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resolved: let's tweak how we calculate TTL left

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When responding from DNS cache, let's slightly tweak how the TTL is
lowered: as before let's round down when converting from our internal µs
to the external seconds. (This is preferable, since records should
better be cached too short instead of too long.) Let's avoid rounding
down to zero though, since that has special semantics in many cases (in
particular mDNS). Let's just use 1s in that case.
This commit is contained in:
Lennart Poettering 2021-03-15 21:18:52 +01:00
parent b974211acb
commit a1acc6e332
1 changed files with 11 additions and 2 deletions
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13
src/resolve/resolved-dns-cache.c
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@ -937,9 +937,18 @@ static int answer_add_clamp_ttl(
assert(rr);
if (FLAGS_SET(query_flags, SD_RESOLVED_CLAMP_TTL)) {
uint32_t left_ttl;
/* Let's determine how much time is left for this cache entry. Note that we round down, but
* clamp this to be 1s at minimum, since we usually want records to remain cached better too
* short a time than too long a time, but otoh don't want to return 0 ever, since that has
* special semantics in various contexts — in particular in mDNS */
left_ttl = MAX(1U, LESS_BY(until, current) / USEC_PER_SEC);
patched = dns_resource_record_ref(rr);
r = dns_resource_record_clamp_ttl(&patched, LESS_BY(until, current) / USEC_PER_SEC);
r = dns_resource_record_clamp_ttl(&patched, left_ttl);
if (r < 0)
return r;
@ -947,7 +956,7 @@ static int answer_add_clamp_ttl(
if (rrsig) {
patched_rrsig = dns_resource_record_ref(rrsig);
r = dns_resource_record_clamp_ttl(&patched_rrsig, LESS_BY(until, current) / USEC_PER_SEC);
r = dns_resource_record_clamp_ttl(&patched_rrsig, left_ttl);
if (r < 0)
return r;