = MDNS_POINTER_DIFF(data, buffer);
                remain = capacity - service_offset;
                data = mdns_string_make(data, remain, service, service_length);
                local_offset = MDNS_POINTER_DIFF(data, buffer) - 7;
        }
        remain = capacity - MDNS_POINTER_DIFF(data, buffer);
        if (!data || (remain <= 10))
                return -1;
        udata = (uint16_t*)data;
        *udata++ = htons(MDNS_RECORDTYPE_PTR);
        *udata++ = htons(rclass);
        *(uint32_t*)udata = htonl(ttl);
        udata += 2;
        uint16_t* record_length = udata++;  // length
        data = udata;
        // Make a string <hostname>.<service>.local.
        full_offset = MDNS_POINTER_DIFF(data, buffer);
        remain = capacity - full_offset;
        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));
 
        // Fill in additional records
        // SRV record for <hostname>.<service>.local.
        data = mdns_string_make_ref(data, remain, full_offset);
        remain = capacity - MDNS_POINTER_DIFF(data, buffer);
        if (!data || (remain <= 10))
                return -1;
        udata = (uint16_t*)data;
        *udata++ = htons(MDNS_RECORDTYPE_SRV);
        *udata++ = htons(rclass);
        *(uint32_t*)udata = htonl(ttl);
        udata += 2;
        record_length = udata++;  // length
        *udata++ = htons(0);      // priority
        *udata++ = htons(0);      // weight
        *udata++ = htons(port);   // port
        // Make a string <hostname>.local.
        data = udata;
        host_offset = MDNS_POINTER_DIFF(data, buffer);
        remain = capacity - host_offset;
        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 for <hostname>.local.
        if (use_ipv4) {
                data = mdns_string_make_ref(data, remain, host_offset);
                remain = capacity - MDNS_POINTER_DIFF(data, buffer);
                if (!data || (remain <= 14))
                        return -1;
                udata = (uint16_t*)data;
                *udata++ = htons(MDNS_RECORDTYPE_A);
                *udata++ = htons(rclass);
                *(uint32_t*)udata = htonl(a_ttl);
                udata += 2;
                *udata++ = htons(4);       // length
                *(uint32_t*)udata = ipv4;  // ipv4 address
                udata += 2;
                data = udata;
                remain = capacity - MDNS_POINTER_DIFF(data, buffer);
        }
 
        // AAAA record for <hostname>.local.
        if (use_ipv6) {
                data = mdns_string_make_ref(data, remain, host_offset);
                remain = capacity - MDNS_POINTER_DIFF(data, buffer);
                if (!data || (remain <= 26))
                        return -1;
                udata = (uint16_t*)data;
                *udata++ = htons(MDNS_RECORDTYPE_AAAA);
                *udata++ = htons(rclass);
                *(uint32_t*)udata = htonl(a_ttl);
                udata += 2;
                *udata++ = htons(16);     // length
                memcpy(udata, ipv6, 16);  // ipv6 address
                data = MDNS_POINTER_OFFSET(udata, 16);
                remain = capacity - MDNS_POINTER_DIFF(data, buffer);
        }
 
        // TXT record for <hostname>.<service>.local.
        if (use_txt) {
                data = mdns_string_make_ref(data, remain, full_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);
                *udata++ = htons(rclass);
                *(uint32_t*)udata = htonl(ttl);
                udata += 2;
                *udata++ = htons((unsigned short)(txt_length + 1));  // length
                char* txt_record = (char*)udata;
                *txt_record++ = (char)txt_length;
                memcpy(txt_record, txt, txt_length);  // txt record
                data = MDNS_POINTER_OFFSET(txt_record, txt_length);
                // Unused until multiple txt records are supported
                // remain = capacity - MDNS_POINTER_DIFF(data, buffer);
        }
 
        size_t tosend = MDNS_POINTER_DIFF(data, buffer);
        if (address_size)
                return mdns_unicast_send(sock, address, address_size, buffer, tosend);
        return mdns_multicast_send(sock, 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