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RFC 6762 section 7.1: a Multicast DNS querier SHOULD NOT include records in the
Known-Answer list whose remaining TTL is less than half of their original TTL
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@ -1245,11 +1245,14 @@ int dns_cache_check_conflicts(DnsCache *cache, DnsResourceRecord *rr, int owner_
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int dns_cache_export_shared_to_packet(DnsCache *cache, DnsPacket *p) {
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unsigned ancount = 0;
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DnsCacheItem *i;
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usec_t t;
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int r;
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assert(cache);
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assert(p);
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t = now(CLOCK_BOOTTIME);
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HASHMAP_FOREACH(i, cache->by_key)
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LIST_FOREACH(by_key, j, i) {
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if (!j->rr)
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@ -1258,6 +1261,11 @@ int dns_cache_export_shared_to_packet(DnsCache *cache, DnsPacket *p) {
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if (!j->shared_owner)
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continue;
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/* RFC6762 7.1: Don't append records with less than half the TTL remaining
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* as known answers. */
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if (usec_sub_unsigned(j->until, t) < j->rr->ttl * USEC_PER_SEC / 2)
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continue;
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r = dns_packet_append_rr(p, j->rr, 0, NULL, NULL);
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if (r == -EMSGSIZE && p->protocol == DNS_PROTOCOL_MDNS) {
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/* For mDNS, if we're unable to stuff all known answers into the given packet,
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