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feat(mdns): Add support for negative answers (a/aaaa types)
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MAJOR CHANGE: According to RFC6762-6.1, mdns must provide NSEC record in the addtional fields, if the host exclusively owns the queried name, but has no answer for the queried type.

Closes espressif#258
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@david-cermak
david-cermak committed Apr 14, 2023
1 parent 8e5a27f commit 4dcb6ca
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Showing 3 changed files with 83 additions and 8 deletions.
81 changes: 78 additions & 3 deletions components/mdns/mdns.c
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Original file line number Diff line number Diff line change
Expand Up @@ -38,6 +38,8 @@ void mdns_debug_packet(const uint8_t *data, size_t len);
// since the ip6_addr_t is defined in lwip and depends on using IPv6 zones
#define _MDNS_SIZEOF_IP6_ADDR (MDNS_ANSWER_AAAA_SIZE)

#define NEGATIVE_ANSWER_BITMAP (0x80000000)

static const char *MDNS_DEFAULT_DOMAIN = "local";
static const char *MDNS_SUB_STR = "_sub";

Expand Down Expand Up @@ -574,6 +576,9 @@ static inline uint8_t _mdns_append_type(uint8_t *packet, uint16_t *index, uint8_
} else if (type == MDNS_ANSWER_AAAA) {
_mdns_append_u16(packet, index, MDNS_TYPE_AAAA);
_mdns_append_u16(packet, index, mdns_class);
} else if (type == MDNS_ANSWER_NSEC) {
_mdns_append_u16(packet, index, MDNS_TYPE_NSEC);
_mdns_append_u16(packet, index, mdns_class);
} else {
return 0;
}
Expand Down Expand Up @@ -1054,6 +1059,60 @@ static uint16_t _mdns_append_srv_record(uint8_t *packet, uint16_t *index, mdns_s
return record_length;
}

static uint16_t _mdns_append_nsec_record(uint8_t *packet, uint16_t *index, uint32_t self_host_queries_answered)
{
const char *str[] = { _mdns_self_host.hostname, MDNS_DEFAULT_DOMAIN };
uint16_t record_length;
uint8_t part_length;

if (_str_null_or_empty(str[0])) {
return 0;
}

part_length = _mdns_append_fqdn(packet, index, str, 2, MDNS_MAX_PACKET_SIZE);
if (!part_length) {
return 0;
}
record_length += part_length;

part_length = _mdns_append_type(packet, index, MDNS_ANSWER_NSEC, false, MDNS_ANSWER_A_TTL);
if (!part_length) {
return 0;
}
record_length += part_length;

uint16_t data_len_location = *index - 2;

part_length = _mdns_append_fqdn(packet, index, str, 2, MDNS_MAX_PACKET_SIZE);
if (!part_length) {
return 0;
}
record_length += part_length;
if (!_mdns_append_u8(packet, index, 0)) {
return 0;
}
uint8_t bitmap_size_location = *index;
*index += 1;
uint8_t bitmap_size = 0;
for (int i = sizeof(self_host_queries_answered) - 1; i >= 0; --i) {
uint8_t bitmap = 0xFF & (self_host_queries_answered >> (i * 8));
if (bitmap == 0) {
break;
}
if (!_mdns_append_u8(packet, index, bitmap)) {
return 0;
}
bitmap_size++;
}
packet[bitmap_size_location] = bitmap_size;
part_length += 2 + bitmap_size;

_mdns_set_u16(packet, data_len_location, part_length);

return 1;
}


/**
* @brief appends A record to a packet, incrementing the index
*
Expand Down Expand Up @@ -1451,10 +1510,19 @@ static void _mdns_dispatch_tx_packet(mdns_tx_packet_t *p)
}
_mdns_set_u16(packet, MDNS_HEAD_QUESTIONS_OFFSET, count);

uint32_t self_host_queries_not_answered = p->self_host_queries;
uint32_t self_host_queries_answered = 0;
count = 0;
a = p->answers;
while (a) {
count += _mdns_append_answer(packet, &index, a, p->tcpip_if);
uint8_t nr_of_answers = _mdns_append_answer(packet, &index, a, p->tcpip_if);
count += nr_of_answers;
// Got a self-hosted answer? -> reset the query bit for potential negative answer
if (nr_of_answers > 0 && a->host == &_mdns_self_host &&
(a->type == MDNS_TYPE_A || a->type == MDNS_TYPE_AAAA)) {
self_host_queries_not_answered &= ~(NEGATIVE_ANSWER_BITMAP >> a->type);
self_host_queries_answered |= NEGATIVE_ANSWER_BITMAP >> a->type;
}
a = a->next;
}
_mdns_set_u16(packet, MDNS_HEAD_ANSWERS_OFFSET, count);
Expand All @@ -1473,6 +1541,9 @@ static void _mdns_dispatch_tx_packet(mdns_tx_packet_t *p)
count += _mdns_append_answer(packet, &index, a, p->tcpip_if);
a = a->next;
}
if (self_host_queries_not_answered) { // if some self-host queries not answered, append negative reply
count += _mdns_append_nsec_record(packet, &index, self_host_queries_answered);
}
_mdns_set_u16(packet, MDNS_HEAD_ADDITIONAL_OFFSET, count);

#ifdef MDNS_ENABLE_DEBUG
Expand Down Expand Up @@ -1752,13 +1823,17 @@ static bool _mdns_create_answer_from_service(mdns_tx_packet_t *packet, mdns_serv
return true;
}

static bool _mdns_create_answer_from_hostname(mdns_tx_packet_t *packet, const char *hostname, bool send_flush)
static bool _mdns_create_answer_from_hostname(mdns_tx_packet_t *packet, const char *hostname, uint16_t type, bool send_flush)
{
mdns_host_item_t *host = mdns_get_host_item(hostname);
if (!_mdns_alloc_answer(&packet->answers, MDNS_TYPE_A, NULL, host, send_flush, false) ||
!_mdns_alloc_answer(&packet->answers, MDNS_TYPE_AAAA, NULL, host, send_flush, false)) {
return false;
}
if (host == &_mdns_self_host) {
packet->self_host_queries |= NEGATIVE_ANSWER_BITMAP >> type;
}

return true;
}

Expand Down Expand Up @@ -1827,7 +1902,7 @@ static void _mdns_create_answer_from_parsed_packet(mdns_parsed_packet_t *parsed_
service = service->next;
}
} else if (q->type == MDNS_TYPE_A || q->type == MDNS_TYPE_AAAA) {
if (!_mdns_create_answer_from_hostname(packet, q->host, send_flush)) {
if (!_mdns_create_answer_from_hostname(packet, q->host, q->type, send_flush)) {
_mdns_free_tx_packet(packet);
return;
}
Expand Down
9 changes: 4 additions & 5 deletions components/mdns/mdns_networking_socket.c
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Original file line number Diff line number Diff line change
Expand Up @@ -347,13 +347,12 @@ static bool create_pcb(mdns_if_t tcpip_if, mdns_ip_protocol_t ip_protocol)
int sock = s_interfaces[tcpip_if].sock;
esp_netif_t *netif = _mdns_get_esp_netif(tcpip_if);
if (sock >= 0) {
mdns_ip_protocol_t other_ip_proto = ip_protocol == MDNS_IP_PROTOCOL_V4 ? MDNS_IP_PROTOCOL_V6 : MDNS_IP_PROTOCOL_V4;
int err = join_mdns_multicast_group(sock, netif, other_ip_proto);
int err = join_mdns_multicast_group(sock, netif, ip_protocol);
if (err < 0) {
ESP_LOGE(TAG, "Failed to add ipv6 multicast group for protocol %d", ip_protocol);
ESP_LOGE(TAG, "Failed to add missing multicast group for protocol %d", ip_protocol);
return false;
}
s_interfaces[tcpip_if].proto |= (other_ip_proto == MDNS_IP_PROTOCOL_V4 ? PROTO_IPV4 : PROTO_IPV6);
s_interfaces[tcpip_if].proto |= (ip_protocol == MDNS_IP_PROTOCOL_V4 ? PROTO_IPV4 : PROTO_IPV6);
return true;
}
sock = create_socket(netif);
Expand All @@ -363,7 +362,7 @@ static bool create_pcb(mdns_if_t tcpip_if, mdns_ip_protocol_t ip_protocol)
}
int err = join_mdns_multicast_group(sock, netif, ip_protocol);
if (err < 0) {
ESP_LOGE(TAG, "Failed to add ipv6 multicast group for protocol %d", ip_protocol);
ESP_LOGE(TAG, "Failed to add first multicast group for protocol %d", ip_protocol);
}
s_interfaces[tcpip_if].proto |= (ip_protocol == MDNS_IP_PROTOCOL_V4 ? PROTO_IPV4 : PROTO_IPV6);
s_interfaces[tcpip_if].sock = sock;
Expand Down
1 change: 1 addition & 0 deletions components/mdns/private_include/mdns_private.h
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Original file line number Diff line number Diff line change
Expand Up @@ -343,6 +343,7 @@ typedef struct mdns_tx_packet_s {
mdns_out_answer_t *additional;
bool queued;
uint16_t id;
uint32_t self_host_queries;
} mdns_tx_packet_t;

typedef struct {
Expand Down

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