resolved: fallback to TCP if UDP fails

This is inspired by the logic in BIND [0], follow-up patches
will implement the reset of that scheme.

If we get a server error back, or if after several attempts we don't
get a reply at all, we switch from UDP to TCP for the given
server for the current and all subsequent requests. However, if
we ever successfully received a reply over UDP, we never fall
back to TCP, and once a grace-period has passed, we try to upgrade
again to using UDP. The grace-period starts off at five minutes
after the current feature level was verified and then grows
exponentially to six hours. This is to mitigate problems due
to temporary lack of network connectivity, but at the same time
avoid flooding the network with retries when the feature attempted
feature level genuinely does not work.

Note that UDP is likely much more commonly supported than TCP,
but depending on the path between the client and the server, we
may have more luck with TCP in case something is wrong. We really
do prefer UDP though, as that is much more lightweight, that is
why TCP is only the last resort.

[0]: <https://kb.isc.org/article/AA-01219/0/Refinements-to-EDNS-fallback-behavior-can-cause-different-outcomes-in-Recursive-Servers.html>

1 files changed, 23 insertions, 3 deletions

diff --git a/src/resolve/resolved-dns-server.h b/src/resolve/resolved-dns-server.h
index 3a78d4a3b5..f82b14e8fb 100644
--- a/src/resolve/resolved-dns-server.h
+++ b/src/resolve/resolved-dns-server.h
@@ -31,8 +31,21 @@ typedef enum DnsServerType {
         DNS_SERVER_LINK,
 } DnsServerType;
 
-#include "resolved-manager.h"
+typedef enum DnsServerFeatureLevel {
+        DNS_SERVER_FEATURE_LEVEL_TCP,
+        DNS_SERVER_FEATURE_LEVEL_UDP,
+        _DNS_SERVER_FEATURE_LEVEL_MAX,
+        _DNS_SERVER_FEATURE_LEVEL_INVALID = -1
+} DnsServerFeatureLevel;
+
+#define DNS_SERVER_FEATURE_LEVEL_WORST 0
+#define DNS_SERVER_FEATURE_LEVEL_BEST (_DNS_SERVER_FEATURE_LEVEL_MAX - 1)
+
+const char* dns_server_feature_level_to_string(int i) _const_;
+int dns_server_feature_level_from_string(const char *s) _pure_;
+
 #include "resolved-link.h"
+#include "resolved-manager.h"
 
 struct DnsServer {
         Manager *manager;
@@ -49,6 +62,11 @@ struct DnsServer {
         usec_t max_rtt;
 
         bool marked:1;
+        DnsServerFeatureLevel verified_features;
+        DnsServerFeatureLevel possible_features;
+        unsigned n_failed_attempts;
+        usec_t verified_usec;
+        usec_t features_grace_period_usec;
 
         /* If linked is set, then this server appears in the servers linked list */
         bool linked:1;
@@ -69,8 +87,8 @@ DnsServer* dns_server_unref(DnsServer *s);
 void dns_server_unlink(DnsServer *s);
 void dns_server_move_back_and_unmark(DnsServer *s);
 
-void dns_server_packet_received(DnsServer *s, usec_t rtt);
-void dns_server_packet_lost(DnsServer *s, usec_t usec);
+void dns_server_packet_received(DnsServer *s, DnsServerFeatureLevel features, usec_t rtt);
+void dns_server_packet_lost(DnsServer *s, DnsServerFeatureLevel features, usec_t usec);
 
 DnsServer *dns_server_find(DnsServer *first, int family, const union in_addr_union *in_addr);
 
@@ -86,4 +104,6 @@ void manager_next_dns_server(Manager *m);
 
 DEFINE_TRIVIAL_CLEANUP_FUNC(DnsServer*, dns_server_unref);
 
+DnsServerFeatureLevel dns_server_possible_features(DnsServer *s);
+
 extern const struct hash_ops dns_server_hash_ops;