resolved: add reference to negative caching RFC

diff --git a/src/resolve/resolved-dns-cache.c b/src/resolve/resolved-dns-cache.c
index eb51b4b..751e961 100644 (file)
--- a/src/resolve/resolved-dns-cache.c
+++ b/src/resolve/resolved-dns-cache.c
@@ -458,6 +458,10 @@ int dns_cache_put(
                 if (r > 0)
                         continue;
 
+                /* See https://tools.ietf.org/html/rfc2308, which
+                 * say that a matching SOA record in the packet
+                 * is used to to enable negative caching. */
+
                 r = dns_answer_find_soa(answer, q->keys[i], &soa);
                 if (r < 0)
                         goto fail;