diff options
| -rw-r--r-- | src/resolve/resolved-dns-server.c | 16 |
1 files changed, 13 insertions, 3 deletions
diff --git a/src/resolve/resolved-dns-server.c b/src/resolve/resolved-dns-server.c index fbd1c27c14..0de6c8cec0 100644 --- a/src/resolve/resolved-dns-server.c +++ b/src/resolve/resolved-dns-server.c @@ -326,11 +326,21 @@ DnsServerFeatureLevel dns_server_possible_feature_level(DnsServer *s) { log_info("Grace period over, resuming full feature set for DNS server %s", strna(ip)); } else if (s->possible_feature_level <= s->verified_feature_level) s->possible_feature_level = s->verified_feature_level; - else if (s->n_failed_attempts >= DNS_SERVER_FEATURE_RETRY_ATTEMPTS && - s->possible_feature_level > DNS_SERVER_FEATURE_LEVEL_WORST) { + else if (s->n_failed_attempts >= DNS_SERVER_FEATURE_RETRY_ATTEMPTS) { _cleanup_free_ char *ip = NULL; - s->possible_feature_level --; + /* Switch one feature level down. Except when we are at TCP already, in which case we try UDP + * again. Thus, if a DNS server is not responding we'll keep toggling between UDP and TCP until it + * responds on one of them. Note that we generally prefer UDP over TCP (which is why it is at a higher + * feature level), but many DNS servers support lack TCP support. */ + + if (s->possible_feature_level == DNS_SERVER_FEATURE_LEVEL_TCP) + s->possible_feature_level = DNS_SERVER_FEATURE_LEVEL_UDP; + else { + assert(s->possible_feature_level > DNS_SERVER_FEATURE_LEVEL_WORST); + s->possible_feature_level --; + } + s->n_failed_attempts = 0; s->verified_usec = 0; |