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mDNS: conflict resolution added

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If the host observes a response (after probing) containing RR's
that he thought were unique to him, there is a conflict. If a host
observes such conflict, it resets back to probing and the probing
procedures will resolve the conflict. (RFC6762 section 9)
This commit is contained in:
Jasper Verschueren 2018-11-20 09:01:42 +01:00 committed by Dirk Ziegelmeier
parent 3bd84aba4b
commit ee1bab3411
1 changed files with 130 additions and 5 deletions
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135
src/apps/mdns/mdns.c
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@ -1682,13 +1682,18 @@ mdns_probe_conflict(struct netif *netif)
}
/**
* Handle response MDNS packet
* Only prints debug for now. Will need more code to do conflict resolution.
* Handle response MDNS packet:
* - Handle responses on probe query
* - Perform conflict resolution on every packet (RFC6762 section 9)
*
* @param pkt incoming packet
* @param netif network interface on which packet was received
*/
static void
mdns_handle_response(struct mdns_packet *pkt, struct netif *netif)
{
struct mdns_host* mdns = NETIF_TO_HOST(netif);
u16_t total_answers_left;
/* Ignore responses with a source port different from 5353
* (LWIP_IANA_PORT_MDNS) -> RFC6762 section 6 */
@ -1707,12 +1712,13 @@ mdns_handle_response(struct mdns_packet *pkt, struct netif *netif)
return;
}
}
while (pkt->answers_left) {
/* We need to check all resource record sections: answers, authoritative and additional */
total_answers_left = pkt->answers_left + pkt->authoritative_left + pkt->additional_left;
while (total_answers_left) {
struct mdns_answer ans;
err_t res;
res = mdns_read_answer(pkt, &ans, &pkt->answers_left);
res = mdns_read_answer(pkt, &ans, &total_answers_left);
if (res != ERR_OK) {
LWIP_DEBUGF(MDNS_DEBUG, ("MDNS: Failed to parse answer, skipping response packet\n"));
return;
@ -1722,6 +1728,11 @@ mdns_handle_response(struct mdns_packet *pkt, struct netif *netif)
mdns_domain_debug_print(&ans.info.domain);
LWIP_DEBUGF(MDNS_DEBUG, (" type %d class %d\n", ans.info.type, ans.info.klass));
if (ans.info.type == DNS_RRTYPE_ANY || ans.info.klass != DNS_RRCLASS_IN) {
/* Skip answers for ANY type or if class != IN */
continue;
}
/* "Conflicting Multicast DNS responses received *before* the first probe
* packet is sent MUST be silently ignored" so drop answer if we haven't
* started probing yet. */
@ -1751,9 +1762,123 @@ mdns_handle_response(struct mdns_packet *pkt, struct netif *netif)
if (conflict != 0) {
mdns_probe_conflict(netif);
break;
}
}
/* Perform conflict resolution (RFC6762 section 9):
* We assume a conflict if the hostname or service name matches the answers
* domain. Only if the rdata matches exactly we reset our assumption to no
* conflict. As stated in the RFC:
* What may be considered inconsistent is context sensitive, except that
* resource records with identical rdata are never considered inconsistent,
* even if they originate from different hosts.
*/
else if ((mdns->state == MDNS_STATE_ANNOUNCING) ||
(mdns->state == MDNS_STATE_COMPLETE)) {
struct mdns_domain domain;
u8_t i;
u8_t conflict = 0;
/* Evaluate unique hostname records -> A and AAAA */
res = mdns_build_host_domain(&domain, mdns);
if (res == ERR_OK && mdns_domain_eq(&ans.info.domain, &domain)) {
LWIP_DEBUGF(MDNS_DEBUG, ("mDNS: response matches host domain, assuming conflict\n"));
/* This means a conflict has taken place, except when the packet contains
* exactly the same rdata. */
conflict = 1;
/* Evaluate rdata -> to see if it's a copy of our own data */
if (ans.info.type == DNS_RRTYPE_A) {
#if LWIP_IPV4
if (ans.rd_length == sizeof(ip4_addr_t) &&
pbuf_memcmp(pkt->pbuf, ans.rd_offset, netif_ip4_addr(netif), ans.rd_length) == 0) {
LWIP_DEBUGF(MDNS_DEBUG, ("mDNS: response equals our own IPv4 address record -> no conflict\n"));
conflict = 0;
}
#endif
}
else if (ans.info.type == DNS_RRTYPE_AAAA) {
#if LWIP_IPV6
if (ans.rd_length == sizeof(ip6_addr_p_t)) {
for (i = 0; i < LWIP_IPV6_NUM_ADDRESSES; i++) {
if (pbuf_memcmp(pkt->pbuf, ans.rd_offset, netif_ip6_addr(netif, i), ans.rd_length) == 0) {
LWIP_DEBUGF(MDNS_DEBUG, ("mDNS: response equals our own iPv6 address record, num = %d -> no conflict\n",i));
conflict = 0;
}
}
}
#endif
}
}
/* Evaluate unique service name records -> SRV and TXT */
for (i = 0; i < MDNS_MAX_SERVICES; i++) {
struct mdns_service* service = mdns->services[i];
if (!service) {
continue;
}
res = mdns_build_service_domain(&domain, service, 1);
if ((res == ERR_OK) && mdns_domain_eq(&ans.info.domain, &domain)) {
LWIP_DEBUGF(MDNS_DEBUG, ("mDNS: response matches service domain, assuming conflict\n"));
/* This means a conflict has taken place, except when the packet contains
* exactly the same rdata. */
conflict = 1;
/* Evaluate rdata -> to see if it's a copy of our own data */
if (ans.info.type == DNS_RRTYPE_SRV) {
/* Read and compare to with our SRV record */
u16_t field16, len, read_pos;
struct mdns_domain srv_ans, my_ans;
read_pos = ans.rd_offset;
do {
/* Check priority field */
len = pbuf_copy_partial(pkt->pbuf, &field16, sizeof(field16), read_pos);
if (len != sizeof(field16) || lwip_ntohs(field16) != SRV_PRIORITY) {
break;
}
read_pos += len;
/* Check weight field */
len = pbuf_copy_partial(pkt->pbuf, &field16, sizeof(field16), read_pos);
if (len != sizeof(field16) || lwip_ntohs(field16) != SRV_WEIGHT) {
break;
}
read_pos += len;
/* Check port field */
len = pbuf_copy_partial(pkt->pbuf, &field16, sizeof(field16), read_pos);
if (len != sizeof(field16) || lwip_ntohs(field16) != service->port) {
break;
}
read_pos += len;
/* Check host field */
len = mdns_readname(pkt->pbuf, read_pos, &srv_ans);
mdns_build_host_domain(&my_ans, mdns);
if (len == MDNS_READNAME_ERROR || !mdns_domain_eq(&srv_ans, &my_ans)) {
break;
}
LWIP_DEBUGF(MDNS_DEBUG, ("mDNS: response equals our own SRV record -> no conflict\n"));
conflict = 0;
} while (0);
} else if (ans.info.type == DNS_RRTYPE_TXT) {
mdns_prepare_txtdata(service);
if (service->txtdata.length == ans.rd_length &&
pbuf_memcmp(pkt->pbuf, ans.rd_offset, service->txtdata.name, ans.rd_length) == 0) {
LWIP_DEBUGF(MDNS_DEBUG, ("mDNS: response equals our own TXT record -> no conflict\n"));
conflict = 0;
}
}
}
}
if (conflict != 0) {
/* Reset host to probing to reconfirm uniqueness */
LWIP_DEBUGF(MDNS_DEBUG, ("mDNS: Conflict resolution -> reset to probing state\n"));
mdns->state = MDNS_STATE_PROBE_WAIT;
mdns->sent_num = 0;
sys_timeout(MDNS_INITIAL_PROBE_DELAY_MS, mdns_probe_and_announce, netif);
break;
}
}
}
/* Clear all xxx_left variables because we parsed all answers */
pkt->answers_left = 0;
pkt->authoritative_left = 0;
pkt->additional_left = 0;
}
/**