resolved: let's tweak how we calculate TTL left

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.

diff --git a/src/resolve/resolved-dns-cache.c b/src/resolve/resolved-dns-cache.c
index db2361ae36375d2883ad95fc30c83e693121b8f7..9b2e7115c0a247634f8d59622c8a557694528f07 100644 (file)
--- a/src/resolve/resolved-dns-cache.c
+++ b/src/resolve/resolved-dns-cache.c
@@ -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;