From 6a1a5eec43892dee3ff6e208bceb1931c25c782e Mon Sep 17 00:00:00 2001 From: Lennart Poettering Date: Fri, 8 Jan 2016 02:29:02 +0100 Subject: resolved: when DNS/TCP doesn't work, try DNS/UDP again If we failed to contact a DNS server via TCP, bump of the feature level to UDP again. This way we'll switch back between UDP and TCP if we fail to contact a host. Generally, we prefer UDP over TCP, which is why UDP is a higher feature level. But some servers only support UDP but not TCP hence when reaching the lowest feature level of TCP and want to downgrade from there, pick UDP again. We this keep downgrading until we reach TCP and then we cycle through UDP and TCP. --- src/resolve/resolved-dns-server.c | 16 +++++++++++++--- 1 file changed, 13 insertions(+), 3 deletions(-) (limited to 'src') 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; -- cgit v1.2.3-54-g00ecf