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fix: usr: Prevent spurious SERVFAILs for certain 0-TTL resource records

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Under certain circumstances, BIND 9 can return SERVFAIL when updating
existing entries in the cache with new NS, A, AAAA, or DS records with 0-TTL.

Closes #5294

Merge branch '5294-preserve-ZEROTTL-on-cache-update' into 'main'

See merge request isc-projects/bind9!10897
This commit is contained in:
Ondřej Surý 2025-08-26 21:03:43 +02:00
commit 72189af7bf
1 changed files with 13 additions and 22 deletions
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35
lib/dns/qpcache.c
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@ -2734,29 +2734,22 @@ find_header:
}
/*
* Don't replace existing NS, A and AAAA RRsets in the
* cache if they already exist. This prevents named
* being locked to old servers. Don't lower trust of
* existing record if the update is forced. Nothing
* special to be done w.r.t stale data; it gets replaced
* normally further down.
* Don't replace existing NS in the cache if they already exist
* and replacing the existing one would increase the TTL. This
* prevents named being locked to old servers. Don't lower trust
* of existing record if the update is forced. Nothing special
* to be done w.r.t stale data; it gets replaced normally
* further down.
*/
if (ACTIVE(header, now) &&
top->typepair == DNS_TYPEPAIR(dns_rdatatype_ns) &&
EXISTS(header) && EXISTS(newheader) &&
header->trust >= newheader->trust &&
header->expire < newheader->expire &&
dns_rdataslab_equalx(header, newheader,
qpdb->common.rdclass,
DNS_TYPEPAIR_TYPE(top->typepair)))
{
/*
* Honour the new ttl if it is less than the
* older one.
*/
if (header->expire > newheader->expire) {
setttl(header, newheader->expire);
}
qpcache_hit(qpdb, header);
if (header->noqname == NULL &&
@ -2790,6 +2783,11 @@ find_header:
header->trust <= newheader->trust)
{
if (newheader->expire > header->expire) {
if (ZEROTTL(header)) {
DNS_SLABHEADER_SETATTR(
newheader,
DNS_SLABHEADERATTR_ZEROTTL);
}
newheader->expire = header->expire;
}
}
@ -2801,16 +2799,9 @@ find_header:
top->typepair == DNS_SIGTYPEPAIR(dns_rdatatype_ds)) &&
EXISTS(header) && EXISTS(newheader) &&
header->trust >= newheader->trust &&
header->expire < newheader->expire &&
dns_rdataslab_equal(header, newheader))
{
/*
* Honour the new ttl if it is less than the
* older one.
*/
if (header->expire > newheader->expire) {
setttl(header, newheader->expire);
}
qpcache_hit(qpdb, header);
if (header->noqname == NULL &&