resolved: rework SERVFAIL handling

There might be two reasons why we get a SERVFAIL response from our selected DNS
server: because this DNS server itself is bad, or because the DNS server
actually serving the zone upstream is bad. So far we immediately downgraded our
server feature level when getting SERVFAIL, under the assumption that the first
case is the only possible case. However, this meant we'd downgrade immediately
even if we encountered the second case described above.

With this commit handling of SERVFAIL is reworked. As soon as we get a SERVFAIL
on a transaction we retry the transaction with a lower feature level, without
changing the feature level tracked for the DNS server itself. If that fails
too, we downgrade further, and so on. If during this downgrading the SERVFAIL
goes away we assume that the DNS server we are talking to is bad, but the zone
is fine and propagate the detected feature level to the information we track
about the DNS server. Should the SERVFAIL not go away this way we let the
transaction fail and accept the SERVFAIL.

1 files changed, 1 insertions, 2 deletions

diff --git a/src/resolve/resolved-dns-server.h b/src/resolve/resolved-dns-server.h
index 463c5724a7..03ed85b3b9 100644
--- a/src/resolve/resolved-dns-server.h
+++ b/src/resolve/resolved-dns-server.h
@@ -77,7 +77,6 @@ struct DnsServer {
         unsigned n_failed_udp;
         unsigned n_failed_tcp;
 
-        bool packet_failed:1;
         bool packet_truncated:1;
         bool packet_bad_opt:1;
         bool packet_rrsig_missing:1;
@@ -113,10 +112,10 @@ void dns_server_move_back_and_unmark(DnsServer *s);
 
 void dns_server_packet_received(DnsServer *s, int protocol, DnsServerFeatureLevel level, usec_t rtt, size_t size);
 void dns_server_packet_lost(DnsServer *s, int protocol, DnsServerFeatureLevel level, usec_t usec);
-void dns_server_packet_failed(DnsServer *s, DnsServerFeatureLevel level);
 void dns_server_packet_truncated(DnsServer *s, DnsServerFeatureLevel level);
 void dns_server_packet_rrsig_missing(DnsServer *s, DnsServerFeatureLevel level);
 void dns_server_packet_bad_opt(DnsServer *s, DnsServerFeatureLevel level);
+void dns_server_packet_rcode_downgrade(DnsServer *s, DnsServerFeatureLevel level);
 
 DnsServerFeatureLevel dns_server_possible_feature_level(DnsServer *s);